Blood Flow Velocity is Reduced in a Tumor Micro-dissemination in the Visceral Pleura in Anesthetized Open-chest Rat Lung

Background: Recently we developed a method to observe pulmonary micrometastasis by labeling cancer cells with green fluorescent protein (GFP). We applied the method for observation of micro-dissemination on the visceral pleura. Materials and Methods: RCN9 rat colon cancer cells labeled with GFP were injected into the pleural cavity of Fischer F344 rats. Six weeks after injection, the chest wall was resected under general anesthesia and the lung surface was observed by real-time confocal laser-scanning microscopy. Blood flow was visualized by intravenous injection of fluorescein isothiocyanate-labeled red blood cells, by which blood flow velocity was measured. Results: Dissemination was created in 4 out of 5 rats. Fifteen sites of micro-dissemination were observed (mean diameter, 35.8±13.3 μm). Blood flow velocity was 114.1±26.1 μm/s in the tumor tissue and 183.4±35.0 μm/s out of the tumor tissue. Conclusion: We were able to observe pleural micro-dissemination. Blood flow velocity was significantly lower in the tumor tissue. Cancer dissemination and metastasis have a significant effect on prognosis, but few experimental models are available and the mechanism of spreading is not well understood. Recently, we developed a method to observe pulmonary metastasis at an early phase (1). In this approach, a site of metastasis comprising only a few cancer cells can be visualized by labeling with green fluorescent protein (GFP). Here, we apply this method to visualization of pleural dissemination in an early phase of spreading. In a clinical situation, some cancer cells that are present in the pleural space may attach to the surface of the pleura, while others are eliminated by phagocytosis or lymphatic drainage. Only a few attached cells will grow and disseminate as tumor masses, and an understanding of how cancer cells obtain oxygen and nutrition at an early stage and establish a dissemination might lead to strategies to reduce cancer spreading. To address some aspects of this process, we developed a double- fluorescence technique to observe red blood cell (RBC) flow (2) associated with early dissemination tumors.