SU‐EE‐A3‐04: ROI‐Based Response Assessment Using Pre‐ and Post‐RT 18F‐FDG PET/CT

Purpose: To assess the therapy‐induced changes of head and neck cancers using ¹⁸F‐FDG PET/CT imaging studies performed prior to and following the completion of a course of radiotherapy (RT) through a region‐of‐interest based (ROI) analysis. Method and Materials: As part of an ongoing study, nine patients with carcinoma of the head and neck had PET/CT imaging studies performed prior to the start of treatment (range: 4 – 58 days; median: 36 days) and following the conclusion of treatment (range: 33 – 63 days; median: 59 days). RT contours, created with the Pinnacle treatment planning system and CT simulation images, from physician‐approved plans were collected to align with pre‐ and post‐RT PET/CT images. Utilizing in‐house developed software, the contours were deformed to the pre‐ and post‐RT PET/CT images through a non‐rigid registration. The non‐rigidly aligned RT contours were then used as ROIs to collect data from the pre‐and post‐RT PET images. The standard uptake value (SUV) was calculated assuming that identical structures were contained in the pre‐ and post‐RT deformed volumes, which were altered by anatomic changes as a result of RT. Results: For the GTV contour, the mean pre‐ and post‐RT SUV and standard deviation was (5.6 ± 0.9) and (2.4 ± 0.6), respectively. Within the GTV, the mean maximum SUV prior to RT was (23 ± 4.6), while after the conclusion of RT, the mean maximum SUV was (5.0 ± 1.9). Additionally, the mean ratio of the pre‐ to post‐RT mean SUV was (2.5 ± 0.7), indicating an overall decrease in uptake of the tracer in tissues within the GTV contour. Conclusion: Through the use of deformable image registration, the feasibility of ¹⁸F‐FDG PET/CT ROI‐based analyses of RT‐induced changes in patients with head and neck cancers has been demonstrated.