Bowel cancer screening-generated diagnostic conundrum of the century: pseudoinvasion in sigmoid colonic polyps.

The introduction of bowel cancer screening, in the United Kingdom, United States of America, and many other Western countries, has provided considerable interest and no little diagnostic consternation for pathologists. In the United Kingdom, the universal introduction of bowel cancer screening, initially by fecal occult blood testing and more recently by the introduction of flexible sigmoidoscopy, has provided four main areas of pathological diagnostic difficulty. This is the biopsy diagnosis of adenocarcinoma, serrated pathology, the diagnosis and management of polyp cancer, and, finally, the phenomenon of pseudoinvasion/epithelial misplacement (PEM), particularly in sigmoid colonic adenomatous polyps. The diagnostic difficulties associated with the latter phenomenon have provided particular problems that have led to the institution of a UK national 'Expert Board', comprising three pathologists, who adjudicate on difficult cases. The pathological features favoring PEM are well recognized but there is no doubt that there can be profound mimicry of adenocarcinoma, and, as yet, no adjunctive diagnostic tools have been developed to allow the differentiation in difficult cases. Research in this area is proceeding and some methodologies do show promise in this difficult diagnostic area.