{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,7]],"date-time":"2026-04-07T09:18:47Z","timestamp":1775553527802,"version":"3.50.1"},"reference-count":16,"publisher":"MDPI AG","issue":"7","license":[{"start":{"date-parts":[[2026,4,4]],"date-time":"2026-04-04T00:00:00Z","timestamp":1775260800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["JCM"],"abstract":"<jats:p>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.<\/jats:p>","DOI":"10.3390\/jcm15072733","type":"journal-article","created":{"date-parts":[[2026,4,6]],"date-time":"2026-04-06T02:06:21Z","timestamp":1775441181000},"page":"2733","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["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"],"prefix":"10.3390","volume":"15","author":[{"given":"Eunyeong","family":"Lee","sequence":"first","affiliation":[{"name":"Department of Pathology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7011-6987","authenticated-orcid":false,"given":"Jungsup","family":"Byun","sequence":"additional","affiliation":[{"name":"Department of Pathology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"},{"name":"Department of Pathology, Daegu Fatima Hospital, Daegu 41199, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Moonsik","family":"Kim","sequence":"additional","affiliation":[{"name":"Department of Pathology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jae-Hui","family":"Kim","sequence":"additional","affiliation":[{"name":"Department of Pathology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7571-1064","authenticated-orcid":false,"given":"Ji-Young","family":"Park","sequence":"additional","affiliation":[{"name":"Department of Pathology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9240-4181","authenticated-orcid":false,"given":"Jongmin","family":"Park","sequence":"additional","affiliation":[{"name":"Department of Radiology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6412-3067","authenticated-orcid":false,"given":"An Na","family":"Seo","sequence":"additional","affiliation":[{"name":"Department of Pathology, School of Medicine, Kyungpook National University, Daegu 41405, Republic of Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2026,4,4]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1093","DOI":"10.1634\/theoncologist.2013-0053","article-title":"Current understanding and management of medullary thyroid cancer","volume":"18","author":"Roy","year":"2013","journal-title":"Oncologist"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"362","DOI":"10.4143\/crt.2014.110","article-title":"Age-Period-Cohort Analysis of Thyroid Cancer Incidence in Korea","volume":"47","author":"Oh","year":"2015","journal-title":"Cancer Res. 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