Japanese Journal of Clinical Oncology 32:347-351 (2002)
© 2002 Foundation for Promotion of Cancer Research
Helpful Parameter for Malignant Potential of Gastrointestinal Stromal Tumors (GIST)
1 Department of Pathology, Wakayama Medical University, Wakayama City, Japan and 2 Department of Pathology, School of Medicine, Shandong University, Jinan, China
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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Background: Although a series of histopathological criteria have been suggested, the prediction of the malignant potential of gastrointestinal stromal tumors (GIST) is still difficult. The older literature called all gastrointestinal stromal tumors smooth muscle tumors or mixed GIST with true smooth muscle tumors. Reports on GIST including homogeneous cases were rare.
Methods: We examined 73 cases of GIST, which were immunohistochemically positive for c-kit and/or CD34, and mainly focused on the correlation between mitotic count and the other clinicopathological features to establish any helpful and reproducible parameters to indicate the malignant potential and to be used practically and objectively in the routine histopathological diagnosis of GIST.
Results: The results showed that there was a statistically significant difference in mitotic count between benign and malignant groups. Other proposed parameters, such as high cellularity, tumor size 
5 cm, stomach and intestinal location, hemorrhage, necrosis, p53 expression and Ki-67 labeling index >10%, were frequently observed in tumors with mitotic figure. Three patients with one mitotic figure in 50HPF died from metastasis or recurrence of the tumors.
Conclusions: Ki-67 index and cellularity should be used as predictors for the malignant potential of GIST. When other morphological features appear benign, mitotic count might also be a helpful practical factor in the prediction of the malignant potential of GIST.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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Gastrointestinal stromal tumors (GIST) are the most common non-epithelial type of tumor among those arising from the gastrointestinal tract. The histological origin of this tumor has recently been suggested to be the interstitial cells of Cajal (1) because of its expression of c-kit and/or CD34. A series of histopathological criteria have been reported to predict the malignant potential of these tumors but their conclusions varied. It was reported that the prognosis of the GIST correlated well with mitotic count, tumor size, tumor cellularity, tumor necrosis, anatomical location, invasive growth (2–7) and expression of Ki-67 (8–12) and PCNA index (13). However, diagnosis and prediction of the malignant potential of GIST is still difficult. On the other hand, the older literature called all stromal tumors smooth muscle tumors or mixed GIST with true smooth muscle tumors, because the latter often occur in the esophagus and show clearer biological behavior. The study of homogeneous groups of true GIST has therefore been advocated (14). The aim of this study was to establish any helpful and reproducible parameters to indicate malignant potential and to be used practically and objectively in the histopathological diagnosis of GIST. We examined 73 cases of GIST immunohistochemically positive for c-kit and/or CD34 and mainly focused on the correlation between mitotic count and other proposed clinicopathological features. Those cases negative for c-kit or CD34 were excluded from the study.
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PATIENTS AND METHODS |
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TOPABSTRACTINTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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Seventy-three consecutive cases of GIST were selected from 106 cases of tumors previously diagnosed as leiomyoma, leiomyoblastoma and leiomyosarcoma from the files of the Department of Pathology, Shandong University, China, in the 30-year period from 1969 to 1999, based on immunohistochemical studies on c-kit and CD34. All specimens were from patients who underwent completely surgical excision without record of positive margin. All of these surgical specimens were routinely fixed in formalin and embedded in paraffin. At least 4–5 paraffin blocks were available in each case for study.
Clinical information including tumor size was obtained from surgical and pathological records. Tumors that developed recurrence or metastasis were judged as malignant, including those which induced death of the patient. Tumors with peripheral invasive growth microscopically were also diagnosed as malignant. The other cases without the above evidence of malignancy were classified as benign in this study, although there was still a slight chance of malignancy in the benign group.
No-mitosis group, low-mitosis group and high-mitosis group were defined depending on mitotic numbers as described previously (4,5,13,14). Briefly, 10 high-power fields in one group were observed to find the mitotic figure at high cellularity fields; a total of five groups (50 high-power fields) were observed in each case. No-mitosis cases virtually showed no mitosis, i.e. 0/50HPF, low-mitosis cases showed 1–4 mitotic figures in 50HPF and high-mitosis cases showed >5 mitotic figures in 50HPF. Simultaneously, (1) histological pattern: leiomyoma-like type, Schwannoma-like type, leiomyoblastoma-like type and mixed-type; (2) cellularity: low (I), intermediate (II) and high (III); (3) tumor necrosis: presence or absence; and (4) tumor hemorrhage: presence or absence, were also investigated in each case.
A standard labeled streptavidin–biotin (LSAB) method of immunohistochemistry was used. The dilution, sources and pretreatment of antibodies are summarized in Table 1. Positive and negative controls were used properly.
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For p53, the results were classified into three groups (no staining for negative, distinctly stained cells <50% for one plus positive and >50% for two plus positive). For Ki-67 index, the results were classified into two groups (distinctly stained cells <10% for one group and >10% for another group).
Time-independent continuous variables were evaluated using Student’s t-test and the chi-squared test was used for categorical data. The F statistic was tested to estimate the correlation between mitosis and the other parameters. Survival analysis was done with the StatView-J 5.0 statistical software package (SAS Institute, Cary, NC). For univariate analysis of time-dependent variables, the Kaplan–Meier method and the log-rank test were used to determine significant prognostic factors. In multivariate analysis, a Cox proportional hazards regression model was used for all factors found to be significant in univariate analysis. The observed differences were assumed to be statistically significant if the probability of chance occurrence was <0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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Clinical and Microscopic Features
Of the 73 cases, 38 were classified as benign and 35 as malignant based on the above-described standard. There were 19 males and 19 females in benign cases and 23 males and 12 females in malignant cases. The age of all 73 cases ranged from 6 days to 73 years, with a mean of 48.9 years for benign cases and 51.8 years for malignant cases. The size of tumor ranged between 0.5 and 30 cm, with a mean of 5.31 cm for benign cases and 9.2 cm for malignant cases (P < 0.001). The tumor size >5 cm was more frequently seen in malignant cases (P = 0.001). Thirteen tumors were located in the esophagus, 34 in the stomach, 22 in the small intestine and four in the large intestine. The tumors of the esophagus showed better behavior than those of other locations (P = 0.0015). The tumors of the small intestine showed much more recurrent and metastatic behavior (P = 0.0399).
Microscopically, 24 cases were found to have no mitosis (0/50HPF) and were seen only in the benign group. The low-mitosis group had 12 benign cases and 18 malignant cases. The high-mitosis group had only two benign cases and 17 malignant cases in (Fig. 1). Cases with high mitosis occurred more frequently in the malignant group than the benign group. The difference among the three groups was statistically significant (P < 0.0001). Tumor cellularity, tumor necrosis and tumor hemorrhage also showed significant differences between the benign and malignant groups (Table 2).
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Mitotic count was significantly higher in the malignant group than the benign group. The number of mitotic counts was statistically correlated with the other parameters, which implies a malignant potential of GIST (Table 3).
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Immunohistochemical Findings
All of the cases showed positive expression for c-kit and/or CD34 (Fig. 2), and 48 of them also showed co-expression with SMA or S-100 or both (unpublished observations).
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The over all p53-positive rate was 50.7% and it was significantly more frequent in the malignant cases (25/35, 71.43%) than the benign cases (13/38, 34.21%) (P = 0.0022).
The total cases stained for Ki-67 >10% were 37/73 (50.7%) and were more frequently seen in the malignant group (23/35, 65.71%) than the benign group (14/38, 36.84%) (P = 0.0193).
Clinical Follow-up Data
Clinical follow-up results were available in 44 cases for this study. In 26 cases in the malignant group, 21 died from recurrence or metastasis of this tumor and only five were alive at the time of follow-up. Of the 21 dead cases, 10 had tumors located in the stomach, 10 in the small intestine and one in the large intestine. In 18 cases in the benign group, eight had tumors located in the esophagus, nine in the stomach and one in the large intestine. In survival cases, no peripheral invasive growth was identified in each case, but in deceased cases 17/26 demonstrated invasive growth into the tumor peripheral tissue. The cases with mitotic figure(s) were only found in the deceased cases (P < 0.0001).
Survival Analysis
Univariate analysis demonstrated that mitotic count (P = 0.0246), cellularity (P = 0.0001), hemorrhage (P = 0.0001), tumor necrosis (P = 0.0004), Ki-67 index (P = 0.0002) and p53 immunostaining (P = 0.0021) were correlated significantly with a poor prognosis.
On multivariate analysis, Ki-67 (P = 0.0093) and cellularity (P = 0.0040) were significant independent prognostic factors. However, we failed to obtain a significant P value to indicate mitotic count as an independent predictor for the malignant potential of GIST.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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Histological criteria have not been well established to predict the malignant potential of GIST. So far, pathologists have investigated a series of factors in GIST in trying to predict their prognosis accurately. Among these, mitotic index has especially been emphasized. However, mitotic count criteria are often varied from series to series. The cut-off of mitotic index has been used in the older literatures customarily as >2/50HPF (15) or >5/50HPF (16) for distinguishing benign and malignant cases and they are available for the majority of GIST. However, there are still rare tumors that have metastasis or recurrence, although they are morphologically benign. Additionally, GIST had been historically designated as a smooth muscle tumor or mixed GIST with smooth muscle tumors in the older literature and textbooks.
In the present study, we examined 73 cases of GIST that were positive for c-kit and/or CD34, excluding the cases negative for c-kit or CD34. The cases in our study might seem more homogeneous than those in earlier reports. We divided these 73 cases into three groups by means of the mitotic count proposed originally by Evans and others (5,17,18) in smooth muscle tumors of the gastrointestinal tract to assess whether the method could also be used for true GIST. In the light of follow-up results in the study, there was a statistical difference in survival cases between the no-mitosis group and the mitosis group. Of 15 cases of no-mitosis tumors, 14 are alive and one died from liver carcinoma. Three cases, which were originally diagnosed as benign, died from metastasis and recurrence of the tumors and we therefore classified them into the malignant group in our study. Histopathologically, the three tumors appeared of low cellularity without necrosis, hemorrhage or peripheral invasive growth and for two of them the tumor size was <5 cm. Two cases demonstrated Ki-67 index <10% and one of them >10%. One mitotic figure in 50HPF was identified in each case. The results may suggested that when other morphological features appear benign, even with the Ki-67 index in a low status, the mitotic count might also be a helpful factor to predict the malignant potential in patients with GIST and mitotic count 1/50HPF might be used as a cut-off to distinguish benign and malignant.
There are several clinical and histopathological parameters to predict biological behavior of GIST, as shown in Table 2. Among them, cellularity and Ki-67 labeling index >10% also showed a statistical significance between the benign and malignant groups in our study. Cellularity, tumor size, hemorrhage and necrosis are traditional parameters consistent with malignant potential in many other tumors, and p53 and Ki-67 index also proved to be well correlated with the prognosis in many malignant neoplasms such as lung cancer, gastric cancer, colorectal cancer and breast cancer (19–22). Mitotic count was associated with those parameters and in particular correlated closely with cellularity (P < 0.0001) and Ki-67 (P < 0.0001) (Table 3). Our results, therefore, supported the contention that the mitotic count could be helpful in predicting the malignant potential of GIST. Although cytometric analysis, computerized static image analysis and silver-stained nucleolar organizer regions are currently used to investigate cellular proliferation, for routine pathological practice mitosis is still a histological ‘gold standard’. Among known histopathological parameters, mitotic count may be a convenient method and may be much more practical and reproducible because the mitotic figure is relatively easier to distinguish by pathologists. No mitosis in GIST may be the most convincing evidence to identify a benign tumor, while a significant proportion of tumors with one or more than one mitosis/50HPF should be regarded having a malignant potential. The cases of GIST with mitotic figure should be carefully followed up clinically.
In conclusion, Ki-67 index and cellularity should be used as predictors for the malignant potential of GIST. Mitotic count in the tumor may also be a helpful factor to predict malignant potential in patients with GIST.
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Acknowledgments |
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TOPABSTRACTINTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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We thank Emiko Taniguchi, Tomoko Kagiya and Misako Satoh for preparing histological sections. This work was supported by a Grant-in-Aid for Scientific Research (B), No.10045073, from the Ministry of Education, Japan.
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FOOTNOTES |
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+ For reprints and all correspondence: Xiaojuan Wang, Department of Pathology, Wakayama Medical University, 811–1 Kimiidera, Wakayama City 641-0012, Japan. E-mail: wang-xj@mail.wakayama-med.ac.jp
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TOPABSTRACTINTRODUCTIONPATIENTS AND METHODSRESULTSDISCUSSIONAcknowledgmentsREFERENCES |
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Received October 15, 2001; accepted May 30, 2002
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