(P050) Hypothyroidism After Definitive Radiation Therapy for Oropharyngeal Cancer

Chiaojung J. Tsai, MD, PhD, Guojun Li, PhD, Erich M. Sturgis, MD, Adam S. Garden, MD; UT MD Anderson Cancer Center

Purpose and Objective: We conducted a retrospective cohort study of oropharyngeal cancer patients to evaluate the incidence of hypothyroidism after definitive radiation therapy (RT).

Materials and Methods: Follow-up records of 439 oropharyngeal cancer patients staged I–IVb and treated with definitive RT between January 2000 and October 2008 were reviewed. The occurrence of hypothyroidism was defined as elevated levels of thyroid-stimulating hormone (TSH) above 5.50 mcU/mL. Patients with baseline hypothyroidism before treatment were excluded from the study. Cox proportional hazards analysis was used to compute hazard ratios (HRs) comparing patients with and without hypothyroidism.

Results: A total of 408 out of 439 patients had follow-up data on TSH levels and were included in the analysis; 200 patients (49%) developed hypothyroidism after radiation. The median time to developing hypothyroidism was 12 months (range: 1–69 mo). Of the 200 patients with hypothyroidism, 13 (7%) did not take oral thyroid hormone replacement, and their median TSH was 5.84 mcU/mL (range: 5.53–8.38 mcU/mL). Further, 156 of the 200 patients had information on thyroid hormone replacement use, and their median TSH level was 7.4 mcU/mL (range: 5.55–119 mcU/mL). There was no difference in the occurrence of hypothyroidism between patients treated with 3D-conformal radiation vs intensity-modulated RT ([IMRT] 54% and 48%, respectively; chi-square P = .34). Patients who received ≥ 70 Gy of the prescribed radiation dose to the primary tumor had a higher risk of hypothyroidism compared with those whose prescription dose was < 70 Gy (55% and 44%, respectively; P = .04). In univariate Cox proportional hazards analyses, patients who were never-smokers had a higher risk of hypothyroidism compared with current smokers (HR = 1.6; P = .02). Age, sex, treatment technique (3D vs IMRT), clinical T category, clinical N category, and chemotherapy use were not significantly associated with hypothyroidism risk. Multivariate Cox proportional models demonstrated elevated risk of hypothyroidism among never-smokers (HR = 1.6; P = .02) and those who received a prescribed radiation dose of 70 Gy or more (HR = 1.2; P = .04) after adjusting for age, sex, treatment technique, and chemotherapy use.

Conclusion: Patients who received a radiation prescription dose of 70 Gy or more to the primary tumor and were never-smokers had an increased risk of hypothyroidism after definitive RT for oropharyngeal cancer.