Unusual skeletal muscle metastasis from poorly differentiated thyroid carcinoma: a rare presentation

Fatma Chaltout; Ghada Mani; Imen Mhiri; Yesmine Elloumi; Issam Jardak; Salma Charfeddine; Mohamed Maaloul; Fatma Hamza; Khalil Chtourou

doi:10.1530/endoabs.110.EP646

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Endocrine Abstracts (2025) 110 EP646 | DOI: 10.1530/endoabs.110.EP646

ECEESPE2025 ePoster Presentations Endocrine Related Cancer (100 abstracts)

Unusual skeletal muscle metastasis from poorly differentiated thyroid carcinoma: a rare presentation

Fatma Chaltout ¹ , Ghada Mani ¹ , Imen Mhiri ¹ , Yesmine Elloumi ² , Issam Jardak ¹ , Salma Charfeddine ¹ , Mohamed Maaloul ¹ , Fatma Hamza ¹ & Khalil Chtourou ¹

Author affiliations

¹Habib Bourguiba University Hospital, Nuclear Medicine Department, Sfax, Tunisia; ²Hedi Chaker University Hospital, Sfax, Tunisia

JOINT3898

Poorly differentiated thyroid carcinoma (PDTC) is a rare and aggressive form of thyroid cancer, with metastases typically involving the lungs, bones, and lymph nodes. However, skeletal muscle involvement remains an exceptional occurrence. We report the case of a 51-year-old woman who initially underwent total thyroidectomy with bilateral mediastino-recurrential lymph node dissection for a poorly differentiated carcinoma arising from an invasive follicular thyroid carcinoma. Histopathological examination revealed the presence of vascular emboli and associated chronic lymphocytic thyroiditis. The patient received two courses of radioactive iodine therapy, resulting in complete clinical, biological, and isotopic remission. Four years later, the patient presented with two palpable subcutaneous nodules in the right sternocleidomastoid muscle, measuring 10 mm and 11 mm on ultrasound. A suspicious right submandibular lymphadenopathy was also noted. Serum thyroglobulin levels under TSH suppression showed a mild elevation from 0.1 to 0.46 ng/mL. 18F-FDG PET/CT revealed metabolically active involvement of the right sternocleidomastoid muscle, associated with bilateral subcentimetric cervical lymph nodes with low metabolic activity. Theses findings were suggestive of muscle metastasis from the known primary thyroid carcinoma. As a result, the patient was considered for surgical excision. This case highlights a rare presentation of PDTC with isolated skeletal muscle metastasis, underscoring the importance of considering atypical metastatic sites in recurrent thyroid cancer to guide appropriate therapeutic decisions.