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Pancreatic Mass Due to Chronic Pancreatitis

Correlation of CT and MR Imaging Features with Pathologic Findings

¹ Department of Radiology, D1, Osaka University Medical School. 2-2 Yamadaoka, Suita City, Osaka 565-0871, Japan.
² Department of Surgery and Clinical Oncology, Osaka University Medical School, 2-2 Yamadaoka, Suita City, Osaka 565-0871, Japan.
³ Department of Surgical Oncology, Osaka University Medical School, Suita City, Osaka 565-0871, Japan.
⁴ Department of Pathology, Osaka City University Medical School, Suita City, Osaka 565-0871, Japan.

Received November 2, 2000; accepted after revision February 22, 2001.

 
Address correspondence to T. Kim.

Abstract

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OBJECTIVE. The purpose of this study was to identify helical CT and MR imaging features of pancreatic masses (focal enlargement) due to chronic pancreatitis and their correlation with pathologic findings.

CONCLUSION. When histologic fibrosis is uniformly present through the pancreas in patients with chronic pancreatitis, there is no demarcation of masses due to chronic pancreatitis. When there is a greater degree of histologic fibrosis in the masslike part of the pancreas, the mass is often demarcated from the remaining pancreas, and the enhancement pattern on two-phase helical CT and dynamic gadolinium-enhanced MR imaging mimics that of pancreatic adenocarcinoma.

Introduction

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Chronic pancreatitis is a disease of prolonged pancreatic inflammation and is characterized by irreversible morphologic or functional abnormalities [1]. The histologic changes include fibrosis and atrophy of the glandular elements [2]. The main cause of chronic pancreatitis is alcohol abuse, but 30 % of patients with chronic pancreatitis have no definite cause and are classified as having idiopathic chronic pancreatitis [3]. The diagnosis of chronic pancreatitis based on in vivo measurements can be difficult because findings of laboratory values are frequently normal and physiologic findings are nonspecific [4]. The most pathognomonic feature of chronic pancreatitis is the presence of parenchymal calcification on CT scans [3]. Morphologic changes frequently result in a shrunken and atrophic pancreas. However, the pancreas is occasionally enlarged [2], and when this enlargement is observed focally, it often looks like a pancreatic neoplasm and is sometimes difficult to distinguish from carcinoma or other tumors [5,6,7]. Previous articles [5,6,7,8] have reported on the CT and MR imaging features of a focal pancreatic mass due to chronic pancreatitis. However, no reports correlate the CT and MR imaging features with histologic findings in the mass and in the nonmass portion of the pancreas.

The purpose of this study was to clarify helical CT and MR imaging features of pancreatic mass due to chronic pancreatitis and to correlate these features with pathologic findings.

Materials and Methods

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Between 1994 and 1997, chronic pancreatitis was histologically confirmed for seven pancreatic masses in seven patients who had been referred to our institution because of suspicion of pancreatic tumor. The patients were five men and two women, 41-70 years old (mean age, 54 years) who were histologically diagnosed with chronic pancreatitis. Five patients underwent surgical excision of the pancreatic mass, and chronic pancreatitis was confirmed by histologic examination of the resected specimens. Histologic diagnosis was made by open biopsy in one patient and by percutaneous core biopsy in one patient. One of the seven patients was asymptomatic, and the symptoms of the other six patients were obstructive jaundice (three patients), abdominal pain (two patients), and abdominal mass (one patient). One patient with obstructive jaundice had a metallic stent inserted at another hospital for stenosis of the common bile duct. The cause of chronic pancreatitis was thought to be alcohol abuse for two patients but was not definite for the other five. The pancreatic mass was located in the pancreatic head in five patients and in the pancreatic body and tail in the other two.

All seven patients underwent two-phase helical CT (HiSpeed Advantage; General Electric Medical Systems, Milwaukee, WI) of the pancreas. After we obtained unenhanced CT scans, the pancreatic and portal venous phase helical CT scans were obtained respectively, starting 35 and 70 sec after the initiation of IV injection of 100 mL of noniodinated 300 mg I/mL iohexol (Omnipaque 300; Daiichi Pharmaceutical, Tokyo, Japan) at a rate of 3 mL/sec. Each helical scan was obtained with a single 20- to 25-sec breath-hold with a slice collimation of 5 mm and a table pitch of 1:1.

Six patients (Table 1) underwent MR imaging with a 1.5-T superconducting unit (Signa Advantage or Horizon; General Electric Medical Systems). For the unenhanced study, T1-weighted spin-echo images (TR/TE, 600/28) with and without fat-suppression technique and T2-weighted spin-echo images (2000/80) were acquired. For the dynamic study, fast spoiled gradient-recalled acquisition in the steady state sequence (12.8/2.6; flip angle, 30°; or 6.2/1.4; flip angle, 40°) was used with the three-dimensional Fourier transformation technique. After obtaining unenhanced images, we obtained pancreatic phase images, starting 30 sec after the initiation of IV bolus injection of 0.1 mmol/kg of gadopentetate dimeglumine (Magnevist; Nihon Schering, Osaka, Japan). Portal venous and late phase images were obtained starting 60 and 90 sec later, respectively.

Both CT and MR imaging were performed less than 2 months before surgery or biopsy. Surgical excision involved a Whipple procedure for one of the four patients with a pancreatic mass in the pancreatic head and pylorus-preserving duodenopancreatectomy for the other three patients. Excision of the pancreatic body and tail was performed in one patient who had a pancreatic mass in that region.

Two experienced abdominal radiologists retrospectively reviewed the CT scans and MR images of the seven patients for the imaging findings, including attenuation, signal intensity, and calcifications of the mass and pancreas, pancreatic duct dilatation, and bile duct dilatation, and reached a consensus. The attenuation on CT scans and the signal intensity on MR images of the pancreatic mass due to chronic pancreatitis were compared with those of the nonenlarged portion of the pancreas in each patient.

For the five patients who underwent surgical resection of the pancreatic mass, the pathologic findings in the tissues of the pancreatic mass due to chronic pancreatitis and in the tissues of the nonenlarged portion of the resected pancreas were assessed by an experienced pathologist without knowledge of CT and MR imaging findings. The pathologist examined the histopathologic specimens for pathologic changes and rated the degree of fibrosis, infiltration of lymphocytes, and reduction of glandular elements on a 4-point scale—absent, mild, moderate, and severe—for both the pancreatic mass and the nonenlarged portion of the pancreas. In the two patients who underwent tumor biopsy, only the pathologic findings for the tissues of the pancreatic mass were assessed.

Results

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Table 1 shows CT and MR imaging findings of chronic pancreatitis in the seven patients. On CT scans, sparse calcification was observed in the pancreatic mass in two patients (Fig. 1A,1B,1C,1D,1E,1F), but no calcification was observed anywhere in the pancreata of the other five patients (Figs. 2A,2B and 3A,3B,3C,3D). Dilatation of the pancreatic duct in the pancreatic body and tail proximal to the pancreatic mass in the pancreatic head was observed in four patients, and dilatation of the common bile duct was observed in three patients with masses in the pancreatic head.

View larger version (89K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —69-year-old woman with mass in pancreatic head who had metallic stent inserted at another hospital for obstruction of common bile duct. Unenhanced CT scan of pancreatic mass (thick arrow) in pancreatic head shows central calcification and compression of duodenum and metallic stent (thin arrow) in common bile duct.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —69-year-old woman with mass in pancreatic head who had metallic stent inserted at another hospital for obstruction of common bile duct. Pancreatic phase helical CT scan shows hypoenhanced mass (thick arrow) relative to nonenlarged portion of pancreas (thin arrow). Demarcation between mass and nonenlarged portion of pancreas is visible.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —69-year-old woman with mass in pancreatic head who had metallic stent inserted at another hospital for obstruction of common bile duct. T2-weighted spin-echo MR image (TR/TE, 2000/80) reveals hypointense mass (thick arrow) relative to nonenlarged portion of pancreatic head (thin arrow).

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D. —69-year-old woman with mass in pancreatic head who had metallic stent inserted at another hospital for obstruction of common bile duct. Pancreatic phase image of dynamic MR imaging (6.2/1.4; flip angle, 40°) shows hypoenhanced mass (thick arrow) relative to nonenlarged portion of pancreatic head (thin arrow).

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1E. —69-year-old woman with mass in pancreatic head who had metallic stent inserted at another hospital for obstruction of common bile duct. Photomicrograph of histopathologic specimen of pancreatic mass obtained with surgical resection shows mild fibrosis. (H and E, x2.5)

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1F. —69-year-old woman with mass in pancreatic head who had metallic stent inserted at another hospital for obstruction of common bile duct. Photomicrograph of histopathologic specimen of pancreatic parenchyma away from mass shows no fibrosis. (H and E, x2.5)

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —63-year-old woman with mass due to chronic pancreatitis in pancreatic body and tail. Pancreatic phase helical CT scan shows mass (thick arrow) in pancreatic body, which is hypoenhanced relative to pancreatic head (thin arrow). Demarcation between mass and pancreatic head is visible.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —63-year-old woman with mass due to chronic pancreatitis in pancreatic body and tail. Photomicrograph of histopathologic specimen of mass obtained with percutaneous biopsy shows severe fibrosis. (H and E, x50)

View larger version (94K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —63-year-old man with pancreatic mass in pancreatic head and dilatation of common bile duct and main pancreatic duct. Pancreatic phase helical CT scan of pancreatic body and tail shows dilated main pancreatic duct (arrow).

View larger version (80K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —63-year-old man with pancreatic mass in pancreatic head and dilatation of common bile duct and main pancreatic duct. Pancreatic phase helical CT scan of mass in pancreatic head shows isoenhancement relative to pancreatic body and tail so that mass is not visibly demarcated.

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —63-year-old man with pancreatic mass in pancreatic head and dilatation of common bile duct and main pancreatic duct. Photomicrograph of histopathologic specimen of mass obtained with surgical resection shows mild fibrosis. (H and E, x2.5)

View larger version (126K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D. —63-year-old man with pancreatic mass in pancreatic head and dilatation of common bile duct and main pancreatic duct. Photomicrograph of histopathologic specimen of pancreatic parenchyma other than mass also shows mild fibrosis. (H and E, x2.5)

Four pancreatic masses showed isoattenuation on all CT scans and isointensity on all MR images, relative to the nonenlarged portion of the pancreas (Table 1 and Fig. 3A,3B,3C,3D), and these masses were not clearly demarcated from the nonenlarged portions of the pancreas on any of the CT scans or MR images. The other three masses exhibited hypoattenuation relative to the nonenlarged portion of the pancreas on pancreatic phase CT scans and were visibly demarcated from the nonenlarged portions (Figs. 1A,1B,1C,1D,1E,1F and 2A,2B), although they showed isoattenuation on unenhanced and portal venous phase CT scans. Two of the three demarcated masses underwent MR imaging. One demarcated mass showed isointensity on T1-weighted spin-echo MR images and hypointensity on T2-weighted spin-echo MR images (Fig. 1A,1B,1C,1D,1E,1F), relative to the nonenlarged portion of the pancreas. The other demarcated mass revealed hypointensity on T1-weighted spin-echo MR images and heterogeneous mixed intensity composed of hypo- to hyperintensity on T2-weighted spin-echo MR images (Fig. 2A,2B). These masses showed hypointensity relative to the nonenlarged portion of the pancreas on the pancreatic phase images of dynamic MR imaging and were visibly demarcated from the nonenlarged portions (Figs. 1A,1B,1C,1D,1E,1F and 2A,2B), although the masses showed isointensity on portal venous and late phase images.

Fibrosis, infiltration of lymphocytes, and reduction of glandular elements of the pancreas were pathologically observed in the pancreatic mass due to chronic pancreatitis in all seven patients. Table 2 shows the degree of pathologic changes in the pancreatic masses due to chronic pancreatitis and in the nonenlarged portion of the pancreas in the five patients who underwent surgical resection of the pancreatic masses. Various degrees of fibrosis, infiltration of lymphocytes, or reduction of glandular elements of the pancreas were observed in the pancreatic masses of all five patients (Figs. 1A,1B,1C,1D,1E,1F,2A,2B,3A,3B,3C,3D). Reduction of the glandular elements of the pancreas was observed in the nonenlarged portions of the pancreata in the five patients who underwent surgery. Mild-to-moderate fibrosis in the nonenlarged portion of the pancreas was observed in the three patients with the nondemarcated masses on CT scans or MR images (Fig. 3A,3B,3C,3D), whereas fibrosis was not observed in the nonenlarged portion of the pancreata in the two patients with demarcated masses (Fig. 1A,1B,1C,1D,1E,1F).

Discussion

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Dilatation of the main pancreatic duct, parenchymal atrophy, pancreatic calcification, fluid collection, focal pancreatic enlargement (pancreatic mass due to chronic pancreatitis), biliary ductal dilatation, and changes in attenuation in peripancreatic fat or fascia are frequent findings in patients with chronic pancreatitis [5]. These findings are also often seen as secondary changes in patients with pancreatic carcinoma [6]. These overlaps of imaging findings make it difficult to distinguish a focal pancreatic mass due to chronic pancreatitis from a pancreatic carcinoma or other tumors.

Two-phase helical CT and dynamic MR imaging have been reported useful for detecting and characterizing pancreatic tumors [9,10,11,12]. Pancreatic ductal adenocarcinoma is usually hypovascular and appears as a hypoenhanced lesion relative to surrounding pancreatic parenchyma on pancreatic phase images of helical CT and on dynamic MR imaging [9, 10]. On the other hand, islet cell tumors and acinar cell carcinomas are usually hypervascular, and these tumors appear as hyperenhanced lesions on such images [12].

Johnson and Outwater [11] found that masses of pancreatic carcinoma and those due to chronic pancreatitis showed more gradual progressive enhancement on dynamic MR imaging than did normal pancreatic parenchyma, and they histologically found abundant fibrosis in both pathologic conditions, which was thought to account for the similar imaging appearances of the two kinds of masses. However, they did not assess enhancement or histology of pancreatic parenchyma other than that of masses due to chronic pancreatitis. In our experience, demarcation of a pancreatic mass is more suggestive of a mass due to pancreatic neoplasm, including pancreatic carcinoma, than of a mass due to chronic pancreatitis. It is, therefore, important to compare enhancement and histologic changes in a mass due to chronic pancreatitis with those in a nonenlarged portion of the pancreas.

Our results revealed two different enhancement patterns in pancreatic masses due to chronic pancreatitis as seen on two-phase helical CT and on dynamic MR imaging: pancreatic masses due to chronic pancreatitis appeared as hypoenhanced demarcated masses or as isoenhancing nondemarcated masses on pancreatic phase images. The two different enhancement patterns in masses due to chronic pancreatitis might be explained by the difference in the degree of fibrosis between the mass and the nonenlarged portion of the pancreas. Fibrosis was pathologically identified in the nonenlarged portion of the pancreas in patients with nondemarcated masses on CT scans or MR images, whereas fibrosis was not found in the nonenlarged portion of the pancreas in patients with demarcated masses. Fibrosis is one of the main pathologic changes characteristic of chronic pancreatitis [2]. Therefore, when chronic pancreatitis occurs focally, the inflammatory mass is visibly demarcated on CT scans or on MR images. On the other hand, when chronic pancreatitis occurs throughout the pancreas, the mass is not demarcated.

It is thought that the hypoenhanced and demarcated pancreatic mass due to chronic pancreatitis on pancreatic phase contrast-enhanced helical CT or MR imaging is not distinguishable from pancreatic carcinoma, whereas the isoenhanced and nondemarcated masses may be distinguishable from hypovascular pancreatic adenocarcinoma or other hypervascular pancreatic tumors. However, histologic examination or close interval follow-up is needed for such isoenhanced and nondemarcated pancreatic masses. Tumor-free pancreatic parenchyma located proximally to a pancreatic carcinoma obstructing the pancreatic duct may undergo atrophy and fibrosis and thus may show the same gradual enhancement pattern as pancreatic carcinoma [11], whereas pancreatic carcinoma located in the pancreatic head may show isoenhancement and no demarcation.

In conclusion, two different enhancement patterns on two-phase helical CT and dynamic MR imaging were identified in masses due to chronic pancreatitis. When histologic fibrosis is uniformly present through the gland in patients with chronic pancreatitis, there is no demarcation of masses due to chronic pancreatitis. When there is a greater degree of histologic fibrosis in the masslike part of the gland, the mass is often demarcated from the remaining pancreas, and the enhancement pattern on two-phase helical CT and dynamic gadolinium-enhanced MR imaging mimics that of pancreatic adenocarcinoma. Further work is needed to improve the rate of correct diagnosis of masses due to chronic pancreatitis.

References

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