Incidental colorectal polyps in positron emission tomography

INTRODUCTION

Positron emission tomography (PET) with fluorodeoxy glucose (FDG) is commonly used imaging modality in the workup of various malignancies. Incidental colonic FDG uptake is often observed in patients who undergo PET/CT studies. Diffuse FDG uptake is associated with normal findings at colonoscopy; segmental high uptake suggests inflammation, while focal and nodular uptake is associated with neoplasia. The usefulness of FDG PET/CT for incidental premalignant colonic lesion detection has been previously reported.[1]

CASE REPORTS

Case 1

A 55-year-old lady from Norway was referred to our department for evaluation of Caecal lesion detected by PET/CT [Figures 1a and b]. She had previously undergone surgery 4 years before presentation for carcinoma cheek. A PET/CT 2 years after the first surgery showed local recurrence and a radical hemimandibulectomy was done. She had no family history of colon, breast or uterine cancer. Colonoscopy showed a 1.5 cm flat polyp in the caecum [Figure 2a] which on narrow band imaging and chromoendoscopy with indigo carmine had pit pattern suggestive of adenomatous polyp [Figure 2b]. Endoscopic mucosal resection (EMR) of the lesion was done. Histopathology showed tubular adenoma.

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Figure 1a

Case1: Positron emission tomography computed tomography showing fluorodeoxy glucose avid lesion in caecum

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Figure 1b

Case1: Positron emission tomography computed tomography showing fluorodeoxy glucose avid lesion in caecum

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Figure 2a

Case 1: Colonoscopy showing flat polyp in caecum

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Figure 2b

Case 1: Narrow band imaging showing adenomatous polyp

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Case 2

A 69-year-old lady with L1 vertebral compression fracture underwent screening for malignancy. On evaluation, she had a suspicious left iliac fossa lesion on PET/CT [Figures 3a and 3b]. Ultrasound showed normal ovaries. Colonoscopy was done, which showed a 2 cm polypoidal lesion in sigmoid colon with short stalk [Figure 4]. Polypectomy was done and biopsy showed tubular adenoma.

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Figure 3a

Case 2: Positron emission tomography computed tomography showing Fluorodeoxy glucose avid lesion in sigmoid colon

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Figure 3b

Case 2: Positron emission tomography computed tomography showing fluorodeoxy glucose avid lesion in sigmoid colon

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Figure 4

Case 2: Colonoscopy showing adenomatous polyp in sigmoid colon

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DISCUSSION

A broad range of screening modalities for the detection of colorectal adenomata are available, such as fecal occult blood testing, colonoscopy, barium enema, and virtual colonoscopy of which colonoscopy and biopsy is regarded as the gold standard for the detection of premalignant lesions such as adenomatous polyps. Detection and removal of adenomata results in a decrease in the incidence and mortality from colorectal cancer.[2] FDG PET/CT is used in clinical practice to detect a variety of tumors, including lymphoma, melanoma, lung, and colon cancer.[3]

Increased glucose metabolism has been reported in colorectal cancer and anaerobic glycolysis in vitro in surgically resected colon cancers was demonstrated as early as in 1962.[4] FDG PET studies based on in vivo measurement of glucose metabolism, showed high FDG uptake in both primary and recurrent colorectal cancers. Primary colorectal cancers, as small as 1.4 cm have been detected with PET.[5] In a study done to validate FDG PET findings with colonoscopy in Netherlands, compared with colonoscopy, FDG PET had a sensitivity of 74% and specificity of 84%. The positive predictive value of FDG PET was 78%. FDG PET failed to detect small (diameter 3-10 mm) polyps in four patients. In nine cases abnormal FDG accumulation on PET imaging was the sole reason for performance of an endoscopic procedure. In these cases, endoscopy detected large adenomatous polyps in four patients and carcinomas in two patients, but no abnormalities were detected on endoscopy in the other three patients. There was a good correlation between the location of FDG uptake and endoscopy-positive lesions. FDG PET can detect clinically relevant lesions of the colon. The degree of uptake was proportional to the degree of dysplasia in the adenoma.[6]

There are pitfalls in using PET/CT for routine screening for colonic adenomata and cancer surveillance in high-risk individuals. The large intestine is an established site of physiologic FDG uptake, and intestinal FDG uptake poses a practical problem in the evaluation of PET images. Intestinal uptake can hinder detection of FDG uptake in the adenoma. It can also be the source of a false-positive result.[7] Data regarding the usefulness of PET/CT comes from centers with high incidence of colonic polyps and positive predictive value may be much lower in India where incidence of polyps is low. The advantage of non-invasive nature of this procedure is off-set by its prohibitive cost at present.

Our cases illustrate the fact that with the increasing use of PET/CT for screening and cancer surveillance, we can expect detection of incidental colonic polyps, which are more likely to be adenomatous. A colonoscopy is warranted in these individuals for confirmation and removal of polyps.

CONCLUSION

Increased glucose metabolism is observed in colonic adenomata, which may be detected on FDG PET/CT. The widespread use of FDG PET/CT will increase the number of adenomatous polyps detected. Hence, it is important to recognize that such polyps can be found incidentally with PET/CT and need to be investigated further by colonoscopy.