Endocrine Abstracts

JOINT3016

Introduction: Spindle epithelial tumor with thymus-like differentiation (SETTLE) is a rare thyroid malignancy with fewer than 50 cases reported worldwide. Typically arising in children and young adults, it is believed to originate from remnants of the branchial pouch or thymic tissue. It is often misdiagnosed as medullary thyroid carcinoma (MTC) or differentiated thyroid malignancies. Despite its indolent growth, SETTLE has a notable propensity for late metastasis, necessitating early recognition and long-term monitoring.

Case Description: A 14-year-old boy presented with a left-sided neck swelling that rapidly increased in size over three weeks. Prior to presentation at our center, an ultrasound and fine-needle aspiration cytology (FNAC) were performed, which showed spindle cells and was reported as MTC. On examination, apart from the neck mass, the child had a marfanoid habitus. There was no significant family history. Based on these findings, we suspected Multiple Endocrine Neoplasia type 2B and proceeded with the estimation of calcitonin, carcinoembryonic antigen (CEA), and plasma fractionated metanephrines, all of which were within the normal range. Since non-secretory MTC is rare, we reviewed slides of the FNAC performed outside and also conducted a fresh FNAC, which was again reported as MTC. Imaging done to stage the tumor did not reveal any evidence of metastasis. We proceeded with a total thyroidectomy with central compartment lymph node dissection. Histopathological examination of the resected specimen showed a biphasic tumor with spindle cells and cystic areas lined by mucinous columnar cells, with no extrathyroidal extension. Immunohistochemistry showed positivity for CK7, CK5/6, vimentin, BCL2, and CD117. Tumor cells were negative for TTF1, calcitonin, and chromogranin. This led to a diagnosis of SETTLE, and the child is under follow-up.

Discussion: SETTLE is a rare thyroid neoplasm that poses a significant diagnostic challenge. Despite its indolent course, SETTLE carries a significant risk of late metastases, especially to the lungs, occurring years after the initial diagnosis. Definitive diagnosis often requires immunohistochemical examination of resected tissue. Complete surgical resection with long-term follow-up is the mainstay of treatment. Given the potential for late recurrence, close follow-up with periodic imaging is essential.