Physical characteristics in cone beam computed Tomography (CBCT) imaging for small motion target in different directions

Abstract

Purpose: Fiducial marker has been an effective and intuitive way to localize motion target for lung Stereotactic Body Radiotherapy (SBRT). However, due to the complexity for motion target imaging, the optimal target localization strategy still need to be developed to improve the efficiency and effectiveness of the clinical procedure. In this study, the golden marker moving in different directions was characterized in CBCT images for optimal localization application. Method and Materials: A Visicoil linear fiducial marker was selected for this study. The length and diameter of the marker were 5mm and 1mm. The motion was generated by Real Time Position Management (RPM) phantom from Varian Medical System. The motion was simulated to be 2.3 second breathing period and about 1cm amplitude. The motion was at three directions along the anterior-posterior (AP), left-right (LR), and In-Out (IO) of the couch. Their CBCT images were taken, and targets were done by auto-contouring with HU range from minus 900 to positive 4000. The targets were post-processed with keeping the largest part and converted to high resolution segment. Their characteristics were described by shape, volume, center of the volume and volume pixel information, which were attained by MIM Software. Results: In this CBCT study, the volume of motion visicoil gold maker were 0.283cc, 0.348cc, and 0.271cc as moving along AP, IO and LR direction. They were corresponding to 143%, 163% and 236% of the static volume. The max HUs for each moving target were 3395, 343 and 3097, which were 49 %, 13% and 44% of those of the static visicoil. The center distances comparing to those of the static targets were 0.63cm with standard deviation at 0.02cm. Conclusions: When the gold makers were applied for localization of treatment target in Lung SBRT, the geometric distance can be reflected in accurate level; however, the direction of motions could generate large HU difference, which could be one of factors resulted in local failure due to undefined boundary of tumor. And dosimetric characteristic in region of interest should be further investigated to understand the potential effect of local failure case even when marker tracking or soft tissue technique for localization is used.