title: Model-based feasibility assessment and evaluation of prostate hyperthermia with a commercial MR-guided endorectal HIFU ablation array.
abstract: OBJECTIVE
  Feasibility of targeted and volumetric hyperthermia (40-45 °C) delivery to the prostate with a commercial MR-guided endorectal ultrasound phased array system, designed specifically for thermal ablation and approved for ablation trials (ExAblate 2100, Insightec Ltd.), was assessed through computer simulations and tissue-equivalent phantom experiments with the intention of fast clinical translation for targeted hyperthermia in conjunction with radiotherapy and chemotherapy.
  
  
  METHODS
  The simulations included a 3D finite element method based biothermal model, and acoustic field calculations for the ExAblate ERUS phased array (2.3 MHz, 2.3 × 4.0 cm(2), ∼1000 channels) using the rectangular radiator method. Array beamforming strategies were investigated to deliver protracted, continuous-wave hyperthermia to focal prostate cancer targets identified from representative patient cases. Constraints on power densities, sonication durations and switching speeds imposed by ExAblate hardware and software were incorporated in the models. Preliminary experiments included beamformed sonications in tissue mimicking phantoms under MR temperature monitoring at 3 T (GE Discovery MR750W).
  
  
  RESULTS
  Acoustic intensities considered during simulation were limited to ensure mild hyperthermia (Tmax < 45 °C) and fail-safe operation of the ExAblate array (spatial and time averaged acoustic intensity ISATA < 3.4 W/cm(2)). Tissue volumes with therapeutic temperature levels (T > 41 °C) were estimated. Numerical simulations indicated that T > 41 °C was calculated in 13-23 cm(3) volumes for sonications with planar or diverging beam patterns at 0.9-1.2 W/cm(2), in 4.5-5.8 cm(3) volumes for simultaneous multipoint focus beam patterns at ∼0.7 W/cm(2), and in ∼6.0 cm(3) for curvilinear (cylindrical) beam patterns at 0.75 W/cm(2). Focused heating patterns may be practical for treating focal disease in a single posterior quadrant of the prostate and diffused heating patterns may be useful for heating quadrants, hemigland volumes or even bilateral targets. Treatable volumes may be limited by pubic bone heating. Therapeutic temperatures were estimated for a range of physiological parameters, sonication duty cycles and rectal cooling. Hyperthermia specific phasing patterns were implemented on the ExAblate prostate array and continuous-wave sonications (∼0.88 W/cm(2), 15 min) were performed in tissue-mimicking material with real-time MR-based temperature imaging (PRFS imaging at 3.0 T). Shapes of heating patterns observed during experiments were consistent with simulations.
  
  
  CONCLUSIONS
  The ExAblate 2100, designed specifically for thermal ablation, can be controlled for delivering continuous hyperthermia in prostate while working within operational constraints.
authors.affiliation: Thermal Therapy Research Group, Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, Suite H-1031, San Francisco, California 94143.
authors.forename: Vasant A
authors.initials: VA
authors.lastname: Salgaonkar
authors.affiliation: Department of Electrical and Computer Engineering, Kansas State University, 2077 Rathbone Hall, Manhattan, Kansas 66506.
authors.forename: Punit
authors.initials: P
authors.lastname: Prakash
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 505 Parnassus Avenue, San Francisco, California 94143.
authors.forename: Viola
authors.initials: V
authors.lastname: Rieke
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 505 Parnassus Avenue, San Francisco, California 94143.
authors.forename: Eugene
authors.initials: E
authors.lastname: Ozhinsky
authors.affiliation: Department of Radiology, Stanford University, 1201 Welch Road, Stanford, California 94305.
authors.forename: Juan
authors.initials: J
authors.lastname: Plata
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 505 Parnassus Avenue, San Francisco, California 94143.
authors.forename: John
authors.initials: J
authors.lastname: Kurhanewicz
authors.affiliation: Thermal Therapy Research Group, Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, Suite H-1031, San Francisco, California 94143.
authors.forename: I-C Joe
authors.initials: IC
authors.lastname: Hsu
authors.affiliation: Thermal Therapy Research Group, Radiation Oncology, University of California San Francisco, 1600 Divisadero Street, Suite H-1031, San Francisco, California 94143.
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Diederich
mesh_terms: D000162:Acoustics; D003198:Computer Simulation; D004867:Equipment Design; D005240:Feasibility Studies; D020342:Finite Element Analysis; D057086:High-Intensity Focused Ultrasound Ablation / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D006979:Hyperthermia, Induced / Q000379:methods*; D021621:Imaging, Three-Dimensional; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011467:Prostate / Q000187:drug effects / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000532:radiotherapy / Q000628:therapy*; D013696:Temperature; D014465:Ultrasonics