1006 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 53, NO. 6, JUNE 2006 Automatic Temperature Controller for Multielement Array Hyperthermia Systems Jessi E. Johnson*, Member, IEEE, Paolo F. Maccarini, Member, IEEE, Daniel Neuman, and Paul R. Stauffer, Member, IEEE Abstract—This paper concerns the optimization and perfor- mance analysis of an automatic control algorithm for managing power output of large multielement array hyperthermia applica- tors. Simulation and corresponding measurement of controller performance in a solid tissue equivalent phantom model is utilized for analysis of controller response to dynamically varying thermal load conditions that simulate clinical treatments. The analysis leads to an optimum controller which demonstrates the ability to achieve a uniform and stable temperature profile over a large surface area regardless of surrounding thermal load. This paper presents several advancements to the performance of a previously published control routine, including: 1) simplified simulation tech- niques for thorough characterization of controller performance; 2) an optimization procedure leading to an improved hybrid control algorithm for maintaining optimal performance during periods of both “rising” and “steady-state” temperature; 3) per- formance analysis of a control algorithm tailored for large area hyperthermia treatments with a mulitelement array applicator. The optimized hybrid controller is applied to the conformal mi- crowave array (CMA) hyperthermia system previously developed for heating large area surface disease such as diffuse chestwall recurrence of breast carcinoma, and shown to produce stable, uniform temperatures under the multielement array applicator for all thermal load conditions. Index Terms—Control systems, hyperthermia, microwave an- tenna arrays, temperature control. I. INTRODUCTION T HE significant enhancement of therapeutic efficacy of cancer treatments combining hyperthermia (42 –45 for 60 min) with conventional radiation and/or chemotherapy has led to the development of numerous hyperthermia tech- niques utilizing radio frequency (RF), microwaves and ul- trasound [1], [2]. For superficial disease such as chestwall recurrence of breast cancer, which often spreads over a large surface area, uniform heating can be particularly difficult in terms of conforming to typical body contours and irregularly Manuscript received January 20, 2005; revised September 25, 2005. This work was supported by the National Institute of Health (NIH) under Grant 2 RO1 CA70761-06. Asterisk indicates corresponding author. *J. E. Johnson was with the Department of Radiation Oncology, University of California, San Francisco, CA 94143 USA. He is currently with the Department of Electrical and Electronics Engineering, Nagasaki University, Nagasaki 852- 8521 Japan (e-mail: johnsonj@ieee.org). P. F. Maccarini and P. R. Stauffer were with the Department of Radiation Oncology, University of California, San Francisco, CA 94143 USA. They are now with the Department of Radiation Oncology at Duke University Medical Center, Durham, NC 27710 USA (e-mail: paul.stauffer@duke.edu). D. Neuman is with the Department of Radiation Oncology, University of Cali- fornia at San Francisco Medical Center at Mount Zion, San Francisco, CA 94115 USA. Digital Object Identifier 10.1109/TBME.2006.873559 shaped disease. In recent years, a number of multiple-element hyperthermia applicators have been developed to meet the clinical challenges of applying hyperthermia over increas- ingly larger surface areas [3]–[12]. The largest of these is the conformal microwave array (CMA) applicator which features a flexible structure and up to 32 individually powered dual concentric conductor (DCC) antennas which allow the treat- ment of irregularly shaped disease over a wide variety of body contours. Although this system has been used successfully in the patient clinic for several years, thermal monitoring and control for uniform and stable temperature profiles across large treatment areas are major shortcomings that have limited wide- spread application of the approach [13], [14]. Thus, in spite of major improvements in system hardware that provide more adjustable heating patterns and improved real-time temperature monitoring, control of power levels to these large applicators remains a major challenge. Manual power adjustments are te- dious and too slow for large multiple-element applicators. Thus, an automatic control routine is necessary based on temperature feedback from tissue under each independently powered heat source, which is possible with recent high density temperature monitoring approaches [15]–[19]. In the current literature, there are many groups who have developed automatic controllers for various hyperthermia systems including: RF and ultra- sound [20]–[22]; scanned-focused and phased-array ultrasound [23]–[25]; microwave [26]; and high-temperature hyperthermia [27]. These systems utilize many control techniques ranging from basic proportional integral derivative type control [21] to complex, model predictive control [27]. Zhou et al. [26] present a control system based on a recur- sive algorithm developed from Pennes bioheat equation [28]. The controller demonstrated good accuracy for control of large, multielement hyperthermia applicators, as well as a fast calcu- lation time. Although this system demonstrated success, several aspects of the controller were not analyzed in detail, including: parametric variation in the control parameters; techniques for determining optimum parameter sets; and effects of system per- turbations on the controller performance. In this paper, the recursive control algorithm proposed by Zhou [26] is thoroughly analyzed and tested with simulations and measurements of controller performance in a tissue equiv- alent phantom with variable thermal loading. As a result of this extensive characterization, general trends in the controller re- sponse for variations in the control parameters are determined. Utilizing these trends, the controller performance is optimized for use with the CMA hyperthermia system which is represen- tative of other multielement array hyperthermia applicators. 0018-9294/$20.00 © 2006 IEEE