Assessing tumor response to therapy

Most anticancer drugs are effective only in subgroups of pa-tients, and our current understanding of tumor biology does not allow us to predict accurately which patient will benefit from a specific therapeutic regimen. Various techniques have, therefore, been developed for monitoring tumor response to therapy, but measuring tumor shrinkage on CT represents the current standard. Although response assessment on CT has been refined over many years, fundamental limitations remain. Interobserver variability in tumor size measurements is still high because of difficulties in delineating tumor tissue from secondary changes in the surrounding tissues. Furthermore, CT is inaccu-rate in differentiating viable tumor from necrotic or fibrotic tissue. Consequently, the degree of response may be underestimated on CT. Conversely, if tumor shrinkage is short lived and followed by rapid tumor regrowth, CT may overestimate the beneficia