Inter-Institutional Dosimetric Comparison For Radiotherapy Centres In Ghana Using Cirs Thorax Phantom

Abstract

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.