Quality assurance guidelines for superficial hyperthermia clinical trials : II. Technical requirements for heating devices

Abstract

Quality assurance (QA) guidelines are essential to provide uniform execution of clinical trials with uniform quality hyperthermia treatments. This document outlines the requirements for appropriate QA of all current superficial heating equipment including electromagnetic (radiative and capacitive), ultrasound, and infrared heating techniques. Detailed instructions are provided how to characterize and document the performance of these hyperthermia applicators in order to apply reproducible hyperthermia treatments of uniform high quality. Earlier documents used specific absorption rate (SAR) to define and characterize applicator performance. In these QA guidelines, temperature rise is the leading parameter for characterization of applicator performance. The intention of this approach is that characterization can be achieved with affordable equipment and easy-to-implement procedures. These characteristics are essential to establish for each individual applicator the specific maximum size and depth of tumors that can be heated adequately. The guidelines in this document are supplemented with a second set of guidelines focusing on the clinical application. Both sets of guidelines were developed by the European Society for Hyperthermic Oncology (ESHO) Technical Committee with participation of senior Society of Thermal Medicine (STM) members and members of the Atzelsberg Circle.

Keywords: Applicator; Heating criteria; Hyperthermia, superficial; Phantoms; Quality assurance; Water bolus.

Fig. 1

Example of the thermal effective field size (TEFS). Calculated normalized temperature rise (TR) distribution at 1 cm depth in a two-layered phantom, with fat layer thickness of 10 mm overlying the muscle phantom. Heating time t = 6 min, P = 175 W. The black solid line indicates the applicator aperture, while the black dashed line represents the water bolus. The maximum TR in the 1 cm deep plane in muscle-tissue equivalent phantom is T_max1cm = 7.6 °C. The TEFS isotherm then quantifies the area with TR ≥ 3.8 °C

Fig. 2

Example of the thermal effective penetration depth (TEPD). Simulated temperature increase (blue – left axis) and specific absorption rate (SAR; red – right axis) as a function of depth in the center of two-layered phantom, with fat layer (blue) thickness of 10 mm. Heating time t = 6 min with P = 175 W and bolus surface temperature identical to the initial phantom temperature. The maximum temperature rise (TR) in the 1 cm deep plane in muscle-tissue equivalent phantom is T_max1cm = 7.6 °C. The TEPD is thus the depth where the TR is 3.8 °C, i. e., 39 mm from the tissue surface. The maximum SAR at 1 cm depth in the muscle is approximately 75%. The resulting effective penetration depth (EPD) derived according to the traditional definition [7] is indicated by the arrow. Observe that the effective heat penetration is essentially at nearly the same depth for both definitions

Fig. 3

Calculated normalized specific absorption rate (SAR) distribution of a 10 × 10 cm lucite cone applicator (LCA) at 1 cm depth in a homogeneous muscle tissue phantom. The color scale from blue to dark red represents a 10% SAR increase for every color transition. Adapted from [66]

Fig. 4

Thermal effective field size (TEFS) profiles should be measured in three orthogonal planes crossing the center of the applicator

Fig. 5

Illustration of three alternative options to obtain thermal profile with depth: a thermal camera view of vertical plane containing peak temperature rise (TR); b reconstruction of vertical distribution from thermal camera views of multiple horizontal planes; and c multiple measurements along a single axis depth probe