Risk stratification of follicular variant of papillary thyroid carcinoma

Recent studies have described an encapsulated and an infiltrative form of follicular variant of papillary thyroid carcinoma (FVPTC). While encapsulated tumors have been reported to have virtually no metastatic potential or recurrence risk if angioinvasion and capsular penetration are absent, infiltrative tumors have been found to have a significant metastatic potential and a risk of recurrence. In our experience, a substantial number of FVPTCs are neither fully encapsulated nor infiltrative, but instead are partially-encapsulated (PE) or well-circumscribed (WC). Thus, the aim of this study was to investigate the metastatic potential and recurrence risk of PE/WC FVPTCs in comparison with that of encapsulated and infiltrative tumors.

We studied 77 FVPTCs resected between 2000 and 2002 and characterized the tumors as encapsulated, PE/WC, or infiltrative. Histologic assessment was then correlated with lymph node status and clinical outcome.

In our cohort, 27 (35%) tumors were encapsulated, 35 (45%) were PE/WC, and 15 (19%) were infiltrative. Lymph node status was similar between PE/WC and encapsulated tumors, but was significantly different between encapsulated and infiltrative groups (p<0.001), and PE/WC and infiltrative groups (p<0.001). Lymph node metastases were absent in all 15 cases of encapsulated tumors and all 9 cases of PE/WC tumors with sampled lymph nodes, but were present in 7 of 9 (78%) cases of infiltrative tumors with sampled lymph nodes. For patients with available clinical follow-up (66 cases, 86%), the median follow-up time was 111 months. No patients with encapsulated tumors recurred, one (3%) patient with a PE/WC tumor had recurrent/residual disease, and two (15%) patients with infiltrative tumors had recurrent/residual disease. The one patient with a PE/WC tumor who had recurrent/residual disease had a tumor bed recurrence 7 years after initial resection. Significantly, this was the only patient in the PE/WC group that had a positive resection margin.

Our results demonstrate that PE/WC FVPTCs have a very low metastatic potential/recurrence risk, indicating that they should be distinguished from more aggressive infiltrative FVPTCs.