MO‐A‐137‐04: Robustness of Bone Marrow Sparing in Intensity Modulated Proton Therapy for Cervical Cancer

Purpose: To determine the effectiveness of pelvic bone marrow sparing by utilizing intensity modulated proton therapy (IMPT) in the treatment of cervical cancer, in comparison to that of intensity modulated photon therapy (IMXT); and to investigate the robustness of the bone marrow sparing by IMPT under range uncertainty and patient setup uncertainty. Methods: 3‐field IMPT was planned for five enrolled patients. DVH objectives for PTV and organs‐at‐risk (OAR) aimed to give 45 Gy to the tumor in 25 fractions, matching those of the IMXT plan delivered clinically. The percentage volumes of bone marrow receiving 10 Gy and 20 Gy were set to be reduced in IMPT compared with IMXT. The dose from the IMPT plan was then recalculated in uncertainty scenarios which were combinations of systematic 3% density errors and uniformly random patient setup errors of (0.6cm, 0.6cm, 0.6cm). The robustness of target coverage and bone marrow sparing through the whole course of 25 fractions was then analyzed. Results: In the static scenario, the IMPT plan achieved the same or better PTV coverage and OAR sparing compared to IMXT, and reduced the percentage volume of bone marrow receiving 15 Gy or larger dose (V15Gy) to 32.3% from 60.0% on average. Robustness analysis showed that proper CTV coverage is preserved over the whole course of treatment with the presumed range and positioning errors. The reduction of bone marrow dose is also statistically significant and robust under those uncertainties, with a minimum reduction of V15Gy to 39.9% from 60.0%. Conclusion: IMPT can significantly reduce the volume of active bone marrow receiving low dose radiation, therefore potentially decreasing hematologic toxicity in concurrent chemoradiation therapy for cervical cancer. Bone marrow sparing by IMPT is robust in the context of range and positioning uncertainties.