M. Fargier-Voiron

Impact of probe pressure variability on prostate localization for ultrasound-based image-guided radiotherapy.

PURPOSE To evaluate the impact of transabdominal probe pressure on prostate positioning with an intramodality ultrasound (US) image-guided-radiotherapy system and to quantify pressure variability over the treatment course. MATERIAL AND METHODS 8 prostate cancer patients (group A) and 17 healthy volunteers underwent 3 consecutive US images with increasing probe pressure levels, and 1 CT acquisition for the group A only. Prostate positions were compared after manual registration of the first US image contour projected on 2 others. Group As pressure levels were quantified by measuring skin-to-skin distances between corresponding CT-US images. The same methodology was used on paired CT/CBCT-US images acquired during treatments of 18 prostate cancer patients to determine whether the different pressure levels applied to the group A were close to the clinical practices and to quantify pressure variability along the treatment course. RESULTS 84% of 3D prostate displacements were above 2mm for at least one pressure level. Probe pressures deliberately applied were similar to the ones observed clinically. The latter drastically varied between sessions. CONCLUSION Even with an intramodality system, probe pressure can impact prostate localization because of the pressure variability along the treatment course. Therefore, margins should be expanded from 0.5 to 1.2mm to ensure treatment accuracy.

Evaluation of a new transperineal ultrasound probe for inter-fraction image-guidance for definitive and post-operative prostate cancer radiotherapy.

PURPOSE The aim of this study was to evaluate a new system based on transperineal ultrasound (TP-US) acquisitions for prostate and post-prostatectomy pre-treatment positioning by comparing this device to cone-beam computed tomography (CBCT). METHODS The differences between CBCT/CT and TP-US/TP-US registrations were analyzed on 427 and 453 sessions for 13 prostate and 14 post-prostatectomy patients, respectively. The inter-operator variability (IOV) of the registration process, and the impact and variability of the probe pressure were also evaluated. RESULTS CBCT and TP-US shift agreements at ± 5 mm were 76.6%, 95.1%, 96.3% and 90.3%, 85.0%, 97.6% in anterior-posterior, superior-inferior and left-right directions, for prostate and post-prostatectomy patients, respectively. IOV values were similar between the 2 modalities. Displacements above 5 mm due to strong pressures were observed on both localizations, but such pressures were rarely reproduced during treatment courses. CONCLUSIONS High concordance between CBCT/CT and TP-US/TP-US localization of prostates or prostatic beds was found in this study. TP-US based prepositioning is a feasible method to ensure accurate treatment delivery, and represents an attractive alternative to invasive and/or irradiating imaging modalities.

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), superior-inferior (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.

Semiautomatic registration of 3D transabdominal ultrasound images for patient repositioning during postprostatectomy radiotherapy

PURPOSE The aim of the present work is to propose and evaluate registration algorithms of three-dimensional (3D) transabdominal (TA) ultrasound (US) images to setup postprostatectomy patients during radiation therapy. METHODS Three registration methods have been developed and evaluated to register a reference 3D-TA-US image acquired during the planning CT session and a 3D-TA-US image acquired before each treatment session. The first method (method A) uses only gray value information, whereas the second one (method B) uses only gradient information. The third one (method C) combines both sets of information. All methods restrict the comparison to a region of interest computed from the dilated reference positioning volume drawn on the reference image and use mutual information as a similarity measure. The considered geometric transformations are translations and have been optimized by using the adaptive stochastic gradient descent algorithm. Validation has been carried out using manual registration by three operators of the same set of image pairs as the algorithms. Sixty-two treatment US images of seven patients irradiated after a prostatectomy have been registered to their corresponding reference US image. The reference registration has been defined as the average of the manual registration values. Registration error has been calculated by subtracting the reference registration from the algorithm result. For each session, the method has been considered a failure if the registration error was above both the interoperator variability of the session and a global threshold of 3.0 mm. RESULTS All proposed registration algorithms have no systematic bias. Method B leads to the best results with mean errors of -0.6, 0.7, and -0.2 mm in left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions, respectively. With this method, the standard deviations of the mean error are of 1.7, 2.4, and 2.6 mm in LR, SI, and AP directions, respectively. The latter are inferior to the interoperator registration variabilities which are of 2.5, 2.5, and 3.5 mm in LR, SI, and AP directions, respectively. Failures occur in 5%, 18%, and 10% of cases in LR, SI, and AP directions, respectively. 69% of the sessions have no failure. CONCLUSIONS Results of the best proposed registration algorithm of 3D-TA-US images for postprostatectomy treatment have no bias and are in the same variability range as manual registration. As the algorithm requires a short computation time, it could be used in clinical practice provided that a visual review is performed.

EP-1750: Monitoring of intra-fraction prostate motion with a new 4D ultrasound device

S819 ________________________________________________________________________________ monitored with Calypso (Varian) for gating and tracking treatments, and compensated with the PerfectPitch couch (Varian) for tracking. The dose in the moving tumor was measured with Gafchromic EBT2 (ISP) films. Changes in homogeneity indices (ΔH1-99) between the films and the planned dose distributions and their gamma agreement scores using 3%/3mm (GS3%/3mm) were evaluated. The film areas receiving more than the planned minimum dose (A>Dmin) were calculated. OAR doses from the treatment plans were compared.

Development of 2D+T tracking algorithm in ultrasound images for radiotherapy

The aim of this study is to develop and validate a deformable tracking algorithm for monitoring the motion of the target volume on 2D ultrasound (US) images during a radiotherapy fraction. The proposed method is applied on images acquired with a transperineal ultrasound (TP-US) probe on 31 treatment patients sessions, treated with a prostate or after a surgery, called a prostatectomy. The developed algorithm is based on Speeded-Up Robust Features (SURF) to find and match the corresponding salient points in the reference and moving images, and Thin Plate Spline (TPS) to warp the image. The results are promising and show that the proposed algorithm performs well with either artificial transforms, or in comparison with a rigid intensity based algorithm used in clinic.

PO-0987: Ultrasound image guided radiotherapy for prostate cancer using a transperineal probe

Pre-treatment imaging based on ultrasound (US) images was first developed using transabdominal probes but several issues linked to imagequality and probe pressure1 were reported. The aim of this work was to evaluate a non-invasive transperineal (TP) US probe comparing its registration results with cone beam CT (CBCT) on patients treated for a prostate cancer, with prostate in situ or after prostatectomy.

MO‐DE‐210‐04: Repositioning and Monitoring of Prostate Cancer Radiotherapy with a New 4D Ultrasound Intra‐Modality IGRT Device

Purpose: We report our clinical experience using a non-invasive transperineal (TP) ultrasound (US) probe dedicated to pre-positioning and monitoring of prostate cancer patients. The accuracy of pre-treatment positioning was compared to CBCT for prostate and post-prostatectomy patients. Intrafraction motions were recorded for both localizations. The dosimetric impact of these displacements was finally investigated on prostate patients. Methods: Differences between CBCT/CT and TP-US/TP-US registrations were analyzed on 427 and 453 sessions for 13 prostate and 14 post-prostatectomy patients, respectively. Ten prostate patients’ dosimetries were retrospectively planned using 2 different protocols: 80Gy in 40 fractions and 36.25Gy in 5 fractions with a 5mm CTV- to- PTV margin. The delivery time was measured in order to analyze ranges of intrafraction motions related to each protocol. Mean prostate displacements were calculated for each patient and applied to the treatment isocenter coordinates to evaluate the dosimetric impact of these motions. Results: CBCT and TP-US shifts agreements at ±5mm were 76.6%, 95.1%, 96.3% and 90.3%, 85.0%, 97.6% in anterior- posterior, superior- inferior and left-right directions, for prostate and post-prostatectomy patients, respectively. Intrafraction motions were analyzed considering delivery times of 140 and 290s with an additional time of 120s for patient installation for doses of 2 and 7.25Gy, respectively. Intrafraction motions were patient-dependent and were larger as the irradiation time increased. Larger displacements were observed for prostate compared to post-prostatectomy localizations. Shifts above 3mm were observed on 17.6% and 4.5% of the 2Gy sessions against 30.6% and 7.3% of the 7.25Gy sessions in the anterior-posterior direction for prostate and post-prostatectomy localizations, respectively. Preliminary dosimetric results showed that intrafraction motions mainly impact the PTV coverage. Conclusion: 4D TP-US modality is a promising alternative to irradiating and/or invasive IGRT modalities for both inter and intrafraction motions management. Preliminary dosimetric results show that intrafraction monitoring is mandatory especially for hypofractionated treatments. M Fargier-Voiron was supported by a PhD grant from Elekta

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

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.