More than 2.5 million breast cancer survivors live in the US today. This number will continue to grow thanks to early detection and advances in treatment that are making early stage breast cancer an increasingly curable disease. None of these gains in survival, however, have come without cost, especially as treatment complexity increases. Long-term effects of treatment, which are complications that begin during treatment and continue beyond the end of treatment, occur as a result of breast cancer therapies and are increasingly prevalent in breast cancer survivors.
Younger women with breast cancer often receive very complex breast cancer treatment regimens, and thus are at risk for a host of various adverse effects. Nowadays, one in three women newly diagnosed with breast cancer is younger than 55 years of age. One of the most common side effects in women with breast cancer, including younger women, is cancer treatment–induced bone loss (CTIBL), which often leads to osteopenia and/or osteoporosis.
In the general US population, more than 8 million women have osteoporosis, a condition characterized by decreased bone strength. Vertebral, hip, and other bone fractures are the chief clinical manifestations of this disease. A Caucasian woman's lifetime risk of osteoporotic fracture is as high as 45%, with these fractures causing significant morbidity and substantial economic consequences. Women with a breast cancer diagnosis are at increased risk for developing both osteoporosis and related fracture,[8,9] in large part due to the substantial role that estrogen plays in bone metabolism.
In women, estrogen protects against bone loss and reduces bone turnover by decreasing the formation, activation, and lifespan of osteoclasts, the cells responsible for breaking down bone. As levels of estrogen decrease, whether due to natural or iatrogenic declines in ovarian function or other treatment effects, bone resorption exceeds new bone formation, leading to net bone loss. Through a variety of mechanisms, treatments for breast cancer interfere with the positive effects of estrogen on bone metabolism, thereby inducing accelerated rates of bone loss.