Background: Anaplastic thyroid carcinoma (ATC) is occasionally found on postoperative pathological examination of patients with differentiated thyroid carcinoma (DTC). There is no general consensus on how we should treat these incidentally diagnosed ATC (incidental ATC).
Materials and methods: A total of 675 patients with ATC were registered with the ATC Research Consortium of Japan. These patients were treated between 1995 and 2008 in 38 registered institutions. About 81 % of the ATC patients had common-type ATC and about 14 % had ATC co-existing with a metastatic DTC lesion. The remaining 5 % had incidental ATC. Among the patients with incidental ATC, we investigated 25 patients whose clinical data were fully available. We examined the clinical profile of incidental ATC, and the relationships between treatment and outcome in patients with incidental ATC.
Results: The tumor size was clearly smaller, and patients with extrathyroid invasion or distant metastasis were significantly fewer in incidental ATC than in common-type ATC. Most incidental ATC coexisted with papillary carcinoma. While the clinical course of incidental ATC was favorable compared with common-type ATC, half of the patients had disease-related deaths. The prognostic factors of incidental ATC were nearly the same as those of common-type ATC, but the tumor size alone was an independent factor on multivariate analysis. Regarding treatments, the outcome was more favorable in those who underwent curative resection, and the clinical course showed a slight improvement by the addition of external beam radiotherapy and/or chemotherapy after curative resection, but it did not reach statistical significance.
Conclusion: Incidental ATC is the only curable type of ATC, and further studies are needed to establish the effectiveness of additional postoperative radiotherapy and/or chemotherapy in incidental ATC.