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1
Edward Mallinckrodt Institute of Radiology, Washington University School of
Medicine, 510 S. Kingshighway Blvd., St. Louis, MO 63110.
2
Department of Pathology, Washington University School of Medicine, St. Louis,
MO 63110.
Received May 3, 1999;
accepted after revision September 22, 1999.
Address correspondence to F. Dehdashti.
Abstract
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MATERIALS AND METHODS. A retrospective review of all patients with mucinous carcinoma who had undergone FDG PET at our institution from 1995 through 1998 identified 25 patients with new or recurrent mucinous carcinoma at the time of PET. In 22 of these patients, tissue specimens available from either core biopsy or surgical resection allowed detailed histologic analysis.
RESULTS. FDG PET revealed mucinous carcinoma in only 13 (59%) of 22 patients, resulting in an unusually high percentage (41%) of false-negative results. Two histologic features were found to be predictive of FDG PET results: tumor cellularity (p = 0.011) and the amount of mucin within the tumor mass (p = 0.009). There was a positive correlation between tumor FDG uptake and cellularity but a negative correlation with the amount of mucin.
CONCLUSION. FDG PET is limited in the evaluation of mucinous tumors, particularly in hypocellular lesions with abundant mucin.
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We recently examined a group of patients with mucinous carcinomas that exhibited poor accumulation of FDG. Mucinous carcinomas are commonly found in the gastrointestinal tract and represent approximately 17% of colonic tumors [4]. These tumors contain clear, gelatinous fluid (mucin), which may be intracellular or extracellular. We postulated that FDG PET might be insensitive in showing mucinous carcinomas because of the low cellularity of these tumors caused by the presence of mucin. To test this hypothesis, we retrospectively reviewed our experience with mucinous carcinomas and correlated the results of FDG PET with the histologic features of these tumors.
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Twelve of the 22 patients were examined for new primary carcinomas and the remaining 10 patients were examined for suspected recurrent disease. Either a core biopsy or a surgical specimen was available for these 22 patients, thus allowing detailed histologic analysis.
PET
PET was performed using an ECAT EXACT scanner (Siemens-CTI, Knoxville, TN)
beginning approximately 40 min after IV administration of 10-15 mCi (370-555
MBq) of FDG. A series of three to five overlapping 47-slice emission and
transmission PET images was obtained to include the region from the middle of
the neck to the upper thighs. Activity in kidneys, ureters, and bladder was
minimized by IV hydration, diuretic administration, and bladder
catheterization during the study, as previously described
[7]. The PET images in 12
patients were reconstructed by filtered back projection using a Hanning filter
(frequency cutoff, 0.6 x Nyquist value). In the remaining 10 patients,
the PET images were reconstructed using an ordered-subset
estimation-maximization (OSEM) iterative algorithm and a Butterworth filter
(frequency-cutoff range, 0.2-0.4 cycles per pixel). The emission images were
corrected for measured attenuation using a local threshold for segmented
attenuation [8]. Images were
displayed in three orthogonal projections and as whole-body,
maximum-pixel-intensity reprojection images for visual interpretation.
All images were interpreted using all available clinical information and correlative anatomic studies (CT, MR imaging, or both). All PET images were evaluated qualitatively by the consensus of at least two observers. For the purpose of this study, the original clinical PET interpretation was used. Image interpretation criteria were those routinely used in scintigraphic imaging. On the basis of knowledge of the normal distribution of FDG, lesions were identified as abnormal or as representing tumor, if the accumulation of FDG was moderately to markedly increased relative to comparable normal contralateral structures or surrounding soft tissues. Mildly increased activity or no increased activity (in the case of an abnormality identified on CT or MR imaging with no corresponding PET abnormality) was considered normal or benign disease.
In addition to qualitative visual analysis of FDG uptake, appropriate regions of interest were drawn around the lesions and around the comparable normal tissues for determination of the simple ratio of tumor-to-normal tissue FDG uptake. We were unable to determine the standardized uptake values of lesions because a recent software upgrade of our computer system made reprocessing of most of the older studies, to allow calibration of the images in standardized uptake value units, impossible.
Pathologic Analysis
Tissue specimens were processed by standard technique (all tissues were
fixed in formaldehyde solution, embedded in paraffin, and sectioned; the
sections were then stained with H and E). All surgical pathology specimens
(i.e., core biopsy or surgical specimens) for all 22 patients with adequate
tissue samples were reevaluated in detail for tumor grade on the basis of
mitotic figures, nuclear enlargement, and nuclear pleomorphism; cellularity;
and the amount of mucin using standard pathologic criteria
[9]. The appropriate H and
Estained slides (tissue sections from the tumor mass alone) for each
patient were reevaluated by one observer who was unaware of the results of the
PET studies. The number of slides reviewed from each patient ranged from one
to 10 with a mean of four slides per patient. Each slide was reviewed at low-
and high-power objectives (2x, 4x, and 10x) for assessment
of the percentage of surface area occupied by solid tumor (tumor cells) and
mucin. Each slide was then reexamined at 4x and 10x power
objectives, focusing specifically on the mucinous component. After all
relevant slides had been examined, tumor grade, the percentage of tumor
cellularity, and mucin content for each patient were calculated and
tabulated.
Statistical Analysis
To determine the relationship between tumor FDG uptake and the histologic
features of the tumor, the tumor-to-normal tissue activity ratio was compared
with tumor cellularity, mucin content, and grade using a nonparametric
Spearman's rank correlation test. A p value of less than 0.05 was
considered indicative of a statistically significant correlation. Sensitivity
of FDG PET for detection of mucinous carcinoma was calculated in the standard
manner.
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PET Results
By qualitative evaluation, FDG PET enabled correct identification of tumor
in 13 of the 22 patients. In the remaining nine patients with malignant
lesions (recurrent colorectal cancer, n = 5; primary cancer of the
esophagus and gastroesophageal junction, n = 2; primary lung cancer,
n = 1; and metastatic breast cancer, n = 1), FDG PET showed
no or minimal uptake relative to corresponding normal tissues (Figs.
1A,
1B and
2A,
2B). The mean tumor-to-normal
tissue ratio was 5.3 ± 2.5 (range, 1.5-9.0) in the 13 lesions detected
on PET and 1.2 ± 0.3 (range, 0.5-1.8) in the nine lesions not detected.
All the false-negative tumor sites measured at least 1 cm in diameter (range,
1.0-5.0 cm) on either correlative anatomic imaging studies or pathologic
specimens; only three of the nine lesions were smaller than 2.0 cm.
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In our group of 22 patients, the overall sensitivity of FDG PET for the detection of mucinous neoplasm was 59%.
Pathologic Results
Of the 22 cases in which tumors were pathologically reevaluated, 12
represented primary resections, and 10 were biopsies of recurrent or
metastatic lesions. Ten of 12 primary mucinous adenocarcinomas originated in
the gastrointestinal tract (distal esophagusgastroesophageal junction,
n = 8; pancreas, n = 2) and two originated in the lung. In
one of the 12 patients, pulmonary biopsy revealed mucinous adenocarcinoma, but
whether this was a primary lung cancer or a metastasis from an unknown primary
carcinoma could not be determined. Biopsy sites of metastatic tumors included
the liver (n = 3), soft tissue (n = 3), lung (n =
2), lymph node (n = 1), and bone (n = 1). Nine of these
metastatic lesions had originated in the gastrointestinal tract (colorectal)
and one in the breast.
The failure of PET to reveal tumor foci significantly correlated with low tumor cellularity (p = 0.011) and overall abundance of mucin (p = 0.009). There was no significant correlation between FDG uptake and tumor grade (p = 0.620).
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FDG PET has been shown to be of great value in the detection, staging, and monitoring response to treatment in a variety of tumors. FDG uptake has been correlated with the number of viable tumor cells as well as the grade and the differentiation of some tumors [14, 15]. In recent reports, researchers have shown the limitations of FDG PET in revealing certain tumors with specific histologic characteristics, such as bronchioloalveolar cell carcinoma [2, 16]. Kim et al. [2] showed that bronchioloalveolar cell carcinoma, which often contains abundant mucin, exhibits significantly lower peak standardized uptake values compared with squamous cell carcinoma, adenocarcinoma, and other cell types. In their study population, Kim et al. found that bronchioloalveolar carcinoma was more often well differentiated, showed moderate degrees of nuclear atypia, and had infrequent mitotic figures. Similar results have been reported by Higashi et al. [16]; these investigators found a correlation between FDG uptake and the degree of cell differentiation in adenocarcinoma of the lung. In their study population, Higashi et al. reported that FDG uptake was significantly lower in bronchioloalveolar carcinoma than other carcinomas and that negative FDG PET results were seen in four of seven patients with bronchioloalveolar carcinoma.
In this study, FDG PET had a lower sensitivity for detection of primary and recurrent mucinous carcinoma than that generally reported for tumors of the gastrointestinal tract and lung [17]. False-negative results were seen in nine of 22 patients with mucinous carcinoma. The apparent insensitivity of FDG PET to show mucinous carcinoma is not surprising given the mechanism by which this tracer localizes in tumors. For example, our results indicate that the relative cellularity of the tumor is important in the detection of disease using FDG PET. In an in vitro study of an adenocarcinoma cell line, Higashi et al. [14] showed that differences in FDG uptake strongly correlate with the presence of viable tumor cells. Although FDG uptake by some tumors has been strongly correlated with grade such as tumors of the brain, musculoskeletal system, and breast, we found that tumor grade was not predictive of tumor detectability using FDG PET [15, 18, 19].
This study suffers from the inherent limitations of a retrospective study performed at a single institution, and our results may not necessarily be generalizable to the patient populations in other institutions. However, most of the tumors in this study originated in the gastrointestinal tract, which is significant for two reasons. First, this reflects the fact that mucinous carcinoma is relatively more common in the gastrointestinal tract. Second, FDG PET has proved particularly useful and is frequently applied in both detecting and staging recurrent disease in tumors of the gastrointestinal tract [1, 17, 20].
In addition to qualitative analysis, semiquantitative analysis of FDG uptake has been widely used for characterization of indeterminate lesions. Although semiquantitative analysis has been shown to be slightly superior to qualitative analysis in differentiating benign from malignant lesions, no statistically significant difference has, to our knowledge, been reported [21-23]. In this study, both the qualitative and semiquantitative (simple tumor-to-normal tissue activity ratio) analyses were used for evaluation of the PET images. The semiquantitative results paralleled the qualitative results in this patient population.
Despite the limitations of this study, the results may help to better define the application of FDG PET in patients with mucinous carcinoma. In particular, FDG PET may not be ideal for evaluation of recurrent or metastatic disease in patients with known mucinous carcinoma. Although this information is not always available, knowledge of the limitations of FDG PET in a particular histologic subtype, mucinous carcinoma, may assist in proper application of this technique and in avoiding misdiagnosis.
Acknowledgments
We thank Andrea Sykes for her assistance in preparing this manuscript and
we greatly appreciate the technical assistance of Renee J. Burney, Delynn K.
Silvestros, and Martin A. Schmitt.
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