Resolution of Breast Cancer in a Patient With Thyroid Stimulating Hormone-Secreting Pituitary Neuroendocrine Tumor With the Combination of Chemotherapy and Lanreotide.

Thyroid stimulating hormone-secreting pituitary neuroendocrine tumor (TSH-PitNET) is a rare disease in which pituitary adenomas secrete excessive amounts of TSH, and TSH is not suppressed despite high blood levels of thyroid hormone. Somatostatin analogs (SSAs) like lanreotide are used to control TSH secretion and manage symptoms in cases where surgery is not fully effective or feasible. The treatment of choice for human epidermal growth factor 2 receptor (HER2)-positive metastatic breast cancer is generally chemotherapy and anti-HER2 therapy. A 52-year-old woman was diagnosed with Graves' disease 26 years ago and stopped going to the hospital after several years of treatment with thiamazole. She had a right breast mass two years prior and visited the Department of Breast and Endocrine Surgery in our hospital one year prior, where she was diagnosed with T3N3M1, stage 4 breast cancer with a mass 52 mm in diameter in the right breast and metastasis in the 12th thoracic vertebra. Breast cancer receptor status was negative for the estrogen receptor, negative for the progesterone receptor, and positive for HER2. She was also found to have an enlarged thyroid gland, palpitations, inappropriate TSH secretion, and a 6 mm nodule on the pituitary gland, which was diagnosed as a TSH-PitNET. She was treated for breast cancer with trastuzumab deruxtecan therapy and for TSH-PitNET with lanreotide. One month after starting lanreotide, pituitary, and thyroid function improved to normal, and four months later, the breast mass was significantly reduced to 16 mm in diameter and a mastectomy was performed. The size of the pituitary adenoma remained unchanged during observation. Remarkably, the mastectomy specimen was free of cancer cells and showed a pathologically complete response. Needle biopsy specimens at the time of breast cancer diagnosis were positive for somatostatin receptor 2 (SSTR2) and insulin-like growth factor 1 receptor (IGF-1R) immunostaining. However, both were negative in the mastectomy specimen. Recently, SSTR2 and IGF-1R were reported to be expressed in breast cancer, and several clinical trials of SSAs for breast cancer have been conducted. SSAs are effective in improving pituitary and thyroid functions against TSH-PiTNET, and in combination with chemotherapy, they may have synergistic antitumor effects in patients with SSTR2-positive breast cancer.