Hypothyroidism in Carcinoma of the Tongue with Adjuvant Treatment

An Analytical Study to Determine Dose-Volume Threshold for Radiation Induced Hypothyroidism

Objective: To determine radiation dose volume threshold in predicting the development of hypothyroidism in cancer patients following neck irradiation. Methods: This is a cross sectional follow up study for patients who had been previously irradiated, prior to enrolment in the study. We have done thyroid dose-volumetric analysis on 120 histologically proven cancer patients in the age group of 18-75 years who received neck irradiation as a part of their definitive or adjuvant radiotherapy with three-dimensional conformal or intensity-modulated radiotherapy technique (3D -CRT or IMRT) and completed at least six months post-radiotherapy. Primary tumor sites included carcinoma or lymphoma of the head and neck, breast, cervical, and upper thoracic esophagus, requiring neck irradiation. Results: The proportion of patients who tested positive for Radiation induced hypothyroidism (RIHT) was found to be 40%, with clinical hypothyroidism and subclinical hypothyroidism being 25.8% and 14.2%, respectively. Time to develop hypothyroidism peaks around two years. Mean thyroid gland dose (Dmean) >28 Gy, thyroid gland volume receiving 40 Gy dose (i.e. V40) >49% and age <50 years were found to be significant risk factors for the development of RIHT on binary logistic regression. RT dose >50 Gy and thyroid gland volume spared from 40 Gy (i.e. VS40) < 2.12cm3 were statistically significant predictors for RIHT on chi-square and (Receiver operating characteristic) ROC curve analysis respectively but not on regression analysis. Conclusion: Dose-volume threshold for the thyroid gland as Dmean <28 Gy and V40 <49% may prevent the development of RIHT.

Radiotherapeutic factors affecting the incidence of developing hypothyroidism after radiotherapy for head and neck squamous cell cancer

PURPOSE

The purpose of this study is to determine radiotherapy (RT) dose-volumetric threshold of radiation-induced hypothyroidism (HT) in head and neck squamous cell carcinoma (HNSCC) patients.

PATIENTS AND METHODS

The diagnosis of HT in 78 HNSCC patients treated with RT was based on a thyroid stimulating hormone (TSH) level greater than the maximum value of laboratory range. In all patients, dose-volumetric parameters were analyzed according to their relation to development of HT, and thyroid volumes spared from doses ≥10, 20, 30, 40 and 50 Gy (V10, V20, V30, V40 and V50) were analyzed from the dose volume histograms (DVHs).

RESULTS

Median follow-up duration was 31 months. At the end of study, 33 patients (42.3%) developed HT and the cumulative incidence of HT was 24.6%, 36.5% and 42.3% at one, two and three years, respectively. V30 of 42.1% (P = 0.005) was defined as dose-volumetric threshold of radiation-induced HT in HNSCC patients. Our analysis showed that V30 separates patients into low- and high-risk groups; the incidence of radiation-induced HT in the group with V30 < 42.1% and V30 ≥ 42.1% was 29.4% and 71.4%, respectively (P = 0.002).

CONCLUSIONS

The V30 may predict risk of developing HT after RT for HNSCC patients. V30 of 42.1%, defined as dose-volumetric threshold of radiation-induced HT, can be useful in treatment planning of HNSCC patients.