Metabolic Trending during SBRT for Central Lung Oligomets!

Abstract

SBRT, predominantly, causes indirect cell kill through (1) Vascular apoptosis, starts few hrs after treatment peaking at 3 days and (2) immunological alteration in the tumor microenvironment, process which starts around 3 days & peaks at 10-12 days. Routine Post SBRT metabolic imaging is advised at 3 months. Present study evaluates clinical response of multifraction SBRT during and post-treatment completion with serial FDG PET-CT scans in patients with solitary central lung oligometastases. Five patients with oligometastatic central lung tumors (Primaries: breast - 3, colorectal - 1, and kidney-1) underwent SBRT by DIBH technique using ABC on 6 DOF C-arm Linac using body fix immobilization. Peripheral lesions were excluded from this study (as fractionation protocols ranges from 1-3). A risk adapted multi-fractionated protocol – 6 Gy x 8 fractions was used for treatment delivery. All patients underwent limited metabolic imaging pre-treatment, during treatment, and post-treatment PET-CT scans up to 3mos. During treatment PET-CT imaging after delivery of 3 fractions or at least after 18 Gy dose delivery. Post SBRT serial PET-CTs scans were timed serially at 48hrs, 10 days, 30 days and 60 and 90 days post treatment. PET-CT images were reviewed in order to determine the pre-treatment, during treatment, and post-treatment maximum standardized uptake value (m-SUV) of the lesion, including “complete resolution” of FDG-avidity. Corresponding Morphologic variations of the target lesions were studied on the CT images during the 3-mo. evaluation period. All the 5 patients showed nearly consistent serial regression in metabolic activity during the treatment. The observed metabolic regression was between 60%-65% (median 50%%) after 3 frcs/ 18gy dose scan, 80% (median – 70%) after delivery of 6 frcs/ post-32gy dose scan and 80% (median 80%) at 48hrs post treatment scans, 85-90 % (median - 90%) 10 days post SBRT. After the 10th day Scan, SUV thereupon remained constant ranging between 1.3 - 3.2 (median 2.3) up to 3 months. The earliest morphologic changes on CT in the form of ground glass appearance was detected at 60 days post SBRT scan uniformly in all patients. During the evaluation period (first 3 months) there was no significant change in size of the lesion. The metabolic response precedes morphologic response, with an early metabolic response seen immediately after 24 Gy dose delivery & maximum metabolic response seen at 10 days post SBRT. This is the first study reported in literature which looked into the serial metabolic trending during Lung SBRT for central lesions.