Key words
HIFU - interventional procedures - ablation procedures - prostate - breast - abdomen
Introduction
High-intensity focused ultrasound (HIFU) can be used to noninvasively ablate deep-seated
tissue. The procedure is usually carried out under image guidance using either diagnostic
ultrasound (US-HIFU) or magnetic resonance imaging (MR-HIFU) to provide spatial targeting
and to monitor the ablation process in real time. Both US-HIFU and MR-HIFU are clinically
used and approved for various applications with several emerging applications currently
being evaluated in clinical trials. In the first part of this review (Magnetic Resonance-Guided
High-Intensity Focused Ultrasound (MR-HIFU): Technical Background and Overview of
Current Clinical Applications (Part 1)), we recently discussed the technical details
of HIFU including the two image guidance approaches and subsequently reviewed the
main clinical applications of MR-guided HIFU applications. In short, MR guidance provides
anatomical images with high soft-tissue contrast and offers the additional advantage
of near real-time temperature mapping which allows delivery of well-defined thermal
doses while protecting heat-sensitive structures. In this second part, we will review
several new and emerging applications, however again limited to MR-HIFU. Since important
US-guided HIFU studies exist for some applications, we will refer interested readers
to relevant reviews.
Prostate
In the last decades, prostate-specific antigen (PSA) screening has improved the detection
of prostate cancer, particularly of localized low- and intermediate-risk prostate
cancer (PCa) [1]. Beside the increased incidence of PCa, the precise mortality benefit of early detection
is still unclear and controversially discussed. This observation could be partly explained
by the detection of primarily low- and intermediate-risk prostate carcinomas and the
low disease-specific ten-year mortality or elderly men with a life expectancy of less
than ten years [2]
[3]. The Prostate Cancer Intervention Versus Observation Trial (PIVOT) found that prostatectomy
for PSA-diagnosed low-risk PCa might provide only limited benefit [4], and the Scandinavian Prostate Cancer Group Study Number Four (SPCG-4) found that
prostatectomy is only beneficial for patients under 65 years of age with clinically
diagnosed PCa [5]. Thus, in addition to radical prostatectomy and external-beam radiation therapy
(EBRT), conservative strategies, such as active surveillance and watchful waiting,
are gaining increasing importance for personalized clinical management for low- and
intermediate-risk cancer patients. Even if conservative management is indicated, some
patients (up to 5 – 10 %) still prefer a more radical approach [6], such as a radical prostatectomy, despite its associated complication rates with
remaining incontinence in up to 31 % [7] and erectile dysfunction in up to 50 % [8]. Hence, less invasive interventional therapies, such as HIFU, are gaining importance
for whole-gland or focal treatment of prostate cancer. For example, the 2016 EAU-ESTROSIOG
guidelines state that even though focal therapy for localized PCa remains experimental
due to the lack of convincing long-term results, salvage HIFU is a recommended thermal
ablation option for radiation-recurrent PCa [9]. In addition, despite multifocal occurrence of PCa [10], up to one third of patients with localized prostate cancer have unilateral disease
that may be suitable for focal treatment, e. g. targeted ablation, hemiablation or
zonal ablation. Recently, the French Urological Association evaluated US-HIFU hemiablation
for the primary treatment of low- and intermediate-risk PCa in the prospective multi-institutional
IDEAL study [11]. The results revealed a 95 % absence of clinically significant cancer associated
with a low morbidity after 1 year, and a radical treatment-free survival rate of 89 %
after 2 years. After 1 year, continence and erectile functions were preserved in 97 %
and 78 %, respectively, and severe adverse events (grade ≥ 3) were reported in only
13 %.
So far, the most common HIFU approaches in prostate cancer treatment employ transrectal
ultrasound-guided (US-) HIFU transducers (Ablatherm®, Sonablate®, FocalOne®), with their efficacy being well reviewed by Chaussy CG and Thüroff S [12]. The main drawback of these US-HIFU systems is the missing visualization of the
target lesion and the missing real-time temperature control during ablation. In contrast,
MRI allows visualization of the target lesion and provides real-time temperature mapping
to deliver well-defined and well-controlled thermal doses during HIFU therapy. Thus,
MR-HIFU seems to be the preferred imaging modality for guiding HIFU intervention compared
to US-HIFU [13]
[14]
[15]. The current clinically available MR-HIFU transducers are the ExAblate® system (Insightec, Haifa, Israel) with a transrectal probe and the TULSA-PRO® (Profound Medical, Mississauga, Canada) with a transurethral probe. Both systems
received CE label and are currently under clinical evaluation or collecting post-treatment
data in multi-national clinical trials for localized (up to 50 % of prostate volume)
ablation (ExAblate®) and whole-gland ablation (TULSA-PRO®) in patients with localized, organ-confined prostate cancer (TACT trial inclusion
closed).
In a prospective single-center phase I clinical trial using the MRI-guided ExAblate® transrectal HIFU probe, 8 men with 10 lesions with low- and intermediate-risk prostate
cancer (Gleason 6 – 7) were treated and followed for 6 months [16]. All patients were discharged within 4 hours after treatment. Only one patient developed
a urinary tract infection and prostatitis, the other patients did not show any complications
in the follow-up examinations. In one patient, treatment failed with significant residual
disease and the need for prostatectomy and 4 out of 10 lesions revealed residual malignant
tissue in the biopsy 6 months after treatment that could not be recognized on MRI.
Although this study was limited by a small sample size and short follow-up, it still
showed the transrectal MR-HIFU approach to be safe with low complication rates and
an acceptable oncologic outcome. In a further prospective phase I clinical trial using
the MRI-guided ExAblate® transrectal HIFU probe, 14 men with low-volume low-grade prostate cancer (Gleason
6) underwent treatment with 12 men completing a 2-year follow-up [17]. The mean sonication time was 117 min with all patients tolerating the procedure
well. One patient developed acute urinary retention after treatment, probably due
to a urinary tract infection and another patient developed epididymo-orchitis, both
complications resolved quickly under treatment with antibiotics. The median overall
satisfaction at 6 months was high. At 24 months, sexual function was the only parameter
to show a trend toward decline from baseline, however, not significantly. The median
PSA decreased by 38.8 % and remained low except in one patient who eventually underwent
radiation therapy. After 6 and 24 months, none of the patients had positive multiparametric
MR imaging findings. However, template biopsy at 24 months revealed 2 patients with
residual tumor in the treated prostate tissue. Nevertheless, this study also demonstrated
transrectal HIFU to be feasible with a favorable safety and functional profile, even
though the sample size was small.
For the transurethral TULSA-PRO® system, an initial treat and resect study showed good agreement between the delineated
tissue in the temperature maps acquired during treatment studies and the extent of
necrotic tissue as found in histology. In this pilot study, five men with localized
prostate cancer on multiparametric MRI were treated by focal MR-HIFU prior to radical
prostatectomy [18]. After prostatectomy, whole mount histological sections demonstrated successful
focal MR-HIFU treatment with target volumes of 4 – 20 ml within radii of up to 35 mm
from the urethra with a mean spatial targeting accuracy of 1.5 ± 2.8 mm. The mean
treatment accuracy with respect to histology was 0.4 ± 1.7 mm with all index tumors
being inside the histological outer limit of thermal injury and completely covered
by the targeted focal HIFU ablation. In a subsequent prospective multinational phase
I clinical trial, 30 men with low- and intermediate-risk prostate cancer (Gleason
6 or 7a) were treated using the MRI-guided TULSA-PRO® transurethral HIFU transducer and followed over one year [19]. The median treatment time was 36 min and 29 participants were successfully discharged
already after 24 hours. Treatment-related adverse events included urinary tract infections
(33 %), acute urinary retention (27 %), and epididymitis (3 %). No intraoperative
complications, rectal injuries, fistulas or severe post-therapeutic urinary incontinence
was observed. After 1 year only one patient had low-grade urinary incontinence. The
median pretreatment erectile function remained widely stable with 13 before treatment
(IQR: 6 – 28) compared to 13 (IQR: 5 – 25) at the 12-month follow-up. The median PSA
decreased by 87 % at the 1-month control and was stable at 0.8 ng/ml (IQR: 0.6 – 1.1)
to 12 months. However, in this clinical safety and feasibility study with the therapeutic
intent of conservative whole-gland ablation including a 3 mm safety margin towards
the capsule, residual viable tumor was found in 55 % of the patients at the 1-year
control.
Anyway, the available data for MR-guided HIFU are still very limited and have not
yet been fully analyzed. In general, the majority of prostate cancers are situated
in the posterior aspect of the peripheral zone near by the rectal wall and neurovascular
bundle and some other less frequent tumors are located at the anterior fibromuscular
stroma. In the comparison of both techniques, the theoretical advantage of the transurethral
TULSA-PRO® approach is related to the fact that these lesions might be better ablated up to
the prostate capsule with a lower risk of harming the rectum wall and neurovascular
bundle in the far field and to be closer to the anterior stroma with fewer side effects
in the near field [20]. Moreover, the transurethral approach allows for simultaneous placement of a rectal
cooling device even further lowering the risk of rectal injuries and fistula. A disadvantage
of the transurethral approach is the risk for urethral injuries during insertion of
the catheter or during sonication, which, however, has been shown to be a rare complication.
On the other hand, the transrectal ExAblate® system seems to be easier to place within the rectum and allows overtreatment outside
the prostate capsule, e. g. in the case of extraprostatic infiltration. However, the
safety margin towards the rectal wall in the direct near field of the transducer implicates
the higher risk of residual tumor tissue at the most common PCa location at the posterior
peripheral zone adjacent to the prostate capsule.
An exemplary workflow for prostate ablation using the TULSA-PRO® is shown in [Fig. 1a – h].
Fig. 1 Overview of prostate cancer ablation (PIRADS 4, Gleason 7a) via a transurethral approach
(TULSA-PRO, Profound Medical, Mississauga, Canada). a Transurethral catheter with HIFU transducer elements at the tip mounted on a motor
stage allowing rotation of the catheter. b Catheter is inserted into the urethra with the transducer elements placed inside
the prostate. For ablation, the tip can be rotated to reach all targeted areas. c – e Planning of the therapy using transverse, coronal and sagittal high-resolution MR
images of the prostate in which the tumor was marked. f Delineation of the target tissue is performed manually. g MR thermometry map acquired during therapy showing maximum temperature reached. Automatic
adjustments of rotation speed, power output, and frequency enable precise heating
and therefore accurate ablation of the previously marked tissue. h Post-therapy acquired CE MRI showing the non-perfused volume (NPV).
Abb. 1 Überblick über die Ablation eines Prostatakarzinoms (PI-RADS 4, Gleason 7a) mittels
transurethralem Vorgehen (TULSA-PRO, Profound Medical, Mississauga, Canada). a Ein transurethraler Katheter mit HIFU-Transducer-Elementen an der Spitze wird von
einem Motor angetrieben, der die Rotation des Katheters erlaubt. b Der Katheter wird so in die Urethra eingebracht, dass die HIFU-Elemente innerhalb
der Prostata liegen. Für die Ablation kann die Katheter-Spitze rotieren, um alle zu
therapierenden Areale zu erreichen. c – e Therapieplanung anhand hochaufgelöster transversaler, koronarer und sagittaler MR-Bilder
der Prostata, in denen der Tumor markiert wurde. f Die Bestimmung des zu abladierenden Zielgewebes erfolgt manuell. g MR-Thermometrie-Karte, die während der Untersuchung aufgezeichnet wird und die maximal
erreichte Temperatur darstellt. Die automatisierte Anpassung der Rotationsgeschwindigkeit,
der Leistungsabgabe und der verwendeten Frequenz ermöglicht die präzise Erhitzung
und folglich akkurate Ablation des zuvor markierten Gewebes. h Nach Therapie akquirierte KM-unterstützte Sequenz, die das nicht perfundierte Volumen
(NPV) zeigt.
Altogether, MR-HIFU seems to be a promising treatment option for clinically low- to
intermediate-risk prostate cancer detected on MRI and for salvage ablation of radiation-recurrent
PCa. Nevertheless, further studies are needed to compare the efficacy, local recurrence
and risk of complications of both currently available MR-HIFU procedures for prostate
cancer treatment.
It should be noted that the clinical use of MR-HIFU is accepted in the current S3-guideline
for prostate cancer when performed under study conditions with the patient still needing
active surveillance after treatment.
Breast
Early stage, localized breast cancer presents a potential application for minimally
invasive and local ablative techniques such as radiofrequency, microwave, laser, cryo
and HIFU techniques. The current clinical standard for low-stage invasive breast carcinomas
is local resection in a breast-conserving manner with high cure rates and long-term
survival of more than 90 % [21]. Comparable to local resection, ablative techniques seem feasible as long as a safety
margin of at least 10 mm is observed and other relevant criteria such as histology,
hormone receptor status, assessment of sentinel lymph node and post-interventional
therapy standards are met [22]. A challenge for all ablative techniques is to achieve total tumor ablation including
a safety margin, while no post-interventional histological assessment of the tumor
rim is available for confirmation. Clinical outcomes comparing the above-mentioned
ablative techniques for the treatment of breast cancer were recently summarized and
analyzed by Peek et al. [23]. Overall, few studies have been performed with low patient numbers, which makes
a systematic comparison of all techniques with respect to outcome and side effects
difficult. From the total 1627 patients included in the analysis, 227 patients were
treated with MR-HIFU.
MR-HIFU treatment of breast fibroadenomas was first reported by Hynynen et al. [24], demonstrating 8 complete or partial ablations of a total of 11 treated lesions.
Patients were placed on the HIFU system and sonicated from anterior, while a water
bag provided acoustic coupling. Most clinical MR-HIFU studies to date have been performed
with anterior sonication using the commercial MR-HIFU ExAblate® system (InSightec, Haifa, Israel) [25]
[26]. In the case of anterior sonication, attention has to be paid to prevent structures
in the far field from heating, such as the pectoral muscle, ribs, or lung tissue (tissue/air
interface), all limiting possible target locations, while near field heating led to
skin burns in some cases. Clinical data showed that several tumors could not be entirely
ablated, possibly due to the fact that not enough acoustic energy could be deposited
due to the above-mentioned limitations or due to misplacement of the HIFU focus point
[24]. Nevertheless, the outcome achieved with MR-HIFU was comparable to other ablation
techniques, while the side effects were lower. A different approach is based on the
lateral sonication of the tumor using dedicated large aperture breast HIFU transducers
that have been developed with a different geometry ([Fig. 2a – d]) [27]
[28]
[29]. Merckel et al. demonstrated partial ablation of breast cancer lesions in a treat
and resect study with such a dedicated breast HIFU system (prototype, Profound Medical,
Mississauga, Canada) [30]. Partial ablation was intended in order to compare treatment planning to temperature
maps obtained during ablation and to ablated margins in overlay with histology sections.
Results showed that lesion size correlated with applied overall acoustic energy although
the achieved absolute temperatures differed per patients. Temperature feedback during
ablation and calculation of thermal dose was complicated by poor temperature maps
obtained by proton-resonance frequency shift (PRFS) thermometry for reasons such as
breathing-induced motion and the presence of mostly fatty tissue ([Fig. 3a – h]). Overall, MR-HIFU for the treatment of breast cancer can be considered a technically
safe and patient-friendly method. However, before MR-HIFU could be considered as a
treatment option for early stage and localized tumors, the technology needs to be
further developed to guarantee complete tumor ablation including the necessary safety
margin by improving image guidance and temperature mapping [31].
Fig. 2 a, b Dedicated MR-HIFU breast platform (prototype, Profound Medical, Mississauga, Canada)
allowing sparing of the thoracic wall during treatment. c Schematic view of laterally mounted ultrasound transducers with the green spot indicating
the sonication cell within the middle of the breast. Source: Merckel LG, Bartels LW,
Köhler M, van den Bongard HJ, Deckers R, Mali WP, Binkert CA, Moonen CT, Gilhuijs
KG, van den Bosch MA. MR-guided high-intensity focused ultrasound ablation of breast
cancer with a dedicated breast platform. Cardiovasc Intervent Radiol 2013; 36: 292 – 301
[rerif].
Abb. 2 a, b Dedizierte MR-HIFU-Brustplattform (prototype, Profound Medical, Mississauga, Canada)
mit der Möglichkeit zur Schonung der Thoraxwand während der Therapie. c Schematische Darstellung der lateral angebrachten Ultraschall-Transducer, wobei der
grüne Punkt eine Sonikations-Zelle in der Mitte der Brust darstellt. Quelle: Merckel
LG, Bartels LW, Köhler M et al. MR-guided high-intensity focused ultrasound ablation
of breast cancer with a dedicated breast platform. Cardiovasc Intervent Radiol 2013;
36: 292 – 301 [rerif].
Fig. 3 MR thermometry data obtained during breast sonication (prototype, Profound Medical,
Mississauga, Canada) overlaid on anatomical coronal a – d and sagittal e – h images. Source: Merckel LG, Knuttel FM, Deckers R, van Dalen T, Schubert G, Peters
NH, Weits T, van Diest PJ, Mali WP, Vaessen PH, van Gorp JM, Moonen CT, Bartels LW,
van den Bosch MA. First clinical experience with a dedicated MRI-guided high-intensity
focused ultrasound system for breast cancer ablation. Eur Radiol 2016; 26: 4037 – 4046
[rerif].
Abb. 3 MR-Thermometrie-Daten aufgezeichnet während der Brusttherapie (prototype, Profound
Medical, Mississauga, Canada), die anatomische koronare a – d und sagittale e – h Bilder überlagern. Quelle: Merckel LG, Knuttel FM, Deckers R et al. First clinical
experience with a dedicated MRI-guided high-intensity focused ultrasound system for
breast cancer ablation. Eur Radiol 2016; 26: 4037 – 4046 [rerif].
Abdominal Applications
US-HIFU has been used in several studies for the ablation of benign and malignant
tumors in abdominal organs with good success rates, thus establishing an alternative
to other minimally invasive thermal ablation methods for a selected group of patients.
In general, treatments reveal partial or complete tumor ablation and are associated
with comparably low side effects and complications [32]
[33]
[34]. However, especially for liver tumors, HIFU ablation has yet to be benchmarked against
other minimally invasive procedures such as radiofrequency and microwave ablation,
or transarterial chemoembolization in a multi-arm study which is currently not available.
For MR-HIFU, only a few case studies have been reported so far for the ablation of
liver or pancreatic tumors [35]. MR-HIFU treatment of malignancies in abdominal organs, such as the liver, pancreas
or kidney, is complicated by motion and often the acoustic beam path is obstructed
by the thoracic cage or air-filled cavities in the bowel system. Respiratory motion,
organ motion and bowel motion exacerbate focusing of the target tissue and the reliability
of MR thermometry [36]. However, with new experimental approaches, respiratory excursion can be controlled
by high-frequency jet ventilation [37] or apnea breaks for gated sonication [38]. Image guidance and navigator techniques allow tracking of organ movement to apply
HIFU when the target is in a defined position or also to follow the movement of the
target with the focus point [39]
[40]
[41]. Another technique currently under development is “beam-shaping” which excludes
the ribs from the beam path thus protecting the patient from uncontrolled heating
of the chest wall [42]. For the ablation of lesions in the liver dome, an iatrogenic intrapleural fluid
infusion resulting in a pleural effusion could provide the necessary acoustic window
[43]. While several studies using ultrasound-guided HIFU for the ablation of liver malignancies
exist [44], only few case studies using MR-guided HIFU and demonstrating the feasibility of
this approach have been published ([Fig. 4a – e]).
Fig. 4 MR-HIFU treatment of a subcapsular hepatocellular carcinoma (ExAblate 2100, InSightec,
Haifa, Israel). a Patient in prone position on the MR-HIFU patient bed. The arrow points out the position
of the transducer. b Manual delineation of the target tissue, c skin: red line, sonication area: yellow box. d Temperature maps and profiles acquired with PRFS MR thermometry. Source: Anzidei
M, Napoli A, Sandolo F, Marincola BC, Di Martino M, Berloco P, Bosco S, Bezzi M, Catalano
C. Magnetic resonance-guided focused ultrasound ablation in abdominal moving organs:
a feasibility study in selected cases of pancreatic and liver cancer. Cardiovasc Intervent
Radiol 2014; 37: 1611 – 1617 [rerif].
Abb. 4 MR-HIFU-Therapie eines subkapsulär gelegenen hepatozellulären Karzinoms (ExAblate
2100, InSightec, Haifa, Israel). a Der Patient liegt in Bauchlage auf dem MR-Tisch, der Pfeil zeigt auf den Transducer.
b Manuelle Markierung des Zielgewebes. c Haut: rote Linie, Sonikations-Zone: gelbe Box. d Temperaturkarten und -profile, die mittels PRFS-MR-Thermometrie gemessen wurden.
Quelle: Anzidei M, Napoli A, Sandolo F et al. Magnetic resonance-guided focused ultrasound
ablation in abdominal moving organs: a feasibility study in selected cases of pancreatic
and liver cancer. Cardiovasc Intervent Radiol 2014; 37: 1611 – 1617 [rerif].
Ductal pancreatic cancer still has a particularly bad prognosis with a 2-year survival
rate of less than 10 %. When diagnosed, 40 % of these tumors have already metastasized,
and in 60 % of patients the tumor is locally inoperable due to infiltration of the
portal vein, celiac trunk or superior mesenteric artery. In addition to radio-chemotherapy
and a palliative operation, ablative procedures can help to control local tumor growth
and reduce pain thereby improving the quality of life. So far, more than 1200 pancreatic
cancer patients have been treated with US-HIFU, showing that this ablation method
is safe with very few adverse events, provides pain palliation, and possibly extends
life expectancy [33]
[45]
[46]
[47]. A HIFU therapy of the pancreas is challenging because of air-filled bowel loops
being present in the acoustic path. Thus, preparatory measures such as fasting, clyster
or a nasogastric tube are important. Contact of the tumor with the duodenum or the
bile duct can be a further contraindication for HIFU. While ultrasound guidance has
the advantage that the acoustic beam path and possible obstructions by air pockets
in the stomach or bowel can be directly probed, targeting of the pancreatic lesion
is complicated due to the low intrinsic tissue contrast. Furthermore, heating to an
ablative temperature is difficult to monitor as thermometry methods are lacking. Few
case reports using MR-HIFU showed the feasibility of performing similar treatments
under MR guidance [48]. For MR-HIFU therapy, patients are typically placed in a prone position on the system
using, if necessary, an acoustically transparent dome-shaped gel pad or water-filled
device to displace stomach and bowel that may potentially block the beam path. An
example for MR-HIFU ablation of a pancreatic tumor is shown in [Fig. 5a – e] [48].
Fig. 5 MR-HIFU treatment of a pancreatic tumor (ExAblate 2100, InSightec, Haifa, Israel). a Axial, contrast-enhanced MR image showing a pancreatic tumor within the pancreatic
body determining the encasement of the celiac artery (arrows). b Planning of MR-HIFU sonication for a patient placed in prone position on the HIFU
transducer c MR-thermometry map acquired during ablation. d Temperature profile within the focal spot over the time course of ablation, e contrast-enhanced MR image after treatment revealing the presence of non-perfused
tissue corresponding to the ablated area within the tumor tissue. Source: Anzidei
M, Marincola BC, Bezzi M, Brachetti G, Nudo F, Cortesi E, Berloco P, Catalano C, Napoli
A. Magnetic resonance-guided high-intensity focused ultrasound treatment of locally
advanced pancreatic adenocarcinoma: preliminary experience for pain palliation and
local tumor control. Invest Radiol 2014; 49: 759 – 765 [rerif].
Abb. 5 MR-HIFU-Therapie eines Pankreastumors (ExAblate 2100, InSightec, Haifa, Israel).
a Axiale, kontrastunterstützte MR-Bilder zeigen einen Pankreastumor im Pankreaskorpus
mit Umscheidung des Truncus coeliacus (Pfeile). b Planung der MR-HIFU-Therapie für einen Patienten, der bäuchlings auf dem HIFU-Transducer
liegt. c MR-Thermometrie-Daten, aufgezeichnet während der Ablation. d Temperaturprofil innerhalb des Fokuspunktes während der Ablation. e Kontrastunterstützte MR-Bilder nach Therapie mit Nachweis nicht perfundierten Gewebes
analog zum abladierten Gewebe innerhalb des Tumors. Quelle: Anzidei M, Marincola BC,
Bezzi M et al. Magnetic resonance-guided high-intensity focused ultrasound treatment
of locally advanced pancreatic adenocarcinoma: preliminary experience for pain palliation
and local tumor control. Invest Radiol 2014; 49: 759 – 765 [rerif].
Similarly, ablation of renal cancer using US-HIFU has been sporadically performed
indicating that ablation is possible in principle [49]
[50].
Conclusion
Currently, the clinical use of MR-HIFU is restricted to thermal ablation of tissues
and is clinically established for the treatment of uterine fibroids, pain alleviation
of bone metastases and treatment of central tremor. Other promising applications in
oncology either with curative intent or in a palliative setting are currently the
subject of preclinical and clinical studies. One prominent and promising example is
the treatment of prostate cancer using either a transrectal or transurethral probe,
both of which recently received CE approval. As a noninvasive method, MR-HIFU treatments
are patient-friendly with low complication rates and minor side effects and can be
repeated if needed. Integration with MRI provides real-time spatial guidance, which
further improves safety and therapeutic outcome. MRI-based temperature mapping offers
the option for closed-loop temperature feedback for the delivery of well-defined thermal
doses which allows protection of crucial structures from overheating while delivering
a lethal thermal dose to the target tissue. While clinical pilot studies do exist
for several new applications, prospective multi-arm clinical trials are urgently needed
to demonstrate improved outcome compared to standard treatments.
