Implications of Clinical Rf Hyperthermia on Protection Limits in the Rf Range

Abstract

The systemic temperature is meticulously regulated to 37-37.5 degrees C. Organ systems (skin, digestive system, muscles) have a considerable potential to regulate the perfusion for thermal regulation, physical activity, or digestion. While the regulation of the systemic temperature (37.5 degrees C) is quite strict, the tolerance and regulation potential with respect to local heat is more variable. Laboratory studies provided the relationship between thermal doses and cytotoxic effects. Tissue damage for short-term expositions (in the range of minutes) is only possible for temperatures above 50 degrees C. Radiofrequency radiation is utilized in cancer therapy, inducing local tissue temperatures in the range of 40-45 degrees C for 30-60 min. During local hyperthermia (with heated volumes <1 L) specific absorption rates (SARs) of 100-200 W kg, reactive perfusions of 20-40 mL/100 g/min, and tumor temperatures of 42-43 degrees C are achieved. Normally no side effects or damage in the normal tissue, such as muscle or skin, have been seen. During regional hyperthermia, SARs of 30-40 W kg are found in heated volumes of 10 L with temperatures of 41-42 degrees C in tumor-related measurement points. Then the reactive average perfusion is 6-9 mL/100 g/min (mean value 8 mL/100 g/min). Local temperatures even for higher SAR are regulated to values of not more than 40-42 degrees C. For these temperatures no damages in normal tissues have been found after regional hyperthermia in hundreds of patients. We conclude that the thermoregulatory potential for the whole body or large body regions is limited by the cardiac output, which can at least double the output from 5 to 10 L min. Even higher is the potential to compensate in smaller volumes. Here the perfusion in muscle can be increased from the basal value of 2-4 mL/100 g/min more than 5-10-fold.

Monitoring of whole-body hyperthermia with transesophageal echocardiography (TEE)

“…WBH represents the most invasive hyperthermia application, requires a great deal of expenditure and bears the risk of severe toxicity when chemotherapeutic agents are applied [3]. In addition to thermal injuries, cardiovascular stress, rise in cardiac index and decrease of systemic vascular resistance are major problems in WBH [7,10]. Whole-body hyperthermia is associated with profound cardio-vascular alterations [10], such as tachycardia and a decrease of mean arterial blood pressure [6].…”

“…In addition to thermal injuries, cardiovascular stress, rise in cardiac index and decrease of systemic vascular resistance are major problems in WBH [7,10]. Whole-body hyperthermia is associated with profound cardio-vascular alterations [10], such as tachycardia and a decrease of mean arterial blood pressure [6]. Furthermore, evaluation of hemodynamic changes during WBH treatment using TEE showed characteristic signs of a hyperdynamic circulation.…”

Comparison of complex permittivities of isotonic colloids containing single‐wall carbon nanotubes of varying chirality

“…These concentrations of SWCNTs far exceeded those used in the earlier RF hyperthermia studies [Gannon et al, 2007]. Ideally, hyperthermia using external RF fields requires local conductivities 25-50 times greater than normal tissue to achieve SARs of 100-200 W/kg at the target while sparing healthy tissue [Wust et al, 2007]. Although these SARs may be achievable at the nanoscopic scale using conductive nanoparticles, it is unclear if they can be achieved at the macroscopic scale through increases in the bulk conductivity of the colloid.…”

In vivo temperature measurement in the subacromial bursa during arthroscopic subacromial decompression

“…3,7,8 Chondrolysis has also been reported but would appear to relate to higher temperatures in the region of 55 C. 4 Several cadaveric studies in the laboratory have shown that prolonged continuous use of the RF ablation wand without constant fluid flow can result in dangerously high temperatures of greater than 80 Celsius ( C). Much lower temperatures of less than 40 C were witnessed with intermittent use of the RF ablation wand and continuous flow of fluid.…”