How to Prevent Local Recurrence of Sacral Chordoma Treated with Carbon-Ion Radiotherapy: An Analysis of the Risk Factors of Local Failure and an Adequate Disease Margin.

Introduction: Recent reports have described the usefulness of carbon ion radiotherapy (CIRT) for inoperable sacral chordomas. However, its long-term local control rate needs to be improved. The present study identified the risk factors that affect the local relapse of sacral chordomas and the appropriate margins from the tumors.

Methods: Forty-nine patients with sacral chordoma treated with CIRT between 2011 and 2022 were retrospectively analyzed. Factors predicting the risk of local recurrence were evaluated, including age, sex, tumor size, muscle invaded with tumor, and surgery before CIRT. To determine the appropriate margin, the distance between the clinical target volume (CTV) and the out-field recurrent lesions was analyzed.

Results: The patients included 37 males and 12 females with a mean age of 67.1 years. A multivariate analysis showed that a tumor size >8 cm and invasion into the gluteus maximus muscle were significant risk factors with hazard ratios of 5.56 and 15.20 (p = 0.02 and 0.01), respectively. Out-field recurrence occurred in 13 cases, with 6, 3, and 4 relapses occurring in the muscle, bone, and both, respectively. The tumor occurred within 20 mm from the CTV in 60% of relapses in the muscles.

Conclusion: The current study presented novel findings on CIRT for sacral chordomas, although there were several limitations, such as a short follow-up period to investigate slow-growth tumors and a small number of tumor specimens owing to inoperative cases. A tumor size >8 cm and invasion into the gluteus maximus muscle were shown to be risk factors for recurrence in the treatment of sacral chordoma with CIRT. Our findings further suggest that an additional 2-cm margin from the CTV in the muscle fiber direction is recommended during CIRT.