Small Bowel Neoplasms
Malignant Tumors
Neuroendocrine Tumors (NET): NETs, formerly used interchangeably with carcinoid tumors, arise from the cells of the endocrine system in the gastrointestinal tract. They account for 1.2%–1.5% of all gastrointestinal neoplasms. They are classified by histologic morphology (well or poorly-differentiated) and degree of differentiation (using Ki67 index). These tumors may endogenously secrete vasoactive peptides leading to systemic paraneoplastic symptoms known as carcinoid syndrome. NETs most commonly arise in the distal ileum, then jejunum, followed by duodenum.
NETs on CT typically present as an ill-defined submucosal or intraluminal soft tissue mass or bowel wall thickening with an associated often partially calcified mesenteric mass with a stellate appearance caused by desmoplastic reaction (Figure 1, 4A-B). Occasionally identified on ultrasound, they can present as a hypoechoic intraluminal mass (Figure 2A) which would require further characterization by CT, MRI, or 68Ga DOTATATE PET/CT. On MR imaging, this tumor appears as a polypoid hypointense mass with arterial enhancement (Figure 2, 3A). Clinical presentations can vary and include vague abdominal pain, acute on chronic small bowel obstruction, incidental finding, or complicated liver metastasis (Figure 3). 68Ga DOTATATE PET/CT utilizes a tracer that selectively binds to somatostatin receptors, which are over-expressed on well-differentiated NETs (Figure 3C-D). This exam is useful in staging, identifying an occult primary, monitoring response to treatment/restaging, and to detect recurrence. Where available, Ga68 DOTATATE PET/CT has largely replaced conventional somatostatin scintigraphy with 111In octreoscan (Figure 4).
Fig. 1: [A, B] Axial and coronal CT images in an 86-year-old male with abdominal pain and weight loss demonstrate a spiculated soft tissue mass with calcification in the right lower quadrant mesentery (green arrow). There is associated bowel wall thickening of the distal ileum (yellow arrow). [C] Axial CT image more superiorly demonstrates a soft tissue mass in the distal ileum (yellow arrow) and ileocecal mesenteric lymphadenopathy (blue arrow) suggesting regional metastases. Pathology revealed well-differentiated neuroendocrine (carcinoid) tumor.
Fig. 2: 61-year-old female with right lower quadrant pain. [A] Ultrasound, performed to rule out appendicitis, demonstrates a 2.1 cm hypoechoic intraluminal small bowel mass with associated focal bowel wall thickening. [B, C] Coronal FIESTA and T1-weighted post-contrast MR enterography images of the same patient demonstrate an enhancing polypoid lesion in the terminal ileum (yellow arrow) with some mesenteric lymph nodes (not shown in the images). Biopsy revealed ileal carcinoid tumor.
Fig. 3: 77-year-old male with abdominal pain. Outside CT showed liver lesions and an abdominal mass. [A, B] Coronal T2-weighted MR images demonstrate a T2 hypointense mass in distal ileum (yellow arrow) causing small bowel obstruction with dilated proximal loops of bowel (blue arrow). A T2-intermediate metastatic liver lesion is also identified (green arrow). [C] FDG-PET images demonstrates mild FDG uptake in multiple hepatic lesions (green arrows). [D] 68 Ga-Dotatate-PET image demonstrates multiple hepatic lesions with intense uptake (green arrows) consistent with metastatic well-differentiated neuroendocrine tumor. [E] Gross examination showing mucosal elevation with central erosion (black arrow) of the small intestinal carcinoid tumor. [F] Cut-surface of carcinoid tumor (blue arrow). [G] At low power view, carcinoid tumor showing nested tumor cells with area of fibrosis and calcification by H&E stain. [H] At high power view, carcinoid tumor showing typical nested growth pattern and uniform cytology. Original magnification: F: 20x; H: 200x
Fig. 4: [A] Axial CT image demonstrates a heterogeneous mass (yellow arrow) in the distal ileum with surrounding desmoplastic reaction, found in a 53-year-old male with abdominal pain. [B] Axial CT image more superiorly in the same patient demonstrates a calcified mesenteric nodal metastasis with surrounding desmoplastic reaction (yellow arrow), typical of a well-differentiated neuroendocrine tumor (carcinoid). [C] Nuclear medicine octreotide scan shows increased radiotracer uptake in the right lower quadrant (yellow arrow).
GIST: Gastrointestinal stromal tumors (GIST) are mesenchymal in origin, can occur anywhere throughout the gastrointestinal tract, and 20% occur in the small bowel. They arise from the interstitial cells of Cajal and are mediated by the C-Kit pathway. They can be classified into low- and high-grade based on proliferation index. Depending on location and size, presentations range from asymptomatic incidental findings to gastrointestinal bleeding or bowel obstruction.
The usual CT presentation of GIST is a heterogeneously enhancing, exophytic mass (Figure 5). MR imaging displays similar findings of a typically exophytic mass (Figure 7). Larger tumors may demonstrate necrosis, dystrophic calcification, or ulceration with fistulous connection to the bowel, which would suggest an aggressive/high-grade tumor (Figures 6, 8). They may also display local invasion into adjacent organs or metastatic disease, with a high risk of recurrence after surgical resection (Figure 6).
Fig. 5: [A, B] Axial and coronal CT images in a 53-year-old male, presenting with upper gastrointestinal bleeding, demonstrates a heterogeneous, exophytic mass arising from the distal ileum (yellow arrows) with a clear delineation from the mesentery. Pathology revealed a gastrointestinal stromal tumor.
Fig. 6: An 83-year-old patient referred for CT after video capsule endoscopy showed jejunal inflammation. [A] Axial CT image demonstrates a heterogenous, exophytic mass arising from the jejunum with calcification and focal areas of hypodensity suggesting necrosis (yellow arrow). [B] Follow-up CT scan in 2 weeks demonstrates foci of air within the mass (blue arrows) suggesting communication with the bowel lumen. Pathology revealed jejunal GIST. The tumor and a portion of jejunum were surgically resected. [C, D] Axial and coronal surveillance CT images obtained 12 months later demonstrate recurrence as a soft tissue mass in the mid-lower abdomen (yellow arrow) arising within the site of prior resection. [E] Coronal image more superiorly demonstrates metastatic disease, with a new hypodense lesion within the liver (green arrow). [F, G] Gross examination (black arrow) and cut-surface (red arrow) showing GIST arising from the small intestinal wall and sparing the mucosa. [H] Solid growth of spindle to epithelioid GIST tumor cells on H&E stain. [I] GIST tumor cells are diffusely positive for CD117 by immunostain.
Fig. 7: 61-year-old patient with iron deficiency anemia, who was found to have a duodenal polyp on upper endoscopy. [A, B] Axial and coronal CT images demonstrate a hyperattenuating lesion in the posterior superior aspect of the third portion of the duodenum (yellow arrow). [C, D] Axial T2 fat saturated and coronal T2 weighted MR images demonstrate similar findings of an exophytic lesion arising from the third portion of the duodenum posteriorly (yellow arrow). Pathology revealed a duodenal GIST.
Fig. 8: 60-year-old patient who presented to the emergency department with abdominal pain. [A, B] Axial and coronal CT images demonstrate a heterogeneous mass arising from the proximal jejunum (yellow arrow). There is a contained fluid collection adjacent to this mass (blue arrow). There is surrounding free fluid (green arrow) and locules of extraluminal air (orange arrow), consistent with bowel perforation.
Lymphoma: Primary small bowel lymphoma accounts for 20% of small bowel neoplasms. Small bowel lymphomas most commonly involves the ileum, with non-Hodgkin B cell lymphoma as the most common subtype. Risk factors associated with development of lymphoma include celiac disease, Crohn’s disease, or an immunocompromised state (e.g. chemotherapy, HIV).
Imaging findings of small bowel lymphoma vary; however the most common finding is bowel wall thickening with aneurysmal dilatation of the lumen, which is due to destruction of the muscle wall and myenteric plexus (Figure 9, 11). Other presentations may include nodules, polyps, circumferential focal wall thickening (Figure 9C), or an exoenteric mass. There may be associated bulky mesenteric lymphadenopathy (Figure 9, 10) or infiltration into adjacent visceral organs (Figure 9A). Lymphomas are hypercellular tumors which generally exhibit intense FDG uptake on 18FDGPET/CT (Figure 9D).
Fig. 9: [A, B] Axial and coronal CT images in a 40-year-old male, presenting with left flank pain, demonstrate circumferential wall thickening and dilatation of the small bowel in the right upper quadrant (yellow arrow) with proximal partial small bowel obstruction. There is also a confluent soft tissue mass in the left renal hilum (blue arrow) with associated retroperitoneal lymphadenopathy (red arrow). Pathology revealed Burkitt-like lymphoma. [C, D] Axial CT and PET images in another patient, a 72-year-old male, demonstrating circumferential bowel wall thickening of the proximal jejunum with mild dilatation (yellow solid arrow). Corresponding PET image demonstrates intense FDG activity, consistent with a hypercellular mass. Pathology revealed follicular lymphoma.
Fig. 10: 59-year-old patient who presented to the emergency department with abdominal pain. [A, B, C] Axial and coronal CT images demonstrate focal wall thickening of the small bowel in the mid abdomen (yellow arrow), causing upstream small bowel obstruction (blue arrow in C). Mesenteric lymphadenopathy is also present (green arrow). Surgical pathology after resection revealed marginal zone B-cell lymphoma.
Fig. 11: 80-year-old patient with a past medical history of follicular lymphoma of the neck, presented to the emergency department with abdominal pain. [A, B] Axial and coronal CT images demonstrate dilatation of a segment of small bowel with wall thickening (yellow arrow), with upstream small bowel obstruction (not demonstrated on these pictures). Pathology revealed diffuse large B-cell lymphoma. [C] Cut-surface of small intestinal high-grade lymphoma infiltrating the intestinal wall on gross examination (black arrows). [D] H&E stain showing diffuse mass-like growth of lymphoma cells. [E] Immunostain showing the lymphoma cells were diffusely positive for CD 20 and BCL2 [F]. Original magnifications: D: 100x; E, F: 200x.
Adenocarcinoma: Small bowel adenocarcinoma is the most common primary malignancy of the small bowel (45%). They are most commonly found in the duodenum, then jejunum, followed by ileum.
Imaging findings of adenocarcinoma are variable, from circumferential thickening (Figure 12) to stricture (Figure 13) to polypoid or infiltrating mass. Clinical presentation varies as well, including weight loss, gastrointestinal bleeding, or obstruction (Figure 14). Adenocarcinoma may also be associated with ill-defined lymphadenopathy and infiltrative fat stranding (Figure 14D), in contrast to lymphadenopathy with lymphoma, which is generally more well defined and can be bulky. Rarely, adenocarcinoma may coexist with neuroendocrine tumors (Figure 15), known as a mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN).
Fig. 12: [A, B] Axial and coronal CT images in a 42-year-old male with abdominal pain demonstrates circumferential thickening in the proximal jejunum (yellow arrow). Pathology revealed poorly differentiated adenocarcinoma. [C] Axial CT image from a follow-up scan performed 2 months later demonstrates extensive omental caking (blue arrows) consistent with metastatic disease.
Fig. 13: 68-year-old patient who presented with nausea and vomiting. [A, B] Coronal and axial CT images demonstrate marked distension of the stomach and duodenum with a transition point in the third portion of the duodenum (yellow arrows). The patient had partial duodenal obstruction due to stricture. Biopsy was positive for
duodenal adenocarcinoma.
Fig. 14: [A, B] Coronal and sagittal CT images in a 53-year-old patient, presenting to the emergency department with nausea and vomiting, demonstrating a jejunal mass causing small bowel obstruction (yellow arrows). This was found to be a small bowel adenocarcinoma. [C, D] 83-year-old patient who presented with weight loss. Axial and coronal CT images demonstrate marked bowel wall thickening in the ileocecal region (yellow arrows) with associated mesenteric lymphadenopathy (green arrows). Pathology revealed adenocarcinoma.
Fig. 15: 54-year-old male with history of Crohn’s disease, presented with abdominal pain. [A, B] Axial and coronal CT images demonstrate irregular, short segment thickening of the distal ileum (yellow arrow) with associated fat stranding. The degree of thickening is greater than that typically seen in active Crohn’s disease and is suspicious for a malignancy. [C] Axial CT image more superiorly demonstrates a liver lesion (green arrow), suspicious for metastatic disease. Pathology revealed a mixed adenoneuroendocrine carcinoma. [D] Gross examination and cut-surface [E] of infiltrative small intestinal mixed adenoneuroendocrine carcinoma. [F] H&E stain showing mixed gland-forming adenocarcinoma component (right side) and solid growth of high-grade neuroendocrine carcinoma with central necrosis (left side). [G] Immunostain showing 2 populations of cancer cells with synaptophysin negative in adenocarcinoma and positive in neuroendocrine carcinoma. Original magnifications: F: 40x; G: 100x.
Neoplasm in Meckel’s Diverticulum: Meckel’s diverticulum is the most common congenital anomaly of the gastrointestinal tract and is seen in 2% of the population. Neoplasm arising within the diverticulum is rare but accounts for 3% of complicated cases, the most common being neuroendocrine tumor. Other neoplasms arising in Meckel’s diverticulum include leiomyosarcoma, carcinosarcoma, or adenocarcinoma (Figure 16).
Fig. 16: 75-year-old patient who presented with right lower quadrant pain. [A, B] Axial and coronal CT images demonstrate a blind-ending structure arising from the terminal ileum, with wall thickening (yellow arrow), surrounding infiltrative fat stranding (green arrow), and a focal calcification within (enterolith). A normal appendix was identified on this patient (not shown). Meckel’s diverticulitis was suspected. Surgical pathology revealed invasive adenocarcinoma.
Metastases to Small Bowel: Small bowel metastases are the most common small bowel malignancy, accounting for 50% of all small bowel malignancies. Small bowel involvement can occur by hematogenous spread, most often seen with melanoma and renal cell carcinoma or by direct/peritoneal implantation on the surface of bowel, seen with primary tumors of the ovary, appendix, and colon. Metastatic lesions can cause small bowel obstruction (Figure 17), intussusception (Figure 18), or present with abdominal pain (Figure 19).
Fig. 17: 65-year-old with a history of lung adenocarcinoma presented with abdominal pain. [A, B] Axial and coronal CT images demonstrate concentric thickening of a short segment of ileum (yellow arrows) with an adjacent pocket of air (green arrow) and mesenteric fat stranding. This was also a site of transition causing proximal small bowel obstruction. [C, D] In the same patient, there is focal thickening of a segment of jejunum (yellow arrow) with extraluminal foci of air (green arrow), consistent with perforation. Surgical pathology demonstrated metastatic adenocarcinoma.
Fig. 18: 75-year-old patient with a history of non small cell lung cancer presented with anemia and blood per rectum. [A, B, C] Axial and coronal CT images demonstrate an intussusception (yellow arrow) caused by a heterogeneous mass in the mid jejunum (green arrow in C) acting as a lead point. Pathology revealed non-small cell carcinoma metastases.
Fig. 19: 65-year-old patient with a history of melanoma presented with abdominal pain. [A, B] Axial and coronal CT images demonstrate a markedly dilated jejunal loop, a finding typically seen with lymphoma (yellow arrows). [C] FDG-PET CT fused image demonstrates intense FDG activity suggesting a hypermetabolic tumor. Pathology revealed metastatic melanoma.
Benign Tumors and Mimics:
Hamartamatous polyp: A hamartomatous polyp is composed of disorganized glands surrounded by prominent smooth muscle bundles with a central stalk. These are generally asymptomatic and benign, often detected incidentally. Hamartomatous polyps may occur sporadically or may be associated with hamartomatous polyposis syndromes. Imaging findings usually demonstrate a hyper-attenuating intraluminal polypoid mass with a stalk/pedicle (Figure 20).
Fig. 20: [A, B, C] Axial, coronal, and sagittal CT images in a 68-year-old female demonstrate a 2.6 cm hyperattenuating lesion (yellow arrow) in the mid jejunum. On the sagittal image, there appears to be a thin stalk (green arrow), suggesting a polypoid lesion. Pathology revealed a benign hamartomatous polyp.
Lipoma: Lipomas are benign, well circumscribed intramural tumors which can arise anywhere along the gastrointestinal tract. Imaging findings demonstrate a well-circumscribed, homogenous fat density lesion, the imaging appearance of which is diagnostic (Figure 21). Small bowel lipomas are usually asymptomatic but may rarely present with small bowel obstruction or intussusception.
Fig. 21: [A, B] Axial and coronal CT images in a 73-year-old patient with an incidental finding of a fat-density well-circumscribed, oval structure within the mid small bowel (yellow arrow), consistent with a lipoma. [C, D] Axial and coronal FIESTA MR images in another patient demonstrate a similar, well-circumscribed intermediate intensity structure consistent with a lipoma (yellow arrow).
Vascular abnormalities: A wide variety of vascular abnormality subtypes can be seen in small bowel, including hemangiomas, telangiectasias (Figure 22), angioectasias (Figure 23), or arterio-venous malformations (AVMs) (Figure 24). These may be isolated or multiple throughout the gastrointestinal tract. They are usually asymptomatic but may occasionally present as gastrointestinal bleeding or intussusception.
Fig. 22: [A, B] Axial and coronal Ct images in an 87-year-old patient who presented with maroon colored stool. There is a tangle of mildly dilated vessels in the jejunum suggesting jejunal telangiectasia (yellow arrow).
Fig. 23: 80-year-old patient being worked up for hematuria. [A, B] CT images demonstrate a focus of hyper-attenuation within the jejunum (yellow arrow). [C, D, E] Coronal and axial T1-weighted MR images demonstrate a focus of early arterial enhancement within the jejunum (green arrow), with persistent enhancement on the delayed phase (blue arrow). Numerous prominent venous structures are identified in close proximity to this jejunal lesion on the delayed phases (orange arrows). Findings suggest jejunal angioectasia.
Fig. 24: 25-year-old patient who presented to the emergency department with complaints of abdominal pain. [A, B] Coronal and sagittal CT images reveal areas of hyper-attenuation (yellow arrow) within a portion of small bowel with numerous calcifications (green arrows). Surgical pathology came back as AVM with dystrophic calcification.
Adhesions: Small bowel adhesions can result as a subacute or long term complication of abdominal surgery or be sequela of prior radiation or intra-abdominal inflammatory process. Adhesions can lead to strictures and scarring resulting in small bowel obstruction and may mimic adenocarcinoma (Figure 25) or other small bowel tumors.
Fig. 25: [A, B] Axial and coronal CT images in an 85-year-old male presenting with abdominal pain, demonstrate small bowel obstruction due to adhesions at a site of prior surgery with mass-like scar tissue in left lower quadrant mimicking a small bowel mass (yellow arrow).
Intussusception/pseudo-intussusception: Intussusception in an adult patient should always prompt the radiologist to search for an underlying mass serving as a lead point. A small bowel mass might serve as a lead point for intussusception. Often, small bowel intussusceptions in adults can be transient and benign or be mimicked by intraluminal polyps or laminar flow of barium (Figure 26, 27).
Fig. 26: [A, B] Axial and coronal images from a 78-year-old patient with history of lung cancer undergoing CT for restaging, demonstrates at least two areas of bowel-within-bowel configuration (yellow arrows) which can mimic a mass. No lead point is identified and these represent transient small bowel intussusceptions. [C, D] Axial and coronal CT images from a 58-year-old patient with history of endometrial cancer undergoing CT for staging, again demonstrate bowel-within-bowel configuration giving the classic target appearance (yellow arrow) in the jejunum and a sausage-shaped mass within the bowel (green arrow) on the coronal images. This represents transient small bowel intussusception.
Fig. 27: 54-year-old female with a history of melanoma. [A, B] Axial and coronal CT images pelvis demonstrate an apparent small bowel intussusception in the left mid abdomen (yellow arrow). Subsequent MR enterography demonstrated normal bowel without focal lesion. Thus, findings represented laminar flow of barium within the small bowel (“pseudo-intussusception”).
Ectopic Pancreas: Ectopic pancreatic tissue is defined as pancreatic tissue lacking vascular or anatomic connection with the main pancreatic gland. These are mostly asymptomatic and discovered incidentally but may present rarely as abdominal pain due to pancreatitis or gastrointestinal bleeding. On imaging, these lesions manifest as a homogenously enhancing mass with similar enhancement to the main pancreatic gland, and may mimic as a small bowel mass (Figure 28).
Fig. 28: 39-year-old patient who presented with hematuria. [A] Axial CT image demonstrates an incidental hyperattenuating mass (yellow arrow) in the jejunum concerning for tumor. [B] Coronal post contrast T1-weighted MR image again shows the jejunal intraluminal mass (green arrow). There was no associated lymphadenopathy. Pathology revealed ectopic pancreatic tissue.