Statement: the primary clinical trial exclusively accepts adult participants
Primary_evidence: Inclusion Criteria:   Metastatic breast cancer (Stage IV)   Histologically confirmed estrogen receptor positive disease   Female   Serum 25(OH) <30 ng/ml   Age  18 years   Pre or post-menopausal   ECOG Performance status 0-2   Adequate organ function as defined as GFR> 30 mls/min and serum calcium  10.4 mg/dl   Any race/ethnicity   English speaking   No changes to MBC treatments within 30 days of enrollment and/or deemed clinically stable by their treating physician   Willingness to sign a written informed consent and complete questionnaires   Cease ingestion of vitamin D supplementation not study related Exclusion Criteria:   Women with Stage I-III breast cancer   Serum 25(OH)D levels  30 ng/ml   Untreated CNS involvement   History of kidney stones   History of renal failure   History of hyperparathyroidism   History of hypersensitivity to vitamin D   Non-English speaking   Currently pregnant or lactating, or anticipating pregnancy   Unwilling to cease ingestion of calcium supplements (>1000 mg/d)   Unwilling or unable to complete informed consent or study questionnaires   Psychiatric or other clinical conditions that preclude study compliance   Other important medical or safety considerations at the discretion of the investigator and/or study physician, including non-compliance with the study therapy or other activities