Room: Exhibit Hall | Forum 4
Purpose: To investigate the impact of spot size and intensity modulation on the robustness of pencil beam scanning (PBS) proton therapy of prostate cancer to interfractional motion and range uncertainty.
Methods: For 10 patients with low/intermediate risk prostate cancer participating in a randomized clinical trial, repeat CT scans were acquired weekly. Patients were treated with proton therapy to 79.2Gy(RBE) to prostate and 50.4Gy(RBE) to proximal seminal vesicles in 44 fractions with lateral opposed beams. Four treatment plans per patient were created using an in-house planning system without robust optimization, using spot size of 9 and 3mm median sigma at isocenter for 90â€“230MeV range in air, and single-field optimized (SFO) and intensity-modulated proton therapy (IMPT) methods. Planning method/margins were equivalent. The CT Hounsfield-units (HU) for all images was uniformly scaled by Â±2.5% to represent worst-case scenarios for range uncertainty. The planned dose was recomputed on nominal and HU-modified weekly CTs, scaled and accumulated via deformable registration.
Results: Plan robustness was reduced with increasing single-field intensity modulation. The loss of target coverage represented by the reduction of prostate D98 was on average 0.48, 0.51, 0.67 and 1.26 Gy over all patients for large spots with SFO, large spots with IMPT, small spots with SFO and small spots with IMPT, respectively. The maximum target coverage loss for the above scenarios was 0.74, 4.2, 1.68 and 5.45 Gy. Range uncertainty had a larger impact on target coverage loss than interfractional motion. The organs-at-risk dose was mainly influenced by interfractional variations. The average/maximum increase in bladder and rectum Dmean for small spots with IMPT was 0.42/18.87 and 1.58/2.33Gy, respectively. The average/maximum increase in femoral heads Dmean for this scenario was 0.19/4 Gy, respectively.
Conclusion: SFO-optimized plans are sufficiently robust to interfractional variations and range uncertainties, regardless of spot size. Robust optimization is required for IMPT.