Clinical Outcomes of Follicular Thyroid Carcinoma Did Not Significantly Differ according to Tumor Size in an Iodine-Excessive Country.

Background: Follicular thyroid carcinoma (FTC) measuring <2 cm is generally associated with good prognosis, while tumor size ≥4 cm is regarded as being associated with a poor prognosis. This study aimed to evaluate FTC prognosis by tumor size to investigate whether the 2- and 4-cm criteria are appropriate for assessing prognosis.

Methods: Data of 248 patients with FTC diagnosed between August 1995 and June 2021 were retrospectively analyzed. The population was divided into four groups according to tumor size: <2.0, 2.0-3.9, 4.0-5.9, and ≥6.0 cm. Distant metastasis (DM), recurrence and/or structural persistence (R/SP), cancer-specific death (CSD), and frequency of telomerase reverse transcriptase (TERT) promoter mutations based on tumor size were evaluated.

Results: While the rates of DM, R/SP, and CSD and the frequency of TERT promoter mutations did not differ among the size groups <6 cm, they increase sharply in tumor size ≥6 cm, although statistically insignificant (P=0.608, P=0.248, P=0.089, and P=0.165 respectively). Widely invasive subtypes, and TERT promoter mutations were significantly associated with DM (P=0.009 and P<0.001, respectively). Age ≥55 years, gross extrathyroidal extension, synchronous DM, and TERT promoter mutation were independent risk factors for CSD (P=0.005, P=0.003, P<0.001, and P=0.002, respectively).

Conclusion: DM, R/SP, CSD, and TERT promoter mutations were not uncommon in FTCs <2 cm compared to those in larger FTCs, whereas FTCs ≥6 cm showed a sharp decline in prognosis, although this was statistically insignificant.