Department of Medical Physics

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Patient Setup Quality Assurance In External Beam Radiotherapy Using A Third-Party Treatment Setup Verification Software
(University Of Ghana, 2021-12) Schandorf, M.T.
This study sought to assess the performance of a MATLAB algorithm (CORRO) as a quality assurance tool for the verification of treatment setup based on using megavoltage (MV) images from an Electronic Portal Imaging Device (EPID), and the treatment planning system's digitally reconstructed radiographs (DRRs) obtained from patients' three-dimensional (3D) computed tomography (CT) data sets. The individual treatment planning system's DRR was used as a reference radiograph to detect patient setup errors during radiotherapy treatment via the superimposition of the EPID Image on the DRR. Matching points on both images were placed on identical landmarks to facilitate the process. MV and kV (DRR) image pairs of thirty (30) anonymised patients who had received radiotherapy treatment on the Varian Medical System Clinac IX medical linear accelerator at the Komfo Anokye Teaching Hospital patients were employed in the study. Another Matlab algorithm (ASEMPA) was used to load the Anterior-Posterior (AP) and lateral (Lat) MV and kV image pairs for each patient at the same time, and the matching landmark spots on the image pairs were selected. The shifts' root mean square deviation was computed. The systematic error range for Prostate cases was determined to be 0.46 cm – 18.62 cm, while that for Head and Neck cases was found to be 1.57 cm – 11.56 cm. This revealed significant variances. These variables aided in calculating the setup variance and, as a result, the facility's setup accuracy.
Image Quality And Lesion Detection In Mammography: A Comparative Study Between Full-Field Digital Mammography And Computed Radiography Digital Mammography
(University Of Ghana, 2022-12) Bediako, D.
Image quality and lesion detection abilities are primary to accurate diagnosis in medical imaging; hence this study was aimed at examining the image quality and lesion detection abilities in Full-Field digital mammography and Computed Radiography digital mammography using the American College of Radiology Mammography Accreditation Phantom (ACR-MAP). Pre-exposure and exposure tests were conducted to establish the effective performance of the mammography systems used. DICOM images were obtained of the ACR-MAP at varying values mAs and kVp. Qualitative image quality assessment was made using the internationally recommended protocol for detection scoring. Quantitative image quality was also estimated using the ImageJ software and the Albert Rose Model to analyze image quality with reference to the Human Health Series numbers 2 and 17 of the International Atomic Energy Agency (HHS - IAEA). Results of the pre-exposure and exposure texts showed optimal and satisfactory performance of four of the systems. The half value layer result of the system D within the CRDM systems was below the recommended limit; hence the poor quality and detection exhibited by the machine of that facility. The obtained signal-to-noise ratio (SNR) and spatial resolution results indicated standard quality images were achievable at the 20 mm and 45 mm thicknesses within both systems but poor-quality images at 70 mm. Signal-to-noise ratio and spatial resolution decreased with increasing PMMA thickness. SNR was 16 % more in FFDM than that of CRDM, whiles the spatial resolution was 0.5 lp/mm, 1.0 lp/mm and 0.5 lp/mm more in in the FFDM systems compared to the CRDM systems within the respective PMMA thicknesses, indicating adequate quality within FFDM. Both FFDM and CRDM systems produced quality images proportional to increasing the detectability as the technique factors (kVp and mAs) increased with the ACR MAP, with the FFDM system’s average percent visibility at 89.05 % and that of CRDM at 75.00 %. The FFDM proved superiority in image quality and lesion detection over the CRDM.
Dosimetric Study Of Image-Guided Brachytherapy For Cervix Carcinoma Treatment At Komfo Anokye Teaching Hospital
(University of Ghana, 2021-12) Acquah, A.N
The study was conducted purposely to compare the dosimetric characteristics of 2D to 3D LDR; to compare their planning procedures, imaging, and dose distributions in both treatment modalities to provide recommendations for an easy transition from 2D LDR to 3D IGBT at KATH. The lack of spatial information on the tumour and OAR volumes and the inability to visualize the extent of the tumour and applicators in 3D results in suboptimal application technique planning with insufficient dose coverage for large tumours, which is a major limitation of brachytherapy treatment planning at KATH. Perspex and water were used in the construction of a water phantom for data collection of certain parameters such as air kerma strength of the sources and dose distribution across the phantom. Both 2D and 3D imaging modalities, X-ray and CT, were utilized and compared, including treatment planning procedures, to develop measurable data on the differences, advantages, and demerits of 2D LDR and 3D IGBT. To determine dosage distribution, cross-calibration measurement was performed to get the correction factor for the ion chamber used for the data collection, which was unavailable. The values obtained for KQQo, KT,P, KPOL, KSAT for cobalt were 8.892, 1.030, 0.975, and 8.854, respectively and the correction factor (DA, Q) for PTW 31010 was 3.240+0.06 x 106 Gy/C. Furthermore, the dose distribution in 2D is point-based and hence volumetric dose distribution is not considered as done in 3D forrmat, which implies that there may be doses beyond the 2 cm margin which are not accounted for during the treatment planning. This and poor imaging in 2D to visualize the crossing of the urethra artery and ureter verifies that point A is not a necessity in 3D LDR IGBT. 2D LDR may have come with some benefits. With the successful construction of the phantom, collection, and analysis of dosimetric parameters, it can be concluded that Image-guided brachytherapy is observed to be quite advantageous in a radioactive source and organ localization, treatment planning, safety, and even cost.
Clinical Implementation of Lung Stereotactic Body Radiation Therapy (SBRT)
(University of Ghana, 2021-12) Quaye, A.N.M.
The stereotactic Body Radiation Therapy (SBRT) technique was developed to treat lung cancers at the Komfo Anokye Teaching Hospital without a four-dimensional computed tomography (4-D CT) scanner to account for motion in treatment planning and delivery. A tank-type water lung phantom was designed and developed from locally available materials, including wood, perspex and acrylic filled roll-on ball implanted in the wood to simulate the lung field, body, and tumour. A motion platform was developed to mimic breathing. The lung phantom underwent a standard and a slow CT scan to account for motion (0.5 cm and 1 cm). Radiation treatment plans were generated according to the Radiation Therapy Oncology Group (RTOG) 0915 protocol for lung SBRT and delivered with a Varian Clinac iX linear accelerator to the phantom for dosimetric verification with a dose prescription of 50 Gy in five fractions. Clinical tumour sizes of 1 cm and 3 cm were used as case studies to demonstrate the dose to organs at risk (OARs). Average measured doses were compared to treatment planning calculated doses for the phantom with and without motion. Dose deviation for treating static mimicked tumour was 8.43%, -4.31% for phantom in 0.5 cm motion, and -25.96% for phantom in 1 cm motion. Phantom dosimetric analysis for PTV V100%, V99%, V90% at 50 Gy/5 without motion were 86.77%, 91.19% and 99.96%; with 0.5 cm motion were 98.60%, 99.61% and 100%; and with 1 cm motion were 72.72%, 90.47% and 99.65%, respectively. Clinical analysis for PTV V100%, V99%, V90% at 48 Gy/5 were 101.10%, 102.62% and 100.49%, respectively. For a tumour size of 3 cm at 50 Gy/5 were 98.10%, 99.68% and 100%, respectively; and for 1 cm tumour size were 97.36%, 99.19% and 100 %, respectively. SBRT treatment planning and delivery were feasible without 4-D CT from phantom studies at the study site. The statistics propose that the target was sufficiently covered with the dose for all scenarios, thereby achieving the primary radiotherapy goal without excessively dosing OARs to minimise toxicity.
Computed Tomography Protocol Optimisation for Peadiatric Head Trauma: Radiation Dose and Image Quality Assessment
(University of Ghana, 2020-10) Anaafi, E.
Objectives: To optimize the radiation dose and assess the image quality for a set of protocols, by evaluating noise, contrast to noise ratio, modulation transfer function and noise power spectrum. Statement of Problem and Justification: Children are sometimes examined with Computed Tomography protocols designed for adults, leading to radiation dose which is higher than necessary. And this can result in radiation induced cancer. Lack of optimisation could lead to image quality higher than what is needed for diagnostic outcome or value of the procedure with associated high dose to patient. Optimising the protocols for paediatric head trauma CT imaging will reduce radiation dose. Methodology: Somaton Sensation 64 was used to scan the head of an anthropomorphic phantom with a set of protocols. ImageJ software was used to analyse the paediatric head image from the scanner. IMPACTSCAN dosimeter software was used to evaluate the radiation dose to the various organs in the head. MATLAB was used to analyse the Modulation Transfer Function and the Noise Power Spectrum. Results: The estimated Computed Tomography Dose Index volume (CTDIvol) increased with increasing tube current and tube voltage. The high pitch 0.9 gave a lower dose than the 0.5 pitch. Eye lens received the highest radiation dose (39.2 mGy) whiles the thyroid received the least radiation dose (13.7 mGy). There was an increase in noise (62.46) when the H60 kernel was used and a lower noise (8.829) was noticed when the H30 kernel was used. Conclusion: The results obtained in this work showed that the H30 kernel (smooth kernel) gave higher values for noise and CNR than the H60 (sharp kernel) which values were also high for the MTF and N
Verification of Patient Specific Quality Assurance System for Volumetric Modulated Arc Therapy (VMAT)
(University of Ghana, 2020-10) Anokye, A.K.
Background: Volumetric Modulated Arc Therapy (VMAT) plans and its treatment are complex modalities and its clinical implementation requires very accurate acceptance testing and a very comprehensive quality assurance program. Failure in their periodic applications can result in errors in treatment delivery. There is therefore the need to compare the calculated dose distribution to the measured dose distribution of treatment plans to ensure accuracy, efficiency and minimize errors in planned radiation dose delivery to the patient. Methods: 15 patients with pathological pelvic lymph node metastasis were included in this study. Measurements were done using both conventional 6 MV beam with flattening filter and flattening filter free beam (FFF) for all 30 VMAT plans. Each patient thus had a 6 MV plan and an FFF plan. The treatment plans were made using two arcs and then delivered to the delta4 phantom. All 30 VMAT plans were replicated on the Delta4 phantom using three Elekta linear accelerators. Results: After machine optimization, there was an increase in the mean total gamma pass rate for the 6 MV plans from 98.7 to 99.9% and the FFF plans also had an increase in the gamma pass rate from 91.7% to 98.4%. There was significant statistical difference between the pass rates of the 6 MV plans and the FFF plans (p = 0.000488). The total monitor units (MUs) for the FFF plans were significantly greater than the 6 MV plans (p = 6.1x10-5). Conclusion: 6 MV VMAT plans with conventional flattened beams are delivered more accurately and hence more beneficial compared to flattening filter free (FFF) VMAT plans for external radiation of cervical cancer with affected pelvic lymph nodes.
Design and Construction of Heterogeneity Phantom for Comparison of Dose Calculation Algorithms of Treatment Planning Systems
(University of Ghana, 2019-07) Allin, P.
Verification of the output of a treatment planning system (TPS) is considered to be part of quality assurance (QA) requirement for the TPS and need to be performed during commissioning and annually, but this is not given the needed priority due to non-availability of appropriate phantoms to facilitate the process. Phantoms with heterogeneities that can simulate clinical situations are needed to assess the performance of TPSs. This research work sought to design and construct such a phantom from locally available materials to be used as QA/quality control (QC) tool for radiotherapy treatment planning systems. The constructed phantom could be configured to represent tissue-lung-tissue or tissue-bone-tissue anatomical regions. The phantom was composed of Polymethylmethacrylate (PMMA) slabs, slabs of wood and a Perspex compartment completely filled with Portland cement, which were used to mimic tissue, lung and bone, respectively. The Perspex slabs, the slabs of wood and the Perspex compartment had the following dimensions (length width thickness): 30 cm 30 cm 1 cm, 30 cm 30 cm 2 cm and 30 cm 30 cm 5 cm, respectively. A Perspex slab with dimensions (length width thickness): 30 cm 30 cm 2 cm with a hole to accommodate a 0.125 cc PTW Semiflex ionization chamber on one of its sides was also constructed to be part of the phantom. One of the slabs of wood also had a hole on one of its sides similar to that of the Perspex slab. The phantom was formed by stacking the various constituent components together to give the desired configuration. After construction, the phantom was used to assess the performance of three TPSs using different dose calculation algorithms. The phantom was scanned in three different configurations with Toshiba-Aquilion One CT-scanner and the images were loaded onto a CD-ROM. The images were later transferred to a Prowess Panther (version 4.6), Oncentra MasterPlan (version 4.3), and Varian Eclipse (version 13.6) TPSs via available DICOM import tools. Treatment plans were created for the different configurations of the phantom per TPS using the treatment machine in use with the TPS for patient treatment planning. The same treatment plans were created for each of the TPSs. For each TPS, dose distributions were calculated with all available dose calculation algorithms. The TPSs considered used Convolution-Superposition (CS), Collapsed Cone Convolution (CCC), Pencil Beam (PB) and the Analytical Anisotropic Algorithm (AAA) algorithms. The created treatment plans were replicated on the respective treatment machines and doses at calculation points placed within the phantom during the treatment planning processes measured with a 0.125 cc PTW Semiflex ionization chamber. The measured doses were compared to their calculated counterparts by the TPSs, and the differences in the doses expressed as a percentage of the respective measured doses. The results of the dose comparison showed that for lung region, the percentage difference between the measured and the calculated doses ranged from: -0.27 to 4.64% (mean of 1.87±1.72%), -10.22 to 9.24% (mean of 4.33±3.81%), -9.13 to 8.39% (mean of 5.83±2.94%) and -3.83 to 9.28% (mean of 3.09±2.83%) for doses calculated with AAA, CCC, PB, and CS, respectively. And for the bony region, the percentage difference between the measured and the calculated doses ranged from: -1.52 to 4.64% (mean of 1.83±1.52%), -13.61 to 4.39% (mean of 5.85±5.20%), -14.52 to 4.81% (mean of 5.97±5.63%) and -10.55 to -3.48% (mean of 5.91±3.08%) for doses calculated with AAA, CCC, PB, and CS, respectively. For dose calculation points placed within high dose gradient regions, the dose calculation algorithms gave discrepancies in dose greater than 80% with the exception of AAA, which gave percentage difference in dose close to 20%. The constructed phantom had provided a cost effective way of accessing the outputs of dose calculation algorithms of TPSs. Using the phantom it was shown that AAA was the most accurate dose calculation algorithm among those considered in the study. The use of the constructed phantom for clinical practice is recommended.
Design and Construction of a Source Holder for the Determination of Source Strength of Iodine-125 Low Dose Rate Brachytherapy Sources.
(University of Ghana, 2019-07) Tuyizere, S.
Iodine-125 brachytherapy (BT) sources have gained a wide application in the treatment of prostate and intraocular cancers. Determination of the source strength of I-125 seeds is, therefore, mandatory to ensure accurate and optimal dose delivery to a patient undergoing BT treatment. There is the need to verify source strength stated or provided by a manufacturer of any BT source prior to clinical use of the source in fulfillment of American Association of Physicists in Medicine and European Society for Therapeutic Radiology and Oncology (AAPM-ESTRO) recommendations for low energy BT sources. For low energy source, this process is sometimes ignored due to unavailability of appropriate equipment to facilitate the measurement of the source strength under clinical conditions. The present study sought to find ways of measuring source strengths of I-125 BT seed sources (model number STM1251) in use at Korle-Bu Teaching Hospital (KBTH) for permanent low dose rate (LDR) prostate BT with a well-type ionization dosimetry system, which is dedicated to high dose rate (HDR) BT source calibration. The BT seed implants are manufactured by Bard BT Inc., and are loaded in a Mick® cartridge which has maximum loading capacity of 15 seeds. To achieve this, a source holder which could be accommodated within the well-type ionization chamber (WTIC) was designed and constructed from Perspex (PMMA). The constructed source holder was designed to hold a whole cartridge of I-125 BT seeds within the WTIC to facilitate batch source strength measurement. The source holder ensuring reproducibility of I-125 seeds placements during source calibration, also enabled placement of a cartridge such that the point of concentration of activity of I-125 seeds coincided with the most sensitive part of the WTIC. Calibration coefficient was established for the dosimeter system (including source holder) through correlation and regression analysis of source strength measurements performed with the dosimeter system which were compared to source strengths with national institute of standards and technology (NIST) traceability provided by manufacturer of the I-125 seed sources. The obtained calibration coefficient was validated by performing I-125 seed source calibrations with the dosimetry system and comparing the source strengths with their counterparts stated by manufacturer of the I-125 seed sources. The dosimeter system was found to have calibration coefficient of 3.372 ± 0.200 ×1011U/A for the I-125 seed sources. The mean percentage deviation of the source strengths was found to be 2.588% which is in agreement with AAPM-ESTRO recommendations. The constructed low cost source holder is, therefore, recommended to be used for Bard I-125 seed source calibration with a HDR 1000 plus WTIC. This source calibration approach comes in handy for BT departments with limited resources.
Comparison of Methods of μ-Map Generation: MR-Based Method in Pet/MR Imaging Versus Pseudo-CT Method in Radiotherapy Dose Planning
(University of Ghana, 2019-07) Acquah, I.K.
The main objective of this study was to compare the similarities and differences in MR-based and pseudo CT μ-map generation in PET/MR and radiotherapy respectively. The study sought to look at ways attenuation correction could be done on PET hybrid imaging systems and compare them in various ways so as to improve treatment and patient care. The methodology involved patient selection, image processing and generation of both μ-maps for PET-attenuation correction and pseudo CT for radiotherapy dose planning from computational software packages. MR-based μ-maps were successfully been generated and could be used to correct for attenuation in PET. Pseudo CTs were also successfully generated and could be used in radiotherapy for dose planning. For head images, it was observed that MR- based μ-maps over-estimated the bone as compared to CT μ-map for two patients, with deviations of 4.0% and 4.2%. Also, for head images, it was observed that the μ-values of bone for both MR-based and CT μ-maps were dynamic and had continuous values. For the pelvis, it was found that the pseudo-CT underestimated the bone volume as compared to CT μ-map in five patients, with deviations of 18.7%, 21.3%, 9.6%, 14% and 10%. It was observed that the μ-values of bone for pseudo-CT were not dynamic and also did not have continuous values as compared to CT μ-map. It was also observed from the pelvis studies that the μ-values for muscle were more dynamic and had a greater range in CT μ-map than the pseudo CT and MRI μ-map.
Comparison of Methods of µ-Map Generation: MR-Based Method in PET/MR Imaging Versus Pseudo-CT Method in Radiotherapy Dose Planning
(University of Ghana, 2019-07) Acquah, I.K.
The main objective of this study was to compare the similarities and differences in MR-based and pseudo CT μ-map generation in PET/MR and radiotherapy respectively. The study sought to look at ways attenuation correction could be done on PET hybrid imaging systems and compare them in various ways so as to improve treatment and patient care. The methodology involved patient selection, image processing and generation of both μ-maps for PET-attenuation correction and pseudo CT for radiotherapy dose planning from computational software packages. MR-based μ-maps were successfully been generated and could be used to correct for attenuation in PET. Pseudo CTs were also successfully generated and could be used in radiotherapy for dose planning. For head images, it was observed that MR- based μ-maps over-estimated the bone as compared to CT μ-map for two patients, with deviations of 4.0% and 4.2%. Also, for head images, it was observed that the μ-values of bone for both MR-based and CT μ-maps were dynamic and had continuous values. For the pelvis, it was found that the pseudo-CT underestimated the bone volume as compared to CT μ-map in five patients, with deviations of 18.7%, 21.3%, 9.6%, 14% and 10%. It was observed that the μ-values of bone for pseudo-CT were not dynamic and also did not have continuous values as compared to CT μ-map. It was also observed from the pelvis studies that the μ-values for muscle were more dynamic and had a greater range in CT μ-map than the pseudo CT and MRI μ-map.
Comparison of Relative Dose Factors of a 6 MV Beam Measured With Diode, Radiochromic Film and Ionization Chambers in Different Orientations
(University of Ghana, 2019-10) Makuraza, J.
In this research study the output factors of a 6 MV beam were measured by using Razor diode, Gafchromic EBT-3 film and small volume ionization chambers 0.13 cc ionization chamber, 0.01 cc Razor ionization chamber from IBA –dosimetry, BahnhofstraBe, Germany. Both perpendicular and parallel orientations of ionization chambers relative to the propagation of radiation beam were considered. Varian Unique medical system Linac was used to deliver a 6 MV beam. Detectors were placed at the stage attached to the water phantom full of water. The measurements of output factor (OF) were conducted at different clinical set ups for source to surface distances of 100 and 90 cm at depths of dmax and 10 cm, respectively. From the results, relative errors (%) calculated on field size of 1 × 1 cm2 between output factors of 0.13 cc chamber and EBT-3 film when the chamber was positioned parallelly with respect to the beam central axis were -2.23 and 0.57%, and -19.68 and -9.40% at SSD = 90 cm and SSD = 100 cm respectively when the chamber was set perpendicularly relative to the beam central axis. Relative errors (%) calculated on field size of 1 × 1 cm2 between OFs of Razor chamber and EBT-3 film when the chamber was positioned parallel with respect to the beam central axis were 8.45 and 12.53%, and -3.29 and 4.12% at SSD = 90 cm and at SSD = 100 cm respectively when the chamber was set perpendicularly relative to the beam central axis. Relative errors (%) were also calculated between 0.13cc chamber and Razor diode, of which the findings were -7.75 and -9.97% at SSD = 90 cm and at SSD = 100 cm when the chamber was set parallel to the central beam axis. The study has shown that 0.13 cc ionization chamber could be a good choice for measurement of output factor from small to large field sizes when the chamber was positioned parallel with respect to the beam central axis.
Comparison of Treatment Indices between Telecobalt Machine and Linear Accelerator-Based Treatment Plans for Selected Conformal Radiotherapy Cases.
(University Of Ghana, 2018-07) Kamfosi, A.M.
The use of telecobalt machine in radiotherapy is of concern in developing countries where there is a limited resource. As such, the study was to ascertain if telecobalt (cobalt-60) machine could be feasible to generate and deliver treatment plans with optimal treatment indices comparable to those of a linear accelerator (Linac). Retrospective DICOM-Radiotherapy images of patients earmarked for treatment of breast, prostate and lung cancer obtained from the European Society for Radiotherapy and Oncology (ESTRO) were uploaded unto the treatment planning system (TPS) used by National Center for Radiotherapy and Nuclear Medicine (NCRNM) and Sweden Ghana Medical Center (SGMC Cancer Center). Based on departmental protocol, international dose constraints, careful selection and adjustments of beam parameters, treatment plans were generated and delivered. Thereafter, treatment indices were calculated and compared. The plans were also subjected to quality assurance to determine their adequacy for clinical implementation. The results of conformity index (CI) were: breast (0.9705, 0.9303), prostate (0.9881, 0.9948), lung (0.9604, 1.0005) and for homogeneity index (HI) were: breast (1.1452, 1.0910), prostate (1.0226, 1.0141), lung (1.0960, 1.0682) between cobalt-60-based and linac- based treatment plans respectively. Analysis of the treatment indices has shown telecobalt dose conformities and homogeneities to be comparable to those of linac-based despite having higher doses to organ at risk. The end to end test showed a deviation of 1.5% and 0.83% with a tolerance of ±2% between cobalt-60 machine and linac while verification of planned and delivered dose in anthropomorphic phantom showed a deviation of 0.5% and 0% with a tolerance of ±1 % between cobalt-60-based plan and linac-based plan respectively. Hence, both machines were performing according to the calibrated dataset within the tolerance level. Therefore, in situations of inadequate financial capacity and maintenance associated with linear accelerators, the use of telecobalt machines should be encouraged for the benefit of the patients.
Inter-Institutional Dosimetric Comparison for Radiotherapy Centres in Ghana Using Cirs Thorax Phantom
(University of Ghana, 2017-07) Mkanda, O.M.
Quality assurance (QA) in all aspects of radiotherapy reduces the number of instances of accidental dose misadministration. Reduction of such errors and uncertainties plays an important role in the outcome of radiotherapy. As part of the audit supported by the IAEA in Ghana this study was designed to review the whole chain of activities in 3D conformal radiotherapy (3D-CRT) workflow, from patient data acquisition to treatment planning and dose delivery.The study focused on dosimetry part of the treatment planning and delivery processes. The audit was conducted in three (3) radiotherapy centers in Ghana using CIRS anthtropomophic thorax phantom. The phantom was scanned with a computed tomography unit (CT). Treatment plans for seven (7) different test cases involving various beam configurations based on TECDOC 1583 were prepared on local treatment planning systems (TPSs) by local medical physicist. The phantom was irradiated following the treatment plans and absorbed dose to water in specific points of the phantom were determined using TRS 398 protocal with an ion chamber connected to an electrometer. The percentage deviations between the measured and calculated doses in specific measuring points were computed and compared to their corresponding predefined agreement criteria. 37 photon beams with energies 1.25 MV and 6 MV were studied and two calculation algorithms (Collapsed Cone and Superposition/Convolution algorithms) were investigated. Most deviation (42%) were discoverd in the lung equivalent material. A maximum deviations of 52.9% at centre 2 and -8.9% at centre 1 were observed for Convolution/Superposition and Collasped Cone algorithms respectively in test case 4, measuring point 6.
Evaluation of Surface Doses and Effect of Air Gaps Under Bolus during External Photon Beam Radiotherapy
(University of Ghana, 2018-07) Odartey, L.P.N.
This study was performed to evaluate surface doses and the effect of air gaps under bolus on surface dose during external beam radiotherapy for photon beam energies of 6 MV and 15 MV. Surface doses were measured by using calibrated GafChromic EBT3 films and the Roos chamber for various bolus-to-surface distances (including no bolus and bolus placed directly on the RW3 phantom surface) and various field sizes. Dose verification was also done for beam energies of 6 MV and 15 MV on the RANDO phantom by measuring doses at dmax using a diode. The introduction of the bolus increased the surface dose for all field sizes and beam energies. For 5×5 cm2 the surface dose increased by 61% and 72% with the introduction of the bolus for 6 MV and 15 MV respectively using the Roos chamber. For both 6 MV and 15 MV, surface doses increased significantly with the introduction of the bolus. Also, due to more skin sparing in 15 MV than 6 MV, surface doses recorded for the 6 MV photons were generally of higher value than those for the 15 MV photons especially for field sizes that were smaller. As the bolus-to-surface distance increased, the doses recorded at the surface decreased. This decrease however was seen for the small field sizes (5×5 cm2 and 10×10 cm2). For field sizes that were larger (15×15 cm2 and 20×20 cm2) the change in dose to the surface as a result of change in air gap was negligible. For 5×5 cm2, dose to the surface decreased by 21% and 33% with a 5 cm air gap for 6 MV and 15 MV respectively compared to 20×20 cm2 field, where the change was negligible with only a 2% and 1% increase in surface dose recorded for both 6 MV and 15 MV respectively when the ion chamber was used. The field size of 5×5 cm2 showed the most variation of surface dose with changing air gaps showing that change in skin dose as a result of air gaps is only significant for small field sizes.
Patient Dose Assessment and Optimization of Pelvic Radiography with Digital X-Ray Systems
(University Of Ghana, 2018-07) Abbeyquaye, D.
Low doses of ionizing radiation are delivered to the area of clinical interest in projection radiography. However, the application of digital radiography systems (DRS) is characterized by higher doses than film-screen radiography. Radiation exposure of the pelvis is of primary concern because of the radiosensitive reproductive organs in the pelvic region. For the radiation protection of patients, dose monitoring is essential in pelvic radiography and whenever possible, dose and image quality must be optimized to ensure safety of patients. The aim of this study was to assess the entrance surface dose (ESD) and effective dose (ED) to patients undergoing pelvic X-ray examinations with computed radiography (CR) systems, and develop dose and image quality optimization strategies with the aid of an anthropomorphic phantom. Hence, determine the optimum exposure factors for AP pelvic examinations. This was to serve as basis for dose and image quality optimization in pelvic radiography. Patient and phantom radiographic image quality was assessed in terms of signal-to-noise ratio (SNR). The dose and image quality was assessed for 102 patients. Twenty-four different exposures with varying tube potential (kVp), tube current (mAs) and focus-to-detector distance (FDD) were made for the optimization studies. Results of patients‟ dose estimation showed that the 3rd quartile ESD of 2.56 0.60 mGy for pelvis AP radiography was lower than DRLs by at least 24% with ED of 0.27 0.07 mSv. The mean SNR of patients‟ radiographs was 8.5 2.2 which is 70% higher than the recommendation by the Rose model. After the phantom optimization process, the optimum exposure factors were 70 kVp, 12.5 mAs and 100 cm FDD giving ESD and SNR of 1.09 mGy and 16.23 respectively. It was found that the dose could be reduced further by using lower kVp settings than what is currently being used without degradation in image quality of the radiographs. The dose at the optimum exposure factors was reduced by 29.2% with 65 kVp, 12.5 mAs and 105 cm FDD. Thus the SNR of the image reduced by 19.6% to a SNR of 13.0404, which is still of optimum diagnostic quality.
Heat Transfer Analysis of a Proposed Fuel Assembly for Supercritical Water Reactors Using Star-Ccm+ Cfd Code
(University Of Ghana, 2018-07) Silas, C.
Accurate knowledge and understanding of heat transfer characteristics near and above critical conditions is crucial to the successful design of the SCWR. Many studies, both numerical and experimental assessments have been conducted on bare tubes and simple geometries like annuli. However, heat transfer performance studies of the fuel assemblies for SCWR are scarce. Thus, there is still a lack of understanding of heat transfer performance in the fuel assembly designs for SCWR. Moreover, the importance of this study cannot be over-emphasized as it will broaden and better the understanding of the concept of heat transfer in the rod bundle geometry by providing more numerical data. In this study a 3D CFD code STAR CCM+ was used in assessing the performance of heat transfer in the square fuel assembly of a High Performance Light Water Reactor (HPLWR). Utilizing the computational environment and the flexibility of STAR CCM+ code, test analysis was conducted using the four turbulence models, namely AKN low-Re, Standard Lien’s low-Re, Standard Wilcox κ-ω and SST κ-ω to choose a suitable turbulence model yielding satisfying prediction (capabilities) in describing the heat transfer and flow of supercritical water in the fuel assembly “near”, at and after the pseudo-critical region. The analysis was carried out at 25 MPa system operating pressure, mass flow rate of 0.167 kg/s (601.2 kg/h), 300 °C inlet temperature with uniform heat flux of 650 kW/m2. Turbulence sensitivity analysis was performed and SST κ-ω model was selected based on its simplicity and superiority to others especially with regard to numerical stability. Moreover, SST κ-ω model also does not involve damping functions, but allows simple Dirichlet specified boundary conditions Furthermore, using the SST κ-ω with low y+ wall treatment the selected heat transfer correlations were assessed. Overall, the Cheng et al. correlation provided the most satisfying prediction for the wall temperatures in all the sub-channels and captured closely Wataa’s Numerical data. This was followed by the McAdams correlation, but the Dyadyakin and Popov and the Petukhov correlations also yielded acceptable results. Test analysis results of the heat transfer correlation also confirmed the occurrence of heat transfer enhancement and heat transfer deterioration at the pseudo-critical point and after or beyond respectively. The maximum wall temperature was obtained in sub-channel 9, the hottest sub-channel and exceeded the design limit of 620 °C by 60 °C for the Cheng correlation while for the other correlations it was more. The difference in temperature between the hottest and coldest sub-channels was approximately 80 °C. Finally, parametric analysis was conducted in sub-channels 4 and 9 by varying mass flow rates 0.1670 kg/s (601.2 kg/h) and 0.1559 kg/s (561.2 kg/h), pressure 23 MPa and 25 MPa and with or without gravity. Results from this test analysis showed that mass flow rate, pressure and gravity have significant influence. It was observed that at low mass flow rate with varying pressure (23 MPa and 25 MPa), the temperatures significantly increased in the heat transfer deterioration region. Nevertheless, experimental investigations involving rod bundles adopted in this study should be conducted to validate the results obtained numerically and address the inconsistence of the conclusions drawn, “when compared with similar studies”. These experimental studies would also be helpful in validating similar numerical studies in future.
Exit Dose Measurement for Patient Treatment Dose Verification in Co-60 Teletherapy
(University of Ghana, 2018-07) Binadeo, A.A.
This work evaluates the possibility of creating an online patient dose verification system through exit dose measurement on a Co-60 teletherapy machine which lacks an electronic portal imaging device (EPID). Exit dose measurements were done by diode and Barracuda and compared to those of an ionization chamber. The diode was first calibrated and the necessary correction factors (depth and field size corrections) determined using a PMMA phantom. The transmission factor of the couch was also determined at gantry angles relevant to the study (0o and 180o) and at various field sizes. The estimated couch transmission factor was 0.96912. The exit dose measurements were done both with PMMA phantom and an anthropomorphic phantom by calculating a dose of 200 cGy to the various depths and corresponding field sizes. The detectors were placed 30 cm underneath the treatment couch for all the measurements. Readings from both detectors compared well with those of the ionization chamber. The percentage deviations were generally lower with the PMMA phantom than for the anthropomorphic phantom though they were all below 2%. The diode deviations range between 0.098 to -1.322 for measurements with PMMA phantom at a depth of 15 cm while those of the Barracuda were between 0.157 and -1.411 at the same conditions. Diode in vivo dosimetry was found to be suitable for the verification of patient dose through exit dose measurements. However the calibration and correction factors must be determined carefully in order to generate more accurate results.
Effects of Immobilization Devices on Skin Doses and Doses in the Build-Up Regions for High Energy Photon Beams
(University of Ghana, 2018-07) Djan, B.
This study presents the significant increase in skin dose caused by the presence of thermoplastic immobilizing masks used for positioning and immobilization of patients during cancer treatments in the head and neck region. Made from organic materials, thermoplastics become soft and pliable when heated and forms a rigid replication of the patient’s anatomy as it cools down. The use of the mask plays a key role in achieving the therapeutic aim in head and neck treatments but can as well lead to a rise in surface dose. The purpose of this research is to examine and analyze the effects of thermoplastic masks used as immobilization devices on skin and the build-up region doses for a 6 MV Three Dimensional Conformal Radiotherapy plan. Measurements in the solid water phantom coupled with an electrometer and a Roos type ionization chamber at the surface (skin), the dmax (1.6 cm) and a specified depth of 10 cm of the 6 MV photon beam were made and recorded employing the isocentric technique for known varying field sizes with 100 MU. Measurements with or without the mask were also made on the phantom. GafChromic films were also used to assess the skin dose and dmax and compared to the measured values. Surface dose increased significantly due to the presence of the thermoplastic masks. It was observed that the estimated surface doses with the presence of the thermoplastic mask were consistently larger than those without the mask. Due to the presence of the thermoplastic mask during measurements, the dose in the build-up region increased, subsequently shifting the depth of maximum dose, dmax, to shallower depths to the skin surface. For a field size of 10 x 10 cm2, an increase in skin dose of 0.21, 0.13 and 0.05 Gy was estimated for the unstretched mask, the 5 cm stretched mask and the 10 cm stretched mask respectively, which was ascribed to the “bolus” effect of the thermoplastic masks. For the various field sizes, the skin dose increased by an average factor less than 1% for the different mask stretches. However, a variation within 1% was recorded although the skin dose decreased as the mask was extended. The thermoplastic factor at the skin surface was also found to increase with an increasing mask thickness and for a particular mask type (unstretched, 5 cm stretched, or 10 cm stretched) the thermoplastic factor decreased with increasing field size. The discrepancies obtained using the GafChromic films varied significantly with those obtained using the ionization chamber which was associated with the heterogeneous composition of the film arising from the manufacturing process. The skin-sparing effect which is an advantage for megavoltage beams was not achieved due to the presence of the masks.
Design and Fabrication of a Turnable System for Dosimetry Investigation on a Cobalt-60 Tomotherapy Treatment System
(University of Ghana, 2018-07) Arthur, C.
The objective of this research is to ascertain if it is feasible to replicate tomotherapy treatment with telecobalt machine based on phantom studies. To replicate gantry motion around a patient during tomotherapy treatment, the phantom was placed on a fabricated turn-table capable of translational and rotational motions while the gantry of the telecobalt machine was kept constant. The treatment simulations were performed with Cirus cobalt 60 teletherapy machine at the Komfo Anokye Teaching Hospital. The fabricated turn-table was made of a metal steel connected to wiper motors of 22, 35 and 50 rpm to control the translational and rotational motion of water phantom. The Co-60 beam dosimetric characteristics such as beam flatness beam, symmetry beam profile, tissue air ratio, output factors and percentage depth dose were determined with tomotherapy procedure. Measurements were done at source -to -surface distance of 80cm for tomotherapy beam using a Farmer type ionization chamber connected to a UNIDOS electrometer and 30 x 30cm2 water phantom. Measurements were carried out using various field sizes ranged from 4x4 to 30x 30 cm2at depths of 0.5cm, 5cm, 15cm and 20cm. The percentage depth dose increased from the surface to a depth of maximum dose which is 0.5cm for cobalt and decreased as the depth increased It was found that as field size increased the output factor and tissues air ratio increased due to an increased in scatter which increased the dose and decreased as depth increased. The beam flatness and beam symmetry were found to be 0.60 %, and 1.19% (at depth 10cm and width of 3cm from the central axis; and 0.74% at depth 5cm and width of 2 cm from the central axis) which agreed with the International Electrotechnical commission range of 3% and 2% respectively. The fabricated turn table needs refinement in term of high rpm rotational motor to rotate fully-filled 30x30cm2 water phantom and potentiometer to regulate the speed of the rotational motor.
Assessment and Optimization of Occupational Radiation Doses in Fluoroscopy Guided Procedure at Korle - Bu Teaching Hospital, Accra, Ghana
(University of Ghana, 2018-07) Njantang, R.N
International Commission on Radiological Protection (ICRP) publication 85 of 2000 recommended the use of two dosimeters for the monitoring of staff performing fluoroscopically guided procedures. This study aimed to assess the dose to operators performing fluoroscopically guided procedures at Korle – Bu Teaching Hospital and evaluate possibility for dose reduction. Four Radiologists, the Interventional Radiologist and the assistant were monitored for a period of one month and four Cardiologists were monitored for two months. Two electronic dosimeters were used by each worker present in the room and the personal equivalent dose Hp (10) and Hp (0.07) were recorded after each procedure. The Kerma Area Product (KAP) and screening time were also recorded per procedure. The scattered radiation dose rate was measured using a water phantom at 1 m from the focal spot, 160 cm from the floor at 0°, 90°, 120° and 180° for 3 projection of the tube (RAO 30°, LAO 30° and AP 0°). The patient dose and screening time at Radiology Department (over couch tube) were [1.89 - 14.38] Gy.cm², (0.2 -2.5) min; [3.76 – 44.16] Gy.cm², (0.3 – 3.1) min and [8.7 – 60.77] Gy.cm², (1.9 – 10.9) min for Retrograde Urethrogram (RUG), Hysterosalpingography (HSG) and special cases respectively. The patient dose and screening time range at Cardiology (under couch tube) were [14.24 – 120.61] Gy.cm², (1.7 – 16.3) min, [42 – 237] Gy.cm², (9.70 – 43) min and [4.37 – 33.56] Gy.cm², (4.30 – 11.8) min for Coronary Angiogram (CA), Percutaneous Coronary Intervention (PCI) and Right Heart Catheterization (RHC) respectively. The dose range per procedure to Cardiologists was [0.1 – 42.15] μSv, [1.2 – 31.2] μSv and [0.1 – 2.75] μSv for the CA, PCI and RHC respectively. The range of the estimated monthly effective dose, and eye lens dose to Cardiologists and Radiologists were [0.01 – 0.07] mSv, (0.15, 0.30) mSv and [0.03 – 0.12] mSv, (0.53 – 3.39) mSv respectively. The interventional procedures were found to be lengthier with exposure time of (52 – 76.4 min), and delivered relatively high dose (47.13 – 412.23 Gy.cm²) to patients. The Interventional Radiologist and the assistant received an effective dose of 0.09 mSv, 0.03 mSv respectively and eye lens dose of 1.2 mSv, 0.33 mSv respectively. A weak but significant relationship (R² = 0.32, p-value < 0.05) was found to exist between patient dose and staff effective dose, meaning that staff dose is influence by other factors. Generally, the staff effective dose and eye lens dose to Cardiologists and Radiologists were below the acceptable limits (1.67 mSv/month) excerpt for one Radiologist whose eye lens dose exceeded the limit by a factor of 2. Therefore, the use of a ceiling suspended screen is highly recommended in radiology to reduce the eye lens dose. Monte Carlo simulation of the distribution of radiation in the room highlighted the safest position that can be occupied by the staff as dose reduction technique to reduce the dose. The Implementation of the proposed radiation safety programme is encouraged for optimization of protection in both departments.

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