Tumor markers can be used to:

Tumor markers are substances produced by cancer cells or by normal cells in response to cancer in the body. They are primarily used in the diagnosis of cancer, monitoring the effectiveness of treatment, and detecting recurrence. The correct choice highlights their role in helping diagnose a tumor as well as tracking how well the tumor responds to treatment over time. For example, a rising level of a specific tumor marker may indicate that the cancer is not responding to therapy, while a decrease in the marker may suggest a positive response.

While tumor markers can provide valuable information, they are not typically used for locating lymph nodes, marking locations for radiation therapy, or guiding surgeons during tumor removal. These tasks often rely on imaging studies, surgical techniques, or other diagnostic tools that do not involve tumor markers. Thus, the primary function of tumor markers as described in the correct choice is their diagnostic and monitoring capability throughout the treatment process.