What Makes a Tumor Marker
There is great interest in the development of new diagnostic markers that can aid cancer patients and their physicians in the process of clinical decision-making. There are two general categories of diagnostic markers:
Prognostic markers indicate the likelihood of outcome tumor recurrence, patient survivalregardless of the specific treatment the patient receives. For example, in most solid tumors the spread of cancer cells to lymph nodes indicates an increased likelihood of tumor recurrence, no matter which particular form of therapy the patient receives following surgery.
Predictive markers indicate the likelihood of response to a specific therapy. For example, breast cancers that express the estrogen receptor tend to respond to hormonal therapies such as tamoxifen.
Some markers are an aid to the pathologist in confirming the tissue of origin of a tumor. Others can be used to monitor a patient’s response to therapy or to detect the growth of metastases. Markers of precancerous conditions are also sought, in order to serve as the basis for screening strategies or to identify populations at high risk for cancer who might benefit from preventive measures. Finally, there is increasing interest in defining molecular targets for new therapeutic agents. For example, treatment with the monoclonal antibody Herceptin is offered to patients whose tumors have amplified the Her2/neu gene or over-express the Her2/neu gene product.
It is worth noting that not all useful markers are tumor-specific, in the sense that their expression is restricted to tumor cells. Nor are they necessarily secreted into the blood or other body fluids.
The successful outcome of research in cancer diagnostics is the development of a new assay, procedure or technique that provides information useful to physicians and patients in designing the course of cancer treatment. A new diagnostic is obviously significant if it will have a clear impact on those decisions that clinicians struggle with today. The nature of a “difficult decision” depends on the type of cancer, the stage of the disease and the range of treatment options.
Ultimately a useful marker is one that meets two criteria: