Differentiation of tumor recurrence from radiation-induced pulmonary fibrosis after stereotactic ablative radiotherapy for lung cancer: characterization of 18F-FDG PET/CT findings

Abstract

Stereotactic ablative radiotherapy (SABR), also known as stereotactic body radiotherapy (SBRT), is now a standard treatment option for patients with stage I non-small cell lung cancer or oligometastatic lung tumor who are medically inoperable or medically operable but refuse surgery. When mass-like consolidation is observed on follow-up CT after SABR, it is sometimes difficult to differentiate tumor recurrence from SABR-induced pulmonary fibrosis. In this study, we evaluated the role of (18)F-fluorodeoxyglucose positron emission tomography combined with computed tomography (FDG-PET/CT) in differentiating tumor recurrence from radiation fibrosis after SABR. Between June 2006 and June 2009, 130 patients received SABR for stage I non-small cell lung cancer or metastatic lung cancer at our institution. Fifty-nine patients of them were imaged with FDG-PET/CT after SABR. There were a total of 137 FDG-PET/CT scans for retrospective analysis. The FDG uptake in the pulmonary region was assessed qualitatively using a 3-point scale (0, none or faint; 1, mild; or 2, moderate to intense), and the shape (mass-like or non mass-like) was evaluated. For semi-quantitative analysis, the maximum standardized uptake value (SUV(max)) was calculated. Sixteen of 59 patients had local failure. In recurrent tumor, the combination of intensity grade 2 and mass-like shape was most common (21/23; 91%). By contrast, in cases of radiation fibrosis, the combination of intensity grade 0 or 1 and non mass-like shape was most common (48/59; 81%). The SUV(max) of tumor recurrence after 12 months was significantly higher than that of radiation fibrosis (8.0 ± 3.2 vs. 2.1 ± 0.9, p < 0.001), and all tumor recurrence showed the SUV(max) > 4.5 at diagnosis of local failure. At ≥12 months after SABR, these two variables, the combination of intensity 2 and mass-like FDG uptake or SUV(max) > 4.5 acquired a significant high predictive value of local recurrence, finding sensitivity 100% and specificity 100% for both of them. The combination of FDG uptake patterns and SUV(max) was useful for distinguishing tumor recurrence from radiation fibrosis after SABR.