Main Session
Sep
29
PQA 05 - Physics
Presenter(s)
Jingjie Yu, BS - UCSF Comprehensive Cancer Center, San Francisco, CA
J. Yu1, D. P. I. Capaldi2, S. E. Braunstein3, K. Sheng4, and Q. Lyu5; 1UCSF, San Francisco, CA, United States, 2Imaging Research Laboratories, Robarts Research Institute, Western University, London, ON, Canada, 3Department of Radiation Oncology, University of California San Francisco, San Francisco, CA, 4Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, 5University of California, San Francisco, San Francisco, CA
Purpose/Objective(s):
Stereotactic radiosurgery (SRS) treatment for multiple brain metastases on Gamma Knife (GK) requires delivering multiple isocenters, which is typically achieved by moving to isocenters from superior to inferior, ignoring time-dependent biological effects due to sublethal damage repair. We develop a method to explicitly optimizing the spatiotemporal delivery sequence and treatment fractionation to enhance the therapeutic ratio (TR) without altering the physical modulation and physical dose distribution.Materials/Methods:
The isocenter locations and shot configurations were optimized using our previously developed Integrated Isocenter and Sector Duration Optimization (ISDO) framework. A bi-exponential Biologically Effective Dose (BED) model was utilized to account for sublethal damage repair during the single-fraction and multi-fraction delivery. Tissue parameters were assigned as Gy for the tumor and 2.47 Gy for the surrounding normal brain. The optimization objective was to maximize the TR, defined as the ratio of, where the ring is defined as the 0 to 3 mm expansion immediately outside the GTV. A two-stage optimization framework was implemented: (1) an inter-fraction combinatorial algorithm dividing the brain metastases (mets) into multiple fractions, and (2) a Genetic Algorithm (GA) to optimize the intra-fraction shot sequence within each fraction. We tested the method on a multiple brain metastases patient with 7 mets.Results:
Optimizing the spatiotemporal delivery sequence enabled substantial biological gains while keeping comparable total beam-on time. Under the standard 1-day delivery utilizing the default geometry-driven shot sequence, the baseline TR was 0.85. Optimizing the intra-fraction sequence via GA for the 1-day delivery improved the TR to 0.94. By transitioning to a two-day delivery regimen (Day 1: 3 mets, Day 2: 4 mets) while maintaining the default logistical intra-fraction sequence, the inter-fraction grouping alone improved the TR to 0.95. Finally, combining the inter-fraction grouping and the intra-fraction sequencing, the TR is 0.99 (>16% improvement compared to the standard delivery).Conclusion:
The spatiotemporal sequence optimization of GK delivery is a potent, underutilized degree of freedom in GK planning. Integrating an inter-fraction partitioning strategy with an intra-fraction shot sequencing optimization substantially improves the therapeutic ratio over standard delivery.