Fine-needle aspiration cytology of isolated skull nodule: Unfolding the clinical spectrum

Follicular thyroid carcinoma (FTC) has been classified as either minimally invasive or widely invasive carcinoma and shows a propensity for blood-borne metastasis. Most common sites of metastasis are lung and bone followed by brain, liver, and skin. Minimally invasive FTC (MIFTC) is characterized by limited capsular and/or vascular invasion with good long-term outcomes, some cases of which show a poor prognosis because of severe distant metastasis. Skull metastasis in adults commonly arises from the lung, breast, and prostate and uncommonly from the thyroid. In our case, fine-needle aspiration cytology of isolated skull nodule was a reliable tool in the diagnosis of metastasis and suggesting the primary in thyroid thereby prompting early workup of a patient. The case is unique since it represents the rare disseminated metastasis from MIFTC with incomplete capsular penetration alone without angioinvasion that can behave as aggressively as a widely invasive FTC.