Lumbosacral Plexopathy After Carbon-ion Radiation Therapy for Postoperative Pelvic Recurrence of Rectal Cancer: Subanalysis of a Prospective Observational Study (GUNMA 0801)

Purpose Data are lacking on the risk factors for radiation-induced lumbosacral plexopathy (RILSP) after carbon-ion radiation therapy (CIRT) for pelvic tumors, such as postoperative recurrence of rectal cancer. We investigated the incidence of RILSP and the associated dosimetric parameters using data from a prospective study of CIRT for postoperative pelvic recurrence of rectal cancer (GUNMA 0801). Methods and Materials The GUNMA 0801 study included 28 patients, of which we analyzed 20 without lumbosacral plexopathy prior to CIRT. The total dose of CIRT was 73.6 Gy (relative biological effectiveness [RBE]) in 16 fractions. The incidence of RILSP and parameters of the dose-volume histogram were evaluated for the lumbosacral plexuses. RILSP was graded according to the Common Terminology Criteria for Adverse Events version 4.0. Results Median follow-up was 24 months. The incidence of all RILSP (grades 1 and 2) and grade 2 RILSP was 22.5% (9/40) and 10% (4/40) of 40 lumbosacral plexuses in 20 patients, respectively, and no grade >= 3 toxicity was observed. Throughout the dose range, the volumes of the irradiated lumbosacral plexuses were significantly higher in patients with RILSP than in patients without RILSP (P < .001 for Dmax, D0.5 cm(3) - D2 cm(3), V20 Gy(RBE) - V70 Gy(RBE)). D2 cm(3) and V50 Gy(RBE) were considered useful for receiver operating characteristic analysis. Cutoff values for RILSP were 73.82 Gy(RBE) and 33.2% for D2 cm(3) and V50 Gy(RBE), respectively. Conclusions We demonstrated the incidence and predictive dosimetric parameters for RILSP after CIRT and showed that D2 cm(3) >= 73.82 Gy(RBE) and V50 Gy(RBE) = 33.2% are cutoff values for predicting RILSP. These results would improve treatment plans using CIRT for patients with pelvic recurrences of rectal cancer.