SU‐E‐T‐173: Design, Development, and Evaluation of a Modified, Anthropomorphic, Head and Neck, Quality Assurance Phantom for Use in Stereotactic Radiosurgery

Purpose: To design, construct, and evaluate a modified anthropomorphic head phantom for evaluation of stereotactic radiosurgery (SRS) dose planning and delivery, for protocols developed by NCI funded cooperative groups. Methods: A phantom was constructed from a water equivalent, plastic, head‐shaped shell. Modifications from an original phantom design containing only a spherical target included the addition of structures resembling a nonspherical target (pituitary) and an adjacent (within 2mm) organ at risk (OAR) (optic chiasm) simulating structures encountered when treating acromegaly. A separate insert for treatment delivery contained two thermoluminescent dosimeters (TLD) for absolute dosimetry and radiochromic film in the sagittal and coronal planes for relative dosimetry. The target and OAR proximity provided a more realistic treatment planning and dose delivery exercise than the original design. To create an achievable planning criterion, the volume of the OAR receiving ≥8Gy was limited to 10%. The rigor of the treatment planning process, reproducibility of the dosimeters, and agreement with calculated doses were assessed through three GammaKnife irradiations. Results: The TLD results from the three irradiations agreed with the calculated target dose to within 4.3% with a coefficient of variation of 2.0%. Gamma analysis in the film planes showed an average passing rate of 99.9% and 99% using 5%/3mm distance to agreement criteria in the coronal and sagittal planes, respectively. A gamma analysis using 3%/2mm criteria showed the percent of total pixels passing to be 87% and 79% respectively. Conclusions: A modified anthropomorphic SRS phantom has been designed that provides a more realistic clinical planning and delivery challenge that can be used to credential institutions wanting to participate in NCI funded clinical trials. Work supported by PHS CA010953, CA081647 and CA21661 awarded by NCI. DHHS.