Evaluation of two methods for soft tissue registration in online CT guided prostate cancer radiotherapy

Abstract

Our clinic is conducting a feasibility study for CT-guided hypo-fractionated treatment of prostate cancer. Patients will receive a CT scan on our cone-beam CT-equipped linac immediately prior to treatment, the displacement of the prostate gland with respect to its planning position will be determined, and a correction will be applied accordingly. The purpose of this study is to evaluate two methods of performing the soft tissue registration. To simulate the entire course of the treatment we used serial CT data from 28 prostate cancer patients. In addiction to the planning CT, each patient had a pelvic CT scan (3 mm slice thickness) bi-weekly over the entire course of the treatment for a total of 15 CTs. Using a commercial treatment planning system, each daily CT was fused with the planning CT based on bony anatomy. This step eliminates the setup error component of the inter-fraction geometric variation. The prostate was contoured on each scan by a single physician. Subsequently prostatic image-based registration (IBR) was performed. This was accomplished by translating each daily prostate CT image to match the reference CT prostate contour, allowing sagittal rotation only. The registration was then repeated using only the contour of the daily prostate (CBR). A single investigator performed the two types of registration for all 28 patients. Each registration induces a transformation (Δdx,Δdy,Δdz,) that characterizes the motion of the daily prostate per a rigid body formalism, where Θ is the rotation in the sagittal plane. We assessed the IBR accuracy and stability by comparing each parameter of the 420 transformations between IBR and CBR. The average time to perform the correction was on the order of 3 minutes for the IBR and 1 minute for the CBR. While both techniques have uncertainties, we adopted the CBR as a reference both because there is a greater body of experience in contour definition than in soft tissue registration, and because there is less ambiguity in performing the CBR. The average shift magnitude from the AP perspective was 0.18 ± 0.18cm for CBR and 0.12 ± 0.13cm for IBR. From the lateral perspective, the results for the CBR and IBR are 0.37 ± 0.30cm and 0.25 ± 0.25cm, respectively. The analysis shows a larger shift in the AP direction then in any other direction, both in the IBR and CBR, a result consistent with previous studies. The average rotation angle, allowed only in the sagittal view, was 0.7o ± 2.0o for the CBR and 1.3o ± 2.4o for the IBR. The per-patient average discrepancy (Δdx,Δdy,Δdz) between the CBR and IBR results varies with perspective. While the average difference in the LAT direction is within 1mm for all but 6 patients, the results for the AP and SI directions have considerably more variation between patients. Furthermore, a systematic bias of more than 1mm is observed in the AP direction for 20 of the patients; in particular, the IBR indicates a larger posterior shift of the patient than does the CBR. The composite result over all patients reiterates these observations: (Δdx,Δdy,Δdz) =( 0.04 ± 0.12, 0.16 ± 0.22, 0.0 ± 0.26) cm. The CBR-IBR agreement was within 2mm for 69% of results, between 2 and 4mm for 23%, and greater than 4mm for 8%. The CBR was faster and easier to perform. The IBR showed an overestimation of the sagittal rotation angle and an underestimation of the shift magnitude for both the AP and LAT perspectives when compared to the CBR. The discrepancy between IBR and CBR is a measure of the intraobserver uncertainty of the online CT based correction. Due to the potential for significant discrepancies between the CBR and IBR results, we strongly recommend performing offline CBR verification of an online IBR-based correction. The observed IBR-CBR gap will possibly be narrowed by the use of on board KV conebeam device capable of good quality images in the coronal and sagittal views