Dose-volumetric Predictors for Thyroid Dysfunction after 3D Conformal Radiotherapy: An Observational, Prospective Study

Aim: This study was conducted to assess the dose-volumetric threshold of radiation induced hypothyroidism (RIHT) in patients receiving radiotherapy (RT) to the neck.

Study Design: This is a prospective cohort observational study.

Place and Duration of the Study: The study was conducted at Mansoura University Hospital, Mansoura, Egypt, between April 2016 until March 2019.

Methods: We have completed 2 years of follow up to 50 patients with different malignancies who were treated by radiotherapy to the neck. Baseline assessment of the thyroid clinically and radiologically was done prior to the start of radiotherapy. Periodic testing of the cohort through the follow up period was done by clinical examination, measurement of TSH, fT4 and thyroid ultrasonography.

Results: the incidence of RIHT was 26%. No statistical significance for the clinical risk factors. The dose-volumetric risk factors were studied and showed positive results. A mean dose of 5185 cGy was found a significant risk factor. Also, V40 ≥ 89%, V45 ≥ 63.5%, V50 ≥ 22.5% were found to be the cutoff predictors for the threshold radiation dose to induce hypothyroidism. Also decreasing the size of the gland by ≥0.7 cm3, fT4 value by ≥ 3.5 pmol/L and TSH by ≥ 0.75 uIU/L after one year from the end of EBRT is the cutoff value for prediction of occurrence of RIHT within the 2nd year of follow up.

Conclusion: RIHT is a considerable late adverse effect for patients receiving RT to the neck. Mean dose, V40, V45 and V50 were found significantly related to RIHT. Mean dose of ≥ 5185 cGy, V40 ≥ 89%, V45 ≥ 63.5%, V50≥ 22.5% were proven to be the dose-volumetric threshold.