DOSIMETRIC AND EARLY CLINICAL OUTCOMES OF THYROID-SPARING VOLUMETRIC MODULATED ARC RADIOTHERAPY IN LOCALLY ADVANCED HEAD AND NECK CANCERS.

Introduction: Radiation-induced hypothyroidism is a well-recognized entity that occurs after an interval of 15-21 months. However, in the treatment of locally advanced Head and neck Squamous cell carcinoma (HNSCC), thyroid-sparing techniques are infrequently employed.

Aims: To evaluate the dosimetric and early clinical outcomes of thyroid-sparing SIB-VMAT technique (Simultaneous Integrated Boost - Volumetric Modulated Arc Radiotherapy) in patients of locally advanced HNSCC.

Methods: In this two-arm prospective pilot study, patients in the study group received radiotherapy by SIB-VMAT technique with a thyroid constraint to a dose of 70 Gy to the gross disease and 59.4 Gy to nodal and subclinical disease in 33 fractions over 6 ½ weeks with concurrent cisplatin. V50Gy<75% was the thyroid constraint used. The control group was treated with the same dose and technique but without using a thyroid gland constraint. The dose-volume parameters of the thyroid gland, PTV (Planning Target Volume) along with thyroid profile were analyzed.

Results: Twenty-six patients were recruited. Thyroid V50Gy of the study group (65.33 ±6.63 %) was significantly lower than that of control group (80.35 ±13.40 %) (p=0.003). Tumor dose parameters of both groups were compared and revealed no significant difference. At 18 months follow-up, the incidence of any degree of hypothyroidism was 46.15% in the study group and 23.07% in the control group (p=0.216).

Conclusion: In locally advanced HNSCC, it is feasible to spare the thyroid gland without compromising the tumour coverage. This has the potential to reduce the frequency of radiation-induced hypothyroidism.