Colorectal flat neoplasia

Abstract

Background and aims. An attempt has been made to evaluate the clinicopathological characteristics of flat colorectal neoplastic lesions, and analyse the factors associated with the malignancy.

Patients and methods. A total of 115 flat neoplastic lesions, ≥5 mm in size, diagnosed in 87 patients by colonoscopy, were investigated.

Results. The rectum was the most common location. Almost half (49.6%) of the flat neoplasms were small (5–10 mm), 27.8% were 11–20 mm and the remainder (22.6%) larger than 20 mm. The surface was smooth in 55.7%, granular in 20.0% and nodular in 24.3%. Histologically, the flat lesions were tubular, tubulovillous and villous adenomas in 69.6%, 20.9% and 5.2%, respectively. Five lesions (4.3%) were composed of carcinomas without adenoma. High-grade dysplasia, intramucosal carcinoma and invasive carcinoma were diagnosed in 9.6%, 7.8% and 6.1% of all flat neoplasms, respectively. Univariate analysis demonstrated that the location, size, surface pattern and histologic type of the flat lesions were factors associated with malignancy. However, in multivariate analysis, the size of the flat lesions was the only significant risk factor for malignant transformation.

Conclusions. Flat neoplastic lesions of the colorectum have a relatively high rate of malignancy, and size is the most important factor associated with malignancy.

Introduction

It is widely accepted that most colorectal cancers (CRC) develop from pre-existing adenomas, a concept known as the adenoma–carcinoma sequence [1], [2], [3]. Therefore, endoscopic detection and removal of polypoid tumours has been emphasized to prevent the development of advanced CRCs [4], [5], [6], [7], [8].

From meticulous histologic studies on polypoid lesions collected from surgically removed specimens, the size of the adenomas was thought to play the most significant role in the adenoma–carcinoma sequence [1], [2], [9], [10], [11].

Several reports have confirmed that flat adenomas and carcinomas do exist in the human colon [12], [13], [14] and these flat lesions show a high potential for malignancy and frequently invade the submucosa even when they are of small size [13], [14], [15], [16], [17]. In contrast, some investigators recently reported a much lower incidence of malignancy for these lesions [12], [18], [19], [20], [21], [22], [23]. The main reason for the lower rate of malignancy, in recent studies, seems to be the higher rate of detection of small flat elevated lesions in the colon. These smaller flat elevated lesions appear to include comparatively high numbers of low-grade dysplastic lesions or non-neoplastic lesions such as metaplastic polyps [21].

However, it is difficult to distinguish small flat adenomas from small sessile polyps. In addition, small flat adenomas may show a change in morphology over a period of time [24], [25]. In the present study, therefore, the clinicopathological characteristics and the degree of dysplasia of flat neoplastic lesions, which were ≥5 mm in the longest diameter, were evaluated in order to avoid any morphological bias from such small lesions. In addition, the factors associated with the degree of dysplasia and malignancy of the flat neoplastic lesions of the colorectum were analysed.