Metastatic Medullary Thyroid Carcinoma Without Identifiable Primary Tumor Within the Thyroid Gland, Presenting with Initial Lymph Node Metastasis Followed by Distant Peritoneal Metastasis: A Case Report of a Rare Phenomenon

Background: We report a rare case of a metastatic neoplasm in the regional lymph nodes and peritoneum whose histopathologic and immunophenotypic profiles were most consistent with a diagnosis of medullary thyroid carcinoma (MTC), although a primary tumor was not histologically demonstrated in the thyroidectomy specimen. Case presentation: A 64-year-old man presented with abdominal pain and was found to have increased calcitonin level and a 20 mm lesion in the peritoneum. Peritoneum biopsy revealed plasmacytoid tumor cells which were positive for calcitonin and synaptophysin staining. The patient had a past history of neck dissection due to left side neck mass. The histology revealed metastatic carcinoma with a nested pattern surrounded by fibrous stroma with stromal amyloid deposition. With immunohistochemistry, the findings were most consistent with metastatic MTC, but following total thyroidectomy showed no malignancy. Next-generation sequencing identified a pathogenic HRAS mutation, but RET mutation was not identified. Despite vandetanib treatment, the disease progressed and the patient expired. Conclusions: This case highlights a rare presentation of a metastatic neoplasm highly suggestive of RET wild-type MTC with peritoneal involvement, despite the absence of an identifiable primary lesion.