Does a pet scan show colon cancer

A pet scan is a type of imaging that uses a small amount of radiation to create a picture of the inside of the body. It is used to look for problems such as tumors, cysts, and abscesses. A pet scan can also be used to check for colon cancer.

A pet scan is not as good as a traditional medical exam for detecting colon cancer, but it can be helpful in detecting some types of the disease. A pet scan can be used to check for tumors or other abnormalities in the colon. A pet scan is not as good as a traditional medical exam for detecting cancer in the colon, but it can be helpful in detecting some types of the disease.

University of Texas M Department of Diagnostic Radiology.D. 1515 Holcombe Blvd., Anderson Cancer Center., Houston, TX 77030, USA

Corresponding address: Revathy B. Iyer, MD, Diagnostic Radiology Department, Unit 368, The University of Texas M.D. 1515 Holcombe Blvd., Anderson Cancer Center., Houston, TX 77030-4009, USA. [email protected]

2008 International Cancer Imaging Society All Rights Reserved

Abstract

Colorectal cancer is a common cancer that affects many people in the Western world. Imaging studies are frequently used to evaluate patients for colorectal cancer screening, staging, and surveillance. Ultrasound, computed tomography, and magnetic resonance imaging are examples of cross-sectional imaging studies that provide anatomic and morphologic information about tumors and patterns of spread. PET differs from other imaging techniques in that it provides information about tumor metabolism.[18F]Fluorodeoxyglucose PET has been used in the clinical evaluation of patients with a wide range of cancers because most malignancies, including colorectal cancer, have increased glucose metabolism. This review discusses the positron emission tomography/computed tomography imaging findings that may be encountered during the diagnosis, staging, and follow-up of colorectal cancer patients.

Keywords: positron emission tomography, colorectal cancer, computed tomography

Introduction

Colorectal cancer is a common cancer that affects many people in the Western world. In the United States, approximately 150,000 new cases will be diagnosed in 2008, and approximately 50,000 deaths will be attributed to the disease this year[]. The majority of colorectal cancer patients are diagnosed between the ages of 60 and 70[]. Approximately 30% of colorectal cancers occur in the sigmoid, 25% occur in the rectum and 25% occur in the cecum and ascending colon. Colon cancers are histologically adenocarcinomas that form moderately to well-differentiated glands that secrete varying amounts of mucin[].

Imaging studies are frequently used to evaluate patients for colorectal cancer screening, staging, and surveillance. This review discusses the positron emission tomography (PET)/computed tomography (CT) imaging findings that may be encountered during the diagnosis, staging, and follow-up of colorectal cancer patients.

Screening

The adenoma-carcinoma sequence theory, which holds that many colon cancers develop directly from adenomatous polyps, is well established. A polyp's malignant potential is largely determined by its size. Polyps greater than 2 cm in size have a greater than 40% risk of being cancerous, while those less than 0.5 cm have almost no chance of developing cancer. Other characteristics of a polyp that predispose it to cancer include villous architecture and the degree of cellular atypia and dysplasia[]. In unresected polyps, the cumulative risk of developing invasive carcinoma has been reported to be 2 percent.5% at 5 years, 8% at 10 years and 24% at 20 years[].

Given that colon cancers develop slowly over time, most often from preexisting adenomas, screening is critical for colon cancer prevention. The ideal screening test should be safe, accurate, and reasonably priced. While there are currently several screening methods in use, such as fecal occult blood testing, optical colonoscopy, and imaging studies such as barium enema or CT colonography, none of them meet all of these criteria. Unlike most imaging studies, which provide an anatomic or structural snapshot of abnormalities, PET imaging provides information about metabolic activity and function.

[18F]Fluorodeoxyglucose (18F-FDG) PET has been used in the clinical evaluation of patients with a wide range of cancers because most malignancies, including colorectal cancer, have increased glucose metabolism. The presence of physiologic uptake in the gastrointestinal tract is the most difficult aspect of using PET for colonic abnormalities. The exact cause of FDG uptake in the colon is unknown. According to some, increased FDG uptake is caused by uptake into mucosal structures. Regional differences in FDG uptake and standard uptake value (SUV) readings can be caused by differences in the histology of the intestinal glands in the ascending colon, descending colon, rectum, and small intestine. FDG uptake may be aided by the presence of lymphoid tissue in the colon. When compared to other regions, increased activation of glandular structures in the ascending colon could result in increased FDG uptake or even excretion into the lumen[]. Muscular activity with peristalsis may also play a minor role in physiologic colon uptake.

Aside from normal FDG uptake in the colon, FDG-PET can detect both benign and malignant colonic lesions. Yasuda et al.'s research. looked at 110 patients and found that precancerous adenomatous polyps can be detected incidentally on whole body images performed for other indications with a sensitivity of 24%[]. The researchers in this study discovered that benign colonic adenomas with FDG uptake could not be distinguished from FDG avid colon carcinomas. The size of the lesion was an important consideration in this study. The positivity rate for PET rises with increasing polyp size, with 90% positivity in lesions greater than 13 mm[]. Van Kouwen et al. conducted another study. showed similar findings with higher detection rates with increasing size (72% sensitivity with size %3E11 mm) and grade of dysplasia of the adenomatous polyps[].

Because FDG-PET imaging frequently shows colonic uptake, it is critical to determine whether the process is focal or diffuse in order to distinguish physiologic from pathologic activity. While SUV measurement cannot distinguish benign from malignant colon processes, the presence of focal colonic FDG uptake as an incidental finding on PET/CT justifies a colon screening examination, and PET/CT fusion can be especially useful for lesion localization ( Fig. 1 )[]. FDG-PET can also detect colon inflammatory diseases such as inflammatory bowel disease or diverticulitis ( Fig. 2 ).

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Figure 1

Coronal MIP image (a) of an FDG-PET scan in a patient with a history of lymphoma who presented for routine surveillance demonstrates focal uptake in the right lower quadrant (arrow) corresponding to a lesion in the cecum (arrow) on axial fused PET/CT (b), which proved to be a 3-cm adenomatous polyp at colonoscopy.

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Figure 2

An FDG-PET scan's Coronal MIP PET image (a) shows a focal area of uptake in the descending colon (arrow). The corresponding axial CT (b) image shows diverticulosis and surrounding stranding (arrow), both of which are consistent with diverticulitis.

Several studies have looked into the feasibility of combining FDG-PET with CT colonography (CTC), a relatively new technique that provides an endoluminal view of the colon. Gollub et al. conducted a prospective study. After colonic cleansing and carbon dioxide insufflation, 17 patients underwent a combined PET/CT examination. These researchers discovered that PET/CTC was a feasible technique that allowed excellent image correlation in polyps larger than 10 mm in size and showed promise in accurate anatomic correlation of both malignant and premalignant colon lesions[]. Cost, availability, and relative non-specificity make this technique unsuitable for widespread colorectal screening at the moment.

Staging

Once a colorectal cancer diagnosis has been established, staging becomes critical for prognosis and determining appropriate therapy. Complete surgical removal of the tumor, as well as regional lymphatic drainage, provides the best prognosis for patients with colorectal cancer. The initial stage of diagnosis has a large impact on recurrence rates. As a result, neoadjuvant or adjuvant chemotherapy and radiation therapy are increasingly being used to reduce the risk of recurrence[]. Pre-operative therapy in patients with rectal cancer can help to downstage more advanced tumors, allowing sphincter preservation[]. When a tumor becomes invasive, it can spread through the layers of the colonic wall and invade neighboring structures[,]. Lymphatic, hematogenous, and peritoneal spread are all possibilities.

PET scans can help with pre-operative colorectal cancer staging. The greatest benefit of PET is that it provides total body coverage, allowing for the detection of disease at distant sites. Due to limited spatial resolution and the inability to distinguish the layers of the colonic wall, PET and PET/CT are clearly limited for T staging of the primary tumor. Transrectal ultrasound (US) and magnetic resonance imaging (MRI) have much higher anatomic resolution and are more useful for T staging[].

Cross-sectional imaging techniques such as US, CT, and MRI can make nodal staging difficult. Size (greater than 1 cm) remains the primary criterion for predicting nodal metastasis on cross-sectional imaging, despite the fact that size is not an ideal indicator of disease. The advantage of PET is the ability to use metabolic activity to differentiate benign from malignant adenopathy at sites other than the primary tumor. Due to the primary tumor's FDG activity, which may obscure small lymph nodes, nodes in the immediate vicinity of the primary tumor are extremely difficult to detect with PET. Small nodes are also difficult to detect with PET. The overall sensitivity for nodal staging is therefore reported to be quite low, only 29%[,]. It is critical not to confuse urinary physiologic activity with tumor spread in the retroperitoneum or pelvis, and fused PET/CT has an advantage in anatomic localization over PET alone.

Accurate staging also necessitates the detection of distant metastatic disease sites ( Fig. 3 ). This is significant because localized disease spread, such as to the liver, can be resected and cured. Resection of colorectal cancer metastases with or without hepatic arterial perfusion therapy can lead to up to 60% 10-year survival in selected patients[]. As a result, knowing the extent of the tumor prior to surgery is critical in determining whether or not curative resection is possible. FDG-PET may be more sensitive than CT in detecting hepatic and pulmonary metastases as well as identifying other sites of intra-abdominal disease[]. FDG-PET showed greatest accuracy in the detection of liver metastases with reported accuracy up to 99%, sensitivity up to 100% and specificity up to 98%[]. It is critical to remember that lesion size is an important detection criterion, and small hepatic lesions are still difficult to detect due to relatively high background liver activity. Furthermore, the limited spatial resolution of PET makes surgical planning difficult.

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Figure 3

In a patient with newly diagnosed colorectal cancer, a coronal MIP PET image shows a primary FDG avid tumor in the rectosigmoid (thick arrow) with FDG avid metastases to the liver (thin arrows).

PET can also detect disease sites that may preclude surgery or alter the surgical approach. Several studies have shown that PET and PET/CT findings result in a change in stage and, as a result, change in management in up to one-third of patients. In a study of patients with low rectal cancers, FDG-PET/CT altered treatment plans in 38% of patients largely through the detection of unsuspected inguinal adenopathy[].

FDG-PET has also been used to predict response to pre-operative therapy and thus outcome in a variety of cancers, including rectal cancer. Guillem et al. conducted a study on.FDG-PET imaging was performed on 15 patients with locally advanced rectal cancer before and after chemoradiation. Based on pathologic examination, all patients showed some degree of response to pre-operative therapy. The mean percentage decrease in SUVmax was 69% for patients that remained free of disease at a median follow-up of 42 months; the SUVmax decreased by only 37% in patients who eventually developed recurrence[].

Surveillance

Despite the fact that most patients with colorectal carcinoma undergo surgery with the goal of curing the disease, nearly four out of every ten patients experience disease relapse[,]. Over the past decade, aggressive surgical approaches to metastatic disease are being practiced and nearly 30% of patients undergo resection of recurrent disease with increased long term survival[]. Many previously unresectable lesions can be reduced in size with the use of newer chemotherapeutic agents, allowing potentially curative surgery[]. Even in patients with surgically unresectable disease, early administration of newer chemotherapeutic agents improves survival[,].

Nearly 85% of recurrences occur within the first 3 years after surgery and nearly none occur after 5 years[]. As a result, most surveillance strategies concentrate resources on the first three years following surgery. Clinicians use a variety of approaches to surveillance, ranging from the "call me if you have symptoms" strategy to aggressive monitoring with regular clinic visits, periodic tumor marker assays, cross sectional imaging, ultrasound, and endoscopy. Recognizing the benefit of early treatment in resectable metastases, the American Society of Clinical Oncology (ASCO) in 2005 recommended that a carcinoembryonic antigen (CEA) assay be performed every 3 months for the first 3 years, a CT scan of the chest, abdomen, and pelvis be performed every year for the first 3 years, and an endoscopy at 3 years in patients with stage 2 and sta.

When there is a high index of suspicion for recurrence as evidenced by a rising CEA but the routine diagnostic work up is equivocal in patients with a history of colorectal cancer, PET is commonly used as a problem solving tool. Flanagan et al. FDG-PET detected disease in 15 of 22 patients with elevated CEA but negative diagnostic workup. They showed a positive predictive value of 89% and a negative predictive value of 100%[]. Flamen et al. conducted a similar study. report a sensitivity of 75% and a positive predictive value of 79% in a retrospective study of 50 patients[]. These studies, however, were conducted using stand-alone PET instruments between 1993 and 1996 and 1996 and 1999, respectively. Since then, significant technological advances have resulted in hybrid images and images of higher resolution and quality.

Another common use of PET in recurrent disease patients is surgical planning, particularly in patients with resectable metastases in the liver or lungs. The detection of occult metastases in such patients would save many patients from unnecessary surgery and significantly alter management. Wiering and colleagues report in a meta-analysis that FDG-PET changed clinical management in 31 patients.6% of patients[]. PET in pre-surgical planning reduces the number of unnecessary surgeries and may increase survival by allowing for better patient selection[,]. FDG-PET can also help patients with rectal cancer who have been treated with surgery and chemoradiotherapy but have a recurrence in the pelvis. Early detection of pelvic recurrence is required for surgery to be effective, and distinguishing tumor from post-treatment fibrosis can be difficult with conventional cross-sectional imaging studies; FDG-PET can help in this regard ( Fig. 4 )[]. FDG-PET can detect other sites of disease in patients who are candidates for curative resection of local recurrence, avoiding unnecessary surgery.

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Figure 4

On contrast enhanced MRI (b), a fused PET/CT image (a) shows an FDG avid area along the left pelvic sidewall (arrow) with diffuse pre-sacral thickening without a distinct mass in a patient with colorectal cancer treated with chemoradiation and surgery, now with rising tumor markers. A biopsy of the FDG-avid area revealed a recurrence.

It is critical to understand some of PET's limitations. When compared to other cross-sectional imaging modalities such as CT and MRI, FDG-PET has lower spatial resolution. Most commercially available PET scanners currently have resolutions in the 1 megapixel range.3 to 1.Lesions smaller than 5 cm may not be detected due to volume averaging[]. The overall high background hepatic activity also makes PET difficult for assessing small hepatic metastases. Despite a few reports of PET being superior to cross-sectional imaging in detecting hepatic metastases, the lack of clear anatomic landmarks and inability to detect small lesions are major limitations of PET alone. Another limitation of FDG-PET that should be considered when imaging patients with colorectal cancer is the relative insensitivity for detection of mucinous tumors, which is likely due to their paucicellularity[]. The use of neoadjuvant chemotherapy prior to surgery can also reduce the sensitivity of PET in detecting lesions[]. Due to physiological bowel activity, serosal metastases on the surface of the large and small bowel may still be missed. Because of partial volume artifacts amplified by breathing, pulmonary metastases, particularly small ones, may be missed. Aside from the previously mentioned false negatives on FDG-PET, it is important to remember that inflammation can result in false positive FDG uptake.

Conclusion

In summary, FDG-PET has been used in colorectal cancer patients for disease detection, staging, and surveillance. To improve specificity, physiologic activity in the gastrointestinal tract should be carefully correlated with fused CT images. FDG-PET also provides staging information, particularly in the presence of distant metastatic disease. Due to a lack of large randomized trials, there is insufficient data to justify the routine use of FDG-PET in detecting recurrence in patients with colorectal cancer. PET is still regarded as a modality with promising future applications. PET CT may find additional future applications in areas such as post-operative surveillance of colorectal carcinoma, where surgical treatment options and chemotherapy strategies are constantly being redefined, particularly with the development of new, more specific radiotracers.

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