An Aggressive Variant of Follicular Thyroid Carcinoma Metastasizing to the Skull Base: Case Report and Review of the Literature

Introduction: Metastasis to the skull base is rare but occurs most frequently with breast and prostate cancer. Suspicion for skull base metastasis should be elevated in patients with a history of cancer and unexpected ipsilateral cranial nerve findings. In this case report we describe the unanticipated finding of metastatic spread of poorly differentiated thyroid carcinoma to the skull base.

Case Description: The patient is a 61 year old female with a complicated past medical history of rhabdomyosarcoma of the right orbit and neck treated with a right orbital exenteration, right radical neck dissection, and a right thyroid lobectomy, radiation, and chemotherapy at age 7. Four years ago, she had new left sided thyroid nodules and underwent a left completion thyroidectomy which revealed follicular thyroid carcinoma. She was also found to have osseus metastasis of the left hemi-pelvis and was treated by both radiation to the pelvis and radioactive iodine. A repeat PET scan one year later demonstrated uptake in the left mid upper neck. She underwent a bilateral central neck dissection followed by a total laryngectomy for laryngeal invasion. Six months later, she developed new onset V2 numbness and rising thyroglobin levels prompting an MRI and PET-CT which demonstrated a skull base mass within the sphenoid bone with contiguous extension from the region of the right inferior orbital fissure, cavernous sinus, Meckel's cave, foramen ovale, and inferomedial aspect of the right middle cranial fossa. The patient was taken for an endoscopic biopsy which on pathology confirmed metastatic follicular carcinoma. The patient was subsequently treated with radiation therapy to the skull base.

Discussion: A review of the literature suggests that skull base metastases from follicular thyroid cancer is extremely rare with less than 25 total cases reported. In this case report, we present a highly aggressive case of metastatic poorly differentiated follicular thyroid carcinoma invading the skull base. This case is unique from others given the extensive involvement of the pelvis, larynx, and aggressive infiltration of multiple skull base sites including the cavernous sinus, Meckel’s Cave, foramen ovale, and the floor of the right middle fossa. In conclusion, skull base metastasis should be on the differential diagnosis for a patient with a history of aggressive thyroid cancer and new pathologic ipsilateral cranial nerve physical exam findings.