[Radiation therapy in thyroid cancer]

Anaplastic thyroid cancers represent 1-2% of all thyroid tumours and are of very poor prognosis even with multimodality treatment including external beam radiation therapy. Conversely, differentiated thyroid carcinomas (at least 80% of thyroid cancers) hamper good prognosis with surgery with or without radioiodine and there is hardly any room for external beam radiation therapy. Insular and medullar carcinomas have intermediary prognosis and are rarely irradiated. We aimed to update recommendations for external beam irradiation in these different clinical situations and put in light the benefits of new irradiations techniques. A search of the French and English literature was performed using the following keywords: thyroid carcinoma, anaplastic, chemoradiation, radiation therapy, surgery, histology and prognostic. Non-mutilating surgery (often limited to debulking) followed by systematic external beam radiation therapy is the standard of care in anaplastic thyroid cancers (hyperfractionated-accelerated radiation therapy with low-dose weekly doxorubicin with or without cisplatin if possible). Given anaplastic thyroid cancers' median survival of 10 months or less, neoadjuvant and adjuvant chemotherapy may also be discussed. Ten-year survival rates for patients with papillary, follicular and Hürthle-cell carcinomas are 93%, 85%, and 76%, respectively. Massive primary incompletely resected iodine-negative disease indicates external beam radiation therapy. Older age (45 or 60-year-old), poor-prognosis histological variants (including tall cell cancers) and insular cancers are increasingly reported as criteria for external beam radiation therapy. Massive extracapsular incompletely resected nodal medullary disease suggests external beam radiation therapy. Radiation therapy morbidity has been an important limitation. However, intensity modulated radiation therapy (IMRT) offers clear dosimetric advantages on tumour coverage and organ sparing, reducing late toxicities to less than 5%. The role of radiation therapy is evolving for anaplastic thyroid cancers using multimodal strategies and new chemotherapy molecules, and for differentiated cancers using minor criteria, such as histological variants, with IMRT becoming a standard of care.