Collision Tumor of the Thyroid: Coexistence of Medullary and Papillary Microcarcinoma – A Rare Diagnostic and Therapeutic Challenge

Collision tumors of the thyroid are rare entities characterized by the coexistence of two histologically distinct malignancies within the same gland. The combination of medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma presents unique diagnostic and therapeutic challenges. A 65-year-old female presented with a left-sided thyroid swelling and cervical lymphadenopathy. Ultrasonography revealed a TIRADS V lesion in the left lobe with metastatic lymph nodes. Fine-needle aspiration cytology suggested follicular neoplasm. The patient underwent total thyroidectomy with central compartment clearance. Histopathological examination demonstrated a collision tumor comprising MTC in the left lobe and papillary microcarcinoma (columnar cell variant) in the isthmus. Postoperative biochemical markers showed elevated calcitonin and carcinoembryonic antigen. Metastatic workup was negative. Radioiodine ablation was administered for the papillary component. However, persistent elevation of calcitonin during follow-up suggested possible residual medullary disease. This case highlights the limitations of preoperative cytology, the importance of biochemical markers, and the therapeutic dilemma posed by dual thyroid malignancies. Management should be guided by the more aggressive component, typically MTC, with emphasis on long-term biochemical surveillance.