Hormone Therapy for a Cancer of Unknown Primary
Some types of cancer grow in response to hormones in the body. In cases where a cancer of unknown primary (CUP) is likely to be a breast or prostate cancer, hormone therapy may be an effective way to slow the growth of the cancer, or perhaps even shrink it, and may help you live longer.
Most breast cancers have proteins called estrogen receptors and/or progesterone receptors on the surface of their cells. These cancers grow faster when exposed to the hormone estrogen. Likewise, most prostate cancers grow in response to male hormones called androgens, such as testosterone.
If the unknown primary is most likely a breast cancer
For a CUP that is suspected to be breast cancer, hormone therapy should be included in treatment, following guidelines for treatment of metastatic breast cancer. Multiple hormone agents are available, including
- Tamoxifen
- Toremifene (Fareston)
- Fulvestrant (Faslodex)
- Luteinizing hormone-releasing hormone (LHRH) agonists like leuprolide (Lupron) and goserelin (Zoladex)
- Aromatase inhibitors like anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin)
These drugs either lower estrogen levels or prevent cancer cells from being able to use it. For more information on how these drugs are used and their potential side effects, see Hormone Therapy for Breast Cancer.
If the unknown primary is most likely a prostate cancer
For a CUP that is suspected to be prostate cancer, hormonal therapy should follow guidelines for the treatment of advanced prostate cancer. Some commonly used drugs include:
- LHRH agonists such as leuprolide (Lupron, Eligard), goserelin (Zoladex), and triptorelin (Trelstar)
- Anti-androgens such as flutamide (Eulexin), bicalutamide (Casodex), enzalutamide (Xtandi), and apalutamide (Erleada)
These drugs either lower the testosterone level or prevent cancer cells from being able to use it. Surgery to remove the testicles (orchiectomy) is another option. For more information, see Hormone Therapy for Prostate Cancer.
More information about hormone therapy
To learn more about how hormone therapy is used to treat cancer, see Hormone Therapy.
To learn about some of the side effects listed here and how to manage them, see Managing Cancer-related Side Effects.
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- References
Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
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National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Occult Primary (Cancer of Unknown Primary). v.2.2025. Accessed at https://www.nccn.org/professionals/physician_gls/pdf/btc.pdf on April 22, 2025.
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Last Revised: May 27, 2025
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