COW-2021-32

CASE OF THE WEEK

2021-32/August 9
Contributors: Susan Prendeville

A middle aged man underwent partial cystectomy and excision of urachal cyst. Gross evaluation identified a 2cm unilocular cystic lesion with abundant intraluminal mucin.

Quiz

1. What is the correct diagnosis?

a. Mucinous cystadenoma

b. Mucinous cystic tumor of low malignant potential

c. Mucinous cystadenocarcinoma

d. Villous adenoma

e. Cystitis glandularis

Mucinous cystic tumor of low malignant potential

This cystic tumour is lined by mucinous epithelium with areas of tufting and papillary formation and low grade nuclear atypia. Residual urachal remnants are noted. Examination of the entire lesion revealed no evidence of invasive malignancy .

Mucinous cystic tumors comprise an uncommon subset of urachal glandular neoplasms which span a morphologic spectrum from benign to borderline to overtly malignant. Classification of these tumors is similar to ovarian mucinous tumors and includes i) mucinous cystadenoma; ii) mucinous cystic tumor of low malignant potential (MCTLMP) and iii) mucinous cystadenocarcinoma. Among this group of tumors, MCTLMP is the most common.

Mucinous cystadenoma is defined as a cystic tumor lined by a single layer of cytologically bland mucinous epithelium with minimal atypia. As demonstrated in this case, MCTLMP shows a greater degree of architectural complexity which may include tufting/papillary/villous formations and mild to moderate atypia. There may be extravasation of acellular mucin into the cyst wall. In some cases, the epithelial lining may exhibit high grade cytologic atypia, often with cribriform growth and increased mitotic activity, and these are designated MCTLMP with intraepithelial carcinoma. Both mucinous cystadenoma and MCTLMP lack stromal invasion and should be thoroughly sampled prior to definitive diagnosis. Mucinous cystadenocarcinomas are invasive tumors with a prominent cystic component and can be further subclassified into those with microinvasion (<2mm and <5% of the tumor) and those with more extensive invasion. Mucinous cystic tumors share the same immunohistochemical profile as non-cystic urachal adenocarcinomas, with typically diffuse CK20 and CDX2 expression and variable CK7.

It is worth noting that villous adenomas, which usually occur in the bladder proper, may also rarely arise in the urachus. ˇThese are exophytic tumours typically composed of slender elongated villi lined by columnar mucinous epithelium with varying degrees of dysplasia ,which can resemble the epithelial lining in mucinous cystic tumors. Extensive sampling is required to exclude an associated invasive adenocarcinoma. Mucinous cystic tumors are distinguished from villous adenoma on the basis of their gross characteristics, with villous adenomas producing a polypoid exophytic lesion in a urachal remnant while in MCTLMP the mucinous epithelium lines a cystic lesion.

The available literature to date shows an overall favorable prognosis for mucinous cystic tumors following complete excision, including those with intraepithelial carcinoma and minimal stromal invasion, although they can rarely give rise to pseudomyxoma peritonei. This is in contrast to non-cystic urachal adenocarcinomas which generally have a poor prognosis.

1. Amin MB, Smith SC, Eble JN, Rao P, Choi WW, Tamboli P, Young RH. Glandular neoplasms of the urachus: a report of 55 cases emphasizing mucinous cystic tumors with proposed classification. Am J Surg Pathol 2014 Aug;38(8):1033-45

2. Paner GP, Lopez-Beltran A, Sirohi D, Amin MB. Updates in the Pathologic Diagnosis and Classification of Epithelial Neoplasms of Urachal Origin. Adv Anat Pathol. 2016 Mar;23(2):71-83

3. Taylor AS, Mehra R, Udager AM Glandular Tumors of the Urachus and Urinary Bladder: A Practical Overview of a Broad Differential Diagnosis. Arch Pathol Lab Med. 2018 Oct;142(10):1164-1176

4. Wang D , Sule N. Mucinous Cystadenoma of the Urachus and Review of Current Classification of Urachal Mucinous Cystic Neoplasms Arch Pathol Lab Med 2019 143 (2): 258–263

Susan Prendeville
Cork University Hospital
Ireland

Urinary Bladder/Urachus

Urachus, mucinous cystic neoplasm; glandular neoplasia