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

PURPOSE/OBJECTIVE(S)

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.

MATERIALS/METHODS

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.

RESULTS

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.

CONCLUSION

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.

Uteroplacental insufficiency decreases p53 serine-15 phosphorylation in term IUGR rat lungs

Intrauterine growth restriction (IUGR) increases the incidence of chronic lung disease (CLD). The molecular mechanisms responsible for IUGR-induced acute lung injury that predispose the IUGR infant to CLD are unknown. p53, a transcription factor, plays a pivotal role in determining cellular response to stress by affecting apoptosis, cell cycle regulation, and angiogenesis, processes required for thinning of lung mesenchyme. Because thickened lung mesenchyme is characteristic of CLD, we hypothesized that IUGR-induced changes in lung growth are associated with alterations in p53 expression and/or modification. We induced IUGR through bilateral uterine artery ligation of pregnant rats. Uteroplacental insufficiency significantly decreased serine-15-phosphorylated (serine-15P) p53, an active form of p53, in IUGR rat lung. Moreover, we found that decreased phosphorylation of lung p53 serine-15 localized to thickened distal air space mesenchyme. We also found that IUGR significantly decreased mRNA for targets downstream of p53, specifically, proapoptotic Bax and Apaf, as well as Gadd45, involved in growth arrest, and Tsp-1, involved in angiogenesis. Furthermore, we found that IUGR significantly increased mRNA for Bcl-2, an antiapoptotic gene downregulated by p53. We conclude that in IUGR rats, uteroplacental insufficiency induces decreased lung mesenchymal p53 serine-15P in association with distal lung mesenchymal thickening. We speculate that decreased p53 serine-15P in IUGR rat lungs alters lung phenotype, making the IUGR lung more susceptible to subsequent injury.