الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Head and neck cancer is the sixth most common cancer in the world. It is predominantly a loco-regional disease, and radiotherapy (RT) and surgery are major treatment options.
Radiation to the normal tissues is inevitable during RT. The current advanced treatment modalities allow precise calculation of radiation doses to normal tissues and there is a potential to distribute the dose to
Avoid adverse effects from radiation. However, this requires knowledge about the radiation tolerance levels of individual organs.
Radiation-induced hypothyroidism (RIHT) is a well-known late effect of radiation to the thyroid gland, which can develop months to years after RT. The reported incidence of RIHT varies considerably, however, and the tolerance level of the thyroid gland is poorly defined.
Our study included 50 HNSCC patients presented at Clinical Oncology and Nuclear Medicine Department (ACOD) Faculty of Medicine Alexandria University and other specialized oncology centers in Alexandria Governorate. All patients were treated with 3-D conformal radiotherapy with or without concurrent chemotherapy during the period between January 2013 and December 2015.
Analysis of patient characteristics, precise radiation doses to the thyroid gland and follow-up assessments of thyroid function after radiotherapy gave an incidence of RIHT of 26%.
Significant risk factors for the development of RIHT were a female gender, young age, small thyroid volume, high minimum and maximum radiation dose to the gland and thyroid volume irradiated to 30 Gy and 40 Gy (V30, V40), but V30 was more significant.
from ROC (Receiver operating characteristic) analyses V30 of 50% can be considered as a dose-volumetric threshold of radiation-induced HT.
In multivariate analysis there was no correlation found for significant clinical or dose volumetric parameters on univariate analysis.
In our study Patients who developed hypothyroidism in the post-treatment period had non-statistically significant higher RFS compared with patients who did not