Post-menopausal breast cancer patients will be prescribed calcitriol supplementation, in doses indicated to prevent osteoporosis, while they are submitted to biopsy, staging exams and have their breast surgery scheduled (approximately one month). Tumor dimension and proliferation rate (as determined by Ki67 expression), 25(OH)D3 and/or 1,25(OH)2D3 serum concentration, will be evaluated before and after calcitriol supplementation. Tumor gene expression will be evaluated in samples collected before and after supplementation to analyze the differential profile.