Use of 4D CT to estimate the dose given to GTV in VMAT lung SBRT

Abstract

Introduction The use of 4DCT to defining the Internal Target Volume (ITV) is a tool to safely minimize the necessary margins to take into account breathing movement in lung Stereotactic Body Radiation Therapy (SBRT). Treatment planning is performed using a conventional CT with slow table speed, producing a plan accepted for treatment (TP). In this work we study the dosimetric degradation produced when recalculate a TP plan on the 4DCTs. Materials and methods 4DCT consists of a set of 8 CTs obtained by retrospectively sorted image data accordingly with a respiratory cycle. These eight CTs are used for ITV delimitation being the envelope of the GTVs delimited on each individual CT. Seven patients with a significant GTV breathing movement were selected. Plans consisted on two or four VMAT partial arcs with collimator rotation of 30° and 330°. PTV’s planning objective is 98% volume covered by 98% of prescription dose. TP plans were recalculated on the 4DCTs. Hence, eight plans were generated (4DP). Average histograms of GTVs and ITV in 4DP and TP respectively were compared. Results Result showed that the dosimetric coverage of GTV (for 4DP plans) is lower than for ITV (for TP plans). In three patients, averaged GTV histograms (4DP) resulted in a dose covering of 97.5%, 96.7% and 95.7% of the prescription dose to 98% of GTV volume. The inherent dosimetric heterogeneity of these treatments and the blurring effect in GTV due to use a slow CT for planning, could explains these discrepancies. Conclusion Appropriate dose coverage of ITV might not be enough to ensure a GTV appropriate irradiation. Disclosure None.