Imaging characteristics of local recurrences after stereotactic body radiation therapy for stage I non-small cell lung cancer: Evaluation of mass-like fibrosis

Background: This study aimed to evaluate stereotactic body radiation therapy (SBRT) in patients with stage I non-small cell lung cancer (NSCLC) in terms of radiation-induced changes and computed tomography (CT) features of local recurrence by 18F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG-PET). Methods: From January 2006 to December 2012, 81 patients with NSCLC received SBRT. Follow-up consisted of non-contrast enhanced CT scans performed before and every four months after SBRT. In addition, 18F-FDG-PET/CT was conducted before SBRT for each patient, and one year later for each case suspected of recurrence. The CT findings were classified into two categories: mass-like fibrosis and others. The mass-like fibrosis category was subdivided into two patterns: mass-like consolidation (with air bronchogram) and mass-like opacity. Results: Six patients had histologically confirmed local recurrence, including 83% (5/6) with mass-like opacity pattern and one case of modified conventional pattern (P = 0.02). In contrast, the non-recurrent group exhibited only 7% (5/75) with mass-like opacity and 13% (10/75) with mass-like consolidation pattern. Five patients with local recurrence presented with the mass-like opacity pattern, compared with 33% of patients (5/15) from the non-recurrent group (P = 0.01) and showed an increase in maximum diameter at ≥12 months after SBRT. The recurrent group also had a significantly higher standardized uptake value (SUVmax) than the non-recurrent group (P < 0.001), with all values >5 (range: 5.7-25.4). Conclusion: The following characteristics of mass-like fibrosis should be considered indicators of local recurrence after SBRT: opacity pattern, increasing maximum diameter, and SUVmax > 5.