Even when a blood cancer such as multiple myeloma or leukemia is in remission, doctors will continue to search for indications that some cancer cells are still lurking in the bloodstream. A small number of nearly undetectable cancer cells found after treatment is known as minimal residual disease (MRD).
It's important to monitor for MRD, because the presence of any cancer cells means the disease can relapse, says Dr. Matthew Pianko, a hematologist with the Rogel Cancer Center at the University of Michigan in Ann Arbor.
Read MoreSearching for MRD
Recent advances in cancer screening are making it harder for even the smallest number of cancer cells to hide in the bone marrow. "MRD tests that can look for trace amounts of plasma cells in the bone marrow after treatment, can often detect sometimes one in a hundred thousand cells or even one in a million cells in the bone marrow," Dr. Pianko says. "Now more than ever before, we’re using these advanced tools to follow multiple myeloma to see and measure how deep patients' responses are and potentially also identify patients at high risk of relapse." RELATED: How Are Patients Monitored After Multiple Myeloma Treatment?He adds that, based on clinical trials, minimal residual disease is considered a prognostic marker, meaning that patients who are MRD-negative tend to have the best outcomes.
The techniques used to monitor for MRD include next generation genetic sequencing, in which bone marrow samples are tested for genetic mutations. If mutations are discovered, MRD is present. The test examines stretches of RNA or DNA and may be precise enough to find one cancer cell among one million bone marrow cells.
Another screening, called flow cytometry, searches for abnormal proteins on the surfaces of bone marrow cells. The process involves treating the bone marrow sample with special antibodies that stick only to cells that have the abnormal proteins on them. Flow cytometry can detect one cancer cell among 100,000 bone marrow cells. Results are usually available in less than 24 hours.
RELATED: What Are the Treatment Options After First Relapse?
One other MRD test expands DNA or RNA segments so that they are more easily studied. The screening is called a polymerase chain reaction (or PCR, the type of test also widely used to check for an active COVID-19 infection). It can make one cancer cell detectable among 100,000 to one million normal cells. The test may be performed using bone marrow samples or blood samples, but it can take a few weeks for results to become available.
From Monitoring to Therapy
Despite the advances in MRD monitoring, there is still some debate about how to use the results to guide treatment, Dr. Pianko says. "For example, if a patient has induction therapy for multiple myeloma and their blood work becomes normal, but their MRD test is still positive, could we consider changing their treatments to try to make that patient’s disease MRD negative?" Dr. Pianko asks. "These are important questions and currently questions that are being asked in clinical trials. However, making these sorts of decisions outside of clinical trials is controversial."
He adds that MRD monitoring could become a common way to identify patients who might have a relapse. For example, if a patient is MRD negative, but months or years later has an MRD-positive result while on maintenance therapy, that result could help a patient and doctor get an early start planning treatment for a possible relapse.
Understanding the role of MRD monitoring should help you cope with the regular screenings necessary to stay ahead of the disease and be better prepared if a relapse occurs.
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