Polish Journal of Medical Physics and Engineering December 2018 The Journal of Polish Society of Medical Physics Vol 24, Issue 4 ISSN 1898-0309, doi: 10.2478/pjmpe-2018-0024 Scientific Paper Implementation of compensator-based intensity modulated radiotherapy with a conventional telecobalt machine using missing tissue approach Samuel N. A. TAGOE1,2,a, Samuel Y. MENSAH2, John J. FLETCHER3 1National Centre for Radiotherapy and Nuclear Medicine, Korle Bu Teaching Hospital, Accra, Ghana 2Department of Physics, School of Physical Sciences, College of Agriculture and Natural Sciences, University of Cape Coast, Cape Coast, Ghana 3Department of Applied Physics, University for Development Studies, Navrongo Campus, Navrongo, Ghana. aE-mail address: s.tagoe@kbth.gov.gh (received 22 April 2018; revised 14 June 2018; accepted 7 August 2018) Abstract Objectives: The present study aimed to generate intensity-modulated beams with compensators for a conventional telecobalt machine, based on dose distributions generated with a treatment planning system (TPS) performing forward planning, and cannot directly simulate a compensator. Materials and Methods: The following materials were selected for compensator construction: Brass, Copper and Perspex (PMMA). Boluses with varying thicknesses across the surface of a tissue-equivalent phantom were used to achieve beam intensity modulations during treatment planning with the TPS. Beam data measured for specific treatment parameters in a full scatter water phantom with a 0.125 cc cylindrical ionization chamber, with a particular compensator material in the path of beams from the telecobalt machine, and that without the compensator but the heights of water above the detector adjusted to get the same detector readings as before, were used to develop and propose a semi- empirical equation for converting a bolus thickness to compensator material thickness, such that any point within the phantom would receive the planned dose. Once the dimensions of a compensator had been determined, the compensator was constructed using the cubic pile method. The treatment plans generated with the TPS were replicated on the telecobalt machine with a bolus within each beam represented with its corresponding compensator mounted on the accessory holder of the telecobalt machine. Results: Dose distributions measured in the tissue-equivalent phantom with calibrated Gafchromic EBT2 films for compensators constructed based on the proposed approach, were comparable to those of the TPS with deviation less than or equal to ± 3% (mean of 2.29 ± 0.61%) of the measured doses, with resultant confidence limit value of 3.21. Conclusion: The use of the proposed approach for clinical application is recommended, and could facilitate the generation of intensity-modulated beams with limited resources using the missing tissue approach rendering encouraging results. Key words: telecobalt machine; bolus; compensator; Gafchromic film; intensity modulated. Introduction minimized by moving the bolus distance of 15-20 cm from the skin while still achieving the purpose of the bolus [3]. By Dose distribution within a patient has been found to be the virtue of the position of the bolus and its objective, the bolus is most reliable and verifiable quantity that links treatment referred to as missing tissue compensator [3]. Owing to the parameters of any radiotherapy treatment technique to position of the compensator, it may be composed of any treatment outcome [1,2]. It is therefore imperative to choose material. irradiation geometries that will maximise radiation dose to the The aforementioned factors (skin topography and tissue tumour volume while concurrently minimizing doses to normal inhomogeneity) coupled with the often complex shapes of tissues in close proximity to tumour volume during external tumours will require or necessitate the modulation of the beam radiation therapy (EBRT) to achieve favourable fluence distribution across beams from conventional treatment outcome. The dose distribution is influenced very teletherapy machines. This has culminated in the introduction much by skin surface contour (body shape) and tissue of Intensity Modulated Radiotherapy (IMRT), where inhomogeneities. Influence of body shape on dose distribution extensively intensity modulated (IM) beams are used [2]. may be diminished by using individually designed bolus [3]. IMRT is becoming more widespread, and it is usually Use of a bolus leads to an undesirable skin doses, which can be 171 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 implemented using linear accelerator equipped with multileaf · ScanMaker 9800XL plus flatbed scanner (Microtek, USA) collimator (MLC) to facilitate the creation of variable fluence · Pack of EBT2 Gafchromic films (lot # 08221302; distributions within an irradiated region of a patient during International Specialty products, USA) EBRT. Desired dose distributions within the patient prior to · Prowess Panther treatment planning system (version 4.6; treatment delivery are simulated with a treatment planning Prowess Inc., USA) system (TPS) that can perform inverse planning or a forward Beam intensity modulations based on the proposed approach planning TPS with direct aperture optimization (DAO) were performed for an Equinox 100 cobalt 60 teletherapy software [4,5]. These treatment planning approaches help in the machine (Best Theratronics, Canada). The telecobalt machine realization of beam intensity maps to be replicated with the was manufactured in April, 2013. The treatment head of the sequential movements of the various leaves of the MLC system telecobalt machine is mounted isocentrically with source axial or by individual compensators each having varying thicknesses distance (SAD) of 100 cm. In the treatment head is contained a across a beam [2]. Realization of fluence distribution across double encapsulated cobalt 60 source having initial total source beams prior to treatment delivery is paramount to any IMRT activity of 399.0 TBq (measured on August 1, 2013, by source delivery technique (or generation of IM beams). Delivering manufacturer, Nordion Inc., Canada). This gives the IMRT with compensators is receiving renewed interest from teletherapy machine a reference beam output in water at the the radiotherapy community due its static nature and simplicity depth of maximum dose (0.5 cm) of 189.49 cGy/min (on [2]. With reference to the above, some vendors of treatment December 12, 2013), measured after installation of the planning systems are incorporating features to facilitate easy telecobalt machine, based on International Atomic Energy implementation of compensator-based IMRT; referred to as Agency (IAEA) technical series report (TRS) 398 protocol [8]. solid IMRT. The Pre-requirements for modern day IMRT are Percentage depth dose for the reference field size of 10 cm x capital intensity and may be out of reach of many developing 10 cm for a depth of 10 cm in water (PDD10), which is used as countries [6,7]. a beam quality specifier for megavoltage beams [9], is 58.36 % Alternative approach of generating IM beams with for the telecobalt machine. High activity encapsulated cobalt customized compensators for a radiotherapy facility using a 60 source within the treatment head of the teletherapy machine conventional telecobalt machine for EBRT and a forward has a diameter of 2 cm and length of 4 cm, and is classified as planning TPS, that cannot generate intensity maps of beams as C-146 teletherapy source capsules by Canadian Nuclear Safety well as simulate directly a compensator, is being present. In the Commission [10]. The source is embedded in a source drawer present study, boluses of varying thicknesses placed on the mechanism which uses a pneumatic system to bring the source surface of a tissue equivalent phantom are used to provide in and out of treatment position. The circular end of the source beam intensity modulations during treatment planning is towards the direction of propagation of beams from the (simulation) with the forward planning TPS. The generated telecobalt machine. Within the treatment head are treatment plans are replicated on the telecobalt machine with asymmetrical collimators that allow the jaws, which define the compensators composed of medium density materials placed shape of the beam to move independently of each other, certain distances from the phantom such that dose distributions providing more freedom in treatment planning. Attached to the within the phantom are the same as planned. Procedures collimator system is an accessory holder with block tray code adopted to obtain the physical dimensions of a compensator to interlock to prevent the use of a wrong accessory for treatment. provide desired dose distribution within a patient are also The distance of radiation source to the accessory holder (or presented. block tray) is 59.3 cm. The telecobalt machine is configured to have features of a modern medical linear accelerator with the Materials exception of a multileaf collimator system and an electronic portal imager. The field size that can be set on the machine The following equipment were used to facilitate the ranges from 1 x 1 to 43 x 43 cm2 (defined at the machine development of the proposed method for generating IM beams isocenter). The machine features a new Motorized Wedge for a telecobalt machine: (MW) system, which allows one to treat with any wedge angle · Blue Phantom2 three dimensional water phantom (IBA ranging from 0 to 60 degrees. This is made possible with a Dosimetry GmbH, Germany) fixed 60 degrees physical wedge permanently positioned in the · Small stationary water phantom (T41014; PTW-Freiburg, treatment head of the machine, which can be brought Germany) automatically in and out of the path of the radiation beam · Solid water phantom (T2967; PTW-Freiburg, Germany) during treatment delivery, such that combinations of time · 0.125 cc Semiflex cylindrical ionization chamber (TW31010; weighted beams with and without the wedge filter create the PTW-Freiburg, Germany) dosimetric effects of the required wedge. A picture of the · 0.60 cc Farmer type cylindrical ionization chamber (TW telecobalt machine showing setup for the acquisition of 30013; PTW-Freiburg, Germany) necessary beam data to facilitate the development of the · UNIDOS electrometer (T10002- 020427; PTW-Freiburg, proposed approach for beam intensity modulation is shown in Germany) Figure 1. 172 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 The following materials each in the form of both plates (or material. The mass attenuation coefficient,  of an absorber slabs; having dimensions of 22 cm x 22 cm) and cubic blocks, for a particular beam quality is defined as: having varying thicknesses, were selected for compensator  construction based on the proposed approach: Copper, Brass  = Eq. 1  and Perspex. Criteria for the selection of a material were as where  and  are the linear attenuation coefficient of the follows: the material should be of a medium density, non- absorber measured with the required beam quality and the corrosive, environmentally friend, easy to machine into density of the absorber respectively. required shape and must retain the shape once formed, must be readily available and of relatively low cost. The first criterion takes into consideration the energy of the beam to be modulated and the level of modulation required. Also, uncertainties in determining the required thickness of a compensator to apply would translate to relatively low discrepancies in the expected doses within a patient if a medium density material is used. The remaining criteria precinct on physical properties or characteristics of the compensator material. Methods Prior to the experimental measurements to facilitate the development of the proposed method, radiological properties relative to those of water of the various selected compensator materials were measured to ensure reproducibility of the proposed method. For each of the selected materials, linear attenuation coefficient was measured in air with beams from the telecobalt machine. Each beam had a field size of 10 cm x 10 cm, and source to detector distance (SDD) of 100 cm was used. The 0.125 cc ionization chamber having its build-up cap (PMMA, thickness of 3 mm; provided by the manufacturer) on was used for the linear attenuation coefficient measurements. For the measurement of linear attenuation coefficient of water, a specialized graduated water tank (see Figure 2) that could be mounted on the accessories holder of the teletherapy machine, to hold varying volumes of water in the path of beams from the Figure 1. Telecobalt machine with beam measuring equipment 2 telecobalt machine, was fabricated from 0.6 cm Perspex (Blue Phantom water phantom) in the path of beam. (acrylic) sheets. The thicknesses of the various compensator materials used for the measurements ranged from: 0 to 29.61 mm (increment of 3.29 mm), 0 to 18 mm (increment of 3 mm), and 0 to 72.00 mm (increment of 8 mm) for copper, brass and Perspex, respectively. The 22 cm x 22 cm compensator material plates (or slabs) were used for the attenuation measurements. For measurements with water, the height of water within the fabricated tank was adjusted from 0 to 12 cm (increment of 2 cm). Densities of the various compensator materials were also determined. For a specific compensator material, the density was calculated from the ratio of the mass (measured with a digital chemical balance) to the volume (determined from measured physical dimensions of the compensator material with digital caliper) of the compensator material. Since Compton interaction predominates at the megavoltage energy range in use for therapy, it would be prudent to express the linear attenuation coefficient of a Figure 2. Locally fabricated water tank mounted on the compensator material in terms of mass attenuation coefficient accessories holder of telecobalt machine during measurement of to remove the dependence of density of the compensator attenuation coefficient of water. 173 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 The relationship between the thickness, , of an applied bolus To account for beam divergence, a compensator sheet with grid on the surface of a tissue-equivalent phantom and that of the lines (beamlet) having dimensions of 1 cm x 1 cm was representing compensator material,  , placed at certain developed for recording bolus/compensator material thickness distance from the surface of the phantom along a particular ray across a radiation field. The compensator sheet had two bolded line (or beamlet), such that dose at any point within the broken lines running central to the sheet, which were used to phantom remains the same, was obtained using the semi- represent the major axes of a beam. The equivalent dimensions empirical formula proposed by Tagoe et al, which is given as of a grid on the sheet at the teletherapy machine isocenter was [1]: determined to facilitate the realization of the area covered by an applied bolus on the surface of the phantom per grid (or  =   Eq. 2 beamlet) relative to the isocentre. A compensator was where  is the appropriate thickness density ratio of a constructed by stacking and piling blocks (having dimensions particular compensator material relative to that of water of 1 cm x 1 cm) of the required compensator material to obtain measured for a specified reference conditions (field size of 10 compensator material thicknesses indicated on the compensator cm x 10 cm and treatment depth of 5 cm, employing isocentric sheet. The output of a constructed compensator was verified irradiation technique); and are correction factors with the Gafchromic EBT2 film. introduced to account for the effects of treatment depth and To facilitate the use of the Gafchromic EBT2 film as a field size respectively. The various terms in Equation 2 were dosimeter, strips of the film were irradiated to known doses obtained by measuring the output of the telecobalt machine in a (calculated from measured beam output of the telecobalt full scatter water phantom for specific field size and treatment machine) to obtain sensitometric curve for the film. Beam depth with a particular thickness of a compensator material in output calibration of the telecobalt machine to facilitate the the path of the beam, and then repeating the measurement usage of the radiochromic film for dosimetry was done with the without the compensator, but the height of water above the 0.6 cc Farmer type ionization chamber inserted into the detector adjusted to obtain the same detector reading as before stationary water phantom, using the IAEA TRS 398 protocol with the compensator. These measurements were repeated for [8]. During the output measurement, the 0.6 cc ionization various field sizes and treatment depths, and the readings chamber was connected to the UNIDOS electrometer. The obtained normalized to those of the reference conditions to equation of the line of best fit of the sensitometric curve was determine expressions for the correction factors introduced used for converting optical densities of film to doses. The through correlation analyses. The field size used ranged from overall accuracy of EBT2 film measurements was derived 3 cm x 3 cm to 35 cm x 35 cm, and that of treatment depth using the method proposed by van Battum et al. [15], that took ranged from 0.5 to 18 cm. Determination of expression for the into account the most pronounced sources of uncertainties in thickness density ratio, , was achieved by using varying dose determination (scanner, lateral correction, fit accuracy,  thicknesses of a particular compensator material. For the intra-batch variation, background, intrinsic film various compensator materials, the thickness ranges were inhomogeneity), and using error propagation analysis an similar to those used for the linear attenuation coefficient overall uncertainty of less than or equal to 2.0% was observed. measurements. Also, the dimensions of the various The beam output calibration of the telecobalt machine was compensator materials employed in the various measurements found to have an associated overall uncertainty of 1.4%. This to facilitate the development the semi-empirical formula gave the film dosimetry an overall uncertainty of less than or (Equation 2) were similar to those used for the attenuation equal to 2.6%. coefficient measurements. All beam data to facilitate the To verify Outputs of the proposed approach, a number of development of the proposed approach were acquired with the treatment plans using a single field with varying irradiation three dimensional water phantom which was connected to a geometries were created for the solid dry phantom. Bolus in the laptop having the OmniPro-Accept 7 software of the same form of step wedges placed on the surface of the solid phantom manufacturer required for the running of the water phantom. at the point of beam entrance were used to provide intensity The 0.125 cc ionization chamber which was connected to the modulation of beams. For each created bolus, the bolus was UNIDOS electrometer, was used for the measurements in the assigned a density similar to that of water (HU=0), which was water phantom. Details of the measurement procedures could the default for the TPS. Two types of step wedges were created be find in the work of Tagoe et al [1]. These measurements for the assessment. The wedges mounted on the solid phantom were done to account for variations in scatter contribution to are depicted in transverse view of the main planning windows dose at any point within a phantom/ patient for using a shown in Figure 3. The created treatment plans were each compensator to represent the bolus. Effect of beam hardening replicated on the telecobalt machine with a bolus within the was ignored as it was considered minimal for cobalt 60 beam. radiation field represented with a compensator constructed Once the physical dimensions of a compensator were using the cubic pile approach, such that the thickness of the determined through the above procedures, the compensator was compensator along the direction of propagation of the beam at constructed using the cubic pile approach [14]. any portion of the radiation field determined using Equation 2. Compensators were constructed from each of the selected 174 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 compensator materials per treatment plan for the verification. % = 100 ×   . − .⁄. Eq. 3 A constructed compensator was mounted on a block tray where   . and . are the calculated dose by TPS and the similar to what is used in the department for mounting of measured dose with film, respectively. The overall confidence customized shielding blocks, and held on the accessory holder limit for the proposed approach, ∆, was therefore defined as: of the teletherapy machine. A compensator was aligned such that its height was towards the radiation source. For each ∆= | !"#$! &! '#(')*| + 1.5 × - Eq. 4 irradiation with a compensator, it was ensured that the block To accomplish the dose assessment, at the depth of dose tray was at least 15 cm from the surface of the phantom to prescription for each plan, calculation points 2 cm apart from minimize electron contamination and also to prevent the each other were placed along a major axis of a beam to cover compensator from acting as a beam spoiler [16]. Treatment the entire field. The major axis was chosen such that it ran plans were grouped into two case scenarios: case scenario 1 for across steps of a step wedge. The boluses were created such irradiation geometries having the step wedge depicted in that a step ran central to a calculation point. Calculation points Figure 3A, and those with the step wedge shown in Figure 3B were the dose determination point within the phantom occurred were considered as case scenario 2. Dose distributions within outside a step was not accounted. Our primary concern was not the phantom along the depth of dose prescription for the on the resolution of the dose distributions within the phantom, various treatment plans were assessed with the Gafchromic but the magnitude of doses within the phantom for a particular films from the same batch, and compared to those calculated calculation point. The shape of a bolus resulted in the creation with the TPS having boluses. For the film irradiations, the film of varying doses at the various calculation points, due to strips were sandwiched between acrylic slabs of the solid dry varying beam attenuation achieved with the different phantom at the required depth and held on the treatment couch thicknesses of the various steps of a step wedge bolus. of the teletherapy machine under gravity. All exposed films used in this study were scanned with the ScanMaker 9800XL plus flatbed scanner, and images obtained (saved in Tagged Image File Format) analyzed with an ImageJ software (National Institutes of Health, USA) to obtain optical densities of films. The optical density of exposed film under a compensator was converted to dose using the equation of the line of best fit of the sensitometric curve obtained for the Gafchromic film. The difference between the measured and the calculated doses, δ(%), was defined as following: Figure 3. Planning windows showing axial configuration of bolus used for assessment of proposed approach. Table 1. Measured radiological properties of selected compensator materials and water. Material Density (g/cm3 Relative mass attenuation coefficient ) Mass attenuation coefficient (cm2/g) (relative to that of water) Water 1.0000±0.0100 0.0678 1.0000 Perspex 1.1800 ±0.0100 0.0697 1.0287 Brass 8.5500±0.0500 0.0597 0.8798 Copper 8.9400 ±0.0200 0.0570 0.8409 Table 2. Expressions for terms in function for converting bolus thickness to compensator material thickness. Compensator material Thickness Ratio, . Field size correction factor, /0 Treatment depth correction factor, /1 −3.0 × 102<"= + 4.0 × 102="4 − −5.0 × 1023( 4 + 3.0 × 10 24( 6 − 26 6.0 × 10 23&4 − 3.0 × 1024&6 + 26 8 28 : 2.0 × 10  6  Perspex (PMMA) " + 4.2 × 10 28"8 − 6.0 × 10 ( + 4.6 × 10 ( + 28 : 28 8.0 × 10 26&8 − 12.7 × 1028&: + 48.3 × 10 " + 234.2 × 10 " + 3.1 × 1028( + 517.4 × 10 28 28 89.7 × 10 28& + 783.5 × 1028 782.1 × 10  −1.0 × 102<"= + 2.0 × 102="4 − −5.0 × 102<( 4 2= 6 + 2.0 × 10 ( − 24 6 28 8 9.0 × 10 23&4 − 4.0 × 1024&6 + Copper 3.0 × 1024 8 24 : 9.0 × 10 " + 2.2 × 10 " − ( + 8.0 × 10 ( + 28 : 28 7.0 × 10 26&8 − 7.7 × 1028&: + 28 2: 26.8 × 10 " + 130.4 × 10 " +4.7 × 10 ( + 7.1 × 10  28 53.6 × 10 28& + 86.4 × 102: 907.4 × 10  −3.0 × 102<"= + 3.0 × 102= 4 −4.0 × 102< 4 " − ( + 2.0 × 10 2=( 6 − −3.0 × 102=&4 26 6 28 8 + 1.0 × 10 26&6 −  Brass 24 8 26 : 2.0 × 10 " + 3.4 × 10 " − 5.0 × 10 ( + 6.0 × 10 ( + 1.2 × 10 28&8 + 1.2 × 1028&: + 28 28 3.9 × 10 2:": + 183.2 × 1028" + 2.0 × 10 ( + 100.4 × 10  28 49.8 × 10 28& + 810.4 × 1028 861.9 × 10  175 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 Table 3. Calculated and measured doses at the various calculation points for compensators constructed from the selected materials per case scenario. Meas. doses (cGy) Calc. dose (cGy) Plan # Calc. Pt # Perspex Brass Copper Case 1 Case 2 Case 1 Case 2 Case 1 Case 2 Case 1 Case 2 1 100.00 100.00 100.00 101.01 102.38 101.03 100.50 101.78 2 98.38 85.08 99.57 87.07 96.87 83.77 100.70 87.05 1 3 92.06 80.29 94.03 78.29 89.92 78.42 93.50 78.84 4 86.56 69.38 85.51 70.70 89.22 71.51 88.04 71.29 5 82.54 66.79 84.52 68.39 84.66 68.50 82.08 65.12 1 150.00 150.00 151.32 152.86 154.10 152.53 154.64 151.29 2 148.15 125.75 152.73 128.15 152.73 128.64 152.56 129.24 3 146.37 123.47 150.22 125.72 147.37 126.87 150.05 126.44 2 4 129.54 99.70 131.46 101.38 130.82 101.92 133.11 101.69 5 126.30 98.48 128.33 100.16 130.06 101.15 128.60 101.09 6 159.70 94.90 164.10 96.82 162.71 97.70 162.71 97.19 7 121.30 94.80 123.84 97.18 124.88 96.71 125.03 97.71 1 200.00 200.00 198.57 203.50 203.05 203.05 204.08 202.00 2 198.17 169.79 203.48 165.86 202.52 174.27 202.67 174.97 3 196.67 170.48 202.07 175.16 192.21 174.53 201.63 175.01 4 175.36 140.83 173.66 145.16 170.25 137.73 177.22 144.99 5 174.27 139.99 177.97 142.91 179.59 135.91 176.44 143.18 6 220.14 140.00 222.43 141.46 226.04 143.80 226.69 144.17 3 7 173.48 138.45 177.49 141.94 177.87 142.67 178.44 142.41 8 214.61 136.12 220.11 138.83 220.00 139.58 220.43 139.81 9 167.95 117.51 165.01 119.52 172.01 120.82 172.40 120.80 10 206.83 110.05 211.22 112.73 212.94 113.27 212.50 112.28 11 208.25 110.82 213.11 113.41 212.67 113.99 213.90 113.01 12 130.61 107.56 132.20 105.60 134.47 110.85 134.65 110.31 13 123.53 58.01 126.05 59.79 127.35 59.80 127.32 59.79 1 250.00 250.00 252.53 253.96 254.84 254.84 256.41 253.81 2 248.60 213.25 255.76 207.44 252.85 219.12 252.49 219.82 3 246.85 214.02 243.32 210.96 253.91 208.96 251.55 220.30 4 222.63 176.95 228.34 181.60 229.23 182.22 219.21 172.37 5 220.48 176.70 226.60 181.75 226.11 181.58 215.86 172.90 6 277.75 175.81 286.34 178.22 285.16 180.80 285.22 171.32 4 7 219.26 173.65 223.55 177.05 223.96 178.01 223.46 178.98 8 270.00 171.55 276.07 175.41 277.24 176.35 274.31 176.29 9 212.27 148.02 216.51 150.37 215.59 151.91 218.14 152.11 10 259.69 138.92 265.23 136.00 265.40 142.48 267.01 143.13 11 263.04 139.88 260.44 142.73 270.28 135.81 270.26 143.72 12 167.20 135.44 168.70 138.05 170.70 139.27 171.80 139.17 13 158.14 122.27 162.19 125.92 162.96 126.05 162.83 125.77 1 250.00 250.00 252.47 253.24 257.73 254.01 255.39 253.96 2 247.82 227.48 253.65 232.98 255.43 233.36 253.50 233.94 3 247.62 226.01 254.18 230.55 253.03 231.90 254.99 231.73 4 225.60 195.12 227.63 198.82 231.79 200.78 231.62 200.66 5 224.56 194.58 231.51 198.33 230.32 199.63 230.91 200.29 6 266.20 194.76 263.43 190.57 272.19 200.00 273.53 200.33 5 7 221.67 192.11 227.77 195.99 228.53 197.20 227.70 197.14 8 261.36 192.08 269.44 195.50 266.64 197.98 269.14 198.00 9 185.29 171.04 189.94 176.06 189.98 176.08 190.00 175.75 10 251.41 140.66 255.47 145.01 257.80 144.24 258.36 144.62 11 161.59 165.99 165.39 156.88 12 174.62 157.61 176.38 160.29 179.10 161.65 180.02 162.45 13 116.81 119.06 120.29 120.42 176 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 Table 4. Comparison between measured doses with compensators and TPS calculated doses with boluses. Percentage Diff. between Calc. and Meas. doses (%) Plan # Calc. Pt # Perspex Brass Copper Case 1 Case 2 Case 1 Case 2 Case 1 Case 2 1 0.00 1.00 2.32 1.02 0.50 1.75 2 1.20 2.28 -1.56 -1.56 2.30 2.26 1 3 2.10 -2.56 -2.38 -2.38 1.54 -1.84 4 -1.23 1.87 2.98 2.98 1.68 2.68 5 2.34 2.34 2.50 2.50 -0.56 -2.56 1 0.87 1.87 2.66 1.66 3.00 0.85 2 3.00 1.87 3.00 2.25 2.89 2.70 3 2.56 1.79 0.68 2.68 2.45 2.35 2 4 1.46 1.66 0.98 2.18 2.68 1.96 5 1.58 1.68 2.89 2.64 1.79 2.58 6 2.68 1.98 1.85 2.87 1.85 2.36 7 2.05 2.45 2.87 1.97 2.98 2.98 1 -0.72 1.72 1.50 1.50 2.00 0.99 2 2.61 -2.37 2.15 2.57 2.22 2.96 3 2.67 2.67 -2.32 2.32 2.46 2.59 4 -0.98 2.98 -3.00 -2.25 1.05 2.87 5 2.08 2.04 2.96 -3.00 1.23 2.23 6 1.03 1.03 2.61 2.64 2.89 2.89 3 7 2.26 2.46 2.47 2.96 2.78 2.78 8 2.50 1.95 2.45 2.48 2.64 2.64 9 -1.78 1.68 2.36 2.74 2.58 2.72 10 2.08 2.38 2.87 2.84 2.67 1.99 11 2.28 2.28 2.08 2.78 2.64 1.94 12 1.20 -1.86 2.87 2.97 3.00 2.49 13 2.00 2.98 3.00 3.00 2.98 2.98 1 1.00 1.56 1.90 1.90 2.50 1.50 2 2.80 -2.80 1.68 2.68 1.54 2.99 3 -1.45 -1.45 2.78 -2.42 1.87 2.85 4 2.50 2.56 2.88 2.89 -1.56 -2.66 5 2.70 2.78 2.49 2.69 -2.14 -2.20 6 3.00 1.35 2.60 2.76 2.62 -2.62 4 7 1.92 1.92 2.10 2.45 1.88 2.98 8 2.20 2.20 2.61 2.72 1.57 2.69 9 1.96 1.56 1.54 2.56 2.69 2.69 10 2.09 -2.15 2.15 2.50 2.74 2.94 11 -1.00 2.00 2.68 -3.00 2.67 2.67 12 0.89 1.89 2.05 2.75 2.68 2.68 13 2.50 2.90 2.96 3.00 2.88 2.78 1 0.98 1.28 3.00 1.58 2.11 1.56 2 2.30 2.36 2.98 2.52 2.24 2.76 3 2.58 1.97 2.14 2.54 2.89 2.47 4 0.89 1.86 2.67 2.82 2.60 2.76 5 3.00 1.89 2.50 2.53 2.75 2.85 6 -1.05 -2.20 2.20 2.62 2.68 2.78 5 7 2.68 1.98 3.00 2.58 2.65 2.55 8 3.00 1.75 1.98 2.98 2.89 2.99 9 2.45 2.85 2.47 2.86 2.48 2.68 10 1.59 3.00 2.48 2.48 2.69 2.74 11 2.65 2.30 -3.00 12 1.00 1.67 2.50 2.50 3.00 2.98 13 1.89 2.89 0.50 3.00 177 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 Results approach even when the attenuation coefficient of a material is comparable to what is presented in Table 1; as the various In Table 1 are listed measured radiological properties of the expressions determined for the conversion of a bolus thickness various compensator materials used in the study including to compensator material thickness may be dependent on design those of water. Densities and mass attenuation coefficients are of collimator system of a teletherapy machine. The expressions provided for the selected materials earmarked for compensator must therefore be determined for ones teletherapy machine. In construction. Also presented in Table 1 are ratios of mass determining the expressions for the various terms in the attenuation coefficient of a compensator material to that of proposed semi-empirical equation for converting bolus water for the respective compensator materials. Determined thickness to compensator material thickness, one needs to expressions for the various terms in Equation 2 through the incorporate ranges of treatment parameters (field size, correlation analyses using the lines of best fits and regressions, 2 treatment depth and applied bolus thickness) that are likely to R are listed in Table 2. Where the various terms: tb, r and d be used clinically during the experimental measurements. within the expressions listed in Table 2 are applied bolus Ignoring this will lead to uncertainties in the dose distribution thickness during treatment planning with the TPS in cm, obtained with a compensator; owing to the degrees of the equivalent square field size of the actual field size used, and polynomial equations which are used to express the various treatment depth in cm, respectively. In Table 3 are listed doses terms of the semi-empirical equation presented in Table 2. calculated (calc. dose) at the various calculation points (calc. From the dose comparison results in Table 3, it shows that Pt. #) obtained with the TPS having boluses within the none of compensator materials can be favoured over the other radiation fields and their measured counterpart with the boluses for the construction of a compensator. represented with compensators constructed based on the The choice of a compensator material is therefore dependent proposed approach, when the generated treatment plans were on convenience with which the compensator can be constructed replicated on the telecobalt. The percentage differences with a particular material and the level of beam intensity between the calculated and measured doses for the various modulations required. Where high levels of modulations are calculation points for compensators constructed from the required, the compensator should be constructed from materials selected materials are enumerated in Table 4. The various dose with higher density values or lower relative mass attenuation differences are each expressed as a percentage of the respective values. This will minimize the thickness of a constructed measured doses, and are calculated using Equation 3. compensator, thereby reducing the magnitude of penumbra associated with the beam resulting from the introduction of the Discussions compensator in the path of the beam. With reference to the Prior to the use of the proposed approach, it is imperative to dose comparison results in Table 3, compensators constructed determine the radiological properties of the material one wants from Perspex give the most comparable doses to those of the to use for the construction of a compensator. Changes in the TPS, which may be attributed to the closeness of the elemental composition of a material may have significant radiological properties of Perspex to those of water. This also influence on the expressions determined for the various terms goes to support the point that using medium density materials in the semi-empirical equation proposed for the conversion of a for compensator constructing would translate to low bolus thickness to a compensator material thickness. This is discrepancies in dose distributions obtained with a because all the measurements acquired to ensure the compensator for uncertainties in the determination of thickness development of the expressions for the various terms in the of the compensator. semi-empirical equation were based on transmission Although the proposed approach gave favourable results, the measurements with a compensator material in the path of the following limitations were encountered: bolus thickness could beam. The most reliable quantity to determine for the material not be increased beyond 15 cm and for abutting fields, bolus is the mass attenuation coefficient measured with the same could not be entered for individual field. These were inherent beam quality as that to be modulated. It is also verifiable that limitations associated with the TPS used. changes of elemental composition and or impurities within Owing to the inherent uncertainties associated with the film constituent elements of a material would affect the value of dosimetry, there is the need to further study the output of attenuation coefficient measured for the material with a proposed approach with other 2D array detector based on diode particular beam quality [17]. Owing to uncertainties in the or ionization chamber. Notwithstanding this, the output of the experimental setup for the measurement of the attenuation proposed approach is within the ± 5% uncertainty proposed for coefficient, it would be prudent for one to normalize the dose delivery in radiation therapy [18]. Also, one needs to attenuation coefficient of a material to that of water measured institute some form of quality assurance procedures to ensure with the same irradiation geometry. The normalized attenuation effective implementation of the proposed approach. The coefficient which is referred to as relative mass attenuation proposed approach may be used to enhance spatial resolution coefficient in Table 1, should be used to characterize a of dose distribution within irradiated regions having high levels material prior to the use of the proposed approach. Also, one of tissue heterogeneities (eg. treatment of lung cancers) and needs to be circumspective with the use of the proposed tissue deficiencies (eg. treatment of head and neck cancers). 178 Tagoe et al: Compensator-based IMRT with a conventional telecobalt machine Pol J Med Phys Eng 2018;24(4):171-179 Conclusion Brass, Copper and Perspex (PMMA), which were used as compensator materials. Dose distributions measured under a A pilot study using quasi-experimental approach had been compensator (fabricated based on the proposed approach) in a conducted, to propose and develop an approach of generating tissue-equivalent phantom with calibrated radiochromic films, intensity modulated beams for a conventional telecobalt are found to compare favourably well to those of the TPS machine with compensating filters, based on generated dose (where bolus was used to provide beam intensity modulations). distributions of a forward planning TPS (with limitations of Radiological properties are provided for the stipulated realizing beam intensity maps). It had been shown that, it is compensator materials to ensure reproducibility of the practically possible to generate modulated fluence distributions proposed approach. across beams from the telecobalt machine using bolus having varying thicknesses across the surface of a phantom (or patient) during treatment simulation with the forward planning TPS, Acknowledgements and then replicating the treatment with a compensator at the This publication formed part of research work carried out by time of treatment delivery. A proposed semi-empirical equation the principle author to enable him complete his PhD thesis, has been found to be effective in converting bolus thickness to which was submitted to the Department of Physics, School of compensator material thickness such that the dose distribution Physical Sciences, College of Agriculture and Natural within the phantom (or patient) remains the same as planned Sciences, University of Cape Coast, Ghana, in partial with the TPS. 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