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Robust Plan Optimization for Calypso Gated Hypo-Fractionated Prostate Treatment Using Volumetric Modulated Arc Therapy

P Zhang

P Zhang*, M Hunt, L Happersett, B Cox, G Mageras, Mem Sloan-Kettering Cancer Ctr, New York, NY

TH-A-213AB-5 Thursday 8:00:00 AM - 9:55:00 AM Room: 213AB

Purpose: To develop a robust optimization algorithm which incorporates a Calypso-guided gating strategy into the optimization process to maximize clinical target volume (CTV) coverage and organs at risk (OAR) sparing in the presence of intra-fractional motion.

Method: In a computer simulation, intra-fractional motion traces from prior treatments with Calypso were converted to a probability distribution function (PDF), truncated using a patient specific gating window, and renormalized to yield a new PDF with Calypso-gated interventions. In lieu of a conventional PTV, multiple instances of CTV and OARs were replicated and displaced to extreme positions inside the gating window representing possible delivery scenarios. When optimizing the volumetric modulated arc therapy plan, doses to the CTV and OARs were calculated as a sum of doses to the replicas weighted by the PDF to account for motion. A robust plan meeting the clinical constraints was produced and compared to the counterpart conventional margin (PTV) plan. Calypso traces from a separate testing database served to simulate motion during gated delivery. Dosimetric end points extracted from 4D dose accumulations for each trace were utilized to evaluate potential clinical benefit.

Result: Five prostate cases from a hypofractionated protocol (42.5 Gy in 5 fractions) were retrospectively investigated. The patient specific gating window resulted in tight anterior and inferior action levels (~1mm) to protect rectal wall and bladder wall, and resulted in an average of four beam interruptions per fraction in the simulation. The robust-optimized plans achieved the same average CTV D95 coverage of 40.5Gy as the PTV-optimized plans, but with reduced patient-averaged rectum wall D1cc by 2.2 Gy and bladder wall mean dose by 2.9Gy.

Conclusion: Integration of an intra-fractional motion management strategy into the robust optimization process is feasible and may yield improved OAR sparing compared to the standard margin approach.

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