Methods of External Hyperthermic Heating

The development of equipment capable of producing and monitoring safe, effective and predictable hyperthermia treatments represents a major challenge. The main problem associated with any heating technique is the need to adjust and control the distribution of absorbed power in the tissue during treatment. Power distribution is considered adequate only when tumor tissue can be maintained at the required hyperthermic levels while, at the same time, healthy tissue is not overheated. This problem is particularly crucial when external heating devices are used to produce hyperthermia. Ex­ ternal hyperthermia refers to those methods which supply heat to tumor tissue in an external, noninvasive manner, as opposed to internal hyperther­ mia by which heat is supplied to tumor tissue in situ. Until recently, most of the technical developments and clinical trials of ther­ motherapy for superficial and deep tumors have been based on elec­ tromagnetic systems. Presently, there is increasing interest in the use of ultra­ sound to accomplish these goals. Electromagnetic techniques of external thermotherapy include radiative, capacitive, and, to a lesser extent, inductive procedures. Recent designs for radiative applicators have incorporated microstrip structures. These have the advantage of being compact and lightweight compared with dielectrically loaded waveguide applicators.

1 Biophysics and Technology of Electromagnetic Hyperthermia.- 1.1 Electromagnetic Fields and Tissues.- 1.1.1 Introduction.- 1.1.2 Overview of the Chapter.- 1.1.3 Electric and Magnetic Fields.- 1.1.3.1 Time-Invariant Fields.- 1.1.3.2 Time-Varying Fields.- 1.1.4 Electrical Properties of Biological Materials.- 1.1.5 Wave Propagation in Tissues.- 1.1.6 Power Absorption.- 1.1.7 Boundary Conditions.- 1.1.8 Summary.- 1.2 Dosimetry in Electromagnetic Hyperthermia.- 1.2.1 Phantom Materials.- 1.2.2 Methods of Calculating SAR.- 1.2.2.1 Analytical Models.- 1.2.2.2 Numerical Models.- 1.2.3 Summary.- 1.3 Electromagnetic Techniques for Hyperthermia.- 1.3.1 Overview of Techniques.- 1.3.2 Techniques for Local (Superficial) Hyperthermia.- 1.3.3 Techniques for Regional (Deep) Hyperthermia.- 1.3.4 Power Requirements for Hyperthermia Systems.- 1.3.5 Impedance Matching.- 1.3.6 Summary.- 1.4 Applicators for Local Hyperthermia.- 1.4.1 Electric (Capacitive) Applicators.- 1.4.2 Magnetic (Inductive) Applicators.- 1.4.2.1 Coil Applicators.- 1.4.2.2 Distributed Current Applicators.- 1.4.3 Radiating Applicators.- 1.4.3.1 Rectangular Waveguide Applicators.- 1.4.3.2 Ridged Waveguide Applicators.- 1.4.3.3 Dielectric Slab-Loaded Rectangular Applicator.- 1.4.3.4 Micro strip and Other Compact Applicators.- 1.4.4 Applicator Size and Penetration Depth.- 1.4.5 Arrays of Applicators.- 1.4.5.1 Coherent and Incoherent Systems.- 1.4.5.2 Phased Arrays.- 1.4.5.3 Arrays with Large Effective Field Size.- 1.4.6 Summary.- 1.5 Applicators for Regional Hyperthermia.- 1.5.1 Electric (Capacitive) Applicators.- 1.5.1.1 Two-Electrode Systems.- 1.5.1.2 Three-Electrode Systems.- 1.5.1.3 Ring Electrode Systems.- 1.5.2 Magnetic Applicators.- 1.5.2.1 Concentric Coil.- 1.5.2.2 Coaxial Coils.- 1.5.2.3 Helical Coil Applicators.- 1.5.2.4 Other Magnetic Devices.- 1.5.3 Radiative Applicators.- 1.5.3.1 Ridged Waveguides.- 1.5.3.2 Annular Arrays.- 1.5.3.3 Coaxial TEM Applicator.- 1.5.3.4 Segmented Cylindrical Array.- 1.5.4 Summary.- 1.6 Biological Effects of RF/Microwave Fields and Exposure Standards.- 1.6.1 Biological Effects of RF/Microwave Fields.- 1.6.1.1 Macromolecular and Cellular Effects.- 1.6.1.2 Chromosomal Effects.- 1.6.1.3 Carcinogenesis.- 1.6.1.4 Reproduction, Growth and Development.- 1.6.1.5 Haematopoietic and Immune Systems.- 1.6.1.6 Endocrine System.- 1.6.1.7 Cardiovascular Function.- 1.6.1.8 Blood-Brain Barrier.- 1.6.1.9 Nervous System.- 1.6.1.10 Cataractogenesis.- 1.6.2 Exposure Guidelines.- 1.6.3 Safety Procedures for Electromagnetic Hyperthermia.- 1.6.4 Summary.- References.- 2 Biophysics and Technology of Ultrasound Hyperthermia.- 2.1 Introduction.- 2.2 Basic Physics of Ultrasound.- 2.2.1 Physical Aspects of Ultrasound Waves.- 2.2.1.1 Wave Equation.- 2.2.1.2 Acoustic Impedance.- 2.2.1.3 Intensity.- 2.2.1.4 Wave Propagation in a Lossy Medium.- 2.2.1.5 Nonlinear Propagation.- 2.2.2 The Ultrasonic Fields.- 2.2.2.1 Unfocused Ultrasonic Fields.- 2.2.2.2 Focused Ultrasonic Fields.- 2.2.2.3 Acoustic Field Calculations.- 2.3 Acoustic Properties of Tissues.- 2.3.1 Velocity.- 2.3.2 Absorption.- 2.3.3 Attenuation.- 2.3.4 Characteristic Acoustic Impedance.- 2.3.5 Shear Wave Properties.- 2.3.6 Nonlinear Propagation Parameter.- 2.4 Biological Effects of Ultrasound.- 2.4.1 Thermal Effects.- 2.4.2 Mechanical Effects.- 2.4.3 Cavitation.- 2.4.4 Summary of Biological Effects.- 2.5 Generation and Characterization of Ultrasonic Fields.- 2.5.1 Piezoelectric Materials.- 2.5.2 Ultrasonic Transducers.- 2.5.2.1 Resonance Frequency.- 2.5.2.2 Backing of the Transducer.- 2.5.2.3 Q-factor.- 2.5.2.4 Mechanical Matching to the Load.- 2.5.2.5 Electric Matching.- 2.5.2.6 The Structure of an Ultrasound Hyperthermia Transducer.- 2.5.3 Calibration of Ultrasonic Fields.- 2.5.3.1 Hydrophones.- 2.5.3.2 Radiation Force Measurements.- 2.5.3.3 Thermal Methods.- 2.5.3.4 Optical Methods.- 2.5.3.5 In Vivo Measurements of Ultrasonic Fields.- 2.5.3.6 Characterization of Ultrasonic Fields for Hyperthermia Treatments.- 2.5.4 The Use of Ultrasonic Phantoms.- 2.5.4.1 Liquid Phantoms.- 2.5.4.2 Solid Phantoms.- 2.5.4.3 Perfused Phantoms.- 2.6 Ultrasonic Systems for Induction of Hyperthermia.- 2.6.1 Planar Transducer Systems.- 2.6.2 Multiple, Overlapping Nonfocused Fields.- 2.6.3 Focused and Stationary Fields.- 2.6.4 Focused and Scanned Fields.- 2.6.4.1 Scanning Speed.- 2.6.4.2 Scanning Pattern.- 2.6.4.3 Perfusion Effects.- 2.6.4.4 Feedback Control.- 2.6.4.5 Clinical Scanned Focused Ultrasound Hyperthermia Systems.- 2.7 Technical Considerations in Ultrasound Hyperthermia.- 2.7.1 Tissue Interfaces.- 2.7.2 Acoustic Window.- 2.7.3 Nonlinear Propagation.- 2.7.4 Treatment Planning.- 2.7.5 Treatment Execution.- 2.8 Future Developments in Ultrasound Hyperthermia.- 2.8.1 Control.- 2.8.2 Special Applicators.- 2.8.3 High Temperature Hyperthermia.- References.- 3 Physics Evaluation and Quality Control of Hyperthermia Equipment.- 3.1 Common Components of Hyperthermia Equipment.- 3.2 Thermometry Evaluations.- 3.2.1 The Importance of Temperature Accuracy.- 3.2.2 Scope of Laboratory Tests.- 3.2.3 Criteria and Frequency of Testing.- 3.2.4 Testing Procedures.- 3.2.4.1 Accuracy of Calibration Thermometers.- 3.2.4.2 Accuracy of Clinical Thermometers.- 3.2.4.3 Precision of Clinical Thermometers.- 3.2.4.4 Stability of Clinical Thermometers.- 3.2.4.5 Response Time of Clinical Thermometers.- 3.2.4.6 Probe Diameter and Sensor Position.- 3.2.4.7 Perturbations and Artifacts.- 3.2.5 Some Known Sources of Temperature Errors.- 3.2.5.1 Electromagnetically Induced Currents in Metallic Probes.- 3.2.5.2 Viscous Heating and Ultrasonic Absorption.- 3.2.5.3 Linear Mapping with Bare Thermocouples.- 3.2.5.4 Temperature Smearing Across High Gradients.- 3.2.5.5 Cross-talk in Multijunction Probes.- 3.2.5.6 Calibration Inaccuracies.- 3.2.5.7 Extrapolation Errors.- 3.2.5.8 Moisture Artifacts.- 3.2.5.9 Electromagnetic Interference in Electronics.- 3.2.5.10 Probe Damage.- 3.3 Power Evaluations.- 3.3.1 Electromagnetic Shielding and Electric Safety Requirements.- 3.3.2 The Need for Accuracy of Power Readings.- 3.3.3 Scope, Criteria, and Frequency of Power Meter Tests.- 3.3.3.1 Accuracy of Power Indicators.- 3.3.3.2 Estimation of Line Loss.- 3.3.3.3 Net Power at Applicator.- 3.4 Applicator Evaluations.- 3.4.1 Characterization of Applicators by SAR Patterns.- 3.4.2 Test Procedures.- 3.4.2.1 Check of Manufacturer-Supplied SAR Data.- 3.4.2.2 SAR Patterns with Coupling Bolus.- 3.4.2.3 Single Point SAR Reproducibility.- 3.4.3 Some Results of SAR Measurements.- 3.4.3.1 Effect of Phantom Size, Shape, and Composition.- 3.4.3.2 Effect of Phantom Composition.- 3.5 Evaluation of Coupling.- 3.5.1 Coupling Efficiency.- 3.5.2 Effect of Coupling Bolus on Heating Pattern.- 3.6 Evaluation of Skin Cooling Devices.- 3.7 Evaluation of Electromagnetic Hazard.- 3.7.1 Operator Exposure to Electromagnetic Leakage Radiation.- 3.7.2 Patient Exposure to EM Leakage Radiation.- 3.7.3 Hazard Survey Procedure.- 3.7.4 Results of EM Leakage Surveys.- References.