About three-fourths of breast cancers are hormone receptor-positive. This means that hormone receptors are found on the surface of the cancer cells. Estrogen in the body binds to the hormone receptors, activating a chain of signals that cause cancer cells to divide more. Estrogen is abundant in the premenopausal female body, but even postmenopausal women continue to produce estrogen in fat tissue and elsewhere despite the slowing of ovarian function.
Hormone therapy is often used as post-operative adjuvant therapy for early-stage, hormone receptor-positive breast cancer, improving the prognosis for cancer of this type. Commonly used hormonal medicines include the anti-estrogen drug tamoxifen and aromatase inhibitors such as letrozole.
Tamoxifen is a drug that prevents estrogens from binding to cancer cells’ hormone receptors by competing for the same binding sites. Tamoxifen thus counteracts estrogen’s effect in breast cancer tissue. However, tamoxifen also has estrogen-like effects elsewhere in the body, such as the lining of the uterus, the skeletal system and as a regulator of blood lipid levels.
Aromatase inhibitors interfere with estrogen production in parts of the body other than the ovaries, especially in fat and muscle tissue. Aromatase inhibitors do not reduce estrogen production in the ovaries, so they are only suitable for treating breast cancer in postmenopausal women. The ovaries are the primary source of estrogen prior to menopause. In young women, aromatase inhibitors may be combined with medication that inhibits ovarian function (such as Zoladex).
Studies have shown that hormone therapy is most effective for breast cancer if used for five years. According to recent studies, however, some patients benefit from a longer duration of hormone therapy, as much as 10 years. Hormone therapy may consist of a single drug, or a different drug may be used after three or five years of treatment.
Side effects
Hormonal breast cancer drugs cause side effects in some patients. All hormone therapy used as adjuvant treatment for breast cancer may cause menopausal symptoms such as hot or cold flashes, sweating, insomnia, mood swings, low sex drive, or dryness of the vaginal or external genital membranes. Menopausal symptoms may lessen as treatment continues, or with lifestyle changes such as exercise, weight management and avoiding excessive alcohol consumption. If these changes alone are not helpful enough, drug treatment may sometimes be necessary.
For women who have had breast cancer, non-hormonal therapy such as ergotamine or venlafaxine may be tried for severe menopausal symptoms. Oral or transdermal estrogen or progestin may increase the risk of breast cancer recurrence, and therefore is not suitable for treating menopause symptoms in women with a history of breast cancer.
Non-hormonal, prescription-free lubricants available from pharmacies are the primary treatment recommended for vaginal dryness (e.g. Ceridal ointment, Repadina vaginal suppositories).
Estriol-containing vaginal suppositories (e.g. Ovestin, Pausanol) can be used for troublesome mucous membrane symptoms, only in combination with anti-estrogen therapy (tamoxifen) and no more than once a week. These options may be discussed with the treating physician or breast-cancer nurse.
Aromatase inhibitors most commonly cause side effects in the form of joint stiffness in the mornings or with initial movements. This is due to low estrogen, and usually lessens during the day and as the joints are moved. The joint stiffness often develops during the very first weeks of treatment, but becomes milder with time. Joint pain may occur in addition to stiffness. The pain may go away as well, but this does not always happen. Regular pain medication such as ibuprofen (Ibuxin, Burana) may then be helpful. In difficult cases, the oncologist might suggest trying another aromatase inhibitor or switching to tamoxifen hormone therapy.
Aromatase inhibitors accelerate osteoporosis somewhat, so this treatment is combined with calcium and vitamin D supplements. Physical activity is also recommended for all breast cancer patients.
Tamoxifen increases susceptibility to venous thrombosis. Swelling and pain in one leg would be a symptom of this. If a venous blood clot is suspected, care should be sought immediately at the emergency room. If a blood clot is diagnosed, tamoxifen must be discontinued and a breast-cancer nurse should be contacted by phone. Contraceptive precautions should be taken during tamoxifen therapy if applicable. During tamoxifen therapy, minor vaginal bleeding may occur even in women who are no longer menstruating. In this case it is advisable to contact a gynecologist, who will check the condition of the uterine membranes. Tamoxifen could cause thickening of the membranes, and in rare cases also malignant changes.