Case Report of a Rare Collision Tumour of Papillary and Follicular Thyroid Carcinoma

Introduction: Papillary and follicular thyroid carcinomas account for 90–95% of all thyroid cancers. The co-existence of these two cancers is extremely rare and have only been described in case reports. We report a rare case of a collision tumour of papillary and follicular thyroid carcinoma in the same thyroid lobe. Case Presentation A 43-year-old woman presented with an anterior neck mass. A right thyroid nodule was clinically palpable. Ultrasonography revealed bilateral thyroid nodules. Fine needle aspiration (FNA) of the right nodule revealed cyst content, while FNA of the left nodule demonstrated atypia of undetermined significance. The patient was counselled for a left hemithyroidectomy but opted for removal of the enlarged right thyroid nodule. A right hemithyroidectomy was performed and histopathology revealed follicular carcinoma as well as a nodule consisting of papillary carcinoma. These two malignant nodules were separated by a section of normal thyroid tissue. A completion left hemithyroidectomy subsequently revealed papillary carcinoma in the left thyroid lobe. The patient was subsequently treated with radioiodine (131I) and thyroxine suppression therapy. Conclusion Collision tumours have been described as two independent tumours with distinct morphology which occur concurrently at the same site but having a distinct border. They are extremely rare entities. Clinical, radiological and cyto-histopathological evaluation is fundamental in investigating thyroid nodules but may miss a collision tumour. A number of theories and genetic mutations have been implicated in this collision phenomenon, but none have been proven so far. Management of collision tumours need to be individualised.