Incidence of hypothyroidism in head and neck squamous cell carcinoma patients receiving radiotherapy with and without immune checkpoint inhibitors.

e18551 Background: Treatment for locally advanced head and neck squamous cell carcinoma (HNSCC) involves a combination of surgery, chemotherapy, and radiotherapy (RT). RT for HNSCC is a known risk factor for the development of hypothyroidism. Recently, anti-PD1 therapies have been approved for recurrent and metastatic HNSCC and are moving to the forefront of HNSCC care. Similarly, thyroid dysfunction is a common immune-related adverse event following anti-PD1 therapy. Whether the addition of anti-PD1 to RT increases the likelihood of developing hypothyroidism remains unknown. Methods: The rate of hypothyroidism in HNSCC patients receiving RT (+/- chemotherapy and surgery) was compared to HNSCC patients receiving RT + anti-PD1 therapy either concurrently or after RT. Exclusion criteria were preexisting thyroid dysfunction, RT dose < 45 Gy and patients with incomplete treatment records. We defined clinical hypothyroidism as an elevation of TSH with low T3, T4 or elevation of TSH with symptoms requiring levothyroxine initiation. Hypothyroidism incidence was compared using Fisher’s exact test. Results: 153 patients were evaluated. In the RT group (N = 103), patients received RT +/- surgery or chemotherapy. 82/103 (80%) were male, median age was 57 and primary tumor groups included oropharynx 62/103 (60%), larynx 29/103 (28%), oral cavity 9/103 (9%) and other 3/103 (3%). In the RT + anti-PD1 group (N = 50), 36/50 (72%) were males, median age was 57 and primary tumor groups included oral cavity 19/50 (38%), oropharynx 17/50 (34%), larynx 8/50 (16%), and other 6/50 (12%). In the RT group, median follow up after RT was 801 days. In the RT+ anti-PD1 group, median follow up was 595 days from RT and 388 days from anti-PD1. The rate of hypothyroidism was significantly higher in the RT group 22.3% (23/103) versus 6% (3/50)after anti-PD1 therapy (p = 0.011). Multinomial logistical regression found no significant difference in hypothyroidism based on age, sex, or BMI. Larynx as primary tumor location was an independent risk factor for development of hypothyroidism (OR 4.74, p = 0.002). Conclusions: The addition of anti-PD1 therapy to standard HNSCC treatments does not significantly increase the risk of developing hypothyroidism. In fact, this study finds a lower incidence of hypothyroidism in HNSCC patient receiving RT + PD1 therapy which may be due to shorter duration of follow up and lower proportion of laryngeal cancer patients who are at relatively higher risk for surgical hypothyroidism.