Dosimetric Parameters After VMAT Adaptive Radiotherapy in Patients With Locally Advanced NSCLC Undergoing 4DCT.

Background/aim: To investigate dosimetric differences in organs at risk (OARs) and cardiac substructures in patients with locally advanced non-small cell lung cancer (NSCLC) between the adaptive radiotherapy (ART) and non-ART groups.

Patients And Methods: Thirty patients were treated with definitive radiotherapy +/- chemotherapy. Cardiac substructures including the left anterior descending coronary artery (LAD) and large vessels, were contoured. Eight patients experienced tumor shrinkage and were replanned (ART). Cumulative plans after ART were compared to the original plans (not considering volume reduction) in terms of dosimetric parameters. The cumulative plans of the ART group (n=8) and non-ART group (n=22) were compared in terms of the same dosimetric parameters.

Results: Within the ART group, the following parameters were found to be significantly improved after re-planning: mean lung dose (MLD) (13.79 Gy vs. 15.6 Gy), V20Gy both lungs (17.88% vs. 27.38%), ipsilateral MLD (20.87 Gy vs. 24.44 Gy), and esophagus mean dose (20.79 Gy vs. 24.2 Gy). No dosimetric differences were observed in heart substructures. Dosimetric parameters, particularly LAD, were significantly worse in the ART group than in the non-ART group. This is probably because this OAR was not considered in the plan optimization after re-planning, because it was not routinely contoured as an OAR.

Conclusion: Our analysis showed an improvement in dosimetric parameters in the lungs and esophagus in the ART group. This approach may lead to a possible reduction in toxicity. Contouring of cardiac substructures could lead to a plan optimization of their parameters and eventually reduce the risk of cardiac toxicities in these patients.