Breast cancer can relapse more than 25 years after diagnosis. During the time between initial treatment and relapse, cancer cells are considered to be in a state of dormancy. Lisa Willis of University College London, UK, and colleagues performed a mathematical analysis of what happens during the dormancy period.
Circulating tumor cells have been detected in breast cancer patients 8 to 22 years after surgical treatment. These cells remained at a steady concentration over a study period of two years. Willis found that primary tumors seed at most six micrometastases before mastectomy surgery, and in most cases only one remains in long-term dormancy.
Three mechanisms have been proposed for dormant cancer:
1. Solitary dormant cells
2. Micrometastases restricted in size by their lack of blood vessels
3. Micrometastases with blood vessels that are held to a steady size by the immune system
“Breast cancer is infamous for long gaps between primary treatment and recurrence,” Willis noted. Current treatments are only marginally effective against dormant cancers that relapse after five years.
Jonathan W. Uhr of the University of Texas Southwestern Medical Center, Dallas, Texas, and Klaus Pantel of the University Medical Center Hamburg-Eppendorf, Germany provided more insights into dormant cancer. These authors reported that 20 percent of breast cancer patients who are clinically disease-free will experience cancer relapse 7 to 25 years after mastectomy.
Similar dormant patterns are found in thyroid, renal, and prostate cancers, as well as B-cell lymphoma and melanoma. Lung and colon cancer rarely relapse after a long period of dormancy.
Uhr and Pantel reported that the innate immune response does not appear to have a major role in preventing cancer growth. Studies of organ transplant patients on immunosuppressive drugs show little or no increase in the incidence of common types of cancer, including breast, prostate, and lung cancer.
However, the adaptive immune response may be a different story.