Impact of Dosimetric Parameters on Local Control and Toxicity of Head and Neck SBRT

SBRT is potentially useful as a local therapy for head and neck cancer patients who require re-irradiation, as well as those who may not be candidates for surgical resection or a lengthy course of conventionally fractionated radiation therapy. The objective of this study was to assess rates of local control with SBRT, as well as the impact of mucosal dosimetric parameters on rates of grade 3 or higher toxicity in patients treated with head and neck SBRT.We retrospectively reviewed patients within our institution who underwent SBRT for cancers of the oral cavity and oropharynx between 2013 and 2022. Primary endpoints were local control (LC) and grade 3 or higher toxicity potentially attributable to SBRT, based on CTCAE 5.0. We analyzed the following endpoints as potential predictors of toxicity: 1) ratio of total oral mucosal volume/oral mucosal volume outside of PTV, 2) mean dose to oral mucosa, and 3) maximum dose to oral mucosa. We conducted regression analysis to determine predictors of local failure and severe toxicity. We estimated local control using Kaplan-Meier analysis.We treated 66 tumors in 60 patients with a median age of 71 years. 41 patients (68.3%) had oral cavity cancer and 19 (31.6%) had oropharynx cancer. 64 tumors (97.0%) were squamous cell carcinomas. 32 tumors (48.5%) were previously irradiated. Mean PTV volume was 55 cc (range: 39.4-74.1 ccs). Median prescribed radiation dose was 40 Gy given in 5 biweekly fractions. A total of 51 patients received systemic therapy (platinum-based chemotherapy in 52%, Cetuximab in 38%). 10 patients (16.7%) additionally received immunotherapy, constituting 56% of 18 patients treated from 2019-2022. Median pain score at presentation was 3/10. Oral pain non-significantly increased between 3 weeks and 3 months after starting treatment and subsequently returned to baseline after SBRT (P = 0.227). Local control was 68.5%, at a median follow up of 9.8 months. In oral cavity tumors, 1- and 2-year local control rates were 78.7% and 43.1% respectively. 1- and 2-year rates of LC were 78.9% and 50.2% in oropharynx tumors. Grade 3 or higher toxicities were present in 18 patients (30.0%), including osteonecrosis in 6 (10.0%) and ulceration or extensive tissue necrosis in 12 (20.0%). No significant relationship was present between mucosal surface radiation doses and acute oral mucosa toxicity. On regression analysis for both local control and grade 3 or higher toxicity, we did not find any significant association with prior radiation, disease site, age, or PTV volume.SBRT provided comparable local control for tumors of the oropharynx and oral cavity, with slightly higher 2-year local control in tumors of the oropharynx, and comparable rates of toxicity. We appreciated increased use of immunotherapy in our study population from 2019 onwards. We did not find any relationships between dosimetric parameters and rates of grade 3 or higher toxicity, or local control, though our analysis is limited by a small sample size.