Does the presence of capsule influence prognosis in poorly differentiated thyroid carcinoma?

We collected all cases of poorly differentiated thyroid carcinoma at our institution diagnosed between 2007 and 2022 to investigate the role of tumor capsule in these neoplasms along with other histologic factors that may lead to adverse patient outcomes. After the exclusion of those meeting criteria for differentiated high-grade thyroid carcinoma or anaplastic carcinoma, we were left with 65 cases with a poorly differentiated component. Four of those cases (6.2%) were entirely encapsulated with no invasion of the tumor capsule. Unencapsulated tumors showed significantly greater rates of extrathyroidal extension (75.0% versus 41.5%) and death from disease (45.5% versus 12.5%) than encapsulated tumors, regardless of capsular invasion, with no differences in sex, tumor size, angioinvasion, local recurrence, or metastasis. Compared with encapsulated tumors with invasion, encapsulated tumors without capsular invasion showed a strong male predominance (100% versus 38.8%). No encapsulated tumors without capsular invasion demonstrated local recurrence, metastasis, or death from disease. No differences in the percentage of poorly differentiated components were noted among the 3 groups, although there was a trend for encapsulated tumors to have a higher percentage of poorly differentiated components than unencapsulated tumors. We conclude that invasive tumors lacking a capsule demonstrate greater rates of disease-related death despite showing similar adverse histologic features to invasive encapsulated tumors. Moreover, we confirm that encapsulated tumors without capsular invasion have excellent long-term outcomes in terms of recurrences, metastases, and survival.