With the exception of adjuvant chemotherapy, in which there is no apparent cancer present, the effectiveness of chemotherapy on cancer cells is measured in terms of "response." The ways to monitor responses to treatment can be similar to the tests used to diagnose cancer.
Regardless of the test used - whether blood test, scans, or tumor marker test, it is repeated at specific intervals so that the results can be compared to earlier tests of the same type.
Response to cancer treatment is defined several ways:
For a person with newly diagnosed cancer who has been prescribed chemotherapy, the number of treatments is set. For example, an oncologist will prescribe a specific number of chemotherapy cycles based on the treatment protocol for that type of cancer. Responses may be measured during the chemotherapy, but the number of cycles does not generally change unless the cancer grows. If the cancer grows, the chemotherapy will likely be stopped or changed to different drugs.
For a person who has had a recurrence (the cancer has returned) or has advanced disease, a specific number of cycles may not be prescribed. Rather, 2-3 cycles are given and then response is evaluated. If the disease is stable or shrinking, additional chemotherapy may be given as long as responses are maintained, provided the toxicity of the chemotherapy is tolerable. In general, a minimum of 2-3 cycles of chemotherapy is required in order to measure response. One cycle of chemotherapy may not be adequate to evaluate its effectiveness.
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