Electrical Resistivity of K12/TRb Tumors vs. Normal Liver Tissue Dieter Haemmerich S. Tyler Staelin Supan Tungjitkusolmun Jang-Zern Tsai Omer R. Ozkan David Mahvi John G. Webster This research has been supported by NIH grant #1R01 HL56143-01

Motivation Liver is second most common site of metastases next to lymph nodes Usage of resistivity data: Detect tumors via bio-impedance measurements Radio Frequency ablation as treatment: Determine distribution of absorbed electromagnetic energy No known studies on in-vivo tumor resistivity at audio and radiofrequencies We tried to model liver metastases in animal (rat) Explain RF ablation: catheter is inserted into tissue; current flows from catheter to groundpad (placed on back) -> surrounding tissue is heated. Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Experimental Details K12/TRb is a colon cancer line with histologic characteristics similar to human colon cancer Liver tumors were initiated in 40 rats by injecting 106 cells intrahepatically (n=20) or 8·106 cells intraspleenically (n=20) Rats were opened 6 – 9 weeks after injections 24 tumors in 18 rats were measured Cell line used to model liver metastases Intraspleenic injections -> metastases developed in liver Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Tumor resistivity was measured over 4 weeks in 1-week intervals Tumors were 1cm – 2 cm in diameter Normal liver tissue was measured for control Animals were sacrificed after measurement Tumor volume was estimated Histologic slides were created of each tumor % Necrosis and % Fibrosis were estimated at measurement site Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Current-to-voltage converter Experiment Setup Tissue Function RS232 generator Amplifier Current-to-voltage converter Channel 1 RS232 Oscilloscope Channel 2 Computer Probe 4-terminal method minimizes errors from electrode polarization Probe has silver electrodes, 0.38 mm diameter, 1.5 mm apart, 5 mm long, covered with AgCl We measured at 8 frequencies from 1 Hz to 1 MHz Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Resistivity: Tumor vs. Normal Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Current paths in tissue Current paths at DC and low frequencies Current paths at high frequencies It is generally agreed upon… Applies to most types of tissue Why different in tumor? I’ll come back to that later. Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Approximation Equation Resistivity curves approximated by: A ... Resistivity at low frequencies Q = A / (1+eB)... Resistivity at high frequencies C ... Controls rate of decrease Parameters A, Q, C were fit for each tumor and control Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Sample Tumor Resistivity Data and Approximation A = 397, Q = 211, C = -1.01 Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Spearman`s correlation coefficients were determined between each of the parameters (A, Q, C) and: Tumor Volume Tumor Age % Necrosis at measurement site % Fibrosis at measurement site How did we know where measurement site is? Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Probe tracts -> measurement site can be determined Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Typical Tumor Compared to normal liver cells: Cells are very unordered, very diverse, not well differentiated, multiple nuclei Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Tumor-Liver Interface Ordered vs. unordered Liver parenchyma compressed at border Very few blood vessels inside tumor Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Necrosis and Fibrosis Tumor gets blood support from tumor-liver-boundary As tumor grows, inner regions get depleted of nutrients and waste products accumulate This leads to necrosis (cell death), preferably at the tumor center After cells die, (tumor) tissue gets replaced with fibroblastic tissue Blood brings nutrients to the cells and carries metabolic waste products away from cells Fibrosis depends on regenerative capabilities of cell type Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Tumor with Necrosis & Fibrosis Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Central Necrosis Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Significance Probabilities (P-values) Tumor Parameters Age Volume Fibrosis Necrosis A 0.2085 0.0161 0.3527 0.0031  0.900 0.1815 0.9198 0.0018 C 0.5355 0.3416 0.2748 0.0116 Significance probabilities calculated from Spearman’s Correlation Coefficients P-values are underlined where significance is reached Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Parameter A vs. Necrosis Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Parameter Q vs. Necrosis Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Conclusions Results strongly support theory that necrosis is responsible for drop in resistivity: When cells are intact, current is restricted to extracellular room at low frequencies During cell necrosis cell membrane loses integrity. Resistivity drops. Difference in resistivity at frequencies below 100 kHz can be exploited to increase efficacy of electromagnetic tumor heating (RF ablation) Currently RF ablation uses frequ. Around 500 kHz Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Model Geometry Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Temp. distribution Current density Parameter Case 1 Case 2 Case 3 Liver resistivity 102.7 cm 308.0 cm 924.0 cm Tumor resistivity Total energy 574 J 974 J 1732 J Red: 50C-boundary Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu

Acknowledgements Web Site Glen Leverson Thomas F. Warner Web Site Acknowledge two people who had significant contribution to study http://rf-ablation.engr.wisc.edu Dieter Haemmerich, Univ. Wisconsin-Madison email: haemmeri@cae.wisc.edu