Flexible coaxial laser endoscope with arbitrarily selected spots in endoscopic view for photodynamic tumor therapy

Photodynamic therapy (PDT), which aims to directly destruct tumor cells without any damage to proximal healthy tissue, is widely used in clinics. However, the devices most often employed in hospitals do not readily enable exact control of the irradiated target location. As a result, the laser often irradiates not only the tumor, but also neighboring healthy tissues or blood vessels, with more serious consequences resulting from necrosis. In this paper, we propose a novel flexible coaxial laser endoscope, which localizes the laser illumination only to the selected tumor target, with minimal illumination of the surrounding tissue. In this system, visible light is first transmitted into an imaging-fiber bundle and then reflected by a polarizing beam splitter, which permits initial imaging of the tumor. Once the tumor target is confirmed, an automated stage moves the laser fiber head and focusing lens system to the appropriate position. The laser light is then turned on, passed through a beam splitter, and focused onto the imaging-fiber bundle, ultimately irradiating only the specified target. To evaluate the effectiveness of the device, we first examined the endoscope image quality for MTF50, MTF20, and chromatic aberration, finding high contrast and low aberration. We then measured the laser power at four locations within this system, from the laser fiber head to the endoscope tip. Although some power loss is observed, the power density at the endoscope tip satisfies therapy requirements. Finally, the laser positioning accuracy of our system was measured at 21 positions throughout the endoscope image at distances from 20 mm to 50 mm. We find that the maximum error is less than 1.2 mm, well within clinical requirement. Therefore, we have developed an optimal system for PDT that effectively transmits laser light to the desired target with unprecedented precision, which we anticipate will find wide use in the clinic.