Calcitonin-negative neuroendocrine tumor (CNNET) of the thyroid is an extremely rare entity. In some of the previously reported cases within the literature, the terms “atypical medullary thyroid carcinoma,” “calcitonin-free oat cell carcinoma,” and “a distinct clinical entity” were applied to NETs without definitive evidence of calcitonin production. In the English-language literature, not only are there only few reported cases of CNNET, but the criteria for diagnosis in these cases are also controversial. Most of the current published cases were also treated surgically for local disease. We describe a case of NET of the thyroid with calcitonin, chromogranin A and thyroglobulin negativity, synaptophysin and TTF-1 positivity, and a high Ki-67 proliferation index with metastases in the cervical region as well as mediastinal adenopathies. This case was considered an unresectable thyroid carcinoma, and chemotherapy including cisplatin and etoposide was started as neoadjuvant treatment at the department of medical oncology. Total thyroidectomy plus bilateral and central cervical dissection was performed, and the patient underwent 2 cycles of adjuvant radiotherapy. Currently, the patient’s 18F-FDG-PET/CT findings show a complete response 17 months after diagnosis. In conclusion, CNNET of the thyroid is very rare and there is limited evidence regarding treatment in patients with metastases. Chemotherapy including cisplatin and etoposide as well as early aggressive surgical resection appears to positively impact patients’ survival.

Breast metastasis from gastric signet ring cell carcinoma is extremely rare in clinical practice. The estimated incidence is 0.5–1.3%. There are few cases reported in the literature (approx. less than 60) of breast metastasis from gastric signet ring cell carcinoma, and due to the rare association between gastric cancer and its extension to the breast, it is difficult to establish the diagnosis. Clinical history, histological findings, and immunohistochemical markers are helpful in distinguishing primary breast cancer from breast metastasis of gastric cancer. The treatment for breast metastasis from gastric carcinoma remains controversial. The prognosis of breast metastasis from gastric carcinoma is generally poor. We report a case of breast metastasis of gastric signet ring cell carcinoma in a 38-year-old woman. She started chemotherapy with ramucirumab, paclitaxel, and irinotecan. Three months later, a combined 2-[18F]-fluoro-2-deoxy-D-glucose positron emission tomography/computed tomography showed a complete response. This is the first reported case of breast metastasis from gastric signet ring cell carcinoma with a complete response.