Traditionally, the field of oncology was interested in complete partial response and progression. Later stable disease came into use, a measure of chemotherapy success that combined tumor shrinkage and cases in which there had been no change. Any growth , usually greater than a 25% increase in tumor size was called progression. Mixed response refers to a situation in which some metastases are responding and others are growing and/or new metastases are appearing in the same time. It is a reflection of the fact that some cancer cells respond to therapy and others do not. A commonly used definition of types of mixed responses, when precision is required, such as in clinical studies, is that it is defined as >30% difference in individual lesion response, with all lesions showing a similar behavior; true mixed response (TMR) as two lesions showing progression versus response; homogeneous response (HR) is similar behavior of all lesions. Mixed responses are quite common in colorectal cancer; in one study of liver metastases, mixed responses were found in 30% of the patients. Unfortunately, this metric has been somewhat neglected and is not often included in clinical studies. As such, there had been little research and no guidelines to the oncologists on how to handle the mixed response situation. There appears to be a consensus that whether to change therapy depends on other available options, degree of security that the lesions that progressed truly progresses (as opposed to swelling or inflammation; some physicians advocate re-biopsy in unclear cases) and the number and size of non-responsive lesions.
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