Interstitial microwave-induced hyperthermia and iridium brachytherapy for the treatment of obstructing biliary carcinomas

Methods of monitoring thermal ablation of soft tissue tumors - A comprehensive review

Thermal ablation is a form of hyperthermia in which oncologic control can be achieved by briefly inducing elevated temperatures, typically in the range 50-80°C, within a target tissue. Ablation modalities include high intensity focused ultrasound, radiofrequency ablation, microwave ablation, and laser interstitial thermal therapy which are all capable of generating confined zones of tissue destruction, resulting in fewer complications than conventional cancer therapies. Oncologic control is contingent upon achieving predefined coagulation zones; therefore, intraoperative assessment of treatment progress is highly desirable. Consequently, there is a growing interest in the development of ablation monitoring modalities. The first section of this review presents the mechanism of action and common applications of the primary ablation modalities. The following section outlines the state-of-the-art in thermal dosimetry which includes interstitial thermal probes and radiologic imaging. Both the physical mechanism of measurement and clinical or pre-clinical performance are discussed for each ablation modality. Thermal dosimetry must be coupled with a thermal damage model as outlined in Section 4. These models estimate cell death based on temperature-time history and are inherently tissue specific. In the absence of a reliable thermal model, the utility of thermal monitoring is greatly reduced. The final section of this review paper covers technologies that have been developed to directly assess tissue conditions. These approaches include visualization of non-perfused tissue with contrast-enhanced imaging, assessment of tissue mechanical properties using ultrasound and magnetic resonance elastography, and finally interrogation of tissue optical properties with interstitial probes. In summary, monitoring thermal ablation is critical for consistent clinical success and many promising technologies are under development but an optimal solution has yet to achieve widespread adoption.

Interstitial microwave transition from hyperthermia to ablation: historical perspectives and current trends in thermal therapy

This work reviews the transition from hyperthermia to ablation for cancer treatment with interstitial microwave (MW) antennas. Early work utilising MW energy for thermal treatment of cancer tissue began in the late 1970s using single antennas applied interstitially or the use of multiple interstitial antennas driven with the same phase and equal power at 915 or 2450 MHz. The original antenna designs utilised monopole or dipole configurations. Early work in thermal therapy in the hyperthermia field eventually led to utilisation of these antennas and methods for MW ablation of tumours. Efforts to boost the radiated MW power levels while decreasing antenna shaft temperatures led to incorporation of internally cooled antennas for ablation. To address larger tumours, MW treatment utilised arrays that were simultaneously activated by either non-synchronous or synchronous phase operation, benefiting both hyperthermia and ablation strategies. Numerical modelling was used to provide treatment planning guidance for hyperthermia treatments and is expected to provide a similar benefit for ablation therapy. Although this is primarily a review paper, some new data are included. These new data show that three antennas with 2.5 cm spacing at 45 W/channel and 10 min resulted in a volume of 89.8 cm(3) when operated synchronously, but only 53.4 cm(3) non-synchronously. Efficiency was 1.1 (synchronous) versus 0.7 (non-synchronous). MW systems, treatment planning, and image guidance continue to evolve to provide better tools and options for clinicians and patients in order to provide better approach and targeting optimisation with the goal of improved treatment for the patient.

Evaluation of temperature distribution during hyperthermic treatment in biliary tumors: a computational approach

A computational approach is adopted to predict the temperature distribution in the biliary tissue during hyperthermic treatments in biliary tumors. Two different models are developed: an axisymmetric model and a three-dimensional model. In the first model the Pennes bioheat transfer equation is applied. It is aimed at simulating the thermoregulatory effect of the capillary bed and it can also give a pressure criterion to determine whether the blood perfusion term should be included in the mathematical model. The second model is aimed at simulating the convective effect of the large hepatic vessels: A constant Nusselt number is assumed on the sides of the vessels. The simulations of the three-dimensional model have been carried out with and without capillary perfusion in the tissue, i.e., respectively in the worst case and in the best case that may occur during heating. The results show that it is possible to obtain therapeutic temperature values in the tissue for time intervals considered acceptable by physicians. Moreover, the model is able to give more precise information about the volumes of tumoral tissue heated above therapeutic temperatures with the hyperthermic technique considered.

The influence of vasculature on temperature distributions in MECS interstitial hyperthermia: importance of longitudinal control

The quality of temperature distributions that can be generated with the Multi Electrode Current Source (MECS) interstitial hyperthermia (IHT) system, which allows 3D control of the temperature distribution, has been investigated. For the investigations, computer models of idealised anatomies containing discrete vessels, were used. A 7-catheter hexagonal implant geometry with a nearest neighbour distance of 15 mm was used. In each interstitial catheter with a diameter of 2.1 mm a number of 1 up to 4 electrodes were placed along an 'active section' with a length of 50 mm. The electrode segments had lengths of 50, 20, 12 and 9 mm respectively. Both single vessel and vessel network situations were analysed. This study shows that even in situations with discrete vasculature and perfusion heterogeneity it remains possible to obtain satisfactory temperature distributions with the MECS IHT system. Due to its 3D spatial control the temperature homogeneity in the implant can be made quite satisfactory.

Brachytherapy and pancreatic cancer

Advances in diagnostic and therapeutic technology have not appreciably changed the outlook of patients with pancreatic cancer. While those patients presenting with localized resectable disease have the best prognosis, local control and intra-abdominal metastases remain significant obstacles to survival. Localized chemoradiation has modestly improved median survival in localized and locally advanced disease. Patients presenting with locally advanced disease at diagnosis benefit from surgical palliation which includes biliary and gastric bypass. Intraoperative interstitial brachytherapy has been effective when utilized at laparotomy to improve local control in locally advanced disease. Advances in laparoscopic techniques have provided the ability to more accurately stage patients prior to laparotomy and perform palliative procedures without the need for laparotomy. The utilization of high-dose-rate brachytherapy has proven effective in palliating obstructive symptoms with minimal morbidity on an outpatient basis. Recent efforts have focused on preoperative chemoradiation to improve resectability in selected patients and prophylactic hepatic irradiation to reduce metastases for patients with locally advanced disease.

Interstitial radiation and hyperthermia in the Dunning R3327 prostate tumour model: therapeutic efficacy depends on radiation dose-rate, sequence and frequency of heating

To determine the most effective means by which to apply the combined treatments of local tumour hyperthermia (LTH) with interstitial low dose-rate irradiation (IRT) we examined the significance of such factors as dose-rate of radiation, and the sequence and frequency of hyperthermia applications in the anaplastic Dunning prostate tumour subline R3327-AT1. IRT was carried out by the insertion of a single Ir-192 seed into the center of the tumour. For LTH treatments, the tumour-bearing leg was positioned in a circulating constant temperature water bath (43.5 +/- 0.1 degrees C) for 35 min. Neither LTH treatment alone nor the insertion of a dummy seed produced any change in tumour growth compared with sham-treated controls. With regard to the sequence of heating and IRT our results showed that during a 72-h treatment time (30 Gy, 40 cGy/h) a single heat treatment given just before the start of IRT was more efficient (TER = 1.47) in terms of growth delay than LTH given in the middle or the end of radiation treatment (TER approximately 1.0). The growth delay for both the 20 and 40 cGy/h groups appear to be linear with increasing dose for the IRT as well as the IRT + LTH groups. The higher dose-rate was more effective especially with respect to long-term delay in tumour growth in some of the animals. As TER at 40 cGy/h decreased subsequently with increasing treatment time from 1.47 to 1.25 at 60 Gy, we conclude that for treatment times > 72 h, one LTH just before IRT might not be sufficient. If multiple heat treatments are applied during a comparable time course, two LTH treatments one just before the start, the other at the end yielded the greatest local tumour control. In contrast, three LTH given daily were not more effective than the one LTH given just before the start of IRT. These data indicate a clear thermal enhancement of low dose-rate irradiation, with maximal sensitization when hyperthermia was given just before IRT. For multiple heatings a better understanding of the underlying mechanisms of sequencing and timing hopefully provides guidelines how to apply optimally both modalities in the treatment of cancer.