Tumor Selective Hyperthermia Induced by Short-Wave Capacitively-Coupled RF Electric-Fields

<div><p>There is a renewed interest in developing high-intensity short wave capacitively-coupled radiofrequency (RF) electric-fields for nanoparticle-mediated tumor-targeted hyperthermia. However, the direct thermal effects of such high-intensity electric-fields (13.56 MHZ, 600 W) on normal and tumor tissues are not completely understood. In this study, we investigate the heating behavior and dielectric properties of normal mouse tissues and orthotopically-implanted human hepatocellular and pancreatic carcinoma xenografts. We note tumor-selective hyperthermia (relative to normal mouse tissues) in implanted xenografts that can be explained on the basis of differential dielectric properties. Furthermore, we demonstrate that repeated RF exposure of tumor-bearing mice can result in significant anti-tumor effects compared to control groups without detectable harm to normal mouse tissues.</p></div

Thermal dose quantification in Hep3B and MDA PATC-3 xenografts under RF field exposure (13.56 MHz, 600 W).

<p><i>Panel A.</i> Hep3B xenograft and normal mouse liver temperatures were measured in real-time using fiber optic thermography while abdominal surface/skin temperatures were measured using infrared thermography. RF exposure was started at a tumor temperature of 35°C. <i>(n = 6). Panel B.</i> MDA PATC-3 xenograft and intra-peritoneal temperatures were measured in real-time using fiber optic thermography while abdominal surface/skin temperatures were measured using infrared thermography. RF exposure was started at a tumor temperature of 37°C. <i>(n = 9). (Solid line represents mean, dashed line represents standard deviation, and vertical dotted line represents frequency of 13.56 MHz).</i></p

Dielectric properties of normal mouse tissues.

<p><i>Panel A, B. Ex-vivo</i> dielectric spectroscopy of normal mouse tissues was performed. Permittivity (ε’) and imaginary permittivity (ε”) are shown for each tissue. <i>(n = 3–15, Solid line represents mean, dashed line represents standard deviation, and vertical dotted line represents frequency of 13.56 MHz)). Panel C.</i> Variation of tumor dielectric properties with respect to tumor mass. <i>The</i> permittivity (ε’) and imaginary permittivity (ε”) values of MDA PATC-3 xenografts with variable mass are shown at 13.56 MHz. <i>(n = 3–6, data points represent mean, and error bars represent standard deviation). Panel D.</i> Areas of spontaneous necrosis were seen in untreated orthotopic xenografts, which were not seen in adjacent normal liver or pancreas. A representative figure of a HepG2 xenograft is shown at the tumor margin. Similar observations were noted for Hep3B, Panc1 and MDA PATC-3 xenografts <i>(not shown)</i>.</p

Radiofrequency generator and fiber optic probe placement.

<p><i>Panel A.</i> For fiberoptic thermography, a temperature-sensing probe is placed through a 20 G needle. The needle is advanced into the tumor (T) under ultrasound guidance. The probe is then advanced through the needle and the needle is withdrawn. <i>Panel B.</i> Kanzius 13.56 MHz external RF generator system is shown (black box) that is connected to an end-firing antenna in the transmission head (Tx). A spacing of 3.5 inches exists between the Tx head and the receiver head (Rx)/ground plate. <i>Panel C.</i> A CB17 SCID mouse is placed supine on the ground plate of the Rx head. A copper shield made from copper tape is used to ground all mice and prevent electrothermal injury. An abdominal window is created in the middle of the copper shield to allow RF field exposure to the tumor-bearing area.</p

E-field measurements.

<p>Electric field measurements were performed using the setup described in methods. The measurements were performed in air or in the peritoneal cavity of mice while keeping the E-field probe at the same <i>x,y,z</i> location, which approximately corresponded with the location of orthotopic tumors in tumored mice. <i>(n = 3, data points represent mean, and error bars represent standard deviation).</i></p

High-temperature vessel degradation.

<p>(A)–(D) Impact of RF exposure on vessel architecture at four different time-points: 0:22, 6:53, 16:18, and 20:31 minutes, respectively. The tumor temperatures and RF power at those time points are shown in the upper-middle and upper-right hand side sections, respectively. Figure (E) illustrates the change in temperature and power with respect to time. Vessel degradation can be seen for temperatures > 41°C. A complete breakdown of the vessel architecture can be seen for temperatures > 47°C.</p

Portable RF system retrofitted to the IVM.

<p>(A) The RF system integrated into the intravital microscope (IVM) for real-time imaging under RF exposure. (B) Mouse manipulation for imaging–an incision is made to expose and gently manipulate the 4T1 tumor for IVM imaging. (C) 4T1 tumor under IVM illumination with a x4 objective lens.</p

Modulation of tumor temperature using RF exposure.

<p>(A) Thermal fiber optic probe placement. Probes #1–3 are positioned (i) under the skin but above the tumor; (ii) under the skin in between the tumor and the main body; and (iii) under the skin next to the intraperitoneal cavity. (B) Extracted thermal probe data. The recorded temperature of the probes was modulated by turning on and off the RF system (+RF and–RF). The system was turned off once the tumor temperature (probe #1) reached 45°C, 43°C, and 41°C, respectively, and was turned on when all probes had values in the range ~29–31°C. (C) The IR camera simultaneously measured the surface temperature of the points where the thermal probes were located.</p

Thermal probe and IR camera calibration.

<p>(A) Three thermal probes were places in a quartz cuvette filled with phosphate buffered saline (PBS) and exposed to 200 W of RF. The IR camera captured the surface temperature of cursor points located next to the thermal probes for the RF exposure time 0 s—380 s (B and C, respectively). (D) Comparison of thermal probe and IR camera heating data.</p