SU‐E‐T‐169: Modification and Implementation of the RPC Heterogeneous Thorax Phantom for Verification of Proton Therapy Treatment Procedures

Purpose: To modify the existing RPC heterogeneous thorax phantom to assess lung tumor proton beam therapy procedures including patient simulation, treatment planning, and treatment delivery. Methods: A new dosimetry/imaging insert was designed for use with the RPC thorax phantom. The insert was constructed of extra heavy balsa wood with density from 0.30 – 0.34 g/cm3 and average HU of −670. A relative linear stopping power of 0.313 was measured. A tapered compression technique eliminated air gaps around the dosimeters. TLDs were positioned in the superior and inferior aspects of a unit density target and EBT2 radiochromic film was placed in three orthogonal planes. The phantom was imaged with a 16 slice CT scanner. A treatment plan with fields at 10 and 80 degree gantry angles and a 15 degree couch kick was designed with an Eclipse TPS utilizing apertures (expanded 10mm) and compensators (smeared 6mm). The plan was delivered three times with new dosimeters for each irradiation.Results: The ratio of measured dose to calculated dose was 0.947±0.009 (superior point) and 0.937±0.003 (inferior point). Gamma analysis utilizing 5%/5mm criteria showed 3 trial average pass rates of 86.7% (axial), 90.0% (coronal), 90.1% (sagittal) and 88.9% (3 plane average). DTA ranged from 0.9 – 2.4mm (axial), 0.4 –1.8mm (coronal), 0.6 – 1.9mm (sagittal). Conclusions: This work indicated that modifications to the existing RPC thorax phantom allowed the evaluation of proton therapy procedures. The differences in measured/calculated doses were higher than expected but consistent with previous experience with this phantom. Gamma analysis and DTA showed good results, with poor correspondence in the distal low dose regions of the treatment fields. Further analysis is underway to determine if the discrepancies reflect inadequacies of the TPS or limitations of the dosimetry system. Work supported in part by PHS grants CA010953 and CA081647, awarded by NCI, DHHS.