Purpose: Dosimetric studies suggested several advantages to defining the radiotherapy target by using positron-emission tomography (PET)/computed tomography (CT) compared with CT alone. We investigated patterns of treatment failure in patients treated to a PET-defined radiotherapy target and evaluated the effect of standardized uptake value (SUV) on recurrence after radiotherapy.
Methods and materials: Thirty-five patients with non-small-cell lung cancer who underwent PET/CT simulation for definitive radiotherapy were included. The PET/CT scans were obtained with patients in the treatment position with custom immobilization for use in radiation treatment planning. Nine to 11 regions of interest (ROIs) were identified for each patient, including the primary tumor and regional nodes. Maximum SUV, volume, and mean dose received were recorded for each ROI, and follow-up scans were used to evaluate for recurrence in each ROI.
Results: We identified 353 ROIs from 35 patients; 5.7% of patients developed isolated out-of-field recurrences. Recursive partitioning analysis was used to divide ROIs into low, intermediate, and high risk by using volume and SUV. All low-risk ROIs with volumes less than 1.2 cm3 were recurrence free compared with 73% of intermediate-risk ROIs (volume >or=1.2 cm3; SUV <or=13.8) and 29% of high-risk ROIs (SUV > 13.8).
Conclusion: Limiting the target volume to predominantly PET-positive disease resulted in a low rate of isolated out-of-field recurrences. The SUV and volume were predictors of recurrence. Recursive partitioning analysis identified SUVs greater than 13.8 as the best identifier of ROIs at the greatest risk of recurrence; control rates for this subgroup did not show a dose-response relationship within the range of doses administered.