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.