High-risk CT features for detection of local recurrence after stereotactic ablative radiotherapy for lung cancer

Background and purpose Early detection of local recurrences following stereotactic ablative radiotherapy (SABR) for lung cancer may allow for curative salvage treatment, but recurrence can be difficult to distinguish from fibrosis. We studied the clinical performance of CT imaging high-risk features (HRFs) for detecting local recurrence. Materials and methods Patients treated with SABR for early stage lung cancer between 2003 and 2012 who developed pathology-proven local recurrence (n = 12) were matched 1:2 to patients without recurrences (n = 24), based on baseline factors. Serial CT images were assessed by blinded radiation oncologists. Previously reported HRFs were (1) enlarging opacity at primary site; (2) sequential enlarging opacity; (3) enlarging opacity after 12-months; (4) bulging margin; (5) loss of linear margin and (6) air bronchogram loss. Results All HRFs were significantly associated with local recurrence (p < 0.01), and one new HRF was identified: cranio-caudal growth (p < 0.001). The best individual predictor of local recurrence was opacity enlargement after 12-months (100% sensitivity, 83% specificity, p < 0.001). The odds of recurrence increased 4-fold for each additional HRF detected. The presence of ≥3 HRFs was highly sensitive and specific for recurrence (both >90%). Conclusion The systematic assessment of post-SABR CT images for HRFs enables the accurate prediction of local recurrence.