Imaging investigation of small motion target for lung stereotactic body radiotherapy (SBRT)

Abstract

Purpose: Motion management is important in lung tumor SBRT. Small target four-dimensional computed tomography (4DCT) imaging could give significant target motion information. In this study, Quality of 4DCT for small target was investigated to provide information for treatment planning in SBRT of small target, which is in favor of magnifying the target margin characteristics during 4DCT scan. Method and Materials: A Real Time Position Management (RPM) phantom from Varian Medical System and 6 CT-spot pellets from Beekley Medical were employed for this study. Each pellet was equivalent to a 4mm diameter ball. The CT spot pellets were attached to the RPM phantom. Two pellet set planes formed 8cm distance and generated two speeds which were 0.2cm and 0.4mm per second given a period with 5.0 seconds. With a GE LightSpeedTM RT CT scanner, the phantom was executed with helical scan with 1.25mm slice thickness with and without motion. Then a 4D CT scan in axial cine mode was carried on at the same phantom. The image exams were exported the GE Advantage Workstation for process. The exams were reviewed and sorted into 10 phases retrospectively. The 0% phase image set, 50% phase image set, maximum intensity projection (MIP) image set, and average intensity projection (Ave-IP) image set were exported to the Eclipse treatment planning system for analysis. In image sets, multi-dimensional variables such as center coordinate of pellets, distance between pellets, and volume of the pellets were used for analysis. Results: At 350 HU window width and 40HU window level, image sets from helical scan for the static phantom is used as reference, comparing to helical scan from moving phantom, retrospective MIP, average intensity projection, 0% phase and 50% phase, for pellets moving at 0.4 cm per second, the average coordinate deviation are 0.49cm, 0.22cm, 0.19cm, 0.59cm and 0.06cm with standard deviation 0.83cm, 0.29cm, 0.32cm, 0.69cm, and 0.07cm, the percentage distance differences are 10.2%, 3.3%, 7.3%, 0.1%, and 0.1% with standard deviation of 13.2%, 4.4%, 9.6%, 0.1%, and 0.1%. and the percentage volumes changes are 17.4%, 171.5%, 18.4%, 14.2% and 7.8% with standard deviation at 25.4%, 201.8%, 23.7%, 19.85% and 10%. Conclusions: A simple and sensitive 4DCT quality assurance method was developed by combining Varian RPM phantom and CT-spot pellets from Beekley Medical and it shows that the 50% phase image set is better for SBRT planning. Further study will include dosimetric effect on the small target moving in different directions.