Those receiving chemotherapy or radiation are getting treatments that kill both cancer and living normal tissue. One of the important tissues is bone. These individuals and treatments may also be necessary based on the results of the bone density scan. Parathyroid hormone is naturally made in the body. Overproduction will deplete the calcium from the bones, making them brittle, in essence leading to osteoporosis.
Tamoxifen can provide some of the beneficial aspects of estrogen.
It is used mainly for breast cancer treatment. It is generally not used to treat osteoporosis. There have been many studies in cancer patients treated with tamoxifen that have shown the beneficial effects on the bone towards the prevention of osteoporosis. I am taking calcium tablets. Most experts in the area do not feel it will cause calcification of the arteries.