Brainstem Injury in Pediatric Patients With Posterior Fossa Tumors Treated With Proton Beam Therapy and Associated Dosimetric Factors.

PURPOSE Proton radiation therapy is commonly used in young children with brain tumors for its potential to reduce late effects. However, some proton series report higher rates of brainstem injury (0%-16%) than most photon series (2.2%-8.6%). We report the incidence of brainstem injury and a risk factor analysis in pediatric patients with posterior fossa primary tumors treated with proton radiation therapy at our institution. METHODS AND MATERIALS The study included 216 consecutive patients treated between 2000 and 2015. Dosimetry was available for all but 4 patients. Grade 2 to 5 late brainstem toxicity was assessed by the National Cancer Institute Common Terminology Criteria for Adverse Events version 4.0. RESULTS The histologies include medulloblastoma (n=154, 71.3%), ependymoma (n=56, 25.9%), and atypical teratoid rhabdoid tumor (n=6, 2.8%). The median age at irradiation was 6.6 years (range, 0.5-23.1 years); median dose, 54 gray relative biological effectiveness (Gy RBE) (range, 46.8-59.4 Gy RBE); and median follow-up period, 4.2 years (range, 0.1-15.3 years) among 198 survivors. Of the patients, 83.3% received chemotherapy; 70.4% achieved gross total resection. The crude rate of injury was 2.3% in all patients, 1.9% in those with medulloblastoma, 3.6% in those with ependymoma, and 0% in those with atypical teratoid rhabdoid tumor. The 5-year cumulative incidence of injury was 2.0% (95% confidence interval, 0.7%-4.8%). The median brainstem dose (minimum dose received by 50% of brainstem) in the whole cohort was 53.6 Gy RBE (range, 16.5-56.8 Gy RBE); maximum point dose within the brainstem (Dmax), 55.2 Gy RBE (range, 48.4-60.5 Gy RBE); and mean dose, 50.4 Gy RBE (range, 21.1-56.7 Gy RBE). In the 5 patients with injury, the median minimum dose received by 50% of the brainstem was 54.6 Gy RBE (range, 50.2-55.1 Gy RBE); Dmax, 56.2 Gy RBE (range, 55.0-57.1 Gy RBE); mean dose, 51.3 Gy RBE (range, 45.4-54.4 Gy RBE); and median volume of the brainstem receiving ≥55 Gy RBE (V55), 27.4% (range, 0%-59.4%). Of the 5 patients with injury, 4 had a brainstem Dmax in the highest quartile (≥55.8 Gy RBE, P = .016) and a V55 in the highest tertile (>6.0%) of the cohort distribution (P = .047). Of the 5 patients with injury, 3 were aged >6 years (age range, 4.1-22.8 years), and 4 of 5 patients received chemotherapy and achieved gross total resection. CONCLUSIONS The incidence of injury in pediatric patients with posterior fossa tumors is consistent with previous reports in the photon setting. Our data suggest that when Dmax and V55 are kept <55.8 Gy RBE and ≤6.0%, respectively, the 5-year rate of radiation brainstem injury would be <2%.