LAUSR

Carcinoid tumors are neuroendocrine tumors derived from enterochromaffin cells. The majority arise from the midgut, and most commonly within the appendix or terminal ileum [1]. A common metaphor applied to small bowel carcinoid tumors on cross-sectional imaging is the hairpin turn. This refers to acute angles often formed by the involved segment of small bowel, which resembles a tight turn as it might be seen on a mountain road (Fig. 1). Small bowel carcinoid tumors are often small and therefore difficult to detect on routine imaging. Visualization is best facilitated with dedicated multiphase computed tomography enterography or magnetic resonance enterography with the use of negative oral contrast [1, 2]. Classically, they appear as an enhancing polypoid or nodular mucosal mass or as focal asymmetric thickening and enhancement of the bowel wall (Fig. 2). Use of negative oral contrast distends the bowel lumen and provides a neutral background to increase conspicuity of lesion enhancement, which is often best differentiated from the bowel wall on arterial phase of contrast. These masses are often associated with a sharp kink or angulation of the involved small bowel segment producing a characteristic hairpin turn appearance. As a result, patients often present with non-specific chronic abdominal symptoms such as pain and weight loss, although more acute presentations due to obstruction, ischemia, or hemorrhage have also been described [1, 3]. Histopathologic analysis of surgical specimens has demonstrated two characteristics that contribute to this appearance of carcinoid tumors. The first is direct invasion of the intestinal wall resulting in fibrosis and contraction of the tissue. This invasion, as well as paracrine secretion of active compounds by the tumor, results in a desmoplastic reaction within the adjacent mesentery that can compromise mesenteric vasculature and further contribute to the hairpin turn appearance [3]. Consequently, when the hairpin turn is seen on cross-sectional imaging, bowel wall invasion may be inferred, even if not directly visible on the examination [2]. Finally, midgut carcinoid tumors most commonly metastasize to the mesenteric lymph nodes and liver and special attention should be paid to these areas when a carcinoid tumor is suspected (Fig. 3) [1]. Metastatic lesions to the mesentery often produce calcified, spiculated masses with associated mesenteric retraction, which is explained by the same histopathologic features responsible for the hairpin turn appearance of the primary tumor [3].