Treatment delivery verification using a pelvic anthropormophic phantom

Abstract

Aim: A series of measurements in a pelvic anthropomorphic phantom was performed as part of the validation of the entire radiotherapy treatment chain. Two treatment planning calculation algorithms were used: the Pencil Beam (PB) and the Collapsed Cone (CC). The dose calculation algorithms of Radiotherapy Treatment Planning Systems (RTPS) were validated to ensure that the dose delivered to a treatment target was accurately predicted. An anthropomorphic phantom, designed and manufactured locally at the Pretoria Academic Hospital, was employed in this study. The phantom was fabricated with locally available materials as human tissue substitutes based on the attenuation coefficients, electron densities and effective atomic numbers. Materials: Pelvic anthropomorphic phantom, Thermoluminescent Dosimeters (TLD_100 chips), X-Omat V film, film processors, a densitometer and 6 MV and 15 MV linear accelerator photon beams with beam quality (TPR20,10) of 0.674 and 0.763 respectively. Results: Two treatment planning techniques were studied, a four field “box” and parallel opposed beams using a local cancer of the cervix protocol. Point doses calculated by the RTPS were compared with equivalent point dose values measured with thermoluminescent dosimeters (TLDs). Three dose regions emerged for the four field technique, those of low, intermediate and high dose gradient. The four field technique for 6 MV gave a dose deviation from -4.9% to -32.1% and at 15 MV from -1.5% to -20.6%. For 6 MV and 15 MV parallel opposed beams, percentage dose deviations from -2.7% to -11.8% and from +0.2% to -11.5 were observed. The mean value of the ratios of measured to calculated dose values was 0.91±0.05 for the four field technique and 0.94±0.02 for the AP/PA. Radiographic film was used to compare the predicted 2D isodose distributions to the actual dose distribution in the phantom. The 2D isodose distributions obtained were not meaningful in comparing the doses predicted by the planning system. A smaller field size of 7 cm x 7 cm was also employed and results of both TLD and film obtained were comparable to those predicted by the planning system. iv Conclusion: The stated goal of dose delivery accuracy (ICRU, 1987) to within 5% was not generally met in this study. On average the measured doses using TLDs and film at a field size of 7 cm x 7 cm were lower than the point doses predicted by the RTPS dose calculation algorithms whereas the film over-responded when a local cancer of the cervix protocol was employed. At a field size of 7 cm x 7 cm, film dosimetry was comparable to the TLD results. Film and TLDs were calibrated perpendicularly and exposed parallel. The phantom is unsuitable for film dosimetry studies at field sizes more than 14 cm x 14 cm.