PET/CT RESPONSE ANALYSIS IN PRIMARY MEDIASTINAL DIFFUSE LARGE B-CELL LYMPHOMA (PMBL): PRELIMINARY RESULTS OF THE IELSG-26 STUDY

The need for radiotherapy in patients with PMBL responding to immunochemotherapy is an unresolved issue. The IELSG-26 study was designed to obtain prospective data in a single cohort regarding the treatment outcomes following anthracycline-containing immunochemotherapy, with or without mediastinal irradiation, according to the local practice of the participating institutions. The primary endpoint was the 18-F-FDG PET response rate following systemic therapy. Between 2007 and 2010 the study enrolled 125 patients with PMBL who received R-CHOP(-like) or R-MACOP-B(-like) regimens. 123 received consolidation radiotherapy. PET-CT scans were performed at baseline, at 3-4 weeks after the end of immunochemotherapy and > 2 months after completion of radiotherapy, according to a standard protocol. CT-PET Complete Response (CR) was defined according to the criteria of the International Harmonization Project by a negative scan or one having minimal residual uptake (MRU) less than the mediastinal blood pool (MBP) activity in regions which were FDG-PET positive at baseline. Central review has been performed of the CT-PET images at the end of chemotherapy in the first 61 patients. The scans showed metabolic CR in 30 pts (49%): in 5 cases (8%) the CT-PET scan was completely negative but in 25 (41%) there were small residual masses with 18-F-FDG uptake less than MBP. Out of 31 (51%) positive CT-PET scans the residual uptake was > MBP but < liver uptake in 17 (28%) cases, slightly > liver uptake in 10 (16%) and >> liver in 4 (7%). The agreement between central review and local reporting was only 70 % (43/ 61). The rate of positive scans on central review was 51%, compared to 39% on site, mainly due to underestimation of the MRU > MBP < Liver. The proportion of patients with positive CT-PET scans at end of immunochemotherapy is higher than has been reported in diffuse large B-cell lymphoma. This may be attributable partly to relatively short interval between the end of chemotherapy and imaging, and partly to low level residual uptake at the site of previous bulky mediastinal disease. Analysis of the CTPET response and its correlation to clinical outcomes in the whole cohort will be presented.