SU‐GG‐J‐106: Prostate and Pelvic Lymph Node PTV Margins for Treatment of Pelvic IMRT Followed by Proton Prostate Boost

Abstract

Purpose: To evaluate the PTV margins for both prostate and pelvic nodes for patients treated with initial Pelvic‐IMRT and prostate proton boost. Method and Materials: Total 15 prostate patients were treated with IMRT and proton therapy with rectal balloons. CBCT and 2D x‐ray were used for pre‐treatment alignments for IMRT and proton treatment, respectively. Shifts and rotations from bone‐matching, seed‐matching and seed‐bone differences were obtained from CBCT with translation only, translation and rotation. Pre‐treatment and post‐treatment 2D x‐ray images were analyzed for residual setup errors and intra‐fraction organ motion. Translation‐only Pelvic‐LN PTV margins were calculated from bone‐seed shift differences. Translation‐only prostate margins were calculated from x‐ray data. Similarly, PTV margins for bone‐matching were also obtained. Bone to skin‐mark rotations and prostate to bone rotations were also presented and evaluated for margin impact. Results: PTV Margin was calculated using 2.5*Σ&+0.7*σ. For seed‐matching, prostate margins are 1.5, 3.5, 2.7 mm for LR, SI, AP, respectively; Similarly, the margins are 2.3, 7.1, 7.0 mm for pelvic‐LN. For bone‐matching, prostate margins are 2.7, 7.8, 7.5 mm for LR, SI, AP, respectively; The margins are 0.7, 1.5, 1.3 mm for pelvic‐LN. For skin‐mark setup, prostate margins are 7.6, 13.5, 11.5 mm and pelvic LN margins are 8.3, 12.7, 12.5 mm. The bone to skin‐mark rotations are 1.3±1.4, −0.5±1.1, −0.2±0.7 mm for LR, SI, AP axes, respectively. The seeds to skin‐mark rotations are −0.2±3.4, −0.8±3.1, 0.2±1.5 mm for LR, SI, AP axes, respectively. Conclusion: Prostate and pelvic LN PTV margins were both obtained from translation only setup using 2D and 3D x‐ray imaging data. Seed‐matching reduced prostate margins, which may reduce dose to bladder and rectum compared to bone‐matching. However, pelvic‐LN margins increased because of the inter‐fraction prostate motion relative to pelvic bones. Rotations of bone and prostate will increase both prostate and pelvic‐LN margins.