To study the clinicopathological features of collision tumours of the thyroid and to develop a logical management regimen in view of the rarity of these tumours.

A retrospective study on collision tumours of the thyroid, diagnosed over the previous 15 years in a tertiary cancer care centre. The inclusion criteria were proven cases of collision tumours of medullary thyroid carcinoma with papillary thyroid carcinoma and/or follicular thyroid carcinoma.

Among the 470 patients with medullary thyroid carcinoma, 24 were found to harbour collision tumours (5.1 per cent). Amongst 18 patients (75 per cent) with lymph node metastases, 88.8 per cent originated from medullary thyroid carcinoma and 22.2 per cent from the papillary thyroid carcinoma component. Two patients (8.3 per cent) presented with distant metastases. Eight patients underwent radioactive iodine scan, of whom seven demonstrated neck uptake and received radioactive iodine therapy. Fifteen patients (62.5 per cent) were disease free, eight patients (33.33 per cent) harboured biochemical and/or structural residual disease, and one patient died due to an unrelated aetiology.

Thyroid collision tumours are relatively rare entities and are usually undiagnosed pre-operatively. The prognosis of the disease primarily depends on tumour aggressiveness the of medullary thyroid carcinoma component. A combined follow up with tumour markers and imaging, including positron emission tomography/computed tomography molecular imaging approaches should be adopted.