Assessment of tumour response after stereotactic ablative radiation therapy for lung cancer: A prospective quantitative hybrid F-fluorodeoxyglucose-positron emission tomography and CT perfusion study.

Introduction: Stereotactic ablative radiotherapy (SABR) is a guideline-recommended treatment for inoperable stage I non-small cell lung cancer (NSCLC), but imaging assessment of response after SABR is difficult. The goal of this study was to evaluate imaging-based biomarkers of tumour response using dynamic F-18-FDG-PET and CT perfusion (CTP). Methods: Thirty-one patients with early-stage NSCLC participated in this prospective correlative study. Each underwent dynamic F-18-FDG-PET/CTP studies on a PET/CT scanner pre-and 8 weeks post-SABR. The dynamic F-18-FDG-PET measured the tumour SUVmax, SUVmean and the following parameters: K-1, k(2), k(3), k(4) and K-i, all using the Johnson-Wilson-Lee kinetic model. CTP quantitatively mapped BF, BV, MTT and PS in tumours and measured largest tumour diameter. Since free-breathing was allowed during CTP scanning, non-rigid image registration of CT images was applied to minimize misregistration before generating the CTP functional maps. Differences between pre-and post-SABR imaging-based parameters were compared. Results: Tumour size changed only slightly after SABR (median 26 mm preSABR vs. 23 mm post-SABR; P = 0.01). However, dynamic F-18-FDG-PET and CTP study showed substantial and significant changes in SUVmax, SUVmean, k(3), k(4) and K-i. Significant decreases were evident in SUVmax (median 6.1 vs. 2.6; P < 0.001), SUVmean (median 2.5 vs. 1.5; P < 0.001), k(3) (relative decrease of 52%; P = 0.002), K-i (relative decrease of 27%; P = 0.03), whereas there was an increase in k(4) (+ 367%; P < 0.001). Conclusions: Hybrid F-18-FDG-PET/CTP allowed the response of NSCLC to SABR to be assessed regarding metabolic and functional parameters. Future studies are needed, with correlation with long-term outcomes, to evaluate these findings as potential imaging biomarkers of response.