Papillary thyroid carcinoma (PTC) is the most common form of differentiated thyroid carcinoma and typically follows an indolent course. Distant metastases occur in 5–10% of cases, commonly involving the lungs and bones. Calvarial or intracranial metastases are exceedingly rare, accounting for less than 1% of cases, and are often linked to the follicular variant due to its hematogenous spread. We report a case of invasive follicular variant PTC (FVPTC) with extensive calvarial and intracranial involvement—a rarity with significant management challenges.

A 67-year-old female presented with a palpable left temporoparietal mass and a 45-year history of untreated anterior neck mass. One year prior, she developed progressive right-sided weakness and slurred speech. Imaging revealed a large left parieto-temporo-occipital lesion with intracranial extension, consistent with metastasis. Histopathology after total thyroidectomy showed widely invasive, multifocal encapsulated Follicular Variant Papillary Thyroid Carcinoma

Due to high tumor vascularity, preoperative embolization was only partially successful, leading to deferral of cranial mass resection. She received radiotherapy (30 Gy in 10 fractions) followed by 150 mCi radioactive iodine ablation. Multidisciplinary evaluation determined that the risk of post-RAI cerebral edema was minimal given the non-confining calvarium. The patient showed neurological improvement post-RAI, with decreased antiseizure medication needs. Persistently elevated thyroglobulin levels indicated the need for subsequent ablations. This case illustrates an unusual pattern of metastasis likely due to chronic indolent transformation and highlights the role of multimodal management when surgery is not feasible.

Calvarial and intracranial metastasis from FVPTC is an exceptionally rare manifestation of differentiated thyroid carcinoma with only isolated case reports in literature and currently with no documented case in the Philippines. Optimal management requires individualized, multidisciplinary collaboration among otorhinolaryngology, neurosurgery, neurology, endocrinology, oncology, and radiation oncology services. Despite advanced disease, coordinated treatment can provide symptomatic improvement and meaningful disease control.