Thyroid spindle epithelial tumor with thymus-like differentiation (SETTLE): case report and review

Area of Science:

• Endocrinology and Oncology
• Pediatric Pathology
• Surgical Oncology

Background:

• Spindle epithelial tumor with thymus-like element (SETTLE) is an exceptionally rare malignant thyroid neoplasm.
• Predominantly affects pediatric and young adult populations, typically presenting as a localized thyroid mass.
• Origin is hypothesized to involve branchial pouch or thymic remnant tissue with primitive thymic differentiation.

Observation:

• A clinical, cytological, histological, and immunohistochemical analysis of a SETTLE case in a 3-year-old girl is presented.
• Microscopic examination revealed a biphasic neoplasm characterized by a prominent spindle cell component and a minor glandular component.
• The glandular component exhibited mucinous or respiratory-type epithelium.

Findings:

• Immunohistochemical analysis demonstrated strong and diffuse positivity for pan-cytokeratin (pan-CK), vimentin, and smooth muscle actin.
• This case represents the first reported instance of SETTLE in Brazil.
• The patient remained disease-free, with no evidence of recurrence or metastasis five years post-surgery.

Implications:

• This report expands the geographic and clinical understanding of SETTLE.
• The findings underscore the importance of comprehensive histopathological and immunohistochemical evaluation for diagnosing rare pediatric thyroid tumors.
• The favorable long-term outcome in this case suggests that complete surgical resection may be an effective treatment strategy for SETTLE.