Treatment Planning Strategies for Interstitial Ultrasound Ablation of Prostate Cancer

PURPOSE

To develop patient-specific 3D models using Finite-Difference Time-Domain (FDTD) simulations and pre-treatment planning tools for the selective thermal ablation of prostate cancer with interstitial ultrasound. This involves the integration with a FDA 510(k) cleared catheter-based ultrasound interstitial applicators and delivery system.

METHODS

A 3D generalized "prostate" model was developed to generate temperature and thermal dose profiles for different applicator operating parameters and anticipated perfusion ranges. A priori planning, based upon these pre-calculated lethal thermal dose and iso-temperature clouds, was devised for iterative device selection and positioning. Full 3D patient-specific anatomic modeling of actual placement of single or multiple applicators to conformally ablate target regions can be applied, with optional integrated pilot-point temperature-based feedback control and urethral/rectum cooling. These numerical models were verified against previously reported ex-vivo experimental results obtained in soft tissues.

RESULTS

For generic prostate tissue, 360 treatment schemes were simulated based on the number of transducers (1-4), applied power (8-20 W/cm2), heating time (5, 7.5, 10 min), and blood perfusion (0, 2.5, 5 kg/m3/s) using forward treatment modelling. Selectable ablation zones ranged from 0.8-3.0 cm and 0.8-5.3 cm in radial and axial directions, respectively. 3D patient-specific thermal treatment modeling for 12 Cases of T2/T3 prostate disease demonstrate applicability of workflow and technique for focal, quadrant and hemi-gland ablation. A temperature threshold (e.g., Tthres = 52 °C) at the treatment margin, emulating placement of invasive temperature sensing, can be applied for pilot-point feedback control to improve conformality of thermal ablation. Also, binary power control (e.g., Treg = 45 °C) can be applied which will regulate the applied power level to maintain the surrounding temperature to a safe limit or maximum threshold until the set heating time.

CONCLUSIONS

Prostate-specific simulations of interstitial ultrasound applicators were used to generate a library of thermal-dose distributions to visually optimize and set applicator positioning and directivity during a priori treatment planning pre-procedure. Anatomic 3D forward treatment planning in patient-specific models, along with optional temperature-based feedback control, demonstrated single and multi-applicator implant strategies to effectively ablate focal disease while affording protection of normal tissues.

Salvage open radical prostatectomy for recurrent prostate cancer following MRI-guided transurethral ultrasound ablation (TULSA) of the prostate: feasibility and efficacy

Introduction: MRI-guided transurethral ultrasound ablation (TULSA) is a novel modality for minimally invasive ablation in patients with localised prostate cancer (PCa). A multi-national Phase 1 (30 patients) and subsequent Phase 2 (115 patients) study showed TULSA to be feasible, safe and well tolerated. However, technical viability and safety of salvage prostatectomy for those who failed TULSA is unclear. Herein, we report the feasibility and morbidity of salvage radical prostatectomy (sRP) post-TULSA.Methods: Four patients with biopsy-proven residual cancer following TULSA underwent open retropubic sRP within 39 months of TULSA. Peri-and post-operative morbidity were reported. Detailed histopathologic assessment is reported.Results: Median follow-up was 43 months after sRP. Mean operating times, blood loss, and length of stay were 210 min, 866 ml, and 3.5 days, respectively. Intraoperative finding of some fibrotic reaction of endopelvic and Denonvilliers fascia was characteristic. There were no perioperative complications. Whole-mount pathology sections showed one pT2b and three pT3a, suggesting under-staging pre-TULSA. Location of disease was compatible with persistent cancer mostly in the untreated peripheral safety region. One man received an artificial urinary sphincter. All men experienced erectile dysfunction responsive to treatment. Two patients with positive surgical margins had PSA progression requiring salvage radiation, with one requiring long-term androgen deprivation therapy.Conclusions: RP is a viable and safe salvage option if TULSA fails. Technical difficulty and perioperative morbidity were negligible and attributable to minimal peri-prostatic reaction from TULSA.

Dual-sectored transurethral ultrasound for thermal treatment of stress urinary incontinence: in silico studies in 3D anatomical models

The purpose of this study is to investigate the feasibility and performance of a stationary, non-focused dual-sectored tubular transurethral ultrasound applicator for thermal exposure of tissue regions adjacent to the urethra for treatment of stress urinary incontinence (SUI) through acoustic and biothermal simulations on 3D anatomical models. Parametric studies in a generalized tissue model over dual-sectored ultrasound applicator configurations (acoustic surface intensities, lateral active acoustic output sector angles, and durations) were performed. Selected configurations and delivery strategies were applied on 3D pelvic anatomical models. Temperature and thermal dose distributions on the target region and surrounding tissues were calculated. Endovaginal cooling was explored as a strategy to mitigate vaginal heating. The 75-90° dual-sectored transurethral tubular transducer (3.5 mm outer diameter (OD), 14 mm length, 6.5 MHz, 8.8-10.2 W/cm²) and 2-3-min sonication duration were selected from the parametric study for acoustic and biothermal simulations on anatomical models. The transurethral applicator with two opposing 75-90° active lateral tubular sectors can create two heated volumes for a total of up to 1.8 cm³ over 60 EM_43 °C, with at least 10 mm radial penetration depth, 1.2 mm urethral sparing, and no lethal damage to the vagina and adjacent bone (< 60 EM_43 °C). Endovaginal cooling can be applied to further reduce the vaginal wall exposure (< 15 EM_43 °C). Simulations on 3D anatomical models indicate that dual-sectored transurethral ultrasound applicators can selectively heat pelvic floor tissue lateral to the mid-urethra in short treatment durations, without damaging adjacent vaginal and bone tissues, as a potential alternative treatment option for stress urinary incontinence. Graphical abstract Schema for in silico investigation of transurethral ultrasound thermal therapy applicator for minimally invasive treatment of SUI.

Deployable cylindrical phased-array applicator mimicking a concentric-ring configuration for minimally-invasive delivery of therapeutic ultrasound

A novel design for a deployable catheter-based ultrasound applicator for endoluminal and laparoscopic intervention is introduced. By combining a 1D cylindrical ring phased array with an expandable paraboloid or conical-shaped balloon-based reflector, the applicator can be controllably collapsed for compact delivery and deployed to mimic a forward-firing larger diameter concentric ring array with tight focusing and electronic steering capabilities in depth. Comprehensive acoustic and biothermal parametric studies were employed to characterize the capabilities of the applicator design as a function of transducer dimensions, phased array configuration, and balloon reflector geometry. Modeling results indicate that practical balloon sizes (43-57 mm expanded diameter), transducer array configurations (e.g. 1.5 MHz, 10 mm OD  ×  20 mm length, 8 or 16 array elements), and sonication durations (30 s) are capable of producing spatially-localized acoustic intensity focal patterns and ablative thermal lesions (width: 2.8-4.8 mm; length: 5.3-40.1 mm) in generalized soft tissue across a 5-100 mm depth range. Larger focal intensity gain magnitudes and narrower focal dimensions are attainable using paraboloid-shaped balloon reflectors with natural geometric focal depths of 25-55 mm, whereas conical-shaped reflectors (angled 45-55°) produce broader foci and extend electronic steering range in depth. A proof-of-concept phased array applicator assembly was fabricated and characterized using hydrophone and radiation force balance measurements and demonstrated good agreement with simulation. The results of this study suggest that combining small diameter cylindrical phased arrays with expandable balloon reflectors can enhance minimally invasive ultrasound-based intervention by augmenting achievable focal gains and penetration depths with dynamic adjustment of treatment depth.

Transurethral high-intensity ultrasound for treatment of stress urinary incontinence (SUI): simulation studies with patient-specific models

BACKGROUND

Stress urinary incontinence (SUI) is prevalent in adult women, attributed to weakened endopelvic supporting tissues, and typically treated using drugs and invasive surgical procedures. The objective of this in silico study is to explore transurethral high-intensity ultrasound for delivery of precise thermal therapy to the endopelvic tissues adjacent to the mid-urethra, to induce thermal remodeling as a potential minimally invasive treatment alternative.

METHODS

3D acoustic (Rayleigh-Sommerfeld) and biothermal (Pennes bioheat) models of the ultrasound applicator and surrounding tissues were devised. Parametric studies over transducer configuration [frequency, radius-of-curvature (ROC)] and treatment settings (power, duration) were performed, and select cases on patient-specific models were used for further evaluation. Transient temperature and thermal dose distributions were calculated, and temperature and dose metrics reported.

RESULTS

Configurations using a 5-MHz curvilinear transducer (3.5 × 10 mm, 28 mm ROC) with single 90 s sonication can create heated zones with 11 mm penetration (>50 °C) while sparing the inner 1.8 mm (<45 °C) radial depth of the urethral mucosa. Sequential and discrete applicator rotations can sweep out bilateral coagulation volumes (1.4 W power, 15° rotations, 600 s total time), produce large volumetric (1124 mm³ above 60 EM43 °C) and wide angular (∼50.5° per lateral sweep) coverage, with up to 15.6 mm thermal penetration and at least 1.6 mm radial urethral protection (<5 EM43 °C).

CONCLUSION

Transurethral applicators with curvilinear ultrasound transducers can deliver spatially selective temperature elevations to lateral mid-urethral targets as a possible means to tighten the endopelvic fascia and adjacent tissues.

The Effect of Tissue Physiological Variability on Transurethral Ultrasound Therapy of the Prostate

Therapeutic ultrasound is an investigational modality which could potentially be used for minimally invasive treatment of prostate cancer. Computational simulations were used to study the effect of natural physiological variations in tissue parameters on the efficacy of therapeutic ultrasound treatment in the prostate. The simulations were conducted on a clinical ultrasound therapy system using patient computed tomography (CT) data. The values of attenuation, perfusion, specific heat capacity and thermal conductivity were changed within their biological ranges to determine their effect on peak temperature andthermal dose volume. Increased attenuation was found to have the biggest effect on peak temperature with a 6.9%rise. The smallest effect was seen with perfusion with ±0.2% variation in peak temperature. Thermal dose was mostly affected by specific heat capacity which showed a 20.7% increase in volume with reduced heat capacity. Thermal conductivity had the smallest effect on thermal dose with up to 2.1% increase in the volume with reduced thermal conductivity. These results can be used to estimate the interpatient variation during the therapeutic ultrasound treatment of the prostate.

Twelve-month prostate volume reduction after MRI-guided transurethral ultrasound ablation of the prostate

PURPOSE

To quantitatively assess 12-month prostate volume (PV) reduction based on T2-weighted MRI and immediate post-treatment contrast-enhanced MRI non-perfused volume (NPV), and to compare measurements with predictions of acute and delayed ablation volumes based on MR-thermometry (MR-t), in a central radiology review of the Phase I clinical trial of MRI-guided transurethral ultrasound ablation (TULSA) in patients with localized prostate cancer.

MATERIALS AND METHODS

Treatment day MRI and 12-month follow-up MRI and biopsy were available for central radiology review in 29 of 30 patients from the published institutional review board-approved, prospective, multi-centre, single-arm Phase I clinical trial of TULSA. Viable PV at 12 months was measured as the remaining PV on T2-weighted MRI, less 12-month NPV, scaled by the fraction of fibrosis in 12-month biopsy cores. Reduction of viable PV was compared to predictions based on the fraction of the prostate covered by the MR-t derived acute thermal ablation volume (ATAV, 55°C isotherm), delayed thermal ablation volume (DTAV, 240 cumulative equivalent minutes at 43°C thermal dose isocontour) and treatment-day NPV. We also report linear and volumetric comparisons between metrics.

RESULTS

After TULSA, the median 12-month reduction in viable PV was 88%. DTAV predicted a reduction of 90%. Treatment day NPV predicted only 53% volume reduction, and underestimated ATAV and DTAV by 36% and 51%.

CONCLUSION

Quantitative volumetry of the TULSA phase I MR and biopsy data identifies DTAV (240 CEM43 thermal dose boundary) as a useful predictor of viable prostate tissue reduction at 12 months. Immediate post-treatment NPV underestimates tissue ablation.

KEY POINTS

• MRI-guided transurethral ultrasound ablation (TULSA) achieved an 88% reduction of viable prostate tissue volume at 12 months, in excellent agreement with expectation from thermal dose calculations. • Non-perfused volume on immediate post-treatment contrast-enhanced MRI represents only 64% of the acute thermal ablation volume (ATAV), and reports only 60% (53% instead of 88% achieved) of the reduction in viable prostate tissue volume at 12 months. • MR-thermometry-based predictions of 12-month prostate volume reduction based on 240 cumulative equivalent minute thermal dose volume are in excellent agreement with reduction in viable prostate tissue volume measured on pre- and 12-month post-treatment T2w-MRI.

Thermal therapy of pancreatic tumours using endoluminal ultrasound: Parametric and patient-specific modelling

PURPOSE

The aim of this study is to investigate endoluminal ultrasound applicator configurations for volumetric thermal ablation and hyperthermia of pancreatic tumours using 3D acoustic and biothermal finite element models.

MATERIALS AND METHODS

Parametric studies compared endoluminal heating performance for varying applicator transducer configurations (planar, curvilinear-focused, or radial-diverging), frequencies (1-5 MHz), and anatomical conditions. Patient-specific pancreatic head and body tumour models were used to evaluate feasibility of generating hyperthermia and thermal ablation using an applicator positioned in the duodenal or stomach lumen. Temperature and thermal dose were calculated to define ablation (> 240 EM(43 °C)) and moderate hyperthermia (40-45 °C) boundaries, and to assess sparing of sensitive tissues. Proportional-integral control was incorporated to regulate maximum temperature to 70-80 °C for ablation and 45 °C for hyperthermia in target regions.

RESULTS

Parametric studies indicated that 1-3 MHz planar transducers are the most suitable for volumetric ablation, producing 5-8 cm(3) lesion volumes for a stationary 5-min sonication. Curvilinear-focused geometries produce more localised ablation to 20-45 mm depth from the GI tract and enhance thermal sparing (T(max) < 42 °C) of the luminal wall. Patient anatomy simulations show feasibility in ablating 60.1-92.9% of head/body tumour volumes (4.3-37.2 cm(3)) with dose < 15 EM(43 °C) in the luminal wall for 18-48 min treatment durations, using 1-3 applicator placements in GI lumen. For hyperthermia, planar and radial-diverging transducers could maintain up to 8 cm(3) and 15 cm(3) of tissue, respectively, between 40-45 °C for a single applicator placement.

CONCLUSIONS

Modelling studies indicate the feasibility of endoluminal ultrasound for volumetric thermal ablation or hyperthermia treatment of pancreatic tumour tissue.

Magnetic Resonance Imaging-Guided Transurethral Ultrasound Ablation of Prostate Tissue in Patients with Localized Prostate Cancer: A Prospective Phase 1 Clinical Trial

BACKGROUND

Magnetic resonance imaging-guided transurethral ultrasound ablation (MRI-TULSA) is a novel minimally invasive technology for ablating prostate tissue, potentially offering good disease control of localized cancer and low morbidity.

OBJECTIVE

To determine the clinical safety and feasibility of MRI-TULSA for whole-gland prostate ablation in a primary treatment setting of localized prostate cancer (PCa).

DESIGN, SETTING, AND PARTICIPANTS

A single-arm prospective phase 1 study was performed at three tertiary referral centers in Canada, Germany, and the United States. Thirty patients (median age: 69 yr; interquartile range [IQR]: 67-71 yr) with biopsy-proven low-risk (80%) and intermediate-risk (20%) PCa were treated and followed for 12 mo.

INTERVENTION

MRI-TULSA treatment was delivered with the therapeutic intent of conservative whole-gland ablation including 3-mm safety margins and 10% residual viable prostate expected around the capsule.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Primary end points were safety (adverse events) and feasibility (technical accuracy and precision of conformal thermal ablation). Exploratory outcomes included quality of life, prostate-specific antigen (PSA), and biopsy at 12 mo.

RESULTS AND LIMITATIONS

Median treatment time was 36min (IQR: 26-44) and prostate volume was 44ml (IQR: 38-48). Spatial control of thermal ablation was ±1.3mm on MRI thermometry. Common Terminology Criteria for Adverse Events included hematuria (43% grade [G] 1; 6.7% G2), urinary tract infections (33% G2), acute urinary retention (10% G1; 17% G2), and epididymitis (3.3% G3). There were no rectal injuries. Median pretreatment International Prostate Symptom Score 8 (IQR: 5-13) returned to 6 (IQR: 4-10) at 3 mo (mean change: -2; 95% confidence interval [CI], -4 to 1). Median pretreatment International Index of Erectile Function 13 (IQR: 6-28) recovered to 13 (IQR: 5-25) at 12 mo (mean change: -1; 95% CI, -5 to 3). Median PSA decreased 87% at 1 mo and was stable at 0.8 ng/ml (IQR: 0.6-1.1) to 12 mo. Positive biopsies showed 61% reduction in total cancer length, clinically significant disease in 9 of 29 patients (31%; 95% CI, 15-51), and any disease in 16 of 29 patients (55%; 95% CI, 36-74).

CONCLUSIONS

MRI-TULSA was feasible, safe, and technically precise for whole-gland prostate ablation in patients with localized PCa. Phase 1 data are sufficiently compelling to study MRI-TULSA further in a larger prospective trial with reduced safety margins.

PATIENT SUMMARY

We used magnetic resonance imaging-guided transurethral ultrasound to heat and ablate the prostate in men with prostate cancer. We showed that the treatment can be targeted within a narrow range (1mm) and has a well-tolerated side effect profile. A larger study is under way.

TRIAL REGISTRATION

NCT01686958, DRKS00005311.

MRI-controlled ultrasound thermal therapy

Advances in medical imaging have enabled the development of new minimally and completely noninvasive therapies that produce a desired biological effect in a target, such as a tumor, with minimal damage to the surrounding tissue. One means of noninvasively achieving bioeffects in tissue is the use of ultrasound to generate heat. Specialized ultrasound transducers can be used to generate focal regions of heating non invasively, without inserting anything into the body or affecting the tissue outside the target region. Ultrasound thermal therapy can be used with magnetic resonance (MR) imaging (MRI) guidance and MRI temperature feedback to automatically control temperature distributions during heating, producing accurate thermal lesions, or maintaining optimal conditions to enhance drug delivery.

Spatiotemporal filtering of MR-temperature artifacts arising from bowel motion during transurethral MR-HIFU

PURPOSE

Transurethral MR-HIFU is a minimally invasive image-guided treatment for localized prostate cancer that enables precise targeting of tissue within the gland. The treatment is performed within a clinical MRI to obtain real-time MR thermometry used as an active feedback to control the spatial heating pattern in the prostate and to monitor for potential damage to surrounding tissues. This requires that the MR thermometry measurements are an accurate representation of the true tissue temperature. The proton resonance frequency shift thermometry method used is sensitive to tissue motion and changes in the local magnetic susceptibility that can be caused by the motion of air bubbles in the rectum, which can impact the performance of transurethral MR-HIFU in these regions of the gland.

METHODS

A method is proposed for filtering of temperature artifacts based on the temporal variance of the temperature, using empirical and dynamic positional knowledge of the ultrasonic heating beam, and an estimation of the measurement noise. A two-step correction strategy is introduced which eliminates artifact-detected temperature variations while keeping the noise level low through spatial averaging.

RESULTS

The filter has been evaluated by postprocessing data from five human transurethral ultrasound treatments. The two-step correction process led to reduced final temperature standard deviation in the prostate and rectum areas where the artifact was located, without negatively affecting areas distal to the artifact. The performance of the filter was also found to be consistent across all six of the data sets evaluated. The evaluation of the detection criterion parameter M determined that a value of M = 3 achieves a conservative filter with minimal loss of spatial resolution during the process.

CONCLUSIONS

The filter was able to remove most artifacts due to the presence of moving air bubbles in the rectum during transurethral MR-HIFU. A quantitative estimation of the filter capabilities shows a systematic improvement in the standard deviation of the corrected temperature maps in the rectum zone as well as in the entire acquired slice.

Temperature-dependent MR signals in cortical bone: potential for monitoring temperature changes during high-intensity focused ultrasound treatment in bone

PURPOSE

Because existing magnetic resonance thermometry techniques do not provide temperature information within bone, high-intensity focused ultrasound (HIFU) exposures in bone are monitored using temperature changes in adjacent soft tissues. In this study, the potential to monitor temperature changes in cortical bone using a short TE gradient echo sequence is evaluated.

METHODS

The feasibility of this proposed method was initially evaluated by measuring the temperature dependence of the gradient echo signal during cooling of cortical bone samples implanted with fiber-optic temperature sensors. A subsequent experiment involved heating a cortical bone sample using a clinical MR-HIFU system.

RESULTS

A consistent relationship between temperature change and the change in magnitude signal was observed within and between cortical bone samples. For the two-dimensional gradient echo sequence implemented in this study, a least-squares linear fit determined the percentage change in signal to be (0.90 ± 0.01)%/°C. This relationship was used to estimate temperature changes observed in the HIFU experiment and these temperatures agreed well with those measured from an implanted fiber-optic sensor.

CONCLUSION

This method appears capable of displaying changes related to temperature in cortical bone and could improve the safety of MR-HIFU treatments. Further investigations into the sensitivity of the technique in vivo are warranted.

A self-tuning adaptive controller for 3-D image-guided ultrasound cancer therapy

One of the challenges in MRI-controlled hyperthermia cancer treatment for localized tumor is that the tissue properties are dynamic and difficult to measure. Therefore, tuning the optimal gains for a constant gain controller can be challenging. In this paper, a new multi-input single-output adaptive controller strategy is proposed to address these problems. The inputs to the controller block are the frequency, rotation rate, and applied power level of an interstitial applicator, and the output is the boundary temperature during treatment. The time-varying gains of the new controller are updated over time using Lyapunov-based stability analysis. The robustness of the new controller to changes in the parameters of the tissue is investigated and compared to a constant gain controller through simulation studies. Simulations take into account changes in tissue properties and other conditions that may be encountered in a practical clinical situation. Finally, the effectiveness of the proposed controller is validated through an experimental study.

Approaches for modelling interstitial ultrasound ablation of tumours within or adjacent to bone: theoretical and experimental evaluations

PURPOSE

The objectives of this study were to develop numerical models of interstitial ultrasound ablation of tumours within or adjacent to bone, to evaluate model performance through theoretical analysis, and to validate the models and approximations used through comparison to experiments.

METHODS

3D transient biothermal and acoustic finite element models were developed, employing four approximations of 7-MHz ultrasound propagation at bone/soft tissue interfaces. The various approximations considered or excluded reflection, refraction, angle-dependence of transmission coefficients, shear mode conversion, and volumetric heat deposition. Simulations were performed for parametric and comparative studies. Experiments within ex vivo tissues and phantoms were performed to validate the models by comparison to simulations. Temperature measurements were conducted using needle thermocouples or magnetic resonance temperature imaging (MRTI). Finite element models representing heterogeneous tissue geometries were created based on segmented MR images.

RESULTS

High ultrasound absorption at bone/soft tissue interfaces increased the volumes of target tissue that could be ablated. Models using simplified approximations produced temperature profiles closely matching both more comprehensive models and experimental results, with good agreement between 3D calculations and MRTI. The correlation coefficients between simulated and measured temperature profiles in phantoms ranged from 0.852 to 0.967 (p-value < 0.01) for the four models.

CONCLUSIONS

Models using approximations of interstitial ultrasound energy deposition around bone/soft tissue interfaces produced temperature distributions in close agreement with comprehensive simulations and experimental measurements. These models may be applied to accurately predict temperatures produced by interstitial ultrasound ablation of tumours near and within bone, with applications toward treatment planning.

Modelling of endoluminal and interstitial ultrasound hyperthermia and thermal ablation: applications for device design, feedback control and treatment planning

Endoluminal and catheter-based ultrasound applicators are currently under development and are in clinical use for minimally invasive hyperthermia and thermal ablation of various tissue targets. Computational models play a critical role in device design and optimisation, assessment of therapeutic feasibility and safety, devising treatment monitoring and feedback control strategies, and performing patient-specific treatment planning with this technology. The critical aspects of theoretical modelling, applied specifically to endoluminal and interstitial ultrasound thermotherapy, are reviewed. Principles and practical techniques for modeling acoustic energy deposition, bioheat transfer, thermal tissue damage, and dynamic changes in the physical and physiological state of tissue are reviewed. The integration of these models and applications of simulation techniques in identification of device design parameters, development of real time feedback-control platforms, assessing the quality and safety of treatment delivery strategies, and optimisation of inverse treatment plans are presented.

Coagulation of human prostate volumes with MRI-controlled transurethral ultrasound therapy: results in gel phantoms

PURPOSE

The feasibility and safety of magnetic resonance imaging (MRI)-controlled transurethral ultrasound therapy were demonstrated recently in a preliminary human study in which a small subvolume of prostate tissue was treated prior to radical prostatectomy. Translation of this technology to full clinical use, however, requires the capability to generate thermal coagulation in a volume up to that of the prostate gland itself. The aim of this study was to investigate the parameters required to treat a full 3D human prostate accurately with a multi-element transurethral applicator and multiplanar MR temperature control.

METHODS

The approach was a combination of simulations (to select appropriate parameters) followed by experimental confirmation in tissue-mimicking phantoms. A ten-channel, MRI-compatible transurethral ultrasound therapy system was evaluated using six human prostate models (average volume: 36 cm(3)) obtained from the preliminary human feasibility study. Real-time multiplanar MR thermometry at 3 T was used to control the spatial heating pattern in up to nine planes simultaneously. Treatment strategies incorporated both single (4.6 or 8.1 MHz) and dual (4.6 and 14.4 MHz) frequencies, as well as maximum acoustic surface powers of 10 or 20 W cm(-2).

RESULTS

Treatments at 4.6 MHz were capable of coagulating a volume equivalent to 97% of the prostate. Increasing power from 10 to 20 W cm(-2) reduced treatment times by approximately 50% with full treatments taking 26 ± 3 min at a coagulation rate of 1.8 ± 0.4 cm(3) min(-1). A dual-frequency 4.6∕14.4 MHz treatment strategy was shown to be the most effective configuration for achieving full human prostate treatment while maintaining good treatment accuracy for small treatment radii. The dual-frequency approach reduced overtreatment close to the prostate base and apex, confirming the simulations.

CONCLUSIONS

This study reinforces the capability of MRI-controlled transurethral ultrasound therapy to treat full prostate volumes in a short treatment time with good spatial targeting accuracy and provides key parameters necessary for the next clinical trial.

MR imaging-controlled transurethral ultrasound therapy for conformal treatment of prostate tissue: initial feasibility in humans

PURPOSE

To evaluate the feasibility and safety of magnetic resonance (MR) imaging-controlled transurethral ultrasound therapy for prostate cancer in humans.

MATERIALS AND METHODS

This pilot study was approved by the institutional review board and was performed in eight men (mean age, 60 years; range, 49-70 years) with localized prostate cancer (Gleason score≤7, prostate-specific antigen level #15 μg/L) immediately before radical prostatectomy. All patients provided written informed consent. This phase 0 feasibility and safety study is the first evaluation in humans. Transurethral ultrasound therapy was performed with the patient under spinal anesthesia by using a clinical 1.5-T MR unit. Patients then underwent radical prostatectomy, and the resected gland was sliced in the plane of treatment to compare the MR imaging measurements with the pattern of thermal damage. The overall procedure time and coagulation rate were measured. In addition, the spatial targeting accuracy was evaluated, as was the thermal history along the thermal damage boundaries in the gland.

RESULTS

The average procedure time was 3 hours, with 2 or fewer hours spent in the MR unit. The treatment was well tolerated by all patients, and a temperature uncertainty of less than 2°C was observed in the treatments. The mean temperature and thermal dose measured along the boundary of thermal coagulation were 52.3°C±2.1 and 3457 (cumulative equivalent minutes at 43°C)±5580, respectively. The mean treatment rate was 0.5 mL/min, and a spatial targeting accuracy of -1.0 mm±2.6 was achieved.

CONCLUSION

MR imaging-controlled transurethral ultrasound therapy is feasible, safe, and well tolerated. This technology could be an attractive approach for whole-gland or focal therapy.

Investigation of power and frequency for 3D conformal MRI-controlled transurethral ultrasound therapy with a dual frequency multi-element transducer

Transurethral ultrasound therapy uses real-time magnetic resonance (MR) temperature feedback to enable the 3D control of thermal therapy accurately in a region within the prostate. Previous canine studies showed the feasibility of this method in vivo. The aim of this study was to reduce the procedure time, while maintaining targeting accuracy, by investigating new combinations of treatment parameters. Simulations and validation experiments in gel phantoms were used, with a collection of nine 3D realistic target prostate boundaries obtained from previous preclinical studies, where multi-slice MR images were acquired with the transurethral device in place. Acoustic power and rotation rate were varied based on temperature feedback at the prostate boundary. Maximum acoustic power and rotation rate were optimised interdependently, as a function of prostate radius and transducer operating frequency. The concept of dual frequency transducers was studied, using the fundamental frequency or the third harmonic component depending on the prostate radius. Numerical modelling enabled assessment of the effects of several acoustic parameters on treatment outcomes. The range of treatable prostate radii extended with increasing power, and tended to narrow with decreasing frequency. Reducing the frequency from 8 MHz to 4 MHz or increasing the surface acoustic power from 10 to 20 W/cm(2) led to treatment times shorter by up to 50% under appropriate conditions. A dual frequency configuration of 4/12 MHz with 20 W/cm(2) ultrasound intensity exposure can treat entire prostates up to 40 cm(3) in volume within 30 min. The interdependence between power and frequency may, however, require integrating multi-parametric functions in the controller for future optimisations.