8. Use of the Hexamotion system coupled to Delta 4 detector for quality control of treatment plans in blocked inspiration

Abstract

Introduction The blocked inspiration technique limits internal movements during CT acquisition and irradiation. This technique of respiratory control allows, especially for breast and lung cancer, to preserve the OARs (Heart, Lungs). In lung cancers, it improves the coverage and delivers more doses to the tumor without increasing the dose to OARs. Implementation of this technique requires the development of new methods of quality control for treatment plan that take account of these movements and their uncertainties. The aim is to simulate displacements between 2 breath hold inspirations representative of clinical cases and to introduce errors to better represent the uncertainty of repositioning the tumor. Then, the purpose is to evaluate the impact of these displacements on the quality of treatment plans using phantom measurements. Methods For lung and breast cancer, the displacement is quantified between the mean tumor position in respiratory cycle and its position in deep inspiration breath hold. This displacement quantification is done with ImageJ in 3D and is included in «Trajectory» file. Errors randomly generated according to Gaussian distribution are introduced in these displacements to represent intrafraction motion. A homemade program allows to apply these displacements to a 3D motion system (Hexamotion, Scandidos). A 3D phantom (Delta 4, Scandidos) placed on this system provides the evaluation of treatment plan quality and to define tolerance on patient motion. The analysis criteria are defined 3%/3 mm, local analysis and 20% dose threshold. Results Analysis on ImageJ allow to estimate offsets of 2.8 mm along the X axis, 5.3 mm along Y and −4 mm along Z. “Trajectory” files based on this displacements are generated and the uncertainties of repositioning are taken into account according to the Gaussian law. Conclusions This study provides an optimization of virtual simulation and dosimetry protocols, based on quality control results of treatment plan. It allows to define tolerances on intrafraction repositioning of blocked inspiration. Download : Download high-res image (113KB) Download : Download full-size image