SU‐E‐T‐99: A Patient Specific QA Protocol for Verification of 4D Dosimetry
Purpose: Presently, there is no patient specific dosimetric QA for radiation treatment of moving targets. Here we present a patient specific QA protocol for verification of 4D dosimetry delivered to a moving target in SBRT of lung and liver tumors. Methods and Materials: The protocol proceeds as follows. The patient's breathing pattern is recorded during 4D simulation and imported to a dynamic phantom incorporating a target that moves within an artificial lung or liver respectively. The patient's treatment plan (e.g. VMAT) is then recalculated on the free‐breathing CT of the dynamic phantom. The target is replaced with a 3D dosimeter (Presage), which is then irradiated while moving with the patient's breathing pattern. The dose in Presage is determined by optical‐CT, and compared with the planned dose, to generate a 4D dose verification index. An end‐to‐end test of the protocol was performed on a target undergoing known motion (amplitude 1.5cm, frequency 5s). Under‐dose and interplay effects were studied in 3DCRT, IMRT, and VMAT treatment plans, where the static target volume was covered 100% with a prescribed dose of 10Gy. Results: The whole process from sample preparation to completion of analysis takes about 1.5 hours for a non‐interruptive operation in the chain. Measured 3D dose distributions were obtained for moving phantom targets, for all plans, with isotropic resolution of 1mm3. In the control study, where motion was absent, good agreement was observed between planned and measured dose distributions with a 90% 3D gamma pass rate. Clear evidence of interplay and target under‐dosing was observed in all motion deliveries under free‐breathing. The under‐dose at the edge of both ends of the dosimeter along the moving direction was in excess of 30%. Conclusion: Comprehensive patient specific QA of 4D dosimetry for SBRT of moving lung and liver targets is feasible with the Presage/Optical‐CT system. © 2011, American Association of Physicists in Medicine. All rights reserved.
Yin, F; Thomas, A; Yan, H; Vergalasova, I; Adamovics, J; wu, Q; Oldham, M
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