The current study shows that DOT using a low dose of the fluorescent agent Omocyanine is feasible and safe for breast cancer visualization in patients. To our knowledge, this is the first report of the use of a novel fluorescent contrast agent for optical imaging of the breast in women suspected of breast cancer.
The fluorescent signal emitted by the contrast agent after excitation by NIR light could be detected with our DOT system in all patients. Since four different dose levels of the fluorescent agent were administered (ranging from 0.01 to 0.1 mg/kg body weight), the absolute increase of fluorescence signal with higher doses of the contrast agent could be clearly observed. Corlu et al. [19
] were the first to demonstrate fluorescence DOT in patients using ICG. Other investigators did not use the fluorescence, but the absorbance characteristics of ICG for their optical imaging technique [16
]. In contrast to Corlu et al., we did not only investigate the breast with the lesion but also the contralateral healthy breast to obtain more information on the specificity of the technique.
In the present study, five of ten malignant lesions could be detected by fluorescence DOT. Best lesion-to-background ratios (range 1.8–2.8) were obtained after 8 h. Limitations in lesion detection primarily resulted from the reconstruction algorithm and the cup geometry of the current DOT system. Lowest doses of Omocyanine performed best in lesion detection, while higher concentrations were problematic for the reconstruction algorithm. The reason was that at a higher dose level, the contrast agent absorption was no longer negligible compared to the tissue absorption, which is an important assumption of the algorithm. Even at later imaging time points (up to 24 h), this reconstruction problem persisted due to the slow washout of the contrast agent. In two patients, the lesions were located close to their chest walls and could not be measured by the DOT system. Most probably, during the optical scans, these lesions were physically located too far above the upper optical fibers in the cup to influence the light pathways. Advances in cup geometry are feasible and should be realized to improve visualization of these lesions.
The other important limitation in this study was that Omocyanine is a non-specific contrast agent. Ideally, this fluorescent probe would accumulate at the tumor site by extravasation through leaky vessels. However, since it does not specifically bind to a cancer-associated target, enhancement of other normal tissues is also possible. This non-specific enhancement pattern was observed when comparing the uptake of the contrast agent over time for the lesion and mirror images in different patients. Differences were noted between patients whose breasts contained mostly fatty tissue and patients whose breasts contained mostly glandular (heterogeneously dense) tissue. To illustrate this observation, two examples are shown in Fig. . As shown in the graphs, uptake of the contrast agent was much lower in fatty tissue (patient 2) than in glandular tissue (patient 5) if the lesion is compared to the mirror image location. In the patients with a high content of glandular tissue in their breasts, the uptake at the mirror image site vs. the background was quite similar to the uptake at the lesion site vs. the background. Although the sample size is small and we should be cautious in generalizing these observations, it indicates that the distinction between glandular and malignant tissue could be problematic with this non-specific fluorescent contrast agent. Clearly, there is a need for target-specific fluorescent agents to be tested in patient studies using an optical imaging device.
Fig. 3 Dye uptake over time for patient 2 and patient 5. The dye kinetics for patient 2 with mainly fatty breast tissue (BI-RADS density category 1) show lower uptake (reconstructed fluorescence signal) of the mirror image vs. the lesion, while for patient 5 (more ...)
In the absorption mode, DOT showed around twofold higher absorption at the site of the malignant lesions using wavelengths 690 and 730 nm, which is in agreement with previous studies and probably related to increased light absorption by the tumors’ higher hemoglobin content due to angiogenesis [7
To gain more insight in the composition of the breast, i.e., determine relative concentrations of (de)oxyhemoglobin, fat, and water, a spectral analysis which combines data of the different wavelengths in one model could be performed. Potentially, this spectral absorption information could improve the identification of malignant lesions and the specificity of DOT.
In conclusion, in this first feasibility study, we have shown that diffuse optical tomography with the use of the fluorescent contrast agent Omocyanine has the potential to safely visualize malignant breast tumors in patients. Development and especially the clinical translation of fluorescent probes with specific binding affinity for relevant molecular targets will be crucial to translate molecular breast imaging to clinical applications.