Surgery is the most effective treatment for
thyroid cancer; however, in some subsets of patients, the role of
radiotherapy (RT) is important. The main indication for external-beam RT is incomplete surgery. When neoplastic tissue is left behind at operation, RT must be considered, but only if an experienced surgeon feels that everything that can be done has been done. Generally, in those patients, the neoplastic tissue involves the larynx, trachea, esophagus, blood vessels or mediastinum. Of 539 patients with differentiated
thyroid cancer treated at Villejuif, France, until 1976, 97 were treated by external
radiotherapy after an incomplete surgical excision. Fifteen years after irradiation, the survival rate is 57% and is approximately 40% at 25 years. The relapse-free survival is lower (39% at 15 years). In patients irradiated with an adequate dose (greater than or equal to 50 Gy) to residual neoplastic tissue after incomplete surgery, the incidence of local recurrence is low (actuarial probability of local recurrence 11% at 15 years versus 23% for patients treated by surgery alone, although the irradiated patients had larger and more extensive
tumors). This demonstrates the efficacy of external-beam
radiotherapy. The effects of
radiotherapy on a
residual tumor can be monitored by a serum
thyroglobulin assay. With regard to local control of
tumors, the effectiveness of radioiodine administration is clearly lower. However, since radioiodine facilitates early detection of distant
metastases, a combination of external RT and radioiodine is indicated and is well-tolerated. For inoperable patients, the results of RT are limited: although complete remissions are sometimes obtained, the incidence of local recurrence is high. External RT is effective in
medullary carcinoma despite the slow shrinkage of the
tumor after irradiation. Assay of the
calcitonin level helps to monitor the effects of the treatment during follow-up and has demonstrated in some patients the efficacy of cervical RT. In undifferentiated
cancers, the results of RT are poor. Combination of RT and
chemotherapy are being explored despite the disappointing preliminary results of this combination.