Development of a Non-invasive Assessment of Hypoxia and Neovascularization with Magnetic Resonance Imaging in Benign and Malignant Breast Tumors: Initial Results

Abstract

Purpose: To develop a novel magnetic resonance imaging (MRI) approach for the noninvasive assessment of hypoxia and neovascularization in breast tumors.

Procedures: In this IRB-approved prospective study, 20 patients with suspicious breast lesions (BI-RADS 4/5) underwent multiparametric breast MRI including quantitative BOLD (qBOLD) and vascular architecture mapping (VAM). Custom-made in-house MatLab software was used for qBOLD and VAM data postprocessing and calculation of quantitative MRI biomarker maps of oxygen extraction fraction (OEF), metabolic rate of oxygen (MRO₂), and mitochondrial oxygen tension (mitoPO₂) to measure tissue hypoxia and neovascularization including vascular architecture including microvessel radius (VSI), density (MVD), and type (MTI). Histopathology was used as standard of reference. Appropriate statistics were performed to assess and compare correlations between MRI biomarkers for hypoxia and neovascularization.

Results: qBOLD and VAM data with good quality were obtained from all patients with 13 invasive ductal carcinoma (IDC) and 7 benign breast tumors with a lesion diameter of at least 10 mm in all spatial directions. MRI biomarker maps of oxygen metabolism and neovascularization demonstrated intratumoral spatial heterogeneity with a broad range of biomarker values. Bulk tumor neovasculature consisted of draining venous microvasculature with slow flowing blood. High OEF and low mitoPO₂ were associated with low MVD and vice versa. The heterogeneous pattern of MRO₂ values showed spatial congruence with VSI. IDCs showed significantly higher MRO₂ (P = 0.007), lower mitoPO₂ (P = 0.021), higher MVD (P = 0.005), and lower (i.e., more pathologic) MTI (P = 0.001) compared with benign breast tumors. These results indicate that IDCs consume more oxygen and are more hypoxic and neovascularized than benign tumors.

Conclusions: We developed a novel MRI approach for the noninvasive assessment of hypoxia and neovascularization in benign and malignant breast tumors that can be easily integrated in a diagnostic MRI protocol and provides insight into intratumoral heterogeneity.

Keywords: Breast cancer; Imaging biomarkers; MRI; Neovascularization; Oxygen metabolism; Tumor hypoxia.

Fig. 1.

MRI data processing pipeline for combined non-invasive in-situ characterization of neovascularization, oxygen metabolism, and hypoxia, respectively, of human breast tumors. Firstly, an MRI data acquisition protocol was used which was adapted from neuroimaging research to be compatible with clinical routine breast imaging purposes. Secondly, data processing of MRI raw data included calculation of relativity maps (R2 and R2*), apparent diffusion coefficient (ADC), blood flow and volume (BF and BV), and vascular hysteresis loops (VHL), respectively. Finally, we used the equations shown gray box for calculation of MRI biomarker maps of oxygen metabolism and hypoxia using the qBOLD approach and neovascularization using the VAM approach. Note: κ = 4/3·π·γ·Δχ·Hct·B₀ (γ = 2.67502·10⁸ rad/s/T is the nuclear gyromagnetic ratio; Δχ = 0.264·10⁻⁶ is the difference between the magnetic susceptibilities of fully oxygenated and fully deoxygenated haemoglobin; Hct = 0.42 × 0.85 is the microvascular hematocrit fraction); C_a = 8.68 mmol/ml is the arterial blood oxygen content; P₅₀ is the hemoglobin half-saturation tension of oxygen (27 mmHg), h is the Hill coefficient of oxygen binding to hemoglobin (2.7), and L (4.4 μmol/mmHg per min) is the tissue oxygen conductivity; Q_max = max[ΔR_2,GE]/max[(ΔR_2,GE)^3/2]; $\overline{\text{R}}\approx$ 3.0 μm is the mean vessel lumen radius; and b is a numerical constant (b = 1.6781). For guidance of the interpretation of MTI maps: negative MTI values were assigned to cool colors and positive to warm colors. Consequently, maps of MTI enabled differentiation between supplying arterial (areas with warm colors) and draining venous microvasculature (areas with cool colors). More specifically, a voxel with high-arterial blood volume, for example, shows a high positive MTI value and an orange to red color in the MTI maps; a capillary voxel shows a very low MTI value around 0 and a black color in the MTI maps; and a voxel with high venous blood volume shows a high, negative MTI value and a purple color in the MTI maps.

Fig. 2.

Non-invasive synergistic assessment of neovascularization, oxygen metabolism, and hypoxia respectively in a 53-year-old patient with an invasive ductal carcinoma (IDC, patient 14 in Table 1). a Conventional MRI (cMRI) using dynamic contrast-enhanced (DCE) T1-weighted perfusion MR images in coronal, sagittal, and axial orientation (top-down) show lesion size and position. The white line indicates the slice intersection. b Imaging biomarker maps of oxygen extraction fraction (OEF), metabolic rate of oxygen (MRO₂), and mitochondrial oxygen tension (mitoPO2) in coronal orientation (top-down) as well as c microvessel density (MVD), vessel size index (VSI) and micro vessel type indicator (MTI) in coronal orientation (top-down) demonstrates intratumoral spatial heterogeneity. The IDC showed high MRO₂, low mitoPO₂, high MVD and low (i.e. more pathologic) MTI.

Fig. 3.

Non-invasive synergistic assessment of neovascularization, oxygen metabolism, and hypoxia, respectively, in a 31-year-old patient with a benign fibroadenoma (FA, patient 16 in Table 1). a Conventional MRI (cMRI) using dynamic contrast-enhanced (DCE) T1-weighted perfusion MR images in coronal, sagittal, and axial orientation (top-down) show lesion size and position. The white line indicates the slice intersection. b Imaging biomarker maps of oxygen extraction fraction (OEF), metabolic rate of oxygen (MRO₂), and mitochondrial oxygen tension (mitoPO2) in coronal orientation (top-down) as well as c microvessel density (MVD), vessel size index (VSI) and micro vessel type indicator (MTI) in coronal orientation (top-down) demonstrate intratumoral spatial heterogeneity with lower MRO₂, higher mitoPO₂, lower MVD and higher MTI as compared to malignant tumors e.g. Figure 2.

Fig. 4.

Histogram analysis of a oxygen extraction fraction (OEF), b metabolic rate of oxygen (MRO₂), c mitochondrial oxygen tension (mitoPO₂), d microvessel density (MVD), e vessel size index (VSI) and f microvessel type indicator (MTI) for benign (BI-RADS 2/3) and malignant breast lesions (BI-RADS 5). P values show significance of difference between the histograms as calculated with a Mann-Whitney test.

Fig. 5.

Linear regression and Spearman’s correlation analysis between MRI biomarker for neovascularization, oxygen metabolism, and hypoxia, respectively. Benign breast lesions (BI-RADS 2/3) revealed significant correlation between a extraction fraction (OEF) and microvessel density (MVD), and between b metabolic rate of oxygen (MRO₂) and vessel size index (VSI), but not between c mitochondrial oxygen tension (mitoPO₂) and MVD. Breast cancer (BI-RADS 5), however, showed significant correlation between d OEF versus MVD, e MRO₂ versus VSI, and f mitoPO₂ versus MVD, respectively.