Variants and Pitfalls in PET/CT Imaging of Gastrointestinal Cancers

Abstract

In the past two decades, PET/CT has become an essential modality in oncology increasingly used in the management of gastrointestinal (GI) cancers. Most PET/CT tracers used in clinical practice show some degree of GI uptake. This uptake is quite variable and knowledge of common patterns of biodistribution of various radiotracers is helpful in clinical practice. ¹⁸F-Fluoro-Deoxy-Glucose (FDG) is the most commonly used radiotracer and has quite a variable uptake within the bowel. ⁶⁸Ga-Prostate specific membrane antigen (PSMA) shows intense uptake within the proximal small bowel loops. ¹¹C-methyl-L-methionine (MET) shows high accumulation within the bowels, which makes it difficult to assess bowel or pelvic diseases. One must also be aware of technical artifacts causing difficulties in interpretations, such as high attenuation oral contrast material within the bowel lumen or misregistration artifact due to patient movements. It is imperative to know the common variants and benign diseases that can mimic malignant pathologies. Intense FDG uptake within the esophagus and stomach may be a normal variant or may be associated with benign conditions such as esophagitis, reflux disease, or gastritis. Metformin can cause diffuse intense uptake throughout the bowel loops. Intense physiologic uptake can also be seen within the anal canal. Segmental bowel uptake can be seen in inflammatory bowel disease, radiation, or medication induced enteritis/colitis or infection. Diagnosis of appendicitis or diverticular disease requires CT correlation, as normal appendix or diverticulum can show intense uptake. Certain malignant pathologies are known to have only low FDG uptake, such as early-stage esophageal adenocarcinoma, mucinous tumors, indolent lymphomas, and multicystic mesotheliomas. Response assessment, particularly in the neoadjuvant setting, can be limited by post-treatment inflammatory changes. Post-operative complications such as abscess or fistula formation can also show intense uptake and may obscure underlying malignant pathology. In the absence of clinical suspicion or rising tumor marker, the role of FDG PET/CT in routine surveillance of patients with GI malignancy is not clear.

Figure 1:

Maximum intensity projection (MIP) images depicting normal gastrointestinal biodistribution of various radiotracers.

Figure 2:

Physiologic and non-physiologic radiotracer uptake within the esophagus. MIP and fused sagittal images demonstrating mild to moderate diffuse uptake of benign esophagitis on 18F-FDG PET/CT (A), ⁶⁸Ga-PSMA PET/CT (B), ⁶⁸Ga-DOTATATE PET/CT (C), and 18F-FACBC PET/CT (D). MIP and axial fused images of the chest showing benign focal uptake within the distal esophagus (E), moderately intense peripheral uptake within a benign leiomyoma (F), post-radiation inflammation within the mid esophagus (G), and low-grade uptake within a biopsy proven esophageal adenocarcinoma (H).

Figure 3:

Axial CT, PET, and fused images of 18F-FDG PET/CT demonstrating intense, predominantly proximal, physiologic gastric uptake (A); moderately intense diffuse uptake in H. pylori infection (B) and chronic inactive gastritis (C); mild diffuse uptake within the pylorus post-radiation (D).

Figure 4:

Axial CT, PET, and fused images of 18F-FDG PET/CT demonstrating intensely avid gastric schwannoma along the greater curvature of the stomach (A), mildly avid leiomyoma at the gastroesophageal junction (B), diffuse low-grade uptake within an infiltrating poorly differentiated adenocarcinoma with signet ring cell features (blue arrows) and peritoneal carcinomatosis (yellow arrows), (C) and mild diffuse uptake within a biopsy proven gastric MALT lymphoma (D).

Figure 5:

Axial CT, PET, and fused images of 18F-FDG PET/CT demonstrating low-grade uptake within a duodenal tubulovillous adenoma (A), intensely avid gastrointestinal stromal tumor (GIST) of the duodenum(B), and duodenal carcinoma (C).

Figure 6:

Axial CT, PET, and fused images of 18F-FDG PET/CT through the lower abdomen showing intense physiological uptake within the ileocecal junction (A), benign sessile tubular adenoma (B), and metastatic deposit at the ileocecal junction from patient’s known poorly differentiated gastric adenocarcinoma with signet ring cell features (C).

Figure 7:

Variable FDG uptake within the colon. A) Intense diffuse uptake throughout the large bowel in patient on metformin. B) Segmental intense uptake within the sigmoid colon with bowel wall thickening in a patient with known ulcerative colitis without active symptoms. C) Intense uptake secondary to attenuation artifact from oral contrast media. D) Moderately intense diffuse uptake within the distal large bowel in a patient with active ulcerative colitis. E) Post-radiation colitis of the hepatic flexure (E2) in a patient who underwent radiation therapy for a moderately intense right posterior peritoneal metastasis (E1; yellow arrows). Post-radiation, the nodule decreased in size; however, intense uptake is seen within the adjacent hepatic flexure (E2; blue arrows). Bowel uptake resolved on subsequent imaging with residual low-grade uptake within the peritoneal nodule (E3).

Figure 8:

Benign and malignant FDG uptake within the bowel. A) Moderately intense uptake within a gas filled normal appendix. B) Intensely avid diverticulitis within the descending colon (B1), which resolved on follow up imaging two weeks later (B2). C) Intensely avid hepatic flexure tubule-villous adenoma. D) Benign intense uptake within a left anterior abdominal wall stoma. E) Rectal cancer with a mildly avid large mucinous component as seen on axial T2W MRI. F) Mildly avid right lateral wall rectal neuroendocrine tumor. G) Physiologic intense uptake within the anal canal in a patient undergoing 18F-FDG PET/CT for head and neck cancer. H) Moderately avid internal/external hemorrhoids at the anal verge as seen on MRI and also noted on clinical examination. I) Moderately intense radiation proctitis. J) Intense uptake along the rectovaginal fistula as seen on MRI.

Figure 9:

Axial fused and CT images of 18F-FDG PET/CT demonstrating moderately intense mesenteric panniculitis (A), left lower quadrant fat necrosis (B), intense uptake relating to surgical mesh hernia repair (C), moderately intense uptake with a hernia plug (D), mildly avid splenules at the pancreatic tail (E), intensely avid transposed ovary within the left paracolic gutter (F), low-grade malignant ascites from unknown primary (G), intensely avid post-surgical abscess adjacent to the ascending colon (H), minimally avid peritoneal mesothelioma within the right lilac fossa (I), and a minimally avid recurrent peritoneal metastatic deposit at the splenectomy bed from a pancreatic tumor (J).