Adenocarcinomas of the small bowel are relatively uncommon, accounting for approximately 2.4% of all gastrointestinal tumours.8
About 55% of small bowel cancers arise in the duodenum.9
In patients with FAP, a cumulative incidence of duodenal adenomas of 90% is found and about 5% develop duodenal carcinomas.10
Likewise, in patients with AFAP, duodenal adenomas are often observed and duodenal cancer has been described in a few patients11
and in one patient as the presenting manifestation.12
Among 157 patients with MAP reported so far, there were only eight patients with duodenal neoplasms.2,3,4,5,6,13
As endoscopy of the upper gastrointestinal tract is not routinely carried out in patients with between 10 and 100 polyps, the identified 5% (8/157) is most likely to be an underestimation. Here we described two patients with duodenal carcinomas and MAP.
The identification of somatic G→T mutations in codon 12 of the K‐RAS2
gene provides evidence that the duodenal lesions were caused by MUTYH deficiency rather than being a sporadic lesion. We could not detect any protein‐truncating APC
mutations as were initially described in most colorectal MAP lesions.1
However, our observation is compatible with more recent and larger series where protein‐truncating APC
mutations, induced by G→T transversions, were found in only 21% (22/105) of adenomas and in 43% (6/14) of carcinomas.7
Alternatively, carcinoma of the small intestine may follow a different genetic pathway than colorectal carcinoma, as has been suggested recently by Wheeler et al
More studies are needed to identify the risk of developing duodenal adenomas and carcinoma, and to provide evidence‐based recommendations for screening in patients with MAP. In the mean time, upper gastrointestinal screening with careful inspection of the papilla for patients with MAP seems justified. A schedule comparable with FAP seems reasonable until further studies are available: starting from age 25–30 years, with 1–5 year intervals depending on findings.