Changes in muscle temperature induced by 434 MHz microwave hyperthermia

Objective

To investigate the changes in temperature of human muscle during microwave hyperthermia.

Methods

Skin surface and muscle temperatures were measured in 11 healthy adult men (mean (SD) age 24.3 (2.2) years; height 174.2 (6.1) cm; weight 70.0 (5.3) kg) during a 30 min exposure of the thigh to 434 MHz microwave hyperthermia. Skin temperature was maintained at the pilot temperature of 40°C, and the temperature of the water in the bolus was 38°C. The peak power output was set at 60 W and controlled automatically to maintain the pilot temperature. The temperature was measured in the vastus lateralis muscle at an average muscle depth of 2.0 (0.2) cm, using a 23 G Teflon‐shielded thermocouple. Biopsy specimens were obtained for light microscopy from three subjects. A muscle‐equivalent phantom was used to evaluate the vertical heating pattern.

Results

Both skin and muscle temperatures increased from baseline, and muscle temperature was higher than skin temperature (skin temperature 39.2 (0.5)°C, temperature rise 5.0 (1.5)°C; muscle temperature 43.7 (0.8)°C, temperature rise 8.9 (1.4)°C). At the end of the hyperthermia treatment, muscle temperature decreased to 39.8 (0.9)°C, but was still 4.8 (1.5)°C higher than the baseline. No signs of muscle damage were observed on the basis of the blood creatine kinase activity and histological sections.

Conclusions

The results show that the 434 MHz microwave hyperthermia treatment increased and maintained muscle temperature locally by 6.3–11.4°C without muscle damage. These findings suggest that the microwave hyperthermia system provides effective and safe treatment.For many years, heat treatment has been used as a therapy for muscle tissue injured in sports. There are several heating modalities, including silicon gel hot packs, whirlpools, paraffin baths, ultrasound and electromagnetic waves.¹ Most of these modalities are classified as superficial heating, and primarily cause a temperature increase in structures up to 1 cm below the skin surface. Ultrasound and electromagnetic waves (eg, radiofrequency and microwaves) are used for deep heating at tissue depths of 3–5 cm.²To gain therapeutic benefit, the target tissue depth and temperature should be considered when choosing the appropriate thermal modality. When deep skeletal muscle (over a small region) is targeted for heat treatment, both ultrasound and microwaves are used clinically because both modalities are thought to effectively heat the target tissue. Garrett et al³ evaluated the changes in gastrocnemius muscle temperature induced by short‐wave diathermy, and found that 20 min of microwave exposure induced an increase of 4.6°C in muscle temperature. Another study reported that ultrasound produced a mean temperature rise of 3.5°C in the gastrocnemius–soleus complex.⁴ A few comparative studies have examined the use of heat modalities on living tissue, such as muscle, and reported that electromagnetic waves are more effective than ultrasound.^3,5 In general, the therapeutic range for sports medicine is assumed to be from 41°C to 45°C.⁶ However, the increase in temperature may depend on the thermal modality.Recently, 434 MHz hyperthermia has been introduced in physical medicine and rehabilitation. Hyperthermia treatment with this equipment was reported to be a highly innovative and reliable modality for treating acute muscle injuries in sports.^5,7,8 These reports mentioned that 434 MHz hyperthermia treatment significantly reduces the pressure pain and pain on active contraction, but they did not determine muscle temperature during treatment. To our knowledge, only one report⁹ has measured the changes in temperature, and it used a phantom. The therapeutic benefits of hyperthermia treatment are related to the target‐tissue temperature and changes in temperature; so the change in temperature with treatment should be measured to assess the therapeutic benefits. Measuring the temperature can also provide evidence for the safety of the treatment. Nevertheless, the changes in human muscle temperature have not been reported because it is difficult to measure muscle temperature in a clinical setting. Therefore, we investigated the changes in the temperature of human muscle induced by a 434 MHz microwave hyperthermia system to provide evidence for its clinical effectiveness and safety.