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Assessing Tumor Response to Therapy

Department of Nuclear Medicine, University of Freiburg, Freiburg, Germany

Correspondence: For correspondence or reprints contact: Wolfgang. A. Weber, Department of Nuclear Medicine, University of Freiburg, Hugstetterstrasse 55, 79106 Freiburg, Germany. E-mail: Wolfgang.weber{at}uniklinik-freiburg.de

Most anticancer drugs are effective only in subgroups of patients, 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 inaccurate 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 beneficial effects of a treatment. Finally, CT is limited in characterizing responses in tumors that do not change in size during therapy. Because the growth rate of untreated human tumors varies tremendously, an unchanged tumor size after some weeks of therapy may represent a drug effect but may also indicate a slowly growing tumor that was not affected by the applied therapy. Molecular imaging with PET and the glucose analogue ¹⁸F-FDG PET has been shown to improve response assessment in several tumor types. In malignant lymphoma, international criteria for monitoring response to therapy have recently been revised, and the ¹⁸F-FDG signal now plays a central role in defining tumor response. In a variety of solid tumors, single-center studies have indicated that ¹⁸F-FDG PET may provide earlier or more accurate assessment of tumor response than CT, suggesting that ¹⁸F-FDG PET could play a significant role in personalizing the treatment of malignant tumors. However, generally accepted criteria for response assessment in solid tumors are missing, which makes it frequently impossible to compare the results of different studies. International guidelines and criteria for response assessment by ¹⁸F-FDG PET in solid tumors are, therefore, eagerly awaited.

Key Words: oncology • PET • PET/CT • monitoring • therapy • tumor response

COPYRIGHT © 2009 by the Society of Nuclear Medicine, Inc.

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