Dome-Type: A Distinctive Variant of Colonic Adenocarcinoma

Introduction. Ten cases of dome-type adenocarcinoma of the colon have been reported so far. Most of them were presented as early lesions, with endoscopic and microscopic distinguishing features. Methods and Results. A raised plaque was removed from the right colon during colonoscopy in a 56-year-old man. Histopathological examination showed a cancerized adenoma invading the submucosa with several typical features of dome-type adenocarcinoma, in particular the associated prominent lymphoid tissue. Immunohistochemistry showed retention of the mismatch repair proteins MLH-1, MSH-2, MLH-6, and PMS-2. Conclusion. We report an additional case of dome-type adenocarcinoma of the colon as an early, low-risk, and microsatellite stable tumor, indicating that this particular histotype may deserve specific consideration for both classification and management.


Introduction
Since the initial reports back in late 90's & 2000's by De Petris et al. [1] and Jass et al. [2] 10 cases of dome-type (DC) adenocarcinoma of the colon have been reported [1][2][3][4][5][6][7]. DC is considered a rare variant of carcinoma of the colon presenting as a nonpolypoid plaque lesion, it is thought to derive from the specialized columnar M-cells of dome epithelium, which makes up in association with the gutassociated lymphoid tissue the domelike masses that bulge into the gut lumen [2].
Originating from this specific microenvironment, the most important morphological feature of DC is the association with a prominent lymphoid stroma.
We would like to add an additional case of this particular histotype that was recently diagnosed in our institute.

Case Presentation
A 56-years male with amyotrophic lateral sclerosis was colonoscoped because of painful constipation. There was no case of colorectal cancer in his family history.
An 8-mm raised plaque was seen in the right flexure ( Figure 1) and removed.
The cancer, arisen in a flat adenoma with high-grade dysplasia, was ulcerated superficially and the advancing edge appeared quite well circumscribed except for a more submucosa-invading tongue of neoplastic glands where some low-grade budding was observed (Figures 2(a), 2(b) and 2(c)).
The glands were in part cystically dilated containing a pink eosinophilic material, in part cribriform arranged (Figures 2(c) and 2(d)).
A clear space often separates the glandular epithelium from the intraglandular material.
No tumor infiltrating lymphocytes nor goblet cells were observed. Necrosis and desmoplasia also were absent.
The T1 adenocarcinoma was considered a low-risk lesion because of the absence of lymphovascular invasion, the lowdifferentiation grade, and the negative resection margin, therefore no hemicolectomy was performed.   Immunohistochemistry showed retained expression of the mismatch repair proteins MLH-1, MSH-2 MSH-6, and PMS-2 in the neoplastic cells as well in the internal control (Figures 3(a), 3(b), 3(c), and 3(d), resp.).
The patient underwent a follow-up colonoscopy one year later: the mucosal biopsies from the polypectomy site showed at histology mild fibroinflammatory changes.

Discussion
DC may develop as sporadic-type colon cancer or in association with ulcerative colitis, [6] familial adenomatous polyposis, [2] hereditary nonpolyposis colorectal cancer, [1] and other positive family history of colorectal cancer, [2,5] in both right and left colon, therefore DC is not associated to any specific mechanisms of tumour predisposition. Some distinguishing features both macroscopic and microscopic are constantly present: the nonpolypoid appearance, the cell architecture, the cytology, and the presence of prominent lymphoid tissue; other features such as the tumor infiltrating lymphocytes, the intra-acinar necrosis, the remnants of a preexisting adenoma and foci of usual-type adenocarcinoma of the colon may be present or absent.
Most cases are reported in the early growth phase: eight over ten case reported are T1N0, [1][2][3][4][5][6] one is T2N0 [3], and the last one is T3N0 [7]. No recurrence is documented so far and the patient presented with this report is one-year recurrence-free.
The case reported here of DC is another early and lowgrade lesion, lacking features of biological aggressiveness, microsatellite stable tumor, suggesting that this particular histotype may deserve a space in the classification of tumors of the colon and rectum.
In the management of neoplastic colonic polyps DC per se may identify a low-risk malignant lesion, influencing the treatment decision-making process.