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Everything you need to know about Computed Tomography (CT) & CT Scanning


Combined PET/CT Scan in Patients with Colorectal Carcinoma: Is the "CT" Portion Necessary?

Ihab R. Kamel, MD, PhD; Edward Neyman, MD; Christian Cohade, MD; Richard L. Wahl, MD; Elliot K. Fishman, MD

Introduction

Imaging plays an important role in the diagnosis and management of patients with colorectal cancer. The goals of imaging include the detection of the primary lesion, staging of the disease, assessing treatment response, and detecting disease recurrence. Anatomical imaging can detect pathological processes only after they reach a detectable size, become contour-deforming, or if they have abnormal enhancement after IV contrast administration. The availability of new technology that provides anatomical (CT) and functional (PET) imaging could have a significant impact on the diagnosis, staging and management of oncologic patients. The high (>85%) sensitivity of FDG PET for tumor localization and detection is associated by a lower specificity due to the normal physiologic accumulation of FDG in the stomach, bowel, kidneys, ureters and bladder. Overall accuracy can potentially be improved by including anatomical imaging, which would increase specificity of FDG PET uptake, and improve localization of increased metabolic activity detected by PET.

Principles of FDG PET

The most commonly used PET radiopharmaceutical is [18F] fluorine-18-fluoro-2-deoxyglucose (FDG) [1]. Physiologic uptake occurs in normal brain, myocardium and skeletal muscle. Uptake is also physiologic in the liver, spleen, kidney, bowel and stomach, and to a lesser degree in the bone marrow, tonsils, thyroid gland, and breast (Figure 1). High uptake by cancer cells results in high tumor-to-background ratio, especially for lesions greater than 1 cm. High uptake may also occur in inflammatory cells, and may be a cause of false positive test [1 - 3].

Figure 1
Physiologic uptake of [18F] fluorine-18-fluoro-2-deoxyglucose (FDG).

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Technique

Fasting is recommended 4 hours prior to imaging, to lower insulin and blood sugar levels. Strenuous exercise should be avoided 24 hours prior to the study to decrease skeletal muscle uptake. Imaging is performed one hour after the administration of 750 mL 3% iodine solution to opacify the bowel, and 15-25 mCi 18FDG intravenously. They study is performed without IV iodinated contrast. Patients are positioned supine, with their arms to the side. If tolerated, patients should be positioned with their arms up to reduce artifacts over the upper abdomen, particularly the liver. Scans are acquired from skull base to mid thigh level during quiet respiration. Non-contrast axial CT images were obtained at the same anatomical locations, with 5-mm thick sections at 4.25 mm intervals and 80 mAs.

Patients and Methods

A total of 100 PET/CT scans in 90 consecutive patients were retrospectively evaluated. There were 40 males and 50 females, with the mean age of 63 years (range 31-92 years). Imaging was performed on a GE Discovery LS PET/CT scanner. The CT component of the combined study was evaluated separately, and the results were compared to the combined PET/CT report. Scans were evaluated for the presence of local recurrence and distant metastases.

Results

The indication for the exam was to evaluate for recurrence of colorectal cancer in 83 cases, to determine spread of disease in 15 cases, and to evaluate for a possible primary malignancy in 2 patients with rising CEA (Figures 2, 3). CT helped in improving overall PET accuracy by decreasing the number of false negative and false positive scans. In a total of 18 studies (18%) CT improved overall accuracy of the PET/CT study, and provided significant information that could potentially alter patient management. PET imaging has low sensitivity in detecting mucinous tumors, and can result in a false negative PET [4]. In 3 patients with mucinous primary colon cancer, CT demonstrated metastatic disease not detected on PET imaging (Figures 4, 5). Patients that have undergone prior surgical resection or radiofrequency ablation may result in false positive uptake on PET imaging. Demonstration of surgical clips and absence of a mass on CT helps to reduce the false positive cases for malignancy on PET. In the current study, four patients had post surgical/radiofrequnecy ablation changes in the liver (Figures 6, 7), and 2 patients had false positive uptake at the site of rectal resection (Figures 8, 9). Inflammatory pulmonary and hilar uptake on PET was resolved by CT in an additional 3 patients. In 4 patients the etiology of the non-neoplastic increased activity on PET was revealed by CT. These were due to left adrenal myelolipoma (Figure 10), Pagetoid clavicle (Figure 11), left hydronephrosis and urine leak (Figure 12), and a recently placed IVC filter (Figure 13). In 2 cases, CT provided useful anatomical information including delineation of a rectovaginal fistula (Figure 14) and subcutaneous nodules in right upper quadrant, which could be mistaken for hepatic metastases (Figure 15).

Other incidental findings were detected by CT. The most common findings were renal (n=15) and these included cysts (n=6), hydronephrosis (n=3), absent/ectopic kidney (n=2), renal artery aneurysm (n=1) and angiomyolipoma (n=1). Cardiovascular findings were recorded in 11 cases, and included vascular/coronary calcifications (n=7), abdominal aortic aneurysm (n=2), cardiomegaly (n=1) and right-sided aortic arch (n=1). Seven patients had incidental thoracic findings including pleural effusion (n=3) and emphysema (n=4). In addition, gallstones were detected in 5 patients.

The CT portion of the PET/CT study has a limited role in evaluation of the liver. This is because of the low mAs utilized, which increases quantum mottle. Lack of IV contrast administration also limits the value of CT, and results in lower detection rate and poor characterization of liver lesions. In addition, most patients are scanned with their arms on their side, resulting in streak artifacts along the liver (Figure 16). This can be minimized by scanning with the patient’s arms above their head. However, this position may not be tolerated by all patients. In 9 studies (9%) liver masses were not adequately visualized on the CT portion of the combined study.

Figure 2
Primary colon cancer and liver metastasis in a 81-year-old woman. (A) Increased activity in the right lobe of the liver (arrow) and in the left pelvis (arrowhead) on coronal PET. (B) Axial non-contrast CT image through the liver demonstrates large hypodense mass in right lobe (arrow). (C) Axial non-contrast CT image through the pelvis reveals apple core lesion in the rectosigmoid colon (arrowhead). (D) Fused image confirms the mass in the rectosigmoid colon and right lobe liver metastasis.
 

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Figure 3
Abdominal metastases in a 62-year-old woman with history of colon cancer. (A) Increased activity in the right lobe of the liver (arrow) and in multiple foci throughout the abdomen (arrowheads) on coronal PET. (B) Axial non-contrast CT image through the liver demonstrates large hypodense mass in right lobe (arrow). (C) Axial CT image more inferiorly demonstrates small serosal implant on the liver surface (arrowhead). (D) Omental implants (arrowheads) are confirmed on this axial CT image through the abdomen. Also, notice small amount of ascitis.
 

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Figure 4
Nodal recurrence in a 47-year-old man with history of mucinous colon cancer. (A) Axial non-contrast CT image through the pelvis demonstrates large hypodense mass in the right lower quadrant (arrow). (B) No corresponding increased activity in the pelvis on the PET image (arrow). (C) Fused image demonstrates mass with poor uptake (arrow) in right lower quadrant.
 

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Figure 5
Pulmonary metastases in a 55-year-old man with history of colon cancer. (A) Axial non-contrast CT image through the chest demonstrates a large mass in left midlung (arrow). Intense uptake is seen on the corresponding PET image (B) and confirmed on the fused image (C). (D) A second mass is seen in the left apex on CT (arrow). (E) Only minimal increased uptake is noted on PET image, and on the fused image (F). Both nodules continued to increase in size on follow-up CT.
 

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Figure 6
Post-radiofrequency ablation of liver metastases in a 70-year-old man with history of colon cancer. (A) Axial PET image through the abdomen reveals linear increased uptake along the right lobe of the liver posteriorly (arrow). No mass was seen on the corresponding axial non-contrast CT (B). (C) Fused image showing the area of increased uptake in the liver (arrow). No residual tumor was seen on follow-up contrast-enhanced CT.
 

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Figure 7
Post-radiofrequency ablation and resection of liver metastases in a 78-year-old man with history of colon cancer. (A) Axial non-contrast CT image through the liver demonstrates hypodense areas at the site of radiofrequency ablation. Surgical clips are also seen along a right lobe lesion (arrow). (B) Axial PET image through the liver reveals small foci of increased uptake along the margin of the treated lesions (arrowhead). (C) Fusion image confirmed the location of areas of increased uptake, most likely due to reactive granulation tissue. No residual tumor or recurrence was detected on follow-up imaging.
 

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Figure 8
Post-surgical resection and bowel anastomoses in a 71-year-old man with history of rectal cancer. (A) Axial non-contrast CT image through the pelvis demonstrates symmetric peri-rectal fat with no mass seen after resection. Axial PET (B) and fusion (C) images show mild posterolateral peri-rectal increased uptake (arrow), most likely reactive. No residual tumor or recurrence was detected on follow-up imaging.
 

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Figure 9
Post-surgical resection and bowel anastomoses in a 57-year-old man with history of rectal cancer. (A) Increased activity in the pre-sacral region (arrow) was suspicious for local recurrence at the site of surgical anastomosis. No anastomotic recurrence (arrow) was detected on axial CT images through the pelvis (B), nor on follow up CT (C).
 

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Figure 10
Increased uptake of the left adrenal gland in a 68-year-old man with history of colon cancer. Coronal (A) and axial (B) PET CT images demonstrating intense focal increased uptake of the left adrenal gland (arrow). CT documented the presence of fat (arrowhead), compatible with myelolipoma. The lesion remained stable on follow-up CT.
 

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Figure 11
Increased uptake of the right clavicle in an 81-year-old man with history of colon cancer. (A) Axial PET image shows intense increased activity in the region of the right clavicle (arrow), suspicious for metastasis. Axial CT image through the same level in soft tissue (B) and bone (C) windows demonstrates bony expansion and sclerosis compatible with Paget's disease.
 

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Figure 12
Increased uptake in the region of the left kidney in a 46-year-old man with history of colon cancer. Coronal (A) and axial (B) PET images show intense increased activity in the region of the left kidney (arrow), suspicious for metastasis. Axial CT (C) image reveals mild left perinephric fat stranding and urine leak in this patient with history of renal stones. Findings were confirmed on the fused image (D).
 

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Figure 13
Increased uptake in the retroperitoneum in a 48-year-old woman with history of colon cancer. (A) Axial PET image shows intense increased activity in the region of the retroperitoneum and IVC (arrow), suspicious for metastasis. (B) Axial CT image reveals a recently placed IVC filter (arrow). Findings were confirmed on the fused image (C).
 

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Figure 14
Increased uptake in the pelvis in a 59-year-old woman with history of rectal cancer. (A) Axial PET image shows intense foci of increased activity in the pelvis (arrows), suspicious for local recurrence. (B) Axial CT image reveals large soft tissue mass extending anterior to the rectum, and suspicious for rectovaginal fistula. (C) Findings were confirmed on axial T1-weighted image through the pelvis (arrow).
 

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Figure 15
Abdominal metastases in a 66-year-old man with history of colon cancer. (A) Increased activity in the right upper quadrant (arrows) on the coronal PET image could be mistaken for hepatic metastases. (B) Axial non-contrast CT image through the upper abdomen demonstrates 2 subcutaneous nodules (arrows) at the site to prior surgical incision. No recurrence was detected in the liver, as confirmed by contrast-enhanced CT (C).
 

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Figure 16
Hepatic metastasis in a 68-year-old man with history of colon cancer. (A) Axial PET image shows intense foci of increased activity in the liver (arrows). (B) Axial CT image reveals subtle mass in the right lobe, partially obscured due to streak artifacts from the arms. (C) Findings were better delineated on the fusion image (arrow).
 

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Discussion

The incidence of colorectal cancer is the United States is approximately 400,000 cases per year [5]. Hepatic metastases develop in up to 30% of patients, and if untreated, will have a poor 5-year survival rate of less than 3% [6]. The results of treatment with systemic chemotherapy or radiation therapy are disappointing. The only potential for cure in these patients is surgical treatment with partial hepatic resection, which results in 5-years survival of 25-40% [7]. However, only 25% of all colorectal cancer patients are potential candidates for surgery. Extrahepatic metastases are often detected at surgery, and they result in unnecessary laparotomy. In addition, recurrence after surgery is extrahepatic in 50% of cases [8]. Therefore, an important role of preoperative imaging is to detect hepatic and extrahepatic disease.

PET imaging has an overall sensitivity of 97% in the detection of colorectal metastases in the whole body, according to a recently published metaanalysis [9]. Increased glucose uptake in malignant tissue results in increased metabolic uptake of FDG PET. This is valuable in cases where malignancy could be detected by conventional anatomic imaging. The presence of lymph node micrometastases have been readily detected by PET when they are apparently uninvolved by CT. However, focal increased uptake of FDG is nonspecific, and can occur in acute and chronic inflammation. Therefore, relying solely on functional imaging may result in decreased overall accuracy due to false positive lesions. The addition of anatomical imaging, as shown in the current study, improves overall accuracy by correctly identifying cases that has increased FDG uptake due to recent surgery, radiofrequency ablation, or pulmonary infiltrate [3]. Overall accuracy is also improved by the ability of CT to detect mucinous colorectal cancer metastases, that have poor PET detection rate [4].

Conclusion

The CT portion of a combined PET/CT provides valuable anatomical and pathological information to the functional information provided by PET, and could potentially improve the overall accuracy of the combined study. Significant improvement in accuracy of the combined study can be achieved if the CT portion of the combined study is performed with IV contrast.

References

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© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.