P2.01-20 FLT-PET for Detection of Relapse Following Radiotherapy for Lung Cancer. Preliminary Results

Abstract

Differentiation of relapse from radiation induced changes of the normal lung tissue following radiotherapy for lung cancer is challenging. CT and 18F-fluorodeoxyglucose (FDG) PET/CT has low specificity due to radiation induced changes; and invasive procedures might be unfeasible due to small size, difficult location, or poor lung function. 18F-fluorothymidine (FLT) is a PET tracer that correlates with proliferation. FLT-PET is more specific than FDG-PET and does not accumulate in inflammatory tissue. The aim of this study is to investigate if FLT-PET is a key to better diagnosis of relapse in this difficult situation. Patients who had received definitive radiotherapy for lung cancer and who were suspected for having a relapse were included in this prospective clinical study. Patients underwent FDG-PET/CT and FLT-PET/low dose CT within 3 weeks. PET scans were evaluated visually on a 5-level scale, and the worst graded lung lesion in each patient was selected for semi-quantitative measurements. Reference standard was a retrospective expert analysis based on histology, subsequent imaging, conference decisions, and treatment within 6 months after inclusion. Descriptive statistics and diagnostic accuracy tests were conducted. We present the results from the first 18 patients. All patients had been treated with definitive radiotherapy (66 Gy in 24 or 33 fractions), and 17 patients had concomitant or sequential chemotherapy. FLT-PET was performed 34-541 days after radiotherapy. 7 patients had no evidence of relapse during the follow up period. 11 patients were diagnosed with relapse based on positive biopsy (3), further progression on CT within 6 months (4), further metabolic progression on FDG-PET/CT and cytology suspicious for malignancy (1), progression on the initial FDG-PET/CT and treatment with response (1), or disseminated disease/bone metastases (2). Maximum standardized uptake value (SUVmax) of FDG and FLT were higher in the lesions with relapse. Mean FDG SUVmax in lesions with relapse vs benign lesions was 13.7 vs 4.9 (range: 4.0-18.0 vs 3.3-6.7). Mean FLT SUVmax in lesions with relapse vs benign lesions was 4.1 vs 2.7 (range: 1.8-6.3 vs 1.7-3.3). Sensitivity; specificity; positive predictive value (PPV); and negative predictive value of FDG-PET/CT vs FLT-PET were100%;57%;85%;100% vs 73%;100%;100%;78%. With a PPV of 100% FLT-PET is a promising non-invasive tool for diagnosing relapse after radiotherapy with the potential to obviate invasive procedures in some patients. Further validation is needed.