Endocytoscopic Observation of Esophageal Lesions: Our Own Experience and a Review of the Literature

Abstract

This review outlines the process of the development of the endocytoscope (EC) with reference to previously reported studies including our own. The EC is an ultra-high-magnification endoscope capable of imaging at the cellular level. The esophagus is the most suitable site for EC observation because it is amenable to vital staining. The diagnosis of esophageal lesions using EC is based on nuclear density and nuclear abnormality, allowing biopsy histology to be omitted. The observation of nuclear abnormality requires a magnification of ×600 or higher using digital technology. Several staining methods have been proposed, but single staining with toluidine blue or methylene blue is most suitable because the contrast at the border of a cancerous area can be easily identified. A three-tier classification of esophageal lesions visualized by EC is proposed: Type 1 (non-cancerous), Type 2 (endocytoscopic borderline), and Type 3 (cancerous). Since characteristic EC images reflecting pathology can be obtained from non-cancerous esophageal lesions, a modified form of classification with four additional characteristic non-cancerous EC features has also been proposed. Recently, deep-learning AI for analysis of esophageal EC images has revealed that its diagnostic accuracy is comparable to that of expert pathologists.

Keywords: artificial intelligence; endocytoscopy; esophageal cancer; esophagitis; esophagus; vital staining.

Figure 1

Ultra-high magnifying observation using 1st generation EC with methylene blue staining (XEC120U 1125x Olympus Medical systems Co., Tokyo, Japan): (a) Normal esophageal mucosa. Surface epithelial cells have a low nuclear/cytoplasm ratio and no nuclear abnormality. (b) Esophageal squamous cell carcinoma. Cancerous cells show an increase in nuclear density in comparison with the normal squamous epithelium and have prominent nuclear abnormalities.

Figure 2

Type classification: (a) Type 1, surface epithelial cells show a low nucleus/cytoplasm (N/C) ratio and no nuclear abnormality. Nuclear density is low or slightly increased (toluidine blue staining, ×900). (b) Type 2, evident increase in nuclear density. Slight nuclear abnormality is evident, but not sufficiently obvious to be considered type 3 (toluidine blue staining, ×900). (c) Type 3, evidently increased nuclear density and nuclear abnormality, e.g., irregular nuclear size and shape, with hyperchromatism (toluidine blue staining, ×900). (d) Non-malignant EC feature 1 (GERD): circularly arranged squamous cells without nuclear abnormality around epithelial papillae (toluidine blue staining, ×900). (e) Non-malignant EC feature 2 (regenerative squamous epithelium): evident increase in nuclear density with differing nuclear sizes and shapes. Densely stained cytoplasm with a clear cell border. Nuclei are stained weakly, and multiple nucleoli are evident (toluidine blue staining, ×900). (f) Non-malignant EC feature 3 (erosion, granulation): increase in nuclear density with spindle-shaped enlarged nuclei (enlarged fibroblasts in the stroma, toluidine blue staining, ×900). (g) Non-malignant EC feature 4 (white coat): evident aggregation of inflammatory cells (toluidine blue staining, ×500).

Figure 3

Comparison between EC observations and histological findings: (a) EC image of normal esophageal mucosa (Type 1, toluidine blue stain, ×900). (b) Histological section of normal esophageal mucosa (HE, ×400). (c) EC image of esophageal squamous cell carcinoma. Increased nuclear density, nuclear abnormality, and nuclear enlargement are evident (Type 3, toluidine blue stain, ×900). (d) Histological section of esophageal squamous cell carcinoma (histological section of the same lesion as (c)). Atypical cells have replaced the entire epithelial layer (HE, ×400). (e) EC image of esophagitis. Non-malignant cells are circularly arranged around the epithelial papilla (non-malignant EC feature 1, toluidine blue stain, ×500). (f) Histological section of the same area as (e). Elongation of the epithelial papillae is evident just beyond the epithelial surface (HE, ×200). (g) EC image of regenerating squamous epithelium. Weakly staining nuclei and prominent nucleoli are evident. The cytoplasm is darkly stained and intercellular space dilation is clearly recognized (non-malignant EC feature 2, toluidine blue stain, ×900). (h) Histology of the same area as (g). Prominent nucleoli are evident within the nuclei, with clear intercellular borders (HE, ×400). (i) EC image of esophageal erosion. Increased nuclear density, nuclei of variable size, and spindle-shaped enlarged nuclei are evident (non-malignant EC feature 3, toluidine blue stain, ×900). (j) Histological section of the same area as (i). Inflammatory cell infiltration and stromal fibroblasts are observed as nuclear atypia (HE, ×400). (k) EC image showing the white coat of radiation esophagitis. Dense aggregation of small inflammatory cells is evident (non-malignant EC feature 4, toluidine blue stain, ×900). (l) Histopathological image of the same area as (k), showing dense aggregation of inflammatory cells (HE, ×400).

Figure 4

A case in which toluidine blue staining was useful for diagnosing the extent of esophageal cancer: (a) White light observation. Whitish and rough mucosa is evident, but the boundary of the esophageal carcinoma is unclear. (b) Narrow-band imaging (NBI). The lesion is not delineated as a brownish area and its boundary is unclear. (c) Iodine staining. The lesion is a mixture of unstained and weakly stained areas, and the boundary is also unclear. (d) Toluidine blue staining shows the lesion as a dark stained area with a well-demarcated border. (e) Ultra-high-magnification observation using the EC (toluidine blue staining, ×900). The area darkly stained with toluidine blue was confirmed to be esophageal carcinoma because of increased nuclear density with nuclear abnormality (nuclear atypia). (f) Resected specimen after endoscopic treatment (iodine staining). Unstained and weakly stained areas are intermixed. The cancerous area (red line) matches the area darkly stained with toluidine blue. (g) The histological diagnosis was squamous cell carcinoma in situ (0-IIb, T1a-EP, HE staining, ×400).