Collision tumor of a papillary and follicular thyroid carcinoma: A second case report in Saudi Arabia and literature review

A collision tumor is a rare occurrence in which two distinct tumor types coexist within an organ while maintaining distinct borders. A 61-year-old female referred to our clinic for the evaluation of a thyroid mass. Ultrasound revealed a large, solid, isoechoic nodule in the right lobe measuring 4.5 × 3.1 cm (TI-RADS category 3). Fine-needle aspiration biopsy was suspicious for a follicular neoplasm, Hürthle cell (oncocytic) type, Bethesda system class IV. The patient underwent right lobectomy, and the final pathology report revealed minimally invasive follicular carcinoma. Completion thyroidectomy revealed papillary carcinoma of the left lobe. Managing thyroid collision tumors is challenging due to the presence of two distinct tumors with different biological aggressiveness, treatment options, and prognoses. Addressing how these factors influence treatment planning can enhance understanding of patient management and outcomes. • Collision tumors of the thyroid are rare entities where two histologically distinct carcinomas coexist within the same gland, maintaining separate boundaries. • This case represents only the second reported instance in Saudi Arabia of a collision tumor comprising both papillary and follicular thyroid carcinomas. • Diagnosis and management are challenging due to differing biological behavior, requiring a tailored, often multidisciplinary treatment approach. • Fine-needle aspiration may suggest one tumor type, but final histopathology remains essential to identify coexisting malignancies. • Awareness of such rare occurrences can improve clinical suspicion, guide surgical planning, and prompt comprehensive histopathological evaluation.