To the Editor:

Visible light is an electromagnetic radiation with wavelengths between 400 and 700 nm. The result of light–tissue interaction may be wave reflection, scattering, and absorption. Sometimes the irradiated tissue may reemit light at a different wavelength, a phenomenon named fluorescence. Five centuries ago, these observations allowed scientists to use a magnifying glass lens to investigate life at the microscopic level. When Anthony van Leeuwenhoek described “the animalcules” (1) and Robert Hooke defined the cell as the “building block of life” (2), they both used a microscope. Magnifying the image of light–tissue interaction is one of the fundamental principles of modern diagnostic investigations.

Endoscopy is based on visible light, allowing exploration of the human body through its “natural gates.” The flexible fiberoptic instruments may reach the depth of the human body, but their field of view is limited by anatomical obstacles. In an effort to improve diagnostic ability, visible light was replaced by invisible light (infrared and ultraviolet) and various lasers, and the optical lenses were changed to electronic charge-coupled devices. Modern endoscopes also use ultrasound technology to extend the range of exploration beyond the anatomical borders of the examined cavity (endobronchial ultrasound).

These technologies allow the diagnosis of macroscopic (visible) diseased tissue at endoscopy or surgery. But what about microscopic diagnosis in vivo during the same procedures?

The last decade was marked by a constant effort to bring the microscope to the diseased organ in real time. Confocal laser microscopy (CLM) is one of these methods based on the autofluorescence or induced fluorescence of tissues irradiated with laser(s) (3).

CLM is available for endoscopic and surgical use mainly through dedicated probes that may pass through the working channels of instruments. Microscopic imaging of different pulmonary disorders was described, but the real challenge is cancer diagnosis both at airway and alveolar levels (4). Probe CLM (Mauna Kea Technologies, Paris, France) is a “touch-and-see” procedure. Because the probe has to reach the tissue, a combination of navigation and target confirmation techniques has to be used for precise diagnosis (4). Cellular identification is essential, and fluorescent “dyes” are mandatory. Fluorescein is one of the few approved fluorophores for human use. On a gray scale of imaging, it colors the blood vessels in white and produces a contrast enabling the appearance of cells nuclei as black dots. A 488-nm laser is used for irradiation.

In a recent issue of the Journal, Wijmans and colleagues describe the use of a 19-gauge CLM probe able to pass through the biopsy needle of an ultrasound bronchoscope (5). After in vivo imaging and CLM examination, standard ex vivo microscopy confirmed cancer diagnosis (5).

The use of this type of needle for CLM was already described in a pilot proof-of-concept study analyzing the feasibility of the technology in computed tomography–guided biopsy of lung and mediastinal tumors (6). The hypothesis was that CLM may help in improving the biopsy yield in large heterogeneous tumors and eventually replace rapid onsite evaluation. Cytological diagnosis was obtained in mediastinal tumors.

Endomicroscopy is a rapidly evolving chapter in pulmonary endoscopy. The case described by Wijmans and colleagues is another piece in the large puzzle of in vivo microscopic diagnosis. Better imaging with dedicated probes and cell-targeted fluorescent dyes will complete the picture. Near infrared light and molecular imaging of the lung cancer mutation panel are some examples of the ongoing research.

Any new technology must be assessed by its added value over the existing ones. At this stage, we may improve the yield of tissue sampling. To reach the ability of a true “optical biopsy,” there is still a long way to go.

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