Radiation-Induced Hypothyroidism in Post-Mastectomy Patients: A Prospective Cohort Study

Purpose: Our study aims to assess the incidence of radiotherapy-induced hypothyroidism in breast cancer patients and correlate it with the mean dose to the thyroid. Method: Out of 75 patients, who were treated under 3-dimensional conformal radiotherapy field in field {3D CRT(FiF)} by conventional Radiotherapy to post-modified radical mastectomy (MRM) chest wall, a dosing regimen of 200cGy per fraction for a total 5000cGy at our center in the year 2016, 63 patients were randomized and investigated. All patients were treated by Linear accelerator, varian, with the Eclipse version 13.6 treatment planning system. All information on radiotherapy used for each patient was extracted from the Eclipse treatment planning system. All patients were followed up for 7 years after Radiotherapy. TSH and freeT4 levels were measured. The Chi-square test was used to compare the outcome of the disease in terms of nominal and rank qualitative variables. Student t-test was used to compare in terms of quantitative variables. Result: After 7 years since the radiotherapy, according to the results, a total of 10 out of 63, {15.87% (P = 0.008)} of patients had hypothyroidism, of which 11.11% (7 out of 63) were clinical, and 4.76% (3 out of 63) were subclinical. Dmean > 21 Gy compared with Dmean ≤ 21 Gy was associated with a significantly higher incidence of radiation-induced hypothyroidism (RHT) at 7 years after radiotherapy (3.17% vs. 12.7%, P<0.001). Conclusion: We recommend that the Dmean of the thyroid should be kept lower than 21Gy for post-modified radical mastectomy radiotherapy to decrease the incidence of radiation-induced hypothyroidism. Further investigations in larger cohorts are required to confirm our results.