Evaluation of an ultrasound-based imaging system for pelvic cancer localization in radiotherapy

Abstract

Purpose Improvement of prostate cancer treatment in radiotherapy requires reliable, non-invasive and if possible non-irradiating modalities to correct target position before each irradiation session. The goal of this study is to evaluate an ultrasound-based (US) verification system for prostate alignment (CLARITY-ELEKTA). Contrary to other systems based on a cross-modality registration (between reference CT and treatment US images), this system proposes an intra-modality registration that could potentially improve the inter-sessions repositioning quality [1]. Methods and materials 14 patients with primary prostate cancer (P) and 9 patients with prostatectomy (post-P) were included in the first study. For each patient, a US image (US-ref) was acquired during the CT (CT-ref) stage. During the treatment session, a US image (US-T) was acquired just before performing a cone-beam computed tomography (CBCT). The shifts obtained with a US-ref/US-T registration were compared to those obtained with a CT-ref/CBCT registration. In the second study, 14 volunteers underwent three successive US acquisitions with increasing pressure in order to evaluate the impact of probe pressure on prostate localization with the Clarity system. Results For the first study, the average registration differences between US-ref/US-T and CT-ref/CBCT were 2.8 ± 4.1 mm, 1.9 ± 5.8 mm and 3.5 ± 3.8 mm for the P group, and of 0.3 ± 4.4 mm, 2.9 ± 7.5 mm and 1.6 ± 6.7 mm for the post-P group in lateral, superior–inferior and anterior-posterior directions, respectively. The second study showed that probe pressure can imply a prostate displacement up to 4 mm. Conclusion The first study pointed out significant standard deviations discrepancies between these two registration methods. However, both techniques have uncertainties. Soft tissue registration with CBCT imaging modality is not a ground truth, which needs to be considered in the analysis of the results[2,3]. Moreover, as highlighted with the second study, probe pressure can affect prostate localization and therefore can be one of uncertainty sources of US modality. Further studies are in progress to evaluate uncertainties of these two modalities in order to use this US system in clinical routine.