Purpose: Internal margin expansion on the gross tumor volume (GTV) is widely used to account for deep inspiration inter-breath hold (DIBH) variation in tumor positioning in pancreatic stereotactic body radiation (SBRT). This margin expansion promotes better target coverage potentially at the cost of increased dose to the gastrointestinal organs. We investigated whether robust optimization with multiple breath-hold CT sets can account for inter-breath hold variation with organ-at-risk (OAR) sparing in comparison with the margin-based compensation.
Methods: Ten DIBH SBRT pancreatic cancer patients were selected for this retrospective study. At simulation, four DIBH CT sets were acquired. Conventional margin-based plans were generated using volumetric modulated arc therapy (PTV-VMAT), where planning tumor volume (PTV) definition included breath hold variation and a 2-mm setup margin. GTV-based plans (GTV-RO-VMAT) were generated using robust optimization (RO). OAR positioning uncertainties were considered using physician-approved contours on all DIBH CT sets with a 2-mm isotropic setup uncertainty. PTV-VMAT and GTV-RO-VMAT plans were considered matched when GTV coverage was equal at D99%. Target coverage and OAR doses were compared for both approaches. The metrics used for the dosimetric comparison were D99% and V130% for the GTV, and V20Gy and Dmax for duodenum, stomach and bowel.
Results: RO reduced OAR V20Gy by an average of 10%, 20%, and 40% for duodenum, stomach, and bowel, respectively, at the cost of slightly increased Dmax (2-3%). Due to plan matching conditions, GTV D99% was consistent between plans. The V130% increased from an average of 0.4 to 6.4 cc in RO plans, however, these hot spots were well-contained within the GTV.
Conclusion: We have demonstrated that RO techniques can be applied to pancreatic SBRT planning with DIBH to significantly reduce OAR dose while maintaining GTV coverage. RO tools are clinically available and may be applied to enhance the efficacy of photon-based SBRT planning.