Study of Tamoxifen in Well Differentiated Neuroendocrine Tumors and Hormone Receptor Positive Expression

Inclusion Criteria:

* Age greater than or equal to 18 years
* Histological diagnosis of well differentiated NET (typical and atypical lung carcinoids, NET G1, NET G2 of all gastroenteropancreatic sites and pancreatic NET G3 according to WHO 2017 classification) 20 advanced / metastatic, inoperable, with no possibility of curative treatment
* Immunohistochemical expression ≥ 1 percent for estrogen and / or progesterone receptor
* Disease with radiological progression (at least 10 percent tumor volume growth) in the last 12 months before day 1 cycle 1.
* No possibility of established treatments due to lack of access, risk of toxicities or without clinical indication. Patients who meet criteria for watchful waiting (low-dose disease and non-functioning NET) may be included.
* Measurable disease
* ECOG performance scale 0 to 2.
* Adequate organic function as defined by the following criteria:

  * serum aspartate aminotransferase (AST), serum alanine aminotransferase (ALT) ≤ 2.5 times the upper limit of local laboratory normality (LSN-LL);
  * Total serum bilirubin ≤ 2.0 x ULN-LL;
  * Absolute neutrophil count ≥ 1,500 / mm\^3;
  * Platelet count ≥ 80,000 / mm\^3;
  * Hemoglobin ≥ 9.0 g / dL;
  * Estimated creatinine clearance by the MDRD equation ≥ 30ml / min
* Albumin ≥ 3.5 g / dL;
* INR ≤ 1.5
* Term of free and informed consent signed by the patient or legal representative.

Exclusion Criteria:

* Patients already on tamoxifen, but other prior treatment are allowed
* Patients with aggressive disease requiring cytotoxic therapy or locoregional therapies (eg hepatic embolization)
* A history of serious clinical or psychiatric illness that, by clinical judgment, may involve participation risk in this study
* Patients participating in other protocols with experimental drugs.
* Patients with oral food difficulties.
* Patients who underwent major recent surgery less than 4 weeks previously.
* Patients receiving chemotherapy or other oncologic therapy for less than 3 weeks.
* Patients who use oral anticoagulation
* Previous history of deep vein thrombosis or pulmonary embolism in the last 12 months.
* Pregnant or lactating patients.
* Patients with postmenopausal vaginal bleeding with no defined etiology.
* Patients with breast cancer who need to use tamoxifen for this neoplasm
* Another synchronous neoplasm that requires systemic treatment