Ultrasound versus Cone-beam CT image-guided radiotherapy for prostate and post-prostatectomy pretreatment localization

Purpose: To evaluate the accuracy of an intra-modality trans-abdominal ultrasound (TA-US) device against soft-tissue based Cone-Beam Computed tomography (CBCT) registration for prostate and post-prostatectomy pre-treatment positioning. Methods: The differences between CBCT and US shifts were calculated on 25 prostate cancer patients (cohort A) and 11 post-prostatectomy patients (cohort B), resulting in 284 and 106 paired shifts for cohorts A and B, respectively. As a second step, a corrective method was applied to the US registration results to decrease the systematic shifts observed between TA-US and CBCT results. This method consisted of subtracting the mean difference obtained between US and CBCT registration results during the first 3 sessions from the US registration results of the subsequent sessions. Inter-operator registration variability (IOV) was also investigated for both modalities. Results: After initial review, about 20% of the US images were excluded because of insufficient quality. The average differences between US and CBCT were: 2.8 +/- 4.1 mm, -0.9 +/- 4.2 mm, 0.4 +/- 3.4 mm for cohort A and 1.3 +/- 5.0 mm, -2.3 +/- 4.6 mm, 0.5 +/- 2.9 mm for cohort B, in the anterior-posterior (AP), superiorinferior (SI) and lateral (LR) directions, respectively. After applying the corrective method, only the differences in the AP direction remained significant (p < 0.05). The IOV values were between 0.6-2.0 mm and 2.1-3.5 mm for the CBCT and TA-US modalities, respectively. Conclusions: Based on the obtained results and on the image quality, the TA-US imaging modality is not safely interchangeable with CBCT for pre-treatment repositioning. Treatment margins adaptation based on the correction of the systematic shifts should be considered. (C) 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.