Treatment delivery verification using a pelvic anthropormophic phantom
Date
2009-09-02T12:03:20Z
Authors
Mwale, Nsangu Augustine
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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.