SU‐E‐T‐457: Impact of Interfractional Variations On Anterior Vs. Lateral‐Field Proton Therapy of Prostate Cancer

Abstract

Purpose:To investigate the effects of interfractional anatomy and setup variations on plans with anterior‐oblique vs. lateral beams for prostate cancer pencil beam scanning (PBS) and passive scattered (PS) proton therapy.Methods:Six patients with low/intermediate risk prostate cancer treated with PS proton therapy at our institution were selected. All patients underwent weekly verification CT scans. Implanted fiducials were used for localization, and endorectal balloons for prostate immobilization. New PBS plans with lateral beams, as well as PBS and PS plans with anterior‐oblique beams (±35 deg) were created. PBS plans used two different spot sizes: ∼10mm (large) and ∼5mm (medium) sigma at 25cm range and optimized as single‐field‐uniform‐dose with ∼8% non‐uniformity. No range uncertainty margins were applied in PBS plans to maximize rectal sparing. Field‐specific apertures were used when planning with large spots to sharpen the penumbrae. The planned dose was recomputed on each weekly CT with fiducials aligned to the simulation CT, scaled and accumulated via deformable image registration.Results:The dose volume analysis showed that although difference between planned and accumulated dose remains negligible for plans with conventional lateral beams using both PS and PBS, this is not the case for plans with anterior beams. The target coverage in anterior plans was largely degraded due to the variations in the beam path length and the absence of range margins. The average prostate D95 was reduced by 7.5/15.9% (using PS/PBS) after accumulation for anterior plans, compared with 0/0.4% for lateral plans. The average mean dose in organs‐at‐risk decreased by 1% for lateral and 2% for anterior plans, similarly for PS and PBS. Spot size did not affect the dose changes.Conclusion:Prostate plans using anterior beams may undergo clinically relevant interfractional dose degradation. Corrective strategies guided by in‐vivo range measurements should be studied before clinical application of this technique.