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Micropapillary variant of urothelial carcinoma – a review of the literature
The micropapillary variant of urothelial carcinoma is one of the variants of urothelial carcinoma that was added to the World Health Organization (WHO) classification in 2004. This variant of urothelial carcinoma is uncommon and, therefore, a number of practitioners have not yet encountered the tumour in their clinical practice. In view of this, the diagnostic features and the biological behaviour of this tumour are not well known by practitioners. This review documents the diagnostic features, management and outcome of the micropapillary variant of urothelial carcinoma. Various internet search engines were used to identify literature on the micropapillary variant of urothelial carcinoma, including case reports and case series, which formed the basis of the literature review. At least 500 cases of the micropapillary variant of urothelial carcinoma have been reported since it was first described in 1994. The gross morphology is variable and there are no special features to differentiate this variant from other variants or from conventional urothelial carcinoma. The micropapillary variant may be sessile, papillary, ulcerative or polypoid or may manifest as an infiltrative mass of variable size, ranging from microscopic to >10cm. The microscopic features of this variant of urothelial carcinoma exhibit an architecture that is reminiscent of the papillary configuration of ovarian papillary serous tumours. The nuclei of the micropapillary variant cells are commonly of high grade and show reverse polarity to the external surface of the tumour nests. A minority of the tumour-containing spaces represent actual lymphovascular invasion, which is supported by their characteristic immunohistochemical staining with endothelial markers. Lymphovascular invasion can be found in most cases of the invasive micropapillary variant of urothelial carcinoma if the specimens are adequately sampled, but a majority of the tumour-containing lacunae do not have endothelial lining and, therefore, do not represent true lymphovascular invasion. Psammoma bodies, which are common in ovarian papillary serous tumours, are rare in the micropapillary variant of urothelial carcinoma, and most of these tumours exhibit deep muscle invasion. Urine cytology exhibits papillary/spheroid clusters of tumour cells that have a high nuclear grade. The micropapillary variant of urothelial carcinoma is a rare and aggressive variant and may be under-reported as some pathologists may not be aware of its diagnostic features. Reports so far would indicate that this variant is associated with poor prognosis and adjuvant chemotherapy may have a questionable efficacy. Because of the rarity of this variant of urothelial carcinoma, there is no consensus opinion regarding the optimal treatment. Therefore, there is a need for urologists and oncologists throughout the world to report cases of the micropapillary variant of urothelial carcinoma so that the biological behaviour of the tumour can be ascertained. There is also a need for multicentre trials of treatment of this variant of urothelial carcinoma so that lessons can be learnt regarding the biological behaviour of the tumour and a consensus opinion can be established regarding the best treatment modality for such an apparently aggressive tumour.