Given the high incidence and excellent prognosis of many papillary thyroid microcarcinomas, the Porto proposal uses the designation papillary microtumor (PMT) for papillary microcarcinomas (PMCs) without risk factors to minimize overtreatment and patients’ stress. To validate Porto proposal criteria, we examined a series of 190 PMC series, also studying sex hormone receptors and BRAF V600E mutation. Our updated Porto proposal (uPp) reclassifies as PMT incidental PMCs found at thyroidectomy lacking the following criteria: (a) detected under the age of 19 years; (b) with multiple tumors measuring >1 cm adding up all diameters; and (c) with aggressive morphologic features (extrathyroidal extension, angioinvasion, tall, and/or hobnail cells). PMCs not fulfilling uPp criteria were considered “true” PMCs. A total of 102 PMCs were subclassified as PMT, 88 as PMC, with no age or sex differences between subgroups. Total thyroidectomy and iodine-131 therapy were significantly more common in PMC. After a median follow-up of 9.6 years, lymph node metastases, distant metastases, and mortality were only found in the PMC subgroup. No subgroup differences were found in calcifications or desmoplasia. Expression of estrogen receptor-α and estrogen receptor-β, progesterone receptor, and androgen receptor was higher in PMC than in nontumorous thyroid tissue. BRAF mutations were detected in 44.7% of PMC, with no differences between subgroups. In surgical specimens, the uPp is a safe pathology tool to identify those PMC with extremely low malignant potential. This terminology could reduce psychological stress associated with cancer diagnosis, avoid overtreatment, and be incorporated into daily pathologic practice.

The hobnail variant (HV) of papillary thyroid carcinoma (PTC) is an unusual entity recently proposed as an aggressive variant of PTC. We describe the pathologic and molecular features of 2 cases of HV of PTC. Both tumors presented in stage III (pT3 pN1a M0). The first case was diagnosed in a 62-year-old man, whereas the second was in a 53-year-old woman. Both patients were treated with total thyroidectomy and radioactive iodine. The primary tumors showed a hobnail/micropapillary pattern in ≥50% of the neoplasm, and positivity for TTF-1, TTF-2, thyroglobulin (TG), cyclin D1, and p53. The Ki-67 index was 4.6% and 5%, respectively. In case 1, the tumor disclosed BRAFV600E and TERT C228T (124:G>A) promoter gene mutation, negativity for NRAS, HRAS, and KRAS mutations, and negativity for RET/PTC1, RET/PTC3, and PAX8/PPARγ rearrangements. After 11 years the patient died with cervical lymph node, bone, and liver metastases. In the liver metastasis, the tumor displayed columnar cell PTC areas (positive for TTF-1, TG, and BRAFV600E) merging with undifferentiated carcinoma (UC) areas (positive for TTF-1 and BRAFV600E; negative for TG). In case 2, the patient died 6 years after treatment with local recurrence and disseminated metastases to the lung, pleura, bone, and liver. The tumor recurrence showed a UC component (positive for cyclin D1 and p53; negative for TTF-1 and TG) with a residual HV of PTC (positive for cyclin D1, p53, TTF-1, and TG). No BRAF, TERT, NRAS, HRAS, nor KRAS mutations were detected in the primary tumor or recurrence in case 2. Our findings suggest that p53-positive HV is a very aggressive form of PTC prone to progression to UC.

Solid cell nests (SCNs) of the thyroid are single or multiple foci of solid and/or cystic clusters of squamoid cells (main cells) with a minor proportion of C-cells, found in the normal thyroid. The SCNs have also been reported in the heart as an ultimobranchial heterotopia. Here, the authors describe a case of thyroid-type SCNs associated with struma ovarii. Main cells were positive for simple and stratified epithelial-type cytokeratins, carcinoembryonic antigen, carbohydrate antigen 19.9, p63, bcl-2, and galectin-3. The neuroendocrine cell population was positive for chromogranin A and synaptophysin but negative for calcitonin, suggesting a common ancestor cell capable of dual differentiation toward thyroid follicular cells and hindgut-type endocrine cells. The existence of thyroid-type SCNs in struma ovarii could be easily understood by considering the struma ovarii as a teratoma; at the same time, these findings also support the idea of a close histogenetic link between the main cells of SCNs and thyroid tissue.

Cribriform morular thyroid carcinoma (CMTC) is a rare malignant thyroid tumor with a peculiar growth pattern secondary to permanent activation of the WNT/β-catenin pathway. CMTC may be associated with familial adenomatous polyposis or sporadic; it shares morphological features with papillary thyroid carcinoma (PTC) and was considered a variant of PTC in the 2017 WHO classification of tumors of endocrine organs. The new 5th edition of the WHO classification of endocrine and neuroendocrine tumors considered CMTC an independent thyroid neoplasm of uncertain histogenesis. A thymic/ultimobranchial pouch-related differentiation in CMTC has been recently postulated. We, however, have used the pathological and immunohistochemical features of this case of CMTC with 2 novel oncogenic somatic variants (c.3428_3429insA, p.(Tyr1143Ter) and c.3565del, p. (Ser1189Hisfs*76) of the APC gene to propose an origin from follicular cells (or their endodermal precursors). As usual in CMTC, the morular component of this tumor was positive for CDX2. Given the fact that WNT/β-catenin signaling, through CDX2, activates large intestine and small intestine gene expression, we postulate that in CMTC, the tumor cells have their terminal differentiation blocked, thus showing a peculiar primitive endodermal (intestinal-like) phenotype negative for sodium-iodide symporter, thyroperoxidase, and thyroglobulin. Establishing the histogenesis of CMTC is very relevant for the development of appropriate therapies of redifferentiation, particularly in patients where the tumor cannot be controlled by surgery.