To monitor the course of disease with a tumor marker, which must be true?

• A. The laboratory measures the marker with the same method over the entire course of treatment.
• B. The marker must be released from the tumor into a testable body fluid.
• C. The marker's half-life must reflect tumor burden and allow detection of response.
• D. All of the above

Answer: (D)

Monitoring disease course with a tumor marker requires measurements that are comparable over time, obtainable from a testable body fluid, and tied to tumor burden in a way that reflects response. Using the same assay method throughout treatment minimizes changes due to test differences rather than biology, so observed trends truly reflect how the disease is changing. The marker must be present in a fluid you can repeatedly sample, like blood or urine, otherwise you can’t track its levels over time. Finally, the marker’s half-life should be such that increases or decreases in tumor burden lead to detectable changes in marker levels within a clinically useful window; if the marker changes too slowly or too unpredictably due to kinetics, it won’t reliably show response or progression. When these conditions are all met, you can accurately monitor the course of disease with a tumor marker, which is why all of the above is the best choice.