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SU-E-T-384: An Easy Use Anthropomorphic Phantom for Lung IMRT QA.

PURPOSE
Even though lung IMRT plans are calculated based on patient CT with heterogeneity correction, patient-specific validations are often performed by using measurements in homogeneous solid media. In this study we developed heterogeneous phantoms for measurement-based validation of lung IMRT.


METHODS
Three slabs from thorax region of anthropomorphic RANDO phantom and virtual water (VW) slabs were used to construct two versions of heterogeneous phantom, "Hetero-1" and "Hetero-2". The RANDO slabs are 2.5 cm thick. A two-dimensional diode array detector, MapCHECK and an ion chamber were used for measurements. "Hetero-1" consisted of three RANDO slabs placed horizontally on the MapCHECK plus a 3cm VW on the top. Hetero-2 was constructed by inserting a 2 cm VW with a chamber cavity on top of the first RANDO slab above the MapCHECK. Two lung-IMRT plans were generated using Pinnacle TPS, including a plan for the RPC Lung-Phantom. The point doses were compared with ion chamber measurements and planar dose were compared with MapCHECK measurements.


RESULTS
The MapCHECK measurements were performed using "Hetero-1" on a beam-by-beam basis for the two IMRT plans. Passing rates under 3%/3mm Gamma criteria of were from 93.5% to 98.5% for all beams. Accumulative measurements were performed using "Hetero-2" and the passing rates under same criteria for two IMRT plans were 96.9% and 96.3% respectively, and 95.2% and 98.5% for measurements with 5cm lateral-IsoCenter-shift including more lung material. The differences between Ion-chamber- measurements and TPS-calculations were 1.2% and 1.71% for two IMRT plans. In vivo results obtained from RPC-report for two TLDs inside the PTV were 98% and 99% between measurements and calculations.


CONCLUSIONS
An easy-to-setup heterogonous phantom for Lung IMRT QA can be constructed by adopting thorax region slabs of anthropomorphic-type phantoms. It can be implemented as a convenient and efficient tool for lung IMRT QA in a routine clinic.