Minimally invasive follicular thyroid cancer: treat as a benign or malignant lesion?

Follicular thyroid cancer is the second most common thyroid cancer, accounting for 10%-15% of all cases. Follicular thyroid carcinomas (FTCs) can be classified into two subtypes: classic (C), which exhibit both vascular and capsular invasion and minimally invasive (MI), which only has limited capsular invasion. Both types, like most well-differentiated thyroid cancers, are traditionally treated the same: a completion thyroidectomy usually followed by radioiodine ablation. We hypothesize that MI-FTC may behave more like a benign follicular adenoma rather than C-FTC and may not require total thyroidectomy and radioiodine.

A prospective thyroid database was screened for patients with follicular cell tumors. Data on recurrence rates, disease-free survival, and requirement for follow-up surgery and/or radioiodine were compared. Disease-free survival was determined by the Kaplan-Meier method. Analysis of variance and chi-square test were used to evaluate other factors.

In total, there were 419 benign adenomas (87%), 21 MI-FTCs (4.5%), and 41 C-FTCs (8.5%). Patients with adenomas were younger (P = 0.035) and were more likely to be female (P = 0.001). Importantly, the 16-y disease-free survival was 100% in the adenoma group, 100% in the MI-FTC group, and 36.6% in the C-FTC group (P < 0.0001).

MI-FTCs behave similar to adenomas with 100% disease-free survival with up to 16 y of follow-up. These data suggest MI-FTCs could be potentially treated by thyroid lobectomy alone like follicular adenomas and perhaps should be classified as a distinct clinical entity.