Hot flashes and night sweats may be side effects of cancer or its treatment. Sweating is the body's way of lowering body temperature by causing heat loss through the skin. In patients with cancer, sweating may be caused by fever, a tumor, or cancer treatment. Hot flashes can also cause too much sweating. They may occur in natural menopause or in patients who have been treated for breast cancer or prostate cancer. Hot flashes combined with sweats that happen while sleeping are often called night sweats or hot flushes.