Finding disease patterns by shining a light on life: A work in progress

Our expectations about the pretreatment evaluation of malignant diseases have undergone a profound change in the past few years. An ideal situation would be if we could detect, diagnose, and stage the disease with a single procedure. This would save considerable time and patient anxiety arising from waiting for results. In addition to standard histology, we would also prefer to know the cellular and biochemical nature of the disease so that we could personalize the treatment. These expectations are likely to be realized with recent advances in technology.

Biopsy and histological evaluation has been the cornerstone of the diagnostic process for many decades. The term “biopsy” has Greek origins, and means “a sight of life” (bios “life,” and opsis “a sight”). This “sight of life” should ideally be experienced in vivo, but is currently done ex vivo with biopsy specimens, microscopes, and special stains. This is because the resolution required for observing the cellular and subcellular structures could not be achieved in vivo in the past. But the situation is changing, and in vivo or “optical biopsy” is becoming a reality. A variety of approaches are being investigated, based upon the interaction of light with tissue, with and without the help of labeling agents, to evaluate the subcellular structure and biochemical changes in vivo. Some of these techniques such as optical coherence tomography, Raman spectroscopy, and confocal laser endomicroscopy (CLE) are already undergoing evaluation in humans, and some more are progressing to that point. These techniques have been the subject of an earlier extensive review [1].

The evolution of in vivo microscopy has its foundation in the developments of light microscopy. The first use of fluorescent dyes to enhance the autofluorescence of cells was reported almost a century ago [2]. The first patent for a confocal microscope was registered in 1961 in the USA [3]. The concept was introduced to eliminate background signals, and consisted of restricting the illumination and image capture to a small point in the same focal plane. In the 1980 s, laser was added to this concept to achieve illumination and higher resolution. These developments, along with the publication in 1997 of the possibility of labeling the proteins within the cells, ushered in the era of in vivo imaging [4]. A confocal endomicroscope became available followed by a probe which could be passed through the endoscope channel (pCLE), or through endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) needles (nCLE). The lateral resolution of CLE is 3.5 μm and the field of view is 320 μm, allowing a magnification of up to 1000-fold.

What should we expect from this technology? Unlike histology, which can decipher the depth of involvement, CLE currently scans parallel to the area of interest, and provides information about the superficial layers alone. Thus staging information is currently not possible, limiting the potential use for diagnosis. Several publications have evaluated CLE for esophageal and colonic proliferative and premalignant lesions [5]. Attempts have been made to differentiate pancreatic cysts based upon CLE findings, and there have been studies assessing the diagnostic potential of CLE in pancreatic neoplasms and bile duct stenosis [6] [7] [8]. Almost all studies have evaluated the possibility of identifying a malignant morphology and pattern on the visualized surface. These are still early days but the results are encouraging to say the least. The negative predictive value in some studies is in excess of 90 %, outlining CLE’s potential in excluding malignancy in doubtful scenarios.

Where does pCLE fit in the diagnostic algorithm of pancreatic masses? EUS-guided tissue acquisition is the current gold standard for diagnosis of pancreatic neoplasms. The technology and expertise is widely available and a positive tissue diagnosis is to be expected in over 85 % of patients [9]. However, the sampling process may not always target the correct areas, because of the two-dimensional nature of the imaging; multiple passes are needed to minimize false negatives; and an on-site pathologist may be required, to ascertain the adequacy of samples and avoid a second procedure because of a negative result. Additional techniques such as elastography and contrast harmonic EUS (CH-EUS) have shown promise in improving the diagnostic potential of EUS. Both these techniques can accurately diagnose pancreatic adenocarcinoma in more than 90 % of patients, and can also provide an accurate target area for EUS-FNA [10] [11]. However more studies are needed to evaluate the cost – effectiveness of these modalities.

A CLE examination through the EUS-FNA needle (nCLE) could provide us with a diagnosis and also potentially guide us regarding the appropriate target area for sampling, particularly in difficult cases such as patients with pre-existing chronic pancreatitis. However, CLE is still an evolving technique, and standardization and training are important for an appropriate and seamless transition from bench to bedside. The interobserver variations in interpretations should be within acceptable limits for the technology to gain acceptance. Several studies have recently addressed the issue of CLE standardization in different organs and diseases. The Miami classification [12] and the subsequent Paris classification [13] have attempted to standardize the interpretation of pCLE for bile duct strictures, with reasonable success. The INSPECT[1] study, the DETECT[2] study, and some other studies have attempted to standardize the CLE interpretation of pancreatic cysts [14] [15].

In this issue of Endoscopy, Giovannini et al. have published the final results of the CONTACT[3] study [16]. This French multicenter study aimed to describe and validate the nCLE interpretation criteria for pancreatic masses with histological correlation. The first part of this two-stage study involved development of nCLE criteria, for normal pancreas, chronic pancreatitis, pancreatic adenocarcinoma, and neuroendocrine tumours of pancreas, by studying the nCLE characteristics and comparing them with histological features in 12 patients. The second stage consisted of validation of the developed criteria. This was done by showing videos of 32 patients to four endosonographers and a pathologist, not involved in the criteria development process. All were novices for nCLE and an initial training was given by showing 10 videos. The results show a low sensitivity (77 % for adenocarcinoma, 50 % for chronic pancreatitis), but a high specificity and negative predictive value for nCLE. The overall accuracy was 85 % for adenocarcinoma, and 91 % for chronic pancreatitis. The interobserver agreement was low (kappa coefficient 0.55).

This study is an important step in the evolution of nCLE for pancreatic masses. The development and validation of nCLE criteria for pancreatic masses will allow further large studies to evaluate the exact role of nCLE in diagnosis. The low interobserver agreement for nCLE in this study points to complexities in interpretation, which will need to be addressed mostly by additional training. The high specificity and negative predictive value of nCLE makes it potentially valuable for excluding malignancy in highly suspicious lesions where EUS-FNA findings are negative, a role potentially similar to that of elastography and CH-EUS. 

EUS technology is also rapidly evolving. Improvements in needle technology may further improve the yield of EUS-FNA, or reduce the number of passes required for accurate diagnosis. Further studies should attempt to define the complementary roles of elastography, CH-EUS, and nCLE, particularly with regard to cost – effectiveness, in patients with high probability of disease and negative EUS-FNA findings.

Optical biopsy is an attractive and alluring concept, but it will have to meet the current standards of histology and cytology, and probably exceed them, before shining the light in vivo becomes a routine stand-alone diagnostic practice.

• References

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