Tumor regression during radiotherapy as a predictor of response in locally advanced nonsmall cell carcinoma

Aims: To compare the predicted response with observed response to treatment by measuring gross tumor volume-primary (GTVp) using onboard kilovoltage (kV) cone-beam computed tomography (CBCT), to analyze the serial tumor volumes during radiotherapy (RT) with serial tumor volumes during follow-up, and to identify the variables associated with survival outcomes. Materials and Methods: Between June 2017 and December 2019, 23 patients of histologically proven locally advanced nonsmall cell lung cancer (LA-NSCLC) received definitive chemoradiation. Serial kV-CBCT images X-ray volume imaging (XVI) were generated weekly for image guidance and were used to generate serial GTVp. Posttreatment follow-up images were used to generate follow-up GTVp. Relative volume regression (VR) during RT and relative response assessment (RA) during follow-up were defined from Avg Vol, of planning CT. The predicted progression model was generated from VR and analyzed against observed progression events. Regression-response model was generated to analyze VR against RA. Results: The median XVI vol(1), XVI vol(2), and XVI vol3 were 78.123, 56.571, and 48.513 cc during the 2nd, 4th, and 6th weeks of RT, respectively. The median VR0 was 11.777% in the 2nd-week, VR1 was 20.959% in the 4th week, and 33.661% in the 6th week. The predicted responders and progression using the VR were similar to the observed response during the follow-up. The prediction of both RA(0) and RA(1) obtained from VR2 was statistically significant. Predication of RA(0) from VR1 tended towards significant (P=0.084). VR2 was statistically significant in predicting RA2 (P = 0.04). The median progression-free survival (PFS) was not reached and the median overall survival (OS) was 24.2 months (95% confidence interval, 20.3-28.2 months). There was no statistically significant difference in PFS and OS between Avg Vol <= 99.5 cc and > 99.5 cc or other clinical parameters. Conclusions: Tumor regression during RT is a potential predictor of response in LA-NSCLC. kV-CBCT is a strong tool in assessing tumor regression during RT.