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1 Department of Radiology, Lady Davis Medical Center, Michal St. 7, Haifa 34362,
Israel.
2 Department of Diagnostic Radiology, The University of Texas M. D. Anderson
Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030.
3 Department of Biostatistics, The University of Texas M. D. Anderson Cancer
Center, Houston, TX 77030.
4 Department of Pathology, The University of Texas M. D. Anderson Cancer Center,
Houston, TX 77030.
Received March 26, 2003;
accepted after revision September 16, 2003.
Address correspondence to E. M. Loyer.
Abstract
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MATERIALS AND METHODS. Thin-section triple phase (20, 40, and 70 sec after the start of injection) contrast-enhanced helical CT scans of the abdomen in 18 patients with a pancreatic carcinoma that was 2 cm or smaller and 18 patients with a normal pancreas were retrospectively reviewed by two senior radiologists who specialized in oncologic abdominal imaging. Discrepancies were resolved by consensus. The observers were unaware of the clinical information. CT scans were evaluated for the presence of a pancreatic mass, bile, and pancreatic duct stricture. The location and size of tumors as determined on CT were compared with pathologic findings. The CT results were also compared with the prospective CT interpretations derived from the radiology reports and with the endoscopic sonographic reports when available.
RESULTS. The sensitivity of thin-section triple-phase helical CT in the detection of small pancreatic masses was 77%, and the specificity was 100% for the two experienced observers. The sensitivity and specificity were 72% and 100%, respectively, for the prospective interpretations done by 10 observers. There was no correlation between the tumor size at pathology and the CT measurements.
CONCLUSION. Thin-section contrast-enhanced helical CT is sensitive and highly specific for the detection of pancreatic tumors measuring 2 cm or smaller. Improvement in the detection rate of this technique compared with previous techniques lies in the optimization of parenchymal enhancement during the pancreatic phase and the decrease in slice thickness.
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Although CT and MRI perform equally well in the detection of pancreatic carcinoma [3], the availability of helical CT and technologic advances have made it the method of choice for pancreatic imaging [3, 4]. Current reported data on the sensitivity of CT in the detection of pancreatic adenocarcinoma stem from studies in which the mean tumor size was greater than 2 cm and in which CT was performed with technical parameters that would now be considered suboptimal [3, 5–9].
A survival benefit from surgery is achieved only in patients who undergo a
negative-margin pancreaticoduodenectomy
[2]. The fact that increasing
tumor size correlates with an increasing rate of unresectability and
decreasing survival rate [10]
underscores the need to detect tumors while they are small and have not spread
locally. This need prompted us to evaluate the sensitivity and specificity of
a triple-phase contrast-enhanced helical CT protocol for the detection of
small (
2 cm in greatest dimension) pancreatic adenocarcinomas.
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A control group composed of 18 patients with no suspected pancreatic disease who had undergone the same triple-phase CT protocol was identified from the CT logbooks. There were seven men and 11 women who ranged in age from 40 to 78 years (mean, 58.1 years). Among these were patients with hepatic metastasis or primary liver tumors and patients with extrahepatic, extrapancreatic tumors. Patients who had undergone hepatic resection or had massive hepatic involvement by cancer were excluded. In two patients, a biliary stent was present.
CT Protocol
All CT scans were obtained using either a LightSpeed multidetector scanner
or a HiSpeed Advantage single-detector scanner (both General Electric Medical
Systems, Milwaukee, WI). Barium sulfate (2%) (Readi-Cat 2, E-Z-EM, Westbury,
NY) was administered orally. Unenhanced helical scans through the liver and
pancreas were acquired with a pitch of 1 and slice thickness of 10 mm on the
single-detector units. Unenhanced helical scans through the liver and pancreas
were acquired with a pitch of 6 and slice thickness of 10 mm on the
multidetector units. Contrast-enhanced helical scans of the pancreas were
acquired in a craniocaudal direction, during a single breath-hold, using 3-mm
slice thickness and a pitch of 2 on the single-detector units. Because of
limited anatomic coverage with a single-detector unit, the examination was
designed to encompass the entire pancreas and to cover as much liver
parenchyma as possible in a single breath-hold while at the same time
constantly maintaining the parameters set for the pancreas imaging (slice,
delay). Consequently, with the single-detector unit, the entire liver was not
imaged during the arterial and parenchymal phases. With the multidetector
units, contrast-enhanced scans of the entire liver and pancreas were acquired
in a craniocaudal direction with a single breath-hold using 5-mm slice
thickness and a pitch of 6. The multidetector slices were retrospectively
reconstructed at 2.5-mm thickness. There were 11 patients evaluated with the
single-detector unit and seven patients evaluated with the multidetector unit.
The scanning sequences were initiated 20 sec (arterial phase), 40 sec
(pancreatic phase), and 70 sec (portal venous phase) after the start of IV
injection of 150 mL of 60% nonionic contrast material (Optiray 320 [ioversol],
Mallinkrodt, St. Louis, MO) at a rate of 5 mL/sec. The scanning time for each
phase was 10–12 sec. Scans were obtained at 120 kV and 240–280 mA.
Scans were printed with both narrow and soft-tissue window settings.
Interpretation of Images
Each CT study was reviewed in a retrospective blinded fashion by two
radiologists who specialize in oncologic abdominal imaging. Discrepancies over
the presence of a measurable tumor were encountered in two cases and were
resolved by consensus. Only the study performed at the completion of
chemoradiation, just before surgery, was analyzed. Each scan was assessed for
the presence of a pancreatic tumor on the basis of the detection of a focal
abnormality in attenuation or texture or a contour deformity, associated or
not with an abrupt change in the caliber of the bile or pancreatic duct.
Change in texture is defined as a loss of the lobulation usually seen in the
normal pancreatic parenchyma. The location and size of the presumed tumor were
recorded. The observers were not asked to assess resectability or to evaluate
for liver, nodal, or peritoneal metastasis. The false-negative studies were
reviewed a second time by a different radiologist with knowledge of the
surgical and pathologic findings.
The original prospective interpretations of the studies derived from the radiology reports were reviewed and compared with the retrospective evaluations for the detection of tumors. A total of 10 radiologists had participated in the prospective interpretation.
Endoscopic Sonography
Endoscopic sonography was performed in 10 patients within 10 days of the
initial diagnostic CT. Endoscopic sonography reports were reviewed and
correlated with the CT interpretations and the pathology reports.
Pathologic Data
Pathology reports were reviewed in respect to size, location, appearance,
and histologic features of the mass and of the normal pancreatic parenchyma.
For the CT studies with false-negative findings, the slides were reviewed a
second time. A size of 0 cm was recorded when numerous microscopic tumor foci
were reported scattered in the pancreas without a discrete mass. The status of
the surgical margin, which was invariably negative for malignancy, was also
noted.
Statistical Analysis
The sensitivity and specificity of this protocol for the detection of small
pancreatic tumors were calculated. Positive predictive values were calculated
separately for each radiographic criterion indicative of tumor: attenuation,
texture, or contour abnormality and ductal obstruction. Observer's preference
relative to the phase of contrast enhancement was analyzed. A Pearson's
correlation between the tumor size reported by the pathologist and that
radiologically assessed by the observers was also performed.
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The studies with false-negative findings included two patients in whom pathologic examination showed only minimal microscopic residual tumor but no discrete mass. In one of these patients, endoscopic sonography was available and revealed a 1.3-cm mass.
In the remaining two false-negative cases, a mass measuring 2 cm was found in both instances at pathologic examination. In both cases, the tumor involved the head of the pancreas with extension into the peripancreatic adipose tissue. On CT, both patients had strictures of the distal bile duct with stents in place. These studies were interpreted as bile duct strictures without definite mass or evidence of pancreatitis, to be further evaluated on ERCP and endoscopic sonography. In both cases, results of endoscopic sonography were negative (Fig. 3A, 3B, 3C).
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There was no correlation between the tumor size measured by the radiologists and the tumor size reported by the pathologists, nor was there correlation between endoscopic sonography and pathology measurements or between endoscopic sonography and CT measurements. The tumor size varied from 1 to 3 cm by CT measurement and from microscopic foci to 2 cm at pathology. The lack of correlation between the different measurements did not follow a pattern.
Review of the original radiology reports showed that prospectively 10 studies were interpreted as positive for tumor, three as suspicious for a mass, and five as negative for a mass. Only one of the false-negative studies was interpreted as positive for tumor during the retrospective interpretation. The remaining four studies were also interpreted as negative in the retrospective study. The sensitivity of the technique in this setting was 72% (13/18) and specificity was 100%.
The radiographic signs most helpful for the diagnosis of a pancreatic tumor were a focal area of low attenuation or a change in texture at the site of an abrupt interruption of the bile or pancreatic duct (Figs. 1A, 1B, 1C and 2A, 2B). There were individual variations in the radiographic analysis: a texture abnormality was detected in 13 of 14 cases by one observer and in only two cases by the other observer. Other findings such as focal hyperattenuation (1/14 cases) and contour abnormality (2/14 cases) were rarely observed (Figs. 1A, 1B, 1C and 4A, 4B, 4C).
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The pancreatic phase was considered the optimal phase for tumor detection in 12 of 14 patients by one observer and in nine of 14 patients by the other observer.
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The detection of small pancreatic tumors can be challenging. Of the imaging techniques that are available, CT is the most commonly used. Detection is based primarily on the difference in enhancement between the tumor and normal parenchyma. Most ductal adenocarcinomas have an abundant, dense fibroblastic stroma with a decreased number of vessels within the tumor [2]. Consequently, these tumors enhance less than the surrounding normal parenchyma. Additional radiographic findings may include a focal change in texture; abrupt change in the caliber of the pancreatic duct, bile duct, or both, particularly when associated with a density or texture change; and modification of the contours of the pancreas (Figs. 1A, 1B, 1C and 2A, 2B).
The sensitivity of dual-phase contrast-enhanced helical CT in the detection of pancreatic carcinoma has been reported to vary from 76% to 92% [3, 5, 6, 9]. However, the reported detection of small tumors has been poor, with a sensitivity of 67% for tumors smaller than 1.5 cm in one study [6] and 58% for tumors smaller than 2 cm in another study [5].
In our study, triple-phase contrast-enhanced CT performed on a state-of-the-art MDCT or a single-detector helical CT scanner with a contrast injection rate of 5 mL/sec had a sensitivity of 77% and specificity of 100% for the detection of pancreatic cancers smaller than 2 cm in diameter. There were four false-negative studies: two cases in which pathologic examination did not identify a discrete mass but found only microscopic foci of tumor cells, and two in which masses each measuring 2 cm were identified pathologically. In the latter two cases, endoscopic sonography evaluations and CT scans were interpreted as negative for a mass. In both cases, a stricture of the distal common bile duct appeared isolated (Fig. 3A, 3B, 3C). Review of the CT studies with knowledge of the pathologic findings did not change the false-negative interpretation for a mass and did not explain the discrepancy between the imaging and pathologic findings. Interestingly, there were two other cases with only microscopic disease pathologically, and these were interpreted as positive for tumor on CT in both and positive for tumor on endoscopic sonography in one of them. These CT diagnoses were based on a subtle change in texture or density or an abrupt change in the size of the pancreatic duct. One could argue that the setting of the study lent itself to overinterpretation (Fig. 5A, 5B).
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Although specificity was high in our study, it may actually be lower, because there was no patient in our study group with the diagnosis of focal chronic pancreatitis, a condition that may mimic carcinoma on imaging [11].
No correlation existed between the size of the tumor measured pathologically and the size of the tumor measured on CT. One likely reason is the difficulty in determining tumor size pathologically. After chemoradiation, the pancreas is usually fibrotic, making gross detection of the tumor difficult. Lack of correlation was also evident between endoscopic sonography and pathology and between endoscopic sonography and CT. Endoscopic sonography, however, was performed at the time of diagnosis, before chemoradiation, making the size comparison less rigorous. We do not have a clear explanation for these disparities and can only assume that the difficulties arise from the poor definition and heterogeneity of the tissue studied. The number of cases in our studies is not sufficient to compare the sensitivity of endoscopic sonography with that of CT.
As expected, the radiographic findings most helpful for determining the presence of a tumor were a focal area of hypoattenuation, a change in texture associated with an abrupt change in the bile or pancreatic duct caliber, or both. A change in contour was rarely observed because of the small size of the tumors. The presence of a biliary stent is a source of problems because the exact site of obstruction may be obscured if the obstruction is affecting the bile duct only and because of intercurrent inflammation around the stent that can be confused with tumor. Optimally, imaging should be performed before stent placement. False-negative studies were characterized by isolated stricture of the bile duct, pancreatic duct, or both without texture or density change.
In summary, the tumor diagnosis was based on the observation of one or more of these radiographic findings, but there was notable interobserver variation in the perception and analysis of the findings, particularly in the visualization of a texture change. We think that this interobserver variation is because change in texture is perceived more subjectively and was given a different weight by each of the observers.
The pancreatic phase of enhancement was rated superior to the arterial or portal venous phases. The poor performance of the arterial phase has previously been reported [6–9, 12]. Although in our study the pancreatic phase was preferred to the portal phase, McNulty et al. [4], using MDCT, found the pancreatic and portal phases to be equivalent. This difference is not explained by technical differences because our protocol is very similar to theirs. A possible explanation is that the thinner slice thickness in the pancreatic phase positively affected the observers' confidence in our study, particularly because the mean tumor size was 1.66 cm in our study versus 3.3 cm in the study by McNulty et al. In our study, observers were not asked to specifically compare the phases of enhancement but only to rate the one they judged the best. The quantitative assessment by McNulty et al. indicates that although pancreatic enhancement is maximal during the pancreatic phase, tumor conspicuity is equivalent in the pancreatic and portal phases.
A higher injection rate, optimal parenchymal opacification, and thinner slice thickness achieved by a decrease in scanning time are the main technical differences between our protocol and those described in the literature [3, 5–9], with the exception of the study by McNulty et al. [4]. CT protocols to detect and stage pancreatic tumors are designed to optimize enhancement of the pancreas to ensure maximal tumor-versus-pancreas contrast and to optimize enhancement of the peripancreatic vessels and liver for staging. Studies on pancreatic enhancement have defined general principles to follow in imaging the pancreas. Peak pancreatic enhancement, which defines the pancreatic phase, occurs before peak enhancement of the liver [12–14]. The time when peak pancreatic enhancement occurs varies with the rate of injection [11, 13]. Enhancement is also affected by the amount of iodine used [11]. With single-detector CT, the arterial phase is used to assess the vascular anatomy for local staging and the portal phase to detect hepatic metastasis. Experience with MDCT, however, is bringing slight modifications to the protocols used to image the pancreas. The shorter scanning time with MDCT provides a pancreatic phase and a portal phase that are focused on maximal arterial enhancement and maximal venous enhancement, respectively. In this setting, a pure arterial phase becomes obsolete [4].
In this study, we have shown that triple phase contrast-enhanced helical CT is a sensitive method for the detection of small, potentially resectable pancreatic tumors. Our study was done with a triple-phase protocol similar to that reported by McNulty et al. [4]. We— and they—now use a biphasic technique that provides the same tumor-versus-pancreas contrast as the triple-phase technique but allows thinner collimation and multiplanar reconstruction when needed.
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