Quality assurance device for four-dimensional IMRT or SBRT and respiratory gating using patient-specific intrafraction motion kernels

Abstract

Purpose: Emerging technologies such as 4D CT and implanted beacons will allow clinicians to accurately model intra-fraction motion and define internal target volumes (ITV) for radiation therapy involving moving targets. In the case of IMRT/SBRT delivery, clinicians must consider the interplay between the temporal nature of the modulation and the target motion within the ITV. There is a need for a 4D IMRT/SBRT QA device that can incorporate and analyze customized intra-fraction motion as it relates to dose delivery and respiratory gating. Method and Materials: A 4D IMRT/SBRT prototype device was built. (X, Y, Z)(T) coordinates representing a motion kernel were entered into a software application that: 1) transformed the kernel into beam-specific 2D motion “projections”, 2) previewed the motion in real time, and 3) drove a precision X-Y motorized device that had atop it a mounted planar IMRT QA measurement device (MapCHECK). The subset of detector positions that intersected the target in the beam’s-eye-view of a single phase of the breathing cycle were defined as “target detectors” and analyzed for dose uniformity between multiple fractions. To show proof-of-concept, two clinical lung target motion kernels, garnered from 4D CT, were explored. Measurements were made for four different delivery modalities: open fields, Solid IMRT, SMLC, and DMLC. Measurements were acquired for both non-gated and gated delivery. Results and Conclusions: The combined software and hardware solution for patient-customized 4D IMRT/SBRT QA proved to be an effective tool for assessing IMRT delivery under conditions of intra-fraction motion. The 4D IMRT QA device accurately reproduced the projected motion kernels for all beam’s-eye-view motion kernels. The device effectively quantified the degradation in dose uniformity due to a moving target within a static ITV. The device also successfully integrated with a commercial respiratory gating system to ensure that the system was working effectively, and could be used to optimize the duty cycle of gated delivery with respect to practical time considerations.