Classic and Follicular Variant of Papillary Thyroid Microcarcinoma: 2 Different Phenotypes Beyond Tumor Size

Abstract

Context: Despite the wide revision of current guidelines, the management of papillary thyroid microcarcinoma (mPTC) still has to be decided case by case. There is conflicting evidence about the role of more frequent histological subtypes, and no data about potential differences at presentation.

Objective: Our aim was to compare the phenotype of the 2 most frequent mPTC variants, namely, classical papillary thyroid microcarcinoma (mPTCc) and the follicular variant of papillary thyroid microcarcinoma (mFVPTC) .

Methods: Retrospective observational study, from January 2008 to December 2017 of a consecutive series of patients with mPTCc and mFVPTC. All cases were classified according to the 2015 American Thyroid Association (ATA) risk classification. Clinical and preclinical features of mPTCc and mFVPTC at diagnosis were collected. The comparison was also performed according to the incidental/nonincidental diagnosis and differences were verified by binary logistic analysis.

Results: In total, 235 patients were eligible for the analysis (125 and 110 mPTCc and mFVPTC, respectively). Compared with mPTCc, mFVPTCs were more often incidental and significantly smaller (4 vs 7 mm) (P < .001 all), possibly influenced by the higher rate of incidental detection. mFVPTC and incidental (P < .001 both) tumors were significantly more often allocated within the low-risk class. A logistic regression model, with ATA risk class as the dependent variable, showed that both mFVPTC (OR 0.465 [0.235-0.922]; P = .028]) and incidental diagnosis (OR 0.074 [0.036-0.163]; P < .001) independently predicted ATA risk stratification.

Conclusion: mFVPTC shows some differences in diagnostic presentation compared with mPTCc, and seems to retain a significant number of favorable features, including a prevalent onset as incidental diagnosis.

Keywords: papillary thyroid microcarcinoma; phenotype; risk assessment; thyroid neoplasms.

Figure 1.

OncoPrint representation of positive genetic mutations. Graphic representation of positive molecular analyses, from 133 analyzed samples. For 84 patients a mutation in BRAF-V600E and/or H-K-N-RAS genes was found. In particular, 50 mPTCc and 19 mFVPTC showed a BRAF-V600E mutation and 3 mPTCc and 14 mFVPTC showed H-K-N-RAS. In 2 cases, 2 simultaneous mutations in BRAF-V600E and H-RAS were found. Negative or inconclusive results are not shown. Abbreviations: mPTC, papillary thyroid microcarcinoma; mPTCc, classical papillary thyroid microcarcinoma; mFVPTC, follicular variant of papillary thyroid microcarcinoma.

Figure 2.

ROC curves plot of AACE/ACE-AME (A) and ACR-TIRADS (B) ultrasound score performance, according to mPTCc histology. (A) AUC of 0.671 (95% CI 0.561-0.781, P = .004); (B) AUC of 0.670 (95% CI 0.552-0.787, P = .004). Abbreviations: ROC, receiver operating characteristic; AACE/ACE-AME, American Association of Clinical Endocrinologists; ACR-TIRADS, American College of Radiology Thyroid Imaging Reporting and Data Systems; mPTCc, classical papillary thyroid microcarcinoma; AUC, area under the curve.