Investigation of the thermal and tissue injury behaviour in microwave thermal therapy using a porcine kidney model

Abstract

Minimally invasive microwave thermal therapies are being developed for the treatment of small renal cell carcinomas (RCC, d<3 cm). This study assessed the thermal history and corresponding tissue injury patterns resulting from microwave treatment of the porcine renal cortex. Three groups of kidneys were evaluated: (1) in vitro treated, (2) in vivo with 2-h post-treatment perfusion (acute) and (3) in vivo with 7-day post-treatment perfusion (chronic). The kidneys were treated with an interstitial water-cooled microwave probe (Urologix, Plymouth, MN) that created a lesion centered in the renal cortex (50 W for 10 min). The thermal histories were recorded at 0.5 cm radial intervals from the probe axis for correlation with the histologic cellular and vascular injury. The kidneys showed a reproducible 2 cm chronic lesion with distinct histologic injury zones identified. The thermal histories at the edge of these zones were found using Lagrangian interpolation. The threshold thermal histories for microvascular injury and stasis appeared to be lower than that for renal epithelial cell injury. The Arrhenius kinetic injury models were fit to the thermal histories and injury data to determine the kinetic parameters (i.e. activation energy and frequency factor) for the thermal injury processes. The resultant activation energies are consistent in magnitude with those for thermally induced protein denaturation. A 3-D finite element thermal model based on the Pennes bioheat equation was developed and solved using ANSYS (V7.0). The real geometry of the kidneys studied and temperature dependent thermal properties were used in this model. The specific absorption rate (SAR) of the microwave probe required for the thermal modelling was experimentally determined. The results from the thermal modelling suggest that the complicated change of local renal blood perfusion with temperature and time during microwave thermal therapy can be predicted, although a first order kinetic model may be insufficient to capture blood flow changes. The local blood perfusion was found to be a complicated function of temperature and time. A non-linear model based on the degree of vascular stasis was introduced to predict the blood perfusion. In conclusion, interstitial microwave thermal therapy in the normal porcine kidney results in predictable thermal and tissue injury behaviour. Future work in human kidney tissue will be necessary to confirm the clinical significance of these results.

Review of Temperature Dependence of Thermal Properties, Dielectric Properties, and Perfusion of Biological Tissues at Hyperthermic and Ablation Temperatures

“…Blood perfusion is temperature and time dependent and depends on the degree of microvascular stasis. The inequality between the normalized blood perfusion and the predicted degree of microvascular stasis found in this study suggests that a first order rate model may be insufficient to model renal blood perfusion response during microwave thermal therapy (123). Schutt et al .…”

“…compared the effect of different microvascular perfusion algorithms on ablation zone dimensions and found that the choice of microvascular perfusion algorithm and base line tissue perfusion have significant effects on final ablation zone dimensions in computer models of thermal ablation (17, 124). It is evident that the choice of modeling the temperature dependence of the electrical as well as thermal and perfusion properties have significant effects on the electrical field and temperature distribution in computer models, respectively (17, 18, 30, 64, 123). Thus, accurate models require the consideration of temperature-dependent properties, and among tissue properties in many cases perfusion and its temperature dependence is often dominating and of primary importance (30, 79, 111, 124).…”

Hyperthermia classic commentary: ‘Arrhenius relationships from the molecule and cell to the clinic’ by William Dewey,Int. J. Hyperthermia, 10:457–483, 1994

“…Some examples include lethal thermal dose thresholds of CEM 43 4210 min for 100% necrosis of breast tumors, 180-240 CEM 43 for prostate [23][24][25][26], 240 CEM 43 for liver during RF ablation [27], and 240 CEM 43 threshold for general soft tissue destruction and treatment of uterine fibroids with MR guided ultrasound [28,29]. The thermal isoeffect dose concept has also been extended to describe regions of thermal fixation produced during ablation, as demonstrated for kidney [30] with temperatures of $65 C; the thermal isoeffect dose that was dependent on the time of damage assay after treatment was associated with activation energies of 61-95 kcal mole…”

Trimodal Therapy: Combining Hyperthermia with Repurposed Bexarotene and Ultrasound for Treating Liver Cancer

“…In another study, researchers used MR-guided RF ablation of liver tumors and observed increase of temperature ranging from 60-100 °C very near the treatment device in the tumor region during ablation 52 . In microwave treatment of porcine renal cortex experiments, a temperature rise of more than 85 °C at the tip of the antenna was observed 53 . In this study, similar temperature rises ranging from 15 °C at 15mm to ~40°C at 5mm were observed in in vivo porcine liver, during ablation with Acoustxs™ applicators in the presence of PBNB.…”