Prostate Shape Research Articles

Whole prostate volume and shape changes with the use of an inflatable and flexible endorectal coil.

Purpose. To determine to what extent an inflatable endorectal coil (ERC) affects whole prostate (WP) volume and shape during prostate MRI. Materials and Methods. 79 consecutive patients underwent T2W MRI at 3T first with a 6-channel surface coil and then with the combination of a 16-channel surface coil and ERC in the same imaging session. WP volume was assessed by manually contouring the prostate in each T2W axial slice. PSA density was also calculated. The maximum anterior-posterior (AP), left-right (LR), and craniocaudal (CC) prostate dimensions were measured. Changes in WP prostate volume, PSA density, and prostate dimensions were then evaluated. Results. In 79 patients, use of an ERC yielded no significant change in whole prostate volume (0.6 ± 5.7%, P = 0.270) and PSA density (−0.2 ± 5.6%, P = 0.768). However, use of an ERC significantly decreased the AP dimension of the prostate by −8.6 ± 7.8% (P < 0.001), increased LR dimension by 4.5 ± 5.8% (P < 0.001), and increased the CC dimension by 8.8 ± 6.9% (P < 0.001). Conclusion. Use of an ERC in prostate MRI results in the shape deformation of the prostate gland with no significant change in the volume of the prostate measured on T2W MRI. Therefore, WP volumes calculated on ERC MRI can be reliably used in clinical workflow.

The accuracy of prostate volume measurement from ultrasound images: A quasi-Monte Carlo simulation study using magnetic resonance imaging

Prostate volume is an important parameter to guide management of patients with benign prostatic hyperplasia (BPH) and to deliver clinical trial endpoints. Generally, simple 2D ultrasound (US) approaches are favoured despite the potential for greater accuracy afforded by magnetic resonance imaging (MRI) or complex US procedures. In this study, different approaches to estimate prostate size are evaluated with a simulation to select multiple organ cross-sections and diameters from 22 MRI-defined prostate shapes. A quasi-Monte Carlo (qMC) approach is used to simulate multiple probe positions and angles within prescribed limits resulting in a range of dimensions. The basic ellipsoid calculation which uses two scanning planes compares well to the MRI volume across the range of prostate shapes and sizes (R=0.992). However, using an appropriate linear regression model, accurate volume estimates can be made using prostate diameters calculated from a single scanning plane.

The use of an active appearance model for automated prostate segmentation in magnetic resonance

Background. The prostate gland is delineated as the clinical target volume (CTV) in treatment planning of prostate cancer. Therefore, an accurate delineation is a prerequisite for efficient treatment. Accurate automated prostate segmentation methods facilitate the delineation of the CTV without inter-observer variation. The purpose of this study is to present an automated three-dimensional (3D) segmentation of the prostate using an active appearance model. Material and methods. Axial T2-weighted magnetic resonance (MR) scans were used to build the active appearance model. The model was based on a principal component analysis of shape and texture features with a level-set representation of the prostate shape instead of the selection of landmarks in the traditional active appearance model. To achieve a better fit of the model to the target image, prior knowledge to predict how to correct the model and pose parameters was incorporated. The segmentation was performed as an iterative algorithm to minimize the squared difference between the target and the model image. Results. The model was trained using manual delineations from 30 patients and was validated using leave-one-out cross validation where the automated segmentations were compared with the manual reference delineations. The mean and median dice similarity coefficient was 0.84 and 0.86, respectively. Conclusion. This study demonstrated the feasibility for an automated prostate segmentation using an active appearance with results comparable to other studies.

TRUS–MRI image registration: a paradigm shift in the diagnosis of significant prostate cancer

Accuracy of multiparametric MRI has greatly improved the ability of localizing tumor foci of prostate cancer. This property can be used to perform a TRUS-MR image registration, new technological advance, which allows for an overlay of an MRI onto a TRUS image to target a prostate biopsy toward a suspicious area Three types of registration have been developed: cognitive-based, sensor-based, and organ-based registration. Cognitive registration consists of aiming a suspicious area during biopsy with the knowledge of the lesion location identified on multiparametric MRI. Sensor-based registration consists of tracking in real time the TRUS probe with a magnetic device, achieving a global positioning system which overlays in real-time prostate image on both modalities. Its main limitation is that it does not take into account prostate and patient motion during biopsy. Two systems (Artemis and Uronav) have been developed to partially circumvent this drawback. Organ-based registration (Koelis) does not aim to track the TRUS probe, but the prostate itself to compute in a 3D acquisition the TRUS prostate shape, allowing for a registration with the corresponding 3D MRI shape. This system is not limited by prostate/patient motion and allows for a deformation of the organ during registration. Pros and cons of each technique and the rationale for a targeted biopsy only policy are discussed.

Investigation of 90Y-avidin for prostate cancer brachytherapy: a dosimetric model for a phase I–II clinical study

A novel method for prostate irradiation is investigated. Similarly to (125)I or (103)Pd seed brachytherapy, (90)Y-avidin could be injected via the perineum under ultrasound image guidance. This study inspects the theoretical feasibility with a dosimetric model based on Monte Carlo simulation. A geometrical model of the prostate, urethra and rectum was designed. The linear-quadratic model was applied to convert (125)I absorbed dose prescription/constraints into (90)Y dose through biological effective dose (BED) calculation. The optimal (90)Y-avidin injection strategy for the present model was obtained. Dose distribution was calculated by Monte Carlo simulation (PENELOPE,GEANT4). Dose volume histograms (DVH) for the prostate, urethra and rectum were compared to typical DVHs of (125)I seed brachytherapy, used routinely in our institute. With (90)Y-avidin, at least 95% of the prostate must receive more than 70 Gy. The absorbed dose to 10% of the urethra (D(10%_urethra)) and the maximum absorbed dose to the rectum (D(max_rectum)) must be lower than 122 Gy. For the present model, the optimum strategy consists in multiple injections of (90)Y-avidin 50 μl drops, for a total volume of 3.1 ml. The minimum activity to deliver the prescribed absorbed dose is 0.7 GBq, which also fully respects urethral and rectal constraints. The resulting dose map has a maximum in the central region with a sharp decrease towards the urethra and the prostate edge. Notably, D(10%_urethra) is 95 Gy and D(max_rectum) is below 2 Gy. Prostate absorbed dose is higher with (90)Y-avidin than (125)I seeds, although the total volume receiving the prescribed absorbed dose is 1-2% lower. Urethral DVH strictly depends on the (90)Y distribution, to be optimized according to prostate shape; in our model, BED(30%_urethra) is 90 Gy with (90)Y-avidin, whereas for patients receiving (125)I seeds it ranges between 150 and 230 Gy. The rectal DVH is always more favourable with (90)Y. The methodology is theoretically feasible and can deliver an effective treatment in T1-T2 prostate cancer. Pharmacokinetic and biodistribution studies in prostate cancer patients are needed for validation.

Automatic prostate MR image segmentation with sparse label propagation and domain-specific manifold regularization.

Automatic prostate segmentation in MR images plays an important role in prostate cancer diagnosis. However, there are two main challenges: (1) Large inter-subject prostate shape variations; (2) Inhomogeneous prostate appearance. To address these challenges, we propose a new hierarchical prostate MR segmentation method, with the main contributions lying in the following aspects: First, the most salient features are learnt from atlases based on a subclass discriminant analysis (SDA) method, which aims to find a discriminant feature subspace by simultaneously maximizing the inter-class distance and minimizing the intra-class variations. The projected features, instead of only voxel-wise intensity, will be served as anatomical signature of each voxel. Second, based on the projected features, a new multi-atlases sparse label fusion framework is proposed to estimate the prostate likelihood of each voxel in the target image from the coarse level. Third, a domain-specific semi-supervised manifold regularization method is proposed to incorporate the most reliable patient-specific information identified by the prostate likelihood map to refine the segmentation result from the fine level. Our method is evaluated on a T2 weighted prostate MR image dataset consisting of 66 patients and compared with two state-of-the-art segmentation methods. Experimental results show that our method consistently achieves the highest segmentation accuracies than other methods under comparison.

Prostatic Inflammation is Determinant for Prostate Overgrowth and Luts Severity in Men with Metabolic Syndrome: Highlights from Two Recently Published Multicentre Studies

Introduction: Several evidences have pointed out the possible association between Metabolic Syndrome (MetS) and low urinary tract symptoms (LUTS)/benign prostate hyperplasia (BPH). Recent epidemiological and histopatological evidences suggested chronic inflammation is a crucial event in BPH pathogenesis. Aim of this study is to demonstrate the correlation among pre-operatory LUTS/BPH severity, MetS features and inflammatory infiltrates in prostatectomy specimens of patients with BPH, highlighting the results of two recently published multicentre studies analyzing all the data from a preclinical and clinical point of view. Materials and methods: We conducted two retrospective study in 271 and 244 consecutive men treated with simple prostatectomy for LUTS/BPH in two tertiary referral centres. Prostate diameters and volume were measured by transrectal ultrasound, LUTS were scored by IPSS, and obstruction diagnosed by uroflowmetry. MetS was defined according to DF &amp; AHA/NHLBI criteria. The inflammatory infiltrate was investigated according to the scoring system of chronic prostatitis (CP-CPPS) and scored as inflammation score (IS) ranging 3 to 9 and glandular disruption (GD). In addition, we investigated the in vitro inflammatory effects of metabolic insults on human prostatic myofibroblast cells isolated from BPH patients (hBPH). Results: Of 271 men, 86 (31.7%) were affected by MetS. Prostatic volume and the anterior-posterior (AP) diameter were positively associated to the number of MetS components. Among MetS determinants, only dyslipidaemia (increased serum triglycerides and reduced serum HDL levels) was significantly associated with an increased risk of having a prostatic volume &gt;60cm3. IS in prostatectomy specimens showed a step- wise association with number of MetS factors (p=0.001). Dyslipidaemia was the only factor significantly associated with IS. Positive significant correlations among MetS, IS, GD and IPSS Scores were observed. In myofibroblastic hBPH oxidized low-density lipoprotein (oxLDL) showed the highest secretion of IL-8 (&gt;10-folds)-a surrogate marker of prostate inflammation-as well as IL-6, and bFGF. Conclusions: MetS and dyslipidaemia are associated with prostate overgrowth and inflammation. In particular, with a selective increase of prostatic AP diameter, leading to a modification of prostatic shape. Hence, MetS can be regarded as a new determinant of prostate inflammation and BPH progression in men with severe LUTS.

A Bayesian nonrigid registration method to enhance intraoperative target definition in image-guided prostate procedures through uncertainty characterization.

This study introduces a probabilistic nonrigid registration method for use in image-guided prostate brachytherapy. Intraoperative imaging for prostate procedures, usually transrectal ultrasound (TRUS), is typically inferior to diagnostic-quality imaging of the pelvis such as endorectal magnetic resonance imaging (MRI). MR images contain superior detail of the prostate boundaries and provide substructure features not otherwise visible. Previous efforts to register diagnostic prostate images with the intraoperative coordinate system have been deterministic and did not offer a measure of the registration uncertainty. The authors developed a Bayesian registration method to estimate the posterior distribution on deformations and provide a case-specific measure of the associated registration uncertainty. The authors adapted a biomechanical-based probabilistic nonrigid method to register diagnostic to intraoperative images by aligning a physician's segmentations of the prostate in the two images. The posterior distribution was characterized with a Markov Chain Monte Carlo method; the maximum a posteriori deformation and the associated uncertainty were estimated from the collection of deformation samples drawn from the posterior distribution. The authors validated the registration method using a dataset created from ten patients with MRI-guided prostate biopsies who had both diagnostic and intraprocedural 3 Tesla MRI scans. The accuracy and precision of the estimated posterior distribution on deformations were evaluated from two predictive distance distributions: between the deformed central zone-peripheral zone (CZ-PZ) interface and the physician-labeled interface, and based on physician-defined landmarks. Geometric margins on the registration of the prostate's peripheral zone were determined from the posterior predictive distance to the CZ-PZ interface separately for the base, mid-gland, and apical regions of the prostate. The authors observed variation in the shape and volume of the segmented prostate in diagnostic and intraprocedural images. The probabilistic method allowed us to convey registration results in terms of posterior distributions, with the dispersion providing a patient-specific estimate of the registration uncertainty. The median of the predictive distance distribution between the deformed prostate boundary and the segmented boundary was ≤3 mm (95th percentiles within ±4 mm) for all ten patients. The accuracy and precision of the internal deformation was evaluated by comparing the posterior predictive distance distribution for the CZ-PZ interface for each patient, with the median distance ranging from -0.6 to 2.4 mm. Posterior predictive distances between naturally occurring landmarks showed registration errors of ≤5 mm in any direction. The uncertainty was not a global measure, but instead was local and varied throughout the registration region. Registration uncertainties were largest in the apical region of the prostate. Using a Bayesian nonrigid registration method, the authors determined the posterior distribution on deformations between diagnostic and intraprocedural MR images and quantified the uncertainty in the registration results. The feasibility of this approach was tested and results were positive. The probabilistic framework allows us to evaluate both patient-specific and location-specific estimates of the uncertainty in the registration result. Although the framework was tested on MR-guided procedures, the preliminary results suggest that it may be applied to TRUS-guided procedures as well, where the addition of diagnostic MR information may have a larger impact on target definition and clinical guidance.

Generating prostate models by means of geometric deformation with application to computerized training of cryosurgery.

As a part of an ongoing project to develop computerized training tools for cryosurgery, the objective of the current study is twofold: to compile literature data on the likelihood of cancer tumor growth and its effect on the prostate shape and to present a deformation scheme for a 3D organ template in order to generate clinically relevant prostate models. The long-term objective of this study is to develop a database of prostate models for computerized training. Cryosurgery is typically performed on patients with localized prostate cancer found in stage T3 or earlier. The distribution of key geometric features likely to be found in the prostate at stage T3 is integrated into a 3D prostate template by employing the extended free-form deformation (EFFD) method. The applied scheme combines two steps: pre-selecting a set of geometric parameter values and manipulating the lattice control points until the prostate model meets the desired criteria. Examples for model generation are displayed, based on two 3D prostate templates previously obtained from ultrasound imaging. These examples include selected cases with unilateral and bilateral stage T3 tumor growth, suitable for incorporation into a training database. EFFD is an efficient method for rapid generation of prostate models. The compiled criteria for model generation do not lead to a unique shape since the contours for template deformation are randomly selected. Nevertheless, these criteria do lead to shapes resembling cancer growth, as various growth histories can lead to a tumor characterized by the same key parameter values.

SU-D-213AB-01: Dosimetry Improvement and Needle Number Reduction in Prostate Brachytherapy Using Electromagnetically Guided Needle Placement.

To investigate dosimetry improvement in prostate brachytherapy by conforming needles to urethral and prostate shape using an electromagnetic tracking device. We have reported a needle tracking system using an electromagnetic sensor embedded inside the tip of the needle to improve needle reconstruction accuracy and efficiency in conventional prostate HDR brachytherapy. Utilizing the same system, we propose to guide needle insertion following pre-optimized tracks based onthe target shape. In this study, we investigate possible dosimetry improvement and needle number reduction by comparing plans using the conventional implant method and the proposed method. Twelve prostate brachytherapy patients were selected and studied retrospectively. New virtual plans were created using the proposed method and conforming needle tracks to the urethral and prostate shapes. Same optimization constraints were applied to both the conventional and the new plans. DVH parameters and total needles used have been analyzed to quantify dosimetry improvement and potential toxicity sparing due to reduction in implant needles. Prostate volumes are 41.16±13.27 cc. Number of needles used for the conventional plan is 16.6±1.2, vs. 13 for all the new plans. The prostate volume receiving 100% (V100), 125% (V125), and 150% (V150) of the prescription dose in conventional plans vs. those in the new plans are 99.48%±0.21% vs. 99.53%±0.20%, 53.90%±5.61% vs. 50.30%±5.23%, and 25.37%±4.91% vs. 20.96%±3.41%, respectively. The corresponding urethra V100 and V110 are 90.96%±3.10% vs. 85.78%±7.76% and 2.06%±1.23 vs. 0.46%±0.28%. The needle numbers and the urethral V110 in the new plans are significantly lower than those in conventional plans(p<0.001 and p=0.004, respectively), with no significant changes in doses tothe prostate. Conformai needle implant following pre-optimized tracks with electromagnetic guidance may significantly reduce acute and late toxicities in prostate brachytherapy by reducing the number of needles and theurethral doses.

MO‐G‐BRA‐03: Semi‐Automatic Segmentation of the Prostate Midgland in Magnetic Resonance Images Using Shape and Local Appearance Similarity Analysis

Purpose: To design, develop and test a semi‐automatic segmentation method for the prostate midgland on T2W magnetic resonance (MR) images acquired using an endorectal (ER) coil, based on inter‐subject prostate shape and local boundary appearance similarity.Method and Materials: We used T2W prostate MR images of 33 subjects acquired using an ER coil. From each image, we extracted the closest mid‐gland axial slice to the verumontanum. We partitioned the images into training and test sets using leave‐one‐out cross‐validation. We used the training images to define a point distribution model (PDM) describing shape variability as well as 36 circular ‘mean intensity patches' characterizing the inter‐subject local appearance at anatomically‐defined boundary locations. We chose a patch radius of 17mm based on our assessment of segmentation error, using the mean absolute boundary distance (MAD), on a subset of 13 images during a systematic radius search from 5mm to 25mm. For each test image, we defined 36 homologous rays emanating from the centre of the prostate. We used a radial‐based‐search strategy to translate each mean intensity patch along its corresponding ray and computed the normalized cross‐correlation (NCC) between the mean patch and the underlying patch in the test image at each point. We chose the point with the highest NCC along each ray. We then used the PDM to regularize the selected points. To compare the algorithm's segmentation to manual delineations performed by one operator, we calculated the MAD and Dice similarity coefficient (DSC). Results: We measured a MAD of 1.6 +/− 1.0mm, and a DSC of 89 +/− 6% between the semi‐automatically segmented contours and the manually‐delineated reference standard. Conclusions: NCC of local prostate boundary regions with a learned mean boundary appearance may be suitable for boundary localization and subsequent refinement using a PDM on T2W MR images acquired using an ER coil.

SU‐E‐T‐331: Geometric Variations between Pre Planned and or Planned Implant Procedures for Patients Undergoing I‐125 Prostate Brachytherapy Seed Implantation

Purpose: The limitations of pre‐planned prostate brachytherapy seed implantation can be overcome by intraoperative re‐planning in the OR. Main purpose of this report is to compare geometric parameters between pre‐planed and OR‐panned procedures regarding changes in prostate volume/shape, number of seeds/needles and total activity, to address prostate volume/shape changes observed in the OR when the patient is under anesthesia and to present post‐implant dosimetric results.Methods and materials: In 33 clinical 125I cases, ultra‐sound (US) volume studies and plans were performed for both preplanning and in the OR just before seed implantation. Changes in volume/shape and number of seeds/needles used were compared between the two data‐sets and patient received brachytherapy treatment using the OR plan. For each patient, D90 and V100 were obtained from day one post‐implant CT dosimetry. Results: The mean US volume change observed between pre‐plans and OR‐plans was (5.3 ± 4.4)cc, with a maximum of 20cc. Change in the number of seeds between the plans were (8.9 ± 5.7) on an average and showed excellent correlation with prostate volume change. The number of needles remains relatively unchanged. The average total‐activity changed was (2.7 ± 1.6)mCi, with a maximum of 7mCi. Prostate shape changes (in terms of no. of seeds changed) in the OR was seen mainly in the anterior aspect of prostate. The average D90 and V100 were (90.3 ± 4.6)Gy and (148.7 ± 16.6)% of the prescription dose, respectively. Summary and Discussion: Significant variations of prostate volume changes between pre‐planned and the OR‐planned implants were observed in many cases. Largest change in the no. of seeds was due to change in shape of the prostate in its anterior aspect clearly shows the effect of pelvic muscle relaxation and patient re‐positioning in the OR. This supports the argument in favor of performing intraoperative re‐planning and implant‐dosimetry.

A spline-based non-linear diffeomorphism for multimodal prostate registration

This paper presents a novel method for non-rigid registration of transrectal ultrasound and magnetic resonance prostate images based on a non-linear regularized framework of point correspondences obtained from a statistical measure of shape-contexts. The segmented prostate shapes are represented by shape-contexts and the Bhattacharyya distance between the shape representations is used to find the point correspondences between the 2D fixed and moving images. The registration method involves parametric estimation of the non-linear diffeomorphism between the multimodal images and has its basis in solving a set of non-linear equations of thin-plate splines. The solution is obtained as the least-squares solution of an over-determined system of non-linear equations constructed by integrating a set of non-linear functions over the fixed and moving images. However, this may not result in clinically acceptable transformations of the anatomical targets. Therefore, the regularized bending energy of the thin-plate splines along with the localization error of established correspondences should be included in the system of equations. The registration accuracies of the proposed method are evaluated in 20 pairs of prostate mid-gland ultrasound and magnetic resonance images. The results obtained in terms of Dice similarity coefficient show an average of 0.980±0.004, average 95% Hausdorff distance of 1.63±0.48 mm and mean target registration and target localization errors of 1.60±1.17 mm and 0.15±0.12 mm respectively.

Dosimetric and deformation effects of image‐guided interventions during stereotactic body radiation therapy of the prostate using an endorectal balloon

During stereotactic body radiation therapy (SBRT) for the treatment of prostate cancer, an inflatable endorectal balloon (ERB) may be used to reduce motion of the target and reduce the dose to the posterior rectal wall. This work assessed the dosimetric impact of manual interventions on ERB position in patients receiving prostate SBRT and investigated the impact of ERB interventions on prostate shape. The data of seven consecutive patients receiving SBRT for the treatment of clinical stage T1cN0M0 prostate cancer enrolled in a multi-institutional, IRB-approved trial were analyzed. The SBRT dose was 50 Gy in five fractions to a planning target volume (PTV) that included the prostate (implanted with three fiducial markers) with a 3-5 mm margin. All plans were based on simulation images that included an ERB inflated with 60 cm(3) of air. Daily kilovoltage cone-beam computed tomography (CBCT) imaging was performed to localize the PTV, and an automated fusion with the planning images yielded displacements required for PTV relocalization. When the ERB volume and/or position were judged to yield inaccurate repositioning, manual adjustment (ERB reinflation and/or repositioning) was performed. Based on all 59 CBCT image sets acquired, a deformable registration algorithm was used to determine the dose received by, displacement of, and deformation of the prostate, bladder (BLA), and anterior rectal wall (ARW). This dose tracking methodology was applied to images taken before and after manual adjustment of the ERB (intervention), and the delivered dose was compared to that which would have been delivered in the absence of intervention. Interventions occurred in 24 out of 35 (69%) of the treated fractions. The direct effect of these interventions was an increase in the prostate radiation dose that included 95% of the PTV (D95) from 9.6 ± 1.0 to 10.0 ± 0.2 Gy (p = 0.06) and an increase in prostate coverage from 94.0% ± 8.5% to 97.8% ± 1.9% (p = 0.03). Additionally, ERB interventions reduced prostate deformation in the anterior-posterior (AP) direction, reduced errors in the sagittal rotation of the prostate, and increased the similarity in shape of the prostate to the radiotherapy plan (increased Dice coefficient from 0.76 ± 0.06 to 0.80 ± 0.04, p = 0.01). Postintervention decreases in prostate volume receiving less than the prescribed dose and decreases in the voxel-wise displacement of the prostate, bladder, and anterior rectal wall were observed, which resulted in improved dose-volume histogram (DVH) characteristics. Image-guided interventions in ERB volume and/or position during prostate SBRT were necessary to ensure the delivery of the dose distribution as planned. ERB interventions resulted in reductions in prostate deformations that would have prevented accurate localization of patient anatomy.

The Dosimetric Impact of Prostate Rotations During Electromagnetically Guided External-Beam Radiation Therapy

To study the impact of daily rotations and translations of the prostate on dosimetric coverage during radiation therapy (RT). Real-time tracking data for 26 patients were obtained during RT. Intensity modulated radiation therapy plans meeting RTOG 0126 dosimetric criteria were created with 0-, 2-, 3-, and 5-mm planning target volume (PTV) margins. Daily translations and rotations were used to reconstruct prostate delivered dose from the planned dose. D95 and V79 were computed from the delivered dose to evaluate target coverage and the adequacy of PTV margins. Prostate equivalent rotation is a new metric introduced in this study to quantify prostate rotations by accounting for prostate shape and length of rotational lever arm. Large variations in prostate delivered dose were seen among patients. Adequate target coverage was met in 39%, 65%, and 84% of the patients for plans with 2-, 3-, and 5-mm PTV margins, respectively. Although no correlations between prostate delivered dose and daily rotations were seen, the data showed a clear correlation with prostate equivalent rotation. Prostate rotations during RT could cause significant underdosing even if daily translations were managed. These rotations should be managed with rotational tolerances based on prostate equivalent rotations.

Concurrent segmentation of the prostate on MRI and CT via linked statistical shape models for radiotherapy planning

Prostate gland segmentation is a critical step in prostate radiotherapy planning, where dose plans are typically formulated on CT. Pretreatment MRI is now beginning to be acquired at several medical centers. Delineation of the prostate on MRI is acknowledged as being significantly simpler to perform, compared to delineation on CT. In this work, the authors present a novel framework for building a linked statistical shape model (LSSM), a statistical shape model (SSM) that links the shape variation of a structure of interest (SOI) across multiple imaging modalities. This framework is particularly relevant in scenarios where accurate boundary delineations of the SOI on one of the modalities may not be readily available, or difficult to obtain, for training a SSM. In this work the authors apply the LSSM in the context of multimodal prostate segmentation for radiotherapy planning, where the prostate is concurrently segmented on MRI and CT. The framework comprises a number of logically connected steps. The first step utilizes multimodal registration of MRI and CT to map 2D boundary delineations of the prostate from MRI onto corresponding CT images, for a set of training studies. Hence, the scheme obviates the need for expert delineations of the gland on CT for explicitly constructing a SSM for prostate segmentation on CT. The delineations of the prostate gland on MRI and CT allows for 3D reconstruction of the prostate shape which facilitates the building of the LSSM. In order to perform concurrent prostate MRI and CT segmentation using the LSSM, the authors employ a region-based level set approach where the authors deform the evolving prostate boundary to simultaneously fit to MRI and CT images in which voxels are classified to be either part of the prostate or outside the prostate. The classification is facilitated by using a combination of MRI-CT probabilistic spatial atlases and a random forest classifier, driven by gradient and Haar features. The authors acquire a total of 20 MRI-CT patient studies and use the leave-one-out strategy to train and evaluate four different LSSMs. First, a fusion-based LSSM (fLSSM) is built using expert ground truth delineations of the prostate on MRI alone, where the ground truth for the gland on CT is obtained via coregistration of the corresponding MRI and CT slices. The authors compare the fLSSM against another LSSM (xLSSM), where expert delineations of the gland on both MRI and CT are employed in the model building; xLSSM representing the idealized LSSM. The authors also compare the fLSSM against an exclusive CT-based SSM (ctSSM), built from expert delineations of the gland on CT alone. In addition, two LSSMs trained using trainee delineations (tLSSM) on CT are compared with the fLSSM. The results indicate that the xLSSM, tLSSMs, and the fLSSM perform equivalently, all of them out-performing the ctSSM. The fLSSM provides an accurate alternative to SSMs that require careful expert delineations of the SOI that may be difficult or laborious to obtain. Additionally, the fLSSM has the added benefit of providing concurrent segmentations of the SOI on multiple imaging modalities.

TU‐G‐BRC‐04: The Dosimetric Impact of Prostate Rotations during Electromagnetically Guided External Beam Radiation Therapy

Purpose: To measure the dose decrement to the prostate caused by daily prostate motion and to correlate target coverage with prostate rotations to establish individualized rotational tolerances and PTV margins. Methods: Real—time tracking data for 26 patients were obtained during RT and used to calculate “&amp;#61455;average” prostate rotations and translations. For each patient, IMRT treatment plans meeting RTOG0126 dosimetric criteria were created with 0, 2, 3, and 5 mm PTV margins. Daily translations and rotations were used to calculate prostate delivered (dynamic) dose from the planned (static) dose. Dose volume histograms, D95, and V79 are computed from the dynamic dose to evaluate the coverage and the adequacy of PTV margins under measured target motion. The intra‐fractional effective rotations of the prostate (rotations weighted by rotation lever arm and prostate geometrical shape) were obtained for each patient. Results: Large variations in dosimetric coverage were seen among patients. Adequate coverage to target was met in 39%, 65%, and 84% of the patients for plans with 2, 3, and 5 mm PTV margins, respectively. This dosimetric coverage could not be correlated with measured rotations for any of the PTV margins. Prostate shape and the length of rotation lever‐arm were parameterized and used to obtain effective rotations from the measured rotations. Clear correlations were seen between effective rotations and the prostate delivered dose. Conclusions: Prostate rotations, if left unmanaged, could cause significant underdosing to the target even if daily translational tolerances applied and a 5 mm PTV margin is used. Unlike translations, effective management of prostate rotations requires individualized rotational tolerances due to variations in prostate shape and rotation lever‐arm length among patients. Consequently, individualized rotational tolerances enable PTV margins to be individualized.

Learning Statistical Correlation of Prostate Deformations for Fast Registration.

This paper presents a novel fast registration method for aligning the planning image onto each treatment image of a patient for adaptive radiation therapy of the prostate cancer. Specifically, an online correspondence interpolation method is presented to learn the statistical correlation of the deformations between prostate boundary and non-boundary regions from a population of training patients, as well as from the online-collected treatment images of the same patient. With this learned statistical correlation, the estimated boundary deformations can be used to rapidly predict regional deformations between prostates in the planning and treatment images. In particular, the population-based correlation can be initially used to interpolate the dense correspondences when the number of available treatment images from the current patient is small. With the acquisition of more treatment images from the current patient, the patient-specific information gradually plays a more important role to reflect the prostate shape changes of the current patient during the treatment. Eventually, only the patient-specific correlation is used to guide the regional correspondence prediction, once a sufficient number of treatment images have been acquired and segmented from the current patient. Experimental results show that the proposed method can achieve much faster registration speed yet with comparable registration accuracy compared with the thin plate spline (TPS) based interpolation approach.

3D conformal MRI-controlled transurethral ultrasound prostate therapy: validation of numerical simulations and demonstration in tissue-mimicking gel phantoms

MRI-controlled transurethral ultrasound therapy uses a linear array of transducer elements and active temperature feedback to create volumes of thermal coagulation shaped to predefined prostate geometries in 3D. The specific aims of this work were to demonstrate the accuracy and repeatability of producing large volumes of thermal coagulation (>10 cc) that conform to 3D human prostate shapes in a tissue-mimicking gel phantom, and to evaluate quantitatively the accuracy with which numerical simulations predict these 3D heating volumes under carefully controlled conditions. Eleven conformal 3D experiments were performed in a tissue-mimicking phantom within a 1.5T MR imager to obtain non-invasive temperature measurements during heating. Temperature feedback was used to control the rotation rate and ultrasound power of transurethral devices with up to five 3.5 × 5 mm active transducer elements. Heating patterns shaped to human prostate geometries were generated using devices operating at 4.7 or 8.0 MHz with surface acoustic intensities of up to 10 W cm−2. Simulations were informed by transducer surface velocity measurements acquired with a scanning laser vibrometer enabling improved calculations of the acoustic pressure distribution in a gel phantom. Temperature dynamics were determined according to a FDTD solution to Pennes’ BHTE. The 3D heating patterns produced in vitro were shaped very accurately to the prostate target volumes, within the spatial resolution of the MRI thermometry images. The volume of the treatment difference falling outside ±1 mm of the target boundary was, on average, 0.21 cc or 1.5% of the prostate volume. The numerical simulations predicted the extent and shape of the coagulation boundary produced in gel to within (mean ± stdev [min, max]): 0.5 ± 0.4 [−1.0, 2.1] and −0.05 ± 0.4 [−1.2, 1.4] mm for the treatments at 4.7 and 8.0 MHz, respectively. The temperatures across all MRI thermometry images were predicted within −0.3 ± 1.6 °C and 0.1 ± 0.6 °C, inside and outside the prostate respectively, and the treatment time to within 6.8 min. The simulations also showed excellent agreement in regions of sharp temperature gradients near the transurethral and endorectal cooling devices. Conformal 3D volumes of thermal coagulation can be precisely matched to prostate shapes with transurethral ultrasound devices and active MRI temperature feedback. The accuracy of numerical simulations for MRI-controlled transurethral ultrasound prostate therapy was validated experimentally, reinforcing their utility as an effective treatment planning tool.

MRS-guided HDR brachytherapy boost to the dominant intraprostatic lesion in high risk localised prostate cancer

BackgroundIt is known that the vast majority of prostate cancers are multifocal. However radical radiotherapy historically treats the whole gland rather than individual cancer foci.Magnetic resonance spectroscopy (MRS) can be used to non-invasively locate individual cancerous tumours in prostate. Thus an intentionally non-uniform dose distribution treating the dominant intraprostatic lesion to different dose levels than the remaining prostate can be delivered ensuring the maximum achievable tumour control probability.The aim of this study is to evaluate, using radiobiological means, the feasibility of a MRS-guided high dose rate (HDR) brachytherapy boost to the dominant lesion.MethodsComputed tomography and MR/MRS were performed for treatment planning of a high risk localised prostate cancer. Both were done without endorectal coil, which distorts shape of prostate during the exams.Three treatment plans were compared:- external beam radiation therapy (EBRT) only- combination of EBRT and HDR brachytherapy- combination of EBRT and HDR brachytherapy with a synchronous integrated boost to the dominant lesionThe criteria of plan comparison were: the minimum, maximum and average doses to the targets and organs at risk; dose volume histograms; biologically effective doses for organs at risk and tumour control probability for the target volumes consisting of the dominant lesion as detected by MR/MRS and the remaining prostate volume.ResultsInclusion of MRS information on the location of dominant lesion allows a safe increase of the dose to the dominant lesion while dose to the remaining target can be even substantially decreased keeping the same, high tumour control probability. At the same time an improved urethra sparing was achieved comparing to the treatment plan using a combination of EBRT and uniform HDR brachytherapy.ConclusionsMRS-guided HDR brachytherapy boost to dominant lesion has the potential to spare the normal tissue, especially urethra, while keeping the tumour control probability high.