Evaluation of alternative approaches for imaging cellular growth

Uncontrolled growth is a characteristic of malignant tumors. Histochemical techniques to measure tumor growth rate in tissue specimens have proved use ful but are limited because of sampling and the difficulty of following res ponse to therapy. PET imaging offers the opportunity to measure tumor growt h non-invasively and repeatedly as an early assessment of response to thera py. Measuring cellular growth instead of energy metabolism offers significa nt advantages in evaluating therapy. The rationale is that a cell's biosynt hetic machinery, rather than its fuelling process, is more susceptible to c ancer therapy. Cytostatic agents may not reduce the quantity of-viable tumo r; so imaging a change in cellular proliferation may be the only effective way to assess the response to therapy. Radiopharmaceuticals to image growth include labeled amino acids, Lipid precursors, and nucleosides. The bioche mical characteristic that most uniquely distinguishes successfully treated cancer cells is that they no longer synthesize DNA and no longer divide, Th us imaging with labeled thymidine, which is incorporated into DNA hut not i nto RNA, provides definitive evidence of a cell that is proliferating and, therefore, whether it has responded to treatment.