Unusual papillary thyroid carcinoma with hyalinizing trabecular tumor-like feature in a young female patient: a case report

BACKGROUND: Hyalinizing trabecular tumor is a rare follicular cell-derived thyroid neoplasm that is considered to be a borderline tumor with malignant potential rather than a benign tumor. The detection of RET/PTC rearrangements and nuclear cytologic features suggests a relationship between hyalinizing trabecular tumor and papillary thyroid carcinoma. Some recent observations of pathogenic genetic alterations in hyalinizing trabecular tumor have indicated that hyalinizing trabecular tumor is not related to papillary thyroid carcinoma, and should be considered an independent entity. Here we present a case of papillary thyroid carcinoma with hyalinizing trabecular tumor-like features and discuss its interesting aspects and diagnostic issues from a histopathological perspective. CASE PRESENTATION: A 19-year-old Japanese woman with an enlarged thyroid gland was admitted to our hospital. Based on fine-needle aspiration cytology, the enlarged nodule was suspected to be a follicular lesion or follicular tumor. A nodular lesion approximately 3 cm in diameter was detected in the left lobe of the thyroid gland. Histological analysis revealed that the tumor cells were mainly arranged in follicles. Solid nests with occasional trabecular arrangements and papillary structures were intermingled, and the tumor cells showed ground-glass nuclei and occasional nuclear grooving. Petaloid and block-like periodic-acid-Schiff and periodic-acid-methenamine-positive basement membrane components were observed in the interstitium of the solid portions of the tumor. Incomplete membranous immunoreactivity of MIB-1 (Ki-67 (cell prolferation marker)) was also observed in the cells within the solid areas. Moreover, this tumor displayed extracapsular invasion and metastasis to the perithyroidal lymph nodes, suggesting that it may be a malignant tumor. However, BRAF(V600E) mutation, RET/PTC rearrangements, and PAX8/GLIS 1 and PAX8/GLIS 3 rearrangements were not detected. CONCLUSION: We diagnosed the tumor as a papillary thyroid carcinoma with characteristic features of hyalinizing trabecular tumor. Importantly, this case may indicate a possible relationship between papillary thyroid carcinoma and hyalinizing trabecular tumor, although specific genetic alterations could not be detected. We also discuss the preoperative diagnostic difficulties with fine-needle aspiration cytology and the unusual pathological findings in this case.