SU-E-T-414: Clinical Implementation of Anthropomorphic Lung Phantom for Patient-Specific SAbR QA

Dose calculations for lung stereotactic ablative radiotherapy (SAbR) are challenged by the presence of extremely heterogeneous tissue and small treatment volumes. In this work, an anthropomorphic chest phantom has been constructed for the purpose of commissioning treatment planning systems (TPS) and for patient-specific SAbR QA.

A CT scan of a realistic chest phantom containing tissue equivalent materials for the spine, ribs, and lungs was imported into a Pinnacle TPS (CCC dose algorithm) where treatment plans were created for right and left-sided lung lesions. The phantom lungs are unique in that they contain embedded unit density spherical targets (2 and 4cm in diameter) that represent lung lesions. Plans directed to both tumors were designed for PTVs ranging from 2-5cm in diameter using 6, 10, and 15MV beams while passing RTOG 0813 dose and conformality criteria. Each plan was then exported to an Eclipse TPS (AAA dose algorithm) for dose calculation. Plans were delivered with a TrueBeam LINAC corrected for machine output. Point dose measurementswere verified with a 0.015 cc air ionization chamber placed in the center of each tumor.

While the majority of plans developed in Pinnacle passed conformality criteria, the dose distribution as calculated in Eclipse failed to meet the RTOG guidelines, particularly for the small tumor at higher photon energies. All point dose measurements matched both TPS within 4%. Both TPS calculated a lower point dose than measured for large PTVs at all energies, improving as PTV size decreased.

After comparing TPS and validating calculations with point dose measurements, the phantom was clinically implemented for patient-specific conformal SAbR QA.