Cytomorphology of monomorphic spindle epithelial tumor with thymus-like elements: A case with local recurrence after subtotal resection

Spindle epithelial tumor with thymus-like elements (SETTLE) is a rare biphasic thyroid tumor with low malignant potential that has a distinct morphology. Despite fine needle aspiration (FNA) being a common method for evaluating thyroid nodules and lymph nodes, there are limited cytologic descriptions of SETTLE in the literature due to its rarity. As a result, SETTLE is frequently underdiagnosed or misdiagnosed as medullary carcinoma, thymoma, teratoma, synovial sarcoma, or solitary fibrous tumor, among others. We present a case of a 28-year-old man with a history of a hemithyroidectomy diagnosed as SETTLE found to have a neck nodule along the strap muscle suspicious for recurrence 5 years post-surgery. The ultrasound-guided FNA cytology specimen of the neck nodule showed loosely cohesive, monomorphous ovoid to spindled cells with scant cytoplasm and nuclei with fine to granular chromatin. In addition, there were occasional clusters of cells with a papillary configuration. The tumor cells were associated with magenta, amorphous extracellular material. Immunocytochemical staining of the cell block material revealed that tumor cells were positive for p63, cytokeratin AE1/3, and CK8/18 and negative for TTF-1 and thyroglobulin. Overall, the morphological and immunocytochemical findings were consistent with a local recurrence of SETTLE. The subsequent left anterior strap mass excision revealed a 4 cm encapsulated tumor consistent with SETTLE. Because ofits rarity and low level of awareness, SETTLE poses a diagnostic and therapeutic challenge. We herein present the cytologic findings of monomorphic SETTLE and highlight the potential cytomorphologic and immunophenotypic pitfalls. We also highlight how tumors with high-risk features can be a therapeutic challenge.