Collision thyroid tumour.

Collision tumours of the thyroid are rare, accounting for less than 1% of all thyroid malignancies. They represent the coexistence of two histologically distinct but adjacent and morphologically independent tumours, most commonly papillary thyroid carcinoma (PTC) and medullary thyroid carcinoma (MTC). We report a case of a diabetic male in his fourth decade of life who presented with neck swelling. Imaging revealed a bulky left thyroid lobe with lymph node involvement. Fine-needle aspiration cytology suggested PTC with lymph node metastasis. However, histopathology following total thyroidectomy, paratracheal dissection and left functional neck dissection revealed a collision tumour comprising both PTC and MTC. Postoperative management included radioiodine therapy, Ga-68 DOTANOC (1,4,7,10-tetraazacyclododecane-tetraacetic acid 1-Nal3-octreotide) scan and external beam radiotherapy. In such cases, the prognosis largely depends on the more aggressive component, typically MTC. The serial monitoring of serum calcitonin and thyroglobulin levels and appropriate imaging are essential for follow-up. Owing to the rarity and complexity of these tumours, multimodal treatment should be considered.