Abstract
INTRODUCTION AND IMPORTANCE: Papillary thyroid carcinoma (PTC) and medullary thyroid carcinoma (MTC) are distinct thyroid malignancies with different cellular origins and management strategies. The coexistence of these two cancers within the same thyroid gland, particularly in separate lobes, is rare and presents unique diagnostic and therapeutic challenges. This case highlights the importance of distinguishing between these malignancies to ensure accurate diagnosis and appropriate treatment. CASE PRESENTATION: A 60-year-old male presented with a progressively enlarging neck mass over 6 months, accompanied by mild discomfort and intermittent dysphagia. Physical examination revealed a firm, fixed mass in the right thyroid lobe. Laboratory tests showed normal thyroid-stimulating hormone, elevated calcitonin (330 pg/mL), and carcinoembryonic antigen (12 ng/mL). Thyroid ultrasound identified a 4.5 cm hypoechoic nodule in the right lobe and a 1.2 cm nodule in the left. Fine needle aspiration cytology confirmed PTC in the right lobe and MTC in the left. CLINICAL DISCUSSION: Total thyroidectomy with central neck dissection was chosen as treatment. Histopathology confirmed classical PTC in the right lobe and MTC in the left, with characteristic amyloid deposition and positive staining for calcitonin and other markers in MTC. RET proto-oncogene mutation testing was negative. The patient received postoperative radioactive iodine therapy and levothyroxine. At six months, there was no evidence of recurrence. CONCLUSION: The coexistence of PTC and MTC in separate thyroid lobes is a rare occurrence. Accurate cytological, histopathological, and immunohistochemical evaluations are essential for proper diagnosis and management.