Thermal characteristics of thermobrachytherapy surface applicators for treating chest wall recurrence

Abstract

Purpose To study temperature and thermal dose distributions of ThermoBrachytherapy Surface Applicators (TBSA) developed for concurrent or sequential high dose rate (HDR) brachytherapy and microwave hyperthermia treatment of chest wall recurrence and other superficial disease. Methods A steady state thermodynamics model coupled with the fluid dynamics of water bolus and electromagnetic radiation of hyperthermia applicator is used to characterize the temperature distributions achievable with TBSA applicators in an elliptical phantom model of the human torso. Power deposited by 915 MHz conformal microwave array (CMA) applicators is used to assess the specific absorption rate (SAR) distributions of rectangular (500 cm2) and L-shaped (875 cm2) TBSA. The SAR distribution in tissue and fluid flow distribution inside the Dual-Input Dual-Output (DIDO) water bolus are coupled to solve the steady state temperature and thermal dose distributions of rectangular TBSA (R-TBSA) for superficial tumor targets extending 10–15 mm beneath the skin surface. Thermal simulations are carried out for a range of bolus inlet temperature (Tb=38–43°C), water flow rate (Qb=2–4 L/min) and tumor blood perfusion (ωb=2–5 kg/m3/s) to characterize their influence on thermal dosimetry. Results Steady state SAR patterns of R- and L-TBSA demonstrate the ability to produce conformal and localized power deposition inside tumor target sparing surrounding normal tissues and nearby critical organs. Acceptably low variation in tissue surface cooling and surface temperature homogeneity was observed for the new DIDO bolus at 2 L/min water flow rate. Temperature depth profiles and thermal dose volume histograms indicate bolus inlet temperature (Tb) to be the most influential factor on thermal dosimetry. A 42 °C water bolus was observed to be the optimal choice for superficial tumors extending 10–15 mm from the surface even under significant blood perfusion. Lower bolus temperature may be chosen to reduce thermal enhancement ratio (TER) in the most sensitive skin where maximum radiation dose is delivered and to extend thermal enhancement of radiation dose deeper. Conclusion This computational study indicates that well-localized elevation of tumor target temperature to 40–44 °C can be accomplished by large surface-conforming TBSA applicators using appropriate selection of coupling bolus temperature.

Conformal microwave array (CMA) applicators for hyperthermia of diffuse chest wall recurrence

“…A waterbolus temperature of 42–42.5°C was used as previous experience with both planar and conformal microwave array applicators demonstrated most balanced heating from skin surface to depths of 1–1.5 cm at this bolus temperature. This observation has subsequently been confirmed with comprehensive thermal modeling (35). Although antenna power levels were adjusted based primarily on thermally mapped surface sensors for all patients, some of the treatments had an interstitial catheter to provide 10–15 temperatures at depth in tumor for correlation of surface and deep temperatures.…”

“…Several multiple element array applicators have been reported in the literature, but have not been characterized for heating properties over contoured anatomy. For this effort, electromagnetic (EM) and thermal simulations were performed using commercial software and 3D simulation approaches that have been described previously for EM (63, 64, 66) and thermal (34, 35, 68, 73) modeling of planar applicators. EM simulations were performed with HFSS (Ansoft Corp, Pittsburg PA) for a 42×27cm L-shape CMA to assess the ability to deliver localized heating to diffuse chestwall disease that extends from the sternum over a mastectomy scar and around the side to the back.…”

“…As applicators increase in size to treat larger areas, the need for flexibility and improved patient interface to handle multiple power connections, surface cooling, and temperature monitoring for treatment control are intensified. The largest surface heating applicators to date include the 25 aperture 915 MHz spiral microstrip array developed at Stanford University (20) and its commercial implementation as a 24 antenna microstrip spiral array (SA-24, BSD Medical Corp., Salt Lake City UT), the 434MHz flexible PCB microstrip-based Contact Flexible Microstrip Applicator (CFMA) (21–23) and the emerging dual concentric conductor (DCC) based Conformal Microwave Array (CMA) applicator (24–31) which has subsequently been expanded for use with simultaneously administered radiation from either external beam electrons or scanning HDR brachytherapy source (32–35). This later applicator is the subject of this article.…”

Using a conformal water bolus to adjust heating patterns of microwave waveguide applicators

“…When adjusted appropriately in combination with microwave power level, the bolus temperature will shift the depth of maximum temperature from the surface to 6 mm or more for practical bolus temperatures, as reported previously. 31 This study documents the effects of varying several other parameters that are under the control of treatment personnel: i) coupling medium (e.g., air coupled, direct contact, or waterbolus coupled); ii) waterbolus lateral size, thickness, and uniformity of thickness; and iii) rotation of the impedance matching tuning blades.…”

Overview of bladder heating technology: matching capabilities with clinical requirements

“…Previous studies of temperature gradients in perfused tissue adjacent to a hot (or cold) surface show that the effective heating potential of a 7°C thermal gradient extend no more than 2–3 mm deep. (56, 57). For NMIBC cancers that extend <2 mm into the bladder wall, this thermal gradient induced heating should be sufficient for effective treatment.…”