SU‐G‐JeP4‐09: Impact of Interfractional Motion On Hypofractionated Pencil Beam Scanning Proton Therapy for Prostate Cancer

Abstract

Purpose:To investigate the impact of anatomy/setup variations on standard vs. hypofractionated anterolateral pencil beam scanning (PBS) proton therapy for prostate cancer.Methods:Six prostate cancer patients treated with double‐scattering proton therapy, who underwent weekly verification CT scans were selected. Implanted fiducials were used for localization, and endorectal balloons for immobilization. New PBS plans using combination of lateral and anterior‐oblique (AO) (±35 deg) beams were created. AO beams were added to spare the femoral heads during hypofractionation. Lateral beams delivered 50.4 Gy(RBE) to prostate plus 5‐15mm of seminal vesicles and AO beams 28.8 Gy(RBE) to prostate, in 44 fractions. PTV was laterally expanded by 2.5% to account for range uncertainty. No range margins were applied for AO beams, assuming delivery with in‐vivo range verification. Field‐specific apertures with 1.2cm margin were used. Spot size was ∼9.5mm sigma for 172MeV @isocenter in air. Plans were optimized as single‐field‐uniform‐dose with ∼5% maximum non‐uniformity. The planned dose was recomputed on each weekly CT after aligning the fiducials with the simulation CT, scaled and accumulated via deformable image registration. Hypofractionated treatments with 12 and 5 fractions were considered. Equivalent doses were calculated for prostate (α/β= 1.5Gy), bladder and rectum (α/β= 3Gy).Results:The biological equivalent prostate dose was 86.2 and 92.9 Gyeq for the hypofractionation scenarios at 4.32 and 7.35 Gy/fx, respectively. The equivalent prostate D98 was degraded by on average 2.7 Gyeq for standard, and 3.1 and 4.0 Gyeq for the hypofractionated plans after accumulation. Differences between accumulated and planned Dmean/D2/EUD were generally reduced when reducing the number of fractions for bladder and rectum. The average Dmean/D2/EUD differences over all patients and organs‐at‐risk were 0.74/4.0/9.23, 0.49/3.64/5.51, 0.37/3.21/3.49 Gyeq for 44, 12 and 5 fractions.Conclusion:Hypofractionation makes proton therapy of prostate more susceptible to interfractional motion‐induced target dose degradation compared to the standard fractionation.