Effects of High Dose Calcitriol in Breast Cancer Patients

Inclusion Criteria: * Must be female. * Must have pathologically confirmed incident, primary invasive breast cancer. * Must be awaiting surgical resection. * Women of child-bearing potential (i.e. women who are pre-menopausal or not surgically sterile) must use acceptable contraceptive methods (abstinence, intrauterine device (IUD), or double barrier device) and must have a negative serum or urine pregnancy test within 1 week prior to beginning treatment on this trial. Contraceptive use needs to be continued at least 1 month after the trial has ended. * Must provide informed consent. * Must be willing to discontinue use of calcium and/or vitamin D supplements other than multivitamin supplementation. * Participants must have an ionized serum calcium level within normal limits (1.19-1.29mmol/L) and a total corrected serum calcium of \< 10.2mg/dl. Exclusion Criteria: * Subjects with life-threatening conditions that would preclude them from breast cancer treatment including: chronic cardiac failure, which is unstable despite medication use; uncontrolled hypertension; uncontrolled diabetes mellitus; or unstable coronary artery disease. * Patients with severe metabolic disorders, which includes phenylketonuria (PKU), homocystinuria, and Fabry's disease, that would preclude them from taking calcitriol. * Patients with a previous history of any other cancer except non-melanomous skin cancer within the past 5 years. * Patients with impaired renal function (CRCL \< 60 mL/min) or who had kidney stones (calcium salt) within the past 5 years. * Patients with hypercalcemia (corrected serum CA \> 10.2 mg/dl) or a history of hypercalcemia or vitamin D toxicity. * Patients currently taking calcium supplements or aluminum-based antacids must immediately discontinue their use if they are to enroll in the study. * Patients currently taking vitamin D supplements must immediately discontinue their use if they are to enroll in the study. * Patients with a known sensitivity to calcitriol. * Women who are pregnant or lactating. * Women on antiresorptive drugs (e.g. bisphosphonates) within the past year. * Women currently using oral contraception. * Women with malabsorptive syndromes (i.e. cystic fibrosis, chronic pancreatitis) or taking medications that decrease the absorption of fat soluble vitamins (i.e. Orlistat, Questran). * Participants assigned to calcitriol who are routinely taking a multivitamin supplement may continue the supplement as long as the amount of vitamin D in the supplement is not in excess of the RDA (recommended daily allowance) of 400 IU or 10 μg. If they are not taking a multivitamin supplement, they will be asked to not start supplementation while on study.