A5.1 Tumor response

Topic

Tumor response to treatment is the quantification of the change in a patient's tumor cell count—or its most practical clinical indicator, tumor volume—following a therapeutic intervention.

To quantify this tumor response in a comparable way, response criteria exist; these provide a standardized language that allows results to be compared across different clinical studies and guides decisions regarding ongoing patient care in daily practice.

Within these response criteria, the distinction between a complete response and a partial response is not merely a matter of terminology: a complete response indicates that the tumor cell count has dropped to zero—meaning the treatment has effectively eliminated the tumor—whereas a partial response indicates that the treatment merely slowed the growth of tumor cells without eliminating them entirely.

In practice, measuring this tumor response is a challenge of imaging physics, and different sets of criteria measure different dimensions of the same tumor: some measure the longest diameter of target lesions via computed tomography (CT); others measure the tumor's metabolic activity via positron emission tomography (PET); and still others directly assess the presence of necrosis within the tumor as a response to treatment, rather than focusing solely on size.

Assessing intratumoral necrosis, rather than just size, is particularly useful when tumor size alone can be misleading; with immunotherapy, for example, a tumor may appear larger on scans immediately after treatment—not because it is actually growing, but because immune cells are infiltrating it (a phenomenon known as pseudoprogression). Only by assessing necrosis within the tumor or using criteria specifically adapted for this phenomenon can one distinguish pseudoprogression from actual treatment failure. Thus, each response criteria system captures a distinct dimension of the same process: the extent to which the therapeutic intervention succeeded in driving the patient's tumor cell count toward the state where the tumor ceases to exist.

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