Patterns of Failure in Anaplastic and Differentiated Thyroid Carcinoma Treated with Intensity-Modulated Radiotherapy

Background: The radiotherapy (RT) volumes in anaplastic (ATC) and differentiated thyroid carcinoma (DTC) are controversial. Methods: We retrospectively examined the patterns of failure after postoperative intensity-modulated RT for ATC and DTC. Computed tomography images were rigidly registered with the original RT plans. Recurrences were considered in-field if more than 95% of the recurrence volume received 95% of the prescribed dose, out-of-field if less than 20% received 95% of the dose, and marginal otherwise. Results: Of 30 DTC patients, 4 developed regional recurrence: 1 being in-field (level III), and 3 being out-of-field (all level II). Of 5 ATC patients, all 5 recurred at 7 sites: 2 recurrences being local, and 5 being regional [2 marginal (intramuscular to the digastric and sternocleidomastoid), 3 out-of-field (retropharyngeal, soft tissues near the manubrium, and lateral to the sternocleidomastoid)]. Conclusions: In DTC, locoregional recurrence is unusual after RT. Out-of-field DTC recurrences infrequently occurred in level II. Enlarged treatment volumes to level II must be balanced against a potentially greater risk of toxicity.