(P 326) CARBON-ION RADIOTHERAPY FOR UNRESECTABLE PELVIC SARCOMAS/CHORDOMAS

Objective: Carbon ion radiotherapy (CIRT) is a potentially curative treatment for unresectable pelvic sarcomas/chordomas. In this study we discuss early clinical outcomes of unresectable pelvic sarcomas treated with Carbon-ion radiotherapy and sacral nerve sparing strategies robustness plans.

Methods: Individual sacral-nerve roots (L5–S3 levels) were routinely contoured (Figure 1a). CIRT plans prescribed to dosages of 73.6 (64-76.8) Gy RBE/16 fraction (4 fractions/week) using the local effect model-I(LEM-I). We subsequently recomputed doses based on the modified microdosimetric kinetic model (mMKM). Doses to sacral-nerve-roots and sacral-nerves-to-spare (outside high-dose CTV) were limited to D2% < 73 Gy RBE and D5% < 69 Gy RBE (LEM-I) respectively, and hot spots were avoided (Figure 1b). Dose constraints for rectum were respected by inserting surgically inserted spacer without compromising tumor dose coverage.

Results: Pelvic sarcomas/chordoma patients (n =35) treated with CIRT from August 2019 -February 2022 were analyzed. The median follow up was 15 months (range, 3-42.8). Tumor regression kinetics show median regression of tumor at 12 months upto 43% (figure 2a). Two-year local recurrence free survival was 86% (figure 2b). Three patients (9%) developed grade-2/3 radiation induced sacral neuropathy (CTCAE v5) (figure c-d). All the patients remained ambulatory. Only one patient developed transient motor deficit with neuropathy. One patient developed grade 3 radiation dermatitis.

Conclusion: Initial results Carbon-ion radiotherapy for unresectable pelvic sarcomas/chordomas are promising, attempts to further improve outcomes in large pelvic sarcomas by optimizing treatment strategies with CIRT are underway.