Author: J. Daniel Bourland, Christina K Cramer, Justin M. Napolitano, James D. Ververs 👨🔬
Affiliation: Wake Forest University School of Medicine 🌍
Purpose:
Brain metastases (BM) can be treated with linear accelerator (LINAC)-based stereotactic radiosurgery (SRS). Treatment planning for this modality has evolved over time; treatment plans typically have one, two, or n isocenters, where n is the number of targets. Common isocenter locations include the center of each target, the center of the brain or a region within it, or coordinates determined by an expert clinician. This work aims to determine an optimal quantity and location of isocenters for any given plan, exploring options outside current and historical standards.
Methods:
A spherical structure of 16 cm in diameter representing a human brain with numerous BM was simulated in RayStation™ and Volumetric Modulated Arc Therapy (VMAT) plans were developed for various BM, isocenter, and arc configurations. Each BM configuration had n isocenter configurations (e.g., a 5-BM configuration had plans generated with 1/2/3/4/5 isocenters. Configurations were selected to 1) evaluate how the quantity and location of isocenters impacts plan quality and 2) investigate potential dependencies on BM quantity and location. Following initial testing on the simulated sphere, clinical cases were evaluated to determine 1) if findings are translatable and 2) how patient physiology and pathology impact results.
Results:
It was found that increasing the quantity of isocenters in a plan initially improves conformality, integral dose, max dose, and modulation, however, these metrics begin to fall after a certain quantity. For example, in a 5-BM configuration, 3-4 isocenter plans yield the best results. This phenomenon was persistent in clinical cases.
Conclusion:
The findings of this work indicate the existence of a “sweet spot” isocenter quantity, which is dependent on the BM configuration. Current and future studies aim to further characterize this this phenomenon with regards to target, isocenter, and arc configuration, as well as target size and prescription, and the presence of critical structures.