P1.14-014 Comparative Evaluation of VMAT & 3D-CRT in Locally Advanced NSCLC: Is There Any Radiobiological Advantage?

Abstract

Our aim was comparative dosimetric evaluation of volumetric modulated arc therapy (VMAT) and 3-dimensional conformal radiotherapy (3D-CRT) in patients with locally advanced non-small cell lung cancer (LA-NSCLC) and to confirm whether this dosimetric benefit translates into a radiobiological advantage with respect to tumor control probability (TCP) and normal-tissue complication probability (NTCP) and finally to generate an algorithm and software to integrate the radiobiological component during plan evaluation and quality assurance. A total of 25 3D-CRT naïve patients of LA-NSCLC, treated with radical radiotherapy were included in this study. We used copies of the Eclipse plan data, organized in separate study treatment courses. For each course, new VMAT plans were generated using the standard provincial dose-volume constraints. The DVH parameters for PTV (planning target volume), lung-GTV (gross tumor volume subtracted from lung), heart, esophagus and spine were reviewed and then the TCP and NTCP values for these regions of interest (ROI), cardio-pulmonary toxicity index i.e. C.P.T.I (∑i NTCPi; i=1-2 representing heart, lung), morbidity index i.e. M.I (∑i NTCPi; i=1-4 representing heart, esophagus, lung and spinal cord) and therapeutic gain (defined as the value of TCP when M.I=0; i.e. TG=TCP(1-M.I)) values were generated. The resulting data sets were compared using the Weltch two sample t-test and p <0.05 was accepted as significant. Parameters were calculated using DVH data exported from the Eclipse and software written in the “R” programming language. The user interface has been developed with the “shiny” “R” web library. Dosimetric parameters included V98% and D99% of PTV, V50Gy of esophagus and mean dose to esophagus, V45Gy of spinal cord, V30Gy and mean dose of heart and V20Gy, V10Gy and V5Gy and mean dose of (lung-GTV). The radiobiological metrics included TCP, NTCP of heart, esophagus, lung and spinal cord, C.P.T.I, M.I and therapeutic gain. Dosimetric evaluation did not show any significant difference in V98% (p=0.169) and D99% (p=0.797) of PTV between VMAT and 3D-CRT, however, there was significant improvement in V50Gy of esophagus (p=0.039), V45Gy of spinal cord (p=0.003) and V30Gy of heart (p=0.038) with VMAT. V10Gy of (lung-GTV) was higher with VMAT (p=0.019). The rest of the dosimetric parameters were not significantly different two modalities. TCP was better with 3D-CRT (p<0.0001) while NTCP of heart (p=0.028), esophagus (p=0.053) and spinal cord (p=0.001) was substantially improved with VMAT. There was no difference in NTCP of lung between two modalities. Overall C.P.T.I (p=0.039), M.I (p=0.019) was remarkably lower and therapeutic gain (p=0.005) was significantly better with VMAT. The study reveals that although TCP is higher with 3D-CRT, majority of the NTCP parameters including C.P.T.I and M.I are substantially improved with VMAT even in 3D-CRT naïve patients. The overall therapeutic gain is therefore notably higher with VMAT which emphasizes its potential to achieve superior oncologic outcome in patients with LA-NSCLC.