Papillary thyroid carcinoma (PTC) and follicular thyroid carcinoma (FTC) are common differentiated thyroid cancers, but the detection of a collision tumor is an extremely rare event.

Literature review and a case report of concurrent PTC and FTC in one thyroid lobe, featuring comprehensive radiological and histological images.

A 49-year-old female presented with an anterior neck swelling of 9-month duration with no other associated symptoms. On examination, she was found to have a palpable nodule of the right lobe of thyroid. Her thyroid function tests were normal. Neck ultrasound revealed a homogenous thyroid interrupted by two nodules in the right lobe: one in the lower third (11 mm x 12 mm x 13 mm), and the other in the posterior middle third with macrocalcifications (7 mm x 6 mm x 7 mm). Fine-needle aspiration cytology of the smaller nodule reported a papillary neoplasm. Right hemithyroidectomy was performed without complications. Microscopic examination showed a 7 mm classic papillary microcarcinoma with extracapsular extension and suspected vascular invasion. The largest macroscopically identified lesion was a 16 mm minimally invasive follicular carcinoma with focal capsular invasion. As a result, a completion thyroidectomy was performed without complications. Histopathology showed a 2 mm nodule, a classic papillary microcarcinoma without capsular, vascular, or perineural invasion, and not intersecting the surgical margin. She is currently asymptomatic with normal thyroid function tests and thyroglobulin levels.

Rare and globally reported, collision tumors present diagnostic and therapeutic challenges due to dual pathology. Limited literature hinders specific treatment guidelines, necessitating tailored management based on each carcinoma's extent.