Design and evaluation of a dosimetry system to verify the radiotherapy procedure from treatment planning to treatment delivery

Abstract

In the advent of more radiotherapy centers being set up across the African continent and the introduction of linear accelerators in addition to some of the already existing cobalt radiotherapy machines, it is apparent that verification of treatment delivery is necessary. Also, most audits in radiotherapy concentrated on radiation bean outputs and few exist that check the radiotherapy chain. A dosimetry system has been developed to address need to verify the radiotherapy chain. The aim of this study is to demonstrate dosimetric verification and the feasibility as well as cost effectiveness of manufacture of the dosimetry phantom using material locally available in the radiotherapy department. The phantom is designed and fabricated in the shape of a female pelvis bearing in mind the high incidence of cervical cancer more especially in Africa. It is designed with purpose of accommodating two dosimetric tools: thermoluminescence dosimeters (TLDs) and radiographic film. The delivery of external beam radiation therapy is evaluated for three different modalities by comparison of predicted and measured dose values. Point dose values measured with TLDs were compared with predicted dose values and resulted in differences of ± 5 % were observed for uniform dose regions. Higher discrepancies (up to ± 28%) were observed for points in the penumbra (high dose gradient) of the radiation field. Isodose distribution were determined by radiographic film and compared with those predicted by the treatment planning system. Distance to agreement between predicted and measured isodoses was within ± 5mm. The dosimetric verification methods were quantitative, qualitative and clinically practical. The fabrication of the phantom, dosimetric measurements and analysis demonstrate the feasibility and accuracy of the system for achieving treatment verification. Thus the system provides a means to verify the full radiotherapy chain from the point prior to and after treatment planning and through to treatment delivery.