(18)F-FDG-PET/CT evaluation of response to treatment in lymphoma: when is the optimal time for the first re-evaluation scan?

Assessing the response to treatment as soon after treatment initiation is one of the key reasons for imaging lymphoma patients. The optimal time after initiating treatment for assessing response to treatment has yet to be determined. Therefore, we were prompted to review our experience with serial (18)F-FDG PET/CT in patients undergoing treatment for Hodgkin's disease (HD) and non Hodgkin's lymphoma (NHL). This is a retrospective study (Feb 2003 - Oct 2004) of 20 patients, 11 men and 9 women, with age range of 7-75 years with diagnosis of HD (10) and NHL (10), who had PET/CT at our institution prior, during and at the completion of therapy. Restaging PET/CT was done after 2 cycles of chemotherapy in 10 patients (group A) and after 4 cycles of chemotherapy in 10 pts (group B). A total of 60 scans were reviewed. The DeltaSUV from baseline to first PET/CT was on average 67.6% in group A and 75.1% in group B. This had no statistical significance (P value: 0.31). The DeltaSUV from baseline to post-therapy PET/CT was on average 72.9% in group A and 79.8% in group B. This difference also had no statistical significance (P value: 0.24). The correlation coefficient was 0.98 in group A and 0.80 in group B. Results of PET/CT after 2 cycles of chemotherapy did not statistically differ from the results of PET/CT after 4 cycles of chemotherapy. These results need to be confirmed in larger, prospective, randomized trials.