INVITED COMMENTARY PET Imaging for Planning Cancer Therapy

has been un-der way as a major field of research in nuclear medicine for nearly 20 y. How-ever, advances have been slow, and only now can we expect the first com-mercial products for cancer therapy to soon become available. What have been the obstacles to this development? The agents themselves have variable and often low levels of tumor binding in vivo, and solid tumors are usually ra-dioresistant. Technology for assess-ment of patient dosimetry estimates has not been consistent because of the diffi-culties in determining accurate concen-trations of the radiotracers in normal and tumor tissues by quantitative im-aging (1,2). Up to now, the choices have been to image and treat patients with 131I or to use 111In as a surrogate to estimate the biodistribution of 90Y as the therapeutic radioisotope (3). In both situations, the errors in determin-ing tissue radioisotope concentrations for input into dosimetry estimates for planning treatment and evaluating ob-served toxicity are variable and can be large. Only a few investigators have se-riously attempted to validate these meth-ods and apply the validated techniques to clinical situations (4). Accordingly, optimization of the clinical therapy proto-cols to deliver maximum tumor dose and the push to define normal tissue toxic-ity limits have not proceeded. In our experience, study protocols that prospectively describe true treat-ment toxicity and efficacy in terms of reliable radiation absorbed doses to normal tissues and tumors are usu-ally required at this stage. Radiola-beled antibody therapy will succeed when optimized quantitative imaging and techniques to estimate radiation absorbed dose are the basis for the treatment plan for every patient. At the center of this requirement is an accurate, reliable means of determin-ing the tissue concentration of radio-labeled antibody