Subharmonic-Aided Pressure Estimation for Monitoring Interstitial Fluid Pressure in Tumors: Calibration and Treatment with Paclitaxel in Breast Cancer Xenografts

Abstract

Interstitial fluid pressure (IFP) in rats with breast cancer xenografts was non-invasively estimated using subharmonic-aided pressure estimation (SHAPE) versus an invasive pressure monitor. Moreover, monitoring of IFP changes after chemotherapy was assessed. Eighty-nine rats (calibration n = 25, treatment n = 64) were injected with 5 × 10⁶ breast cancer cells (MDA-MB-231). Radiofrequency signals were acquired (39 rats successfully imaged) with a Sonix RP scanner (BK Ultrasound, Richmond, BC, Canada) using a linear array (L9-4, transmit/receive: 8/4 MHz) after administration of Definity (Lantheus Medical Imaging, North Billerica, MA, USA; 180 μL/kg) and compared with readings from an invasive pressure monitor (Stryker, Berkshire, UK). An inverse linear relationship was established between tumor IFP and SHAPE (y = -1.06x + 28.27, r = -0.69, p = 0.01) in the calibration group. Use of this relationship in the treatment group resulted in r = 0.74 (p < 0.05) between measured (pressure monitor) and SHAPE-estimated IFP (average error: 6.24 mmHg). No significant before/after differences were observed with respect to paclitaxel treatment (5 mg/kg, Mayne Pharma, Paramus, NJ, USA) with either method (p ≥ 0.15).

Keywords: Breast cancer xenografts; Contrast agents; Interstitial fluid pressure; Microbubbles; Subharmonic-aided pressure estimation; Ultrasound.

Figure 1.

An example of the acoustic output curve generated by the modified solution. The green arrow points to the data point selected for scanning. Note the S shape of the curve corresponding to theory.

Figure 2.

Block diagram of the off-line processing performed on the RF data.

Figure 3.

Breast tumor xenograft (arrows) depicted in maximum projection intensity (MIP) SHI mode (a) and the same tumor using a different color scheme for clarity (b). The ROI selected from image (b) is then shown for 100% (c), 115% (d) and 130% (e) thresholds.

Figure 4.

Subharmonic amplitude results compared to the pressure monitor at a 115% threshold. In this case data points with a IFP standard deviation larger than 5 mmHg have been eliminated. Note that for a clearer comparison relative values for subharmonic amplitude (0 dB corresponding to the lowest dB value and then subharmonic amplitude difference relative to that value) are reported.

Figure 5.

Box plot of the subharmonic data per threshold level. All groups are significantly different from each other. Red line indicates the median, blue box indicates first and third quartile.

Figure 6.

Comparison for the 115% threshold of calculated IFP from calibration equations where values below −10 mmHg have been removed and SHAPE data from the treatment phase of the study and the corresponding IFP measured during data acquisition.

Figure 7.

Subharmonic amplitude results compared to the pressure monitor at a 100% threshold. Note that for a clearer comparison relative values for subharmonic amplitude (0 dB corresponding to the lowest dB value and then subharmonic amplitude difference relative to that value) are reported.

Figure 8.

Box plot of the subharmonic data per threshold. All groups are significantly different from each other. Red line indicates the median, blue box indicates first and third quartile. Outliers are beyond +/−2.7σ.

Figure 9.

The difference in IFP from pre to post treatment for each rat in both the control and treatment groups