Dynamic Radiologic Changes and Predictors of Pseudoprogression After Lung Stereotactic Ablative Radiotherapy (SABR)

Post-SABR radiologic appearances in the lung are dynamic and may imitate local failure. Although radiologic changes after SABR have been described, the dynamic evolution and predictors of pseudoprogression remain unclear. We aimed to characterize serial computed tomography (CT) evolution after SABR and examine its association with pseudoprogression.

We retrospectively included patients who underwent SABR and had ≥24 months radiologic follow-up without definite local recurrence. The maximum diameter of the evolving post-SABR lesion was referenced to the initial (pre-SABR) tumor. Pseudoprogression was defined as any >20% increase in maximum diameter during follow-up that was followed by radiographic stabilization and lacked pathologic or clinical evidence of recurrence.

We evaluated 109 lesions from 104 patients (median follow-up, 53 months). Mass-like consolidation developed in 94/109 (86%) lesions. CT evolution followed three phases with typical onset: initiation-reticular infiltrates adjacent to the index tumor (median 3 months; range 1-9); development-increasing density with patchy/streaky opacities and/or fibrotic consolidation (median 4.5 months; range 1.5-15.5); and mass-like consolidation-organization/absorption or retraction culminating in a final mass-like consolidation (median 13 months; range 5.5-39). A total of 74 (68%) lesions showed a >20% increase vs the initial tumor, and 50% occurred during the third phase. Planning target volume (PTV) was an independent predictor of pseudoprogression (adjusted odds ratio 0.98, 95% CI 0.97-0.99; P = 0.002).

Dynamic CT evolution after SABR commonly manifests as mass-like consolidation with high rates of pseudoprogression. PTV independently predicts this phenomenon, underscoring the need for cautious interpretation of radiographic enlargement during follow-up.