SU‐E‐J‐95: Evaluation of Repeat Respiratory Gated PET Scans for the Assessment of Treatment Effectiveness

Abstract

Purpose: Gated (4D) PET/CT scans have been shown to be effective in recovering the signal lost due to respiratory motion. Ungated (3D) and 4D FDG‐PET scans obtained pre and post radiation therapy (RT) for lung cancer were compared to examine the role of 4D PET in assessing FDG uptake changes due to therapy. Methods: Sequential 3D and 4D FDG‐PET/CT scans were acquired approximately two weeks before (pre‐RT) and one month after (post‐RT) treatment for patients receiving definitive RT for lung cancer. Targeted lesions were identified for each patient, and a region of interest (ROI) was defined for each lesion on the 3D and each of the five phases of the 4D pre‐RT scans. The ROIs were generated using a 40% of the maximum standardized uptake value (SUV) threshold, and the maximum SUV for each lesion from the pre—RT scans was recorded. Pre‐RT ROIs were then used to locate the areas of the original FDG uptake on the post‐RT scans using rigid registration. A post‐RT ROI was generated, and the maximum post‐RT SUV was recorded. Results: Sixteen lesions were identified and analyzed for twelve patients. Comparison of pre‐RT to post‐RT 3D PET scans showed a median SUV decrease of 62% (range 36–89%), while comparison of pre‐RT to post‐RT 4D PET scans showed a median SUV decrease of 67% (range 30–89%). The mean difference between the SUV change on 3D versus 4D scans was 4.9% (range 0–15%, p=.07), and the highest quartile had an absolute difference greater than 9% in SUV change.Conclusion: When assessing SUV changes with PET scans done pre and post‐RT, a substantial proportion of lesions had showed differences of over 9% in measurement of SUV change when comparing 3D PET to 4D PET. Such differences could potentially influence classification of PET‐based response assessment.