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Department of Animal Pathology, School of Veterinary Medicine, University of Pisa, 2 I-56124 Pisa, Italy (FM, GL, MM, AP); and Clinic Department, School of Veterinary Medicine, University of Pisa, 1 I-56124 Pisa, Italy (IV)
| Abstract |
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Key words: Cats; grading; immunohistochemistry; mammary dysplasias; mammary tumors; MIB-1; prognostic significance.
Mammary tumors are commonly obsreved in female cats.1 Between 80% and 96% of these tumors are malignant and very aggressive10,18, and can grow rapidly and metastatize to regional lymph nodes, spleen, liver, and lung.11,17 For these reasons, these tumors are an important cause of mortality, especially in middle-aged and elderly females. The interval between feline mammary carcinoma (FMC) detection and death varies from <6 to 12 months.18 Several factors, including age, diameter of the primary lesion, the presence of histologically tumor-positive lymph nodes, mitotic count, extent of necrosis, and complete surgical excision, are independently correlated with postsurgery survival time. A radical surgical treatment instead of a conservative one is significantly related to the disease-free interval.16,23 In the last decades, the interest of veterinary pathologists has been focused on the assessment of markers that may improve the prognosis of FMC, such as morphologic features2,23 and the proliferative activity of the tumor.3,20 In human oncology, the immunohistochemical detection of cell proliferationassociated antigens has been widely investigated. One of the most useful reagents in this field is the antibody to Ki-67 antigen, a nonhistone, highly protease-sensitive nuclear protein, whose expression is strictly associated with cell proliferation.8,22 Ki-67 antigen is expressed in all phases of continuously cycling cells, except G0 and G1.7 Thus, it is an excellent marker for determining the so-called growth fraction of a given cell population. Antibodies directed against this protein are widely used in evaluation of tumors, especially mammary tumors. The Ki-67 labeling index (percentage of positive nuclei) is often correlated with the clinical course of the disease.12,22 The MIB-1 monoclonal antibody (MIB-1) directed against this antigen produces a cross-reaction in frozen and paraffin-embedded feline tissue sections.13
The few studies on the determination of cell proliferation activity of canine and feline mammary tumors by the immunohistochemical detection of Ki-67 antigen have produced contradictory results.4,15,19 We used immunohistochemistry to analyze MIB-1 labeling in a series of feline dysplastic and neoplastic mammary tissues to determine whether this antibody would give more information on the clinical behavior of invasive FMC in a 2-year follow-up study. We also considered the usefulness of tumor grading as a prognostic indicator.
| Materials and Methods |
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± SD = 10.3 ± 2.6 years). The queens included in the study were examined and surgically treated at the Veterinary Teaching Hospital (Clinic Department, School of Veterinary Medicine, University of Pisa) or by practitioners in the surrounding areas. After anamnesis collection, including age, breed, body size, history of ovariohysterectomy, and prevention of estrus with hormonal treatment, a complete physical examination was done. Information on tumor size (maximum length), adherence to underlying tissues, and skin ulceration were also recorded. All the cats included in the study were followed for at least 2 years after surgery by the referring veterinary surgeons to evaluate the postsurgery course of the disease. Overall survival was defined as the time from the day of diagnosis until the day of death or last follow-up.
Ten normal mammary tissues were also studied. These samples were from specific-pathogen-free cats (Iffa Credo, L'Asbrege, France) used as negative controls in a feline immunodeficiency virus (FIV) vaccination program and housed in a climate-controlled animal facility (Techniplast, Gazzada, Italy) of the Retrovirus Centre of the University of Pisa, under conditions stipulated by European Community law.
Representative portions of each mammary sample were fixed in 10% buffered formalin and routinely embedded in paraffin. Five-micrometer-thick sections were stained with hematoxylin and eosin (HE) for histologic evaluation; additional 5-µm sections were used for immunohistochemical studies. The lesions were classified according to the World Health Organization (WHO) classification,9 and tumors displaying different features were classified according to the most pronounced histologic differentiation. Presence of lymphatic or stromal invasion was also recorded.
Histologic grading was performed on HE-stained sections according to the classification proposed by Elston and Ellis5 and previously used to determine the grade of FMCs.2 Histologic grading mainly involved invasive carcinomas; noninvasive tumors were excluded from this part of the study. According to the Elston and Ellis method,5 the overall grade of an individual tumor was derived from the assessment of three morphologic features: degree of tubule formation, degree of nuclear and cellular pleomorphism, and mitotic count. To score the degree of tubular formation, two complete sections from different areas of each tumor were examined, and the proportion of tumor showing tubular formation was determined. When >75% of the examined area displayed definite tubular formation, a score of 1 point was given, from 10% to 75% was given 2 points, and less than 10% was given 3 points. Nuclear pleomorphism was scored qualitatively (1 point when nuclei were small and regular with uniform chromatin, 2 points when cells were larger than normal with vescicular nuclei and clearly visible nucleoli, and 3 points when cells differed in size and shape with prominent and multiple nucleoli). Mitotic count was evaluated quantitatively (1 point was given for >7 mitoses/10 fields; 2 points for 815 mitoses/10 fields, and 3 points for >15 mitoses/10 fields). After adding together the score of each category, the tumor grade was obtained as follows: well-differentiated carcinoma (grade I), 35 points; moderately differentiated carcinoma (grade II), 6 or 7 points; and poorly differentiated carcinoma (grade III), 8 or 9 points.
Five-micrometer-thick sections on poly-L-lysinecoated glass slides were deparaffinised in xylene and rehydrated in alcohol. Endogenous peroxidase activity was blocked by incubating the slides for 5 minutes at 37 C in Endo/Blocker (BiØmeda Corp., Foster City, CA) solution diluted in methanol. To unmask the antigen, the slides were microwaved in 10 mM citrate buffer, pH 6, for 5 minutes at 750 W (three times). After blocking nonspecific staining with normal goat serum, the sections were incubated with the primary mouse monoclonal antibody MIB-1 (DBA, Milano, Italy) diluted 1:50 in phosophate-buffered saline solution (PBS) for 12 hours at room temperature in a moist chamber. Sections were extensively washed in PBS and then incubated with a biotinylated affinity-purified goat anti-mouse secondary antibody (Biogenex Laboratories, San Ramon, CA) for 30 minutes. Section were again washed before incubation for 10 minutes in a streptavidinbiotinylated horseradish peroxidase complex (Biospa, Milan, Italy), and the reaction was developed using a aminoethylcarbazole substrate (AEC, BiØmeda Corp.) for 10 minutes. Finally, the sections were counterstained with hematoxylin, dehydrated, and mounted. Positive controls were included in each staining and consisted of the germinal center of a hyperplastic lymph node from an FIV-infected cat. Negative controls were obtained both by omitting the primary antibody and by using murine subclass-matched (IgG1) unrelatd primary monoclonal antibody. The evaluation of MIB-1 labeling index (MIB-1 I) was assessed at the periphery of the lesions, where the number of positive cells per 1,000 cells examined were counted. This procedure was performed with 10 representative tumor areas from each slide.
Statistical analysis was performed using the statistical package SPSS Advanced Statistics 7.5 (SPSS Inc., Chicago, IL). The analysis of variance (ANOVA) was determined, and the difference between groups was evaluated with Bonferroni's test. Curves for overall survival were estimated by the KaplanMeier method.
| Results |
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Histological grading of the 48 invasive carcinomas revealed that 16 (33.3%) were well-differentiated carcinomas (WDCs; grade I, Fig. 1), 23 (47.9%) were moderately differentiated carcinomas (MDCs; grade II, Fig. 2), and nine (18.7%) were poorly differentiated carcinomas (PDCs; grade III, Fig. 3). The relationship between the results of histologic classification and tumor grading is detailed in Table 1. All the tubular and papillary cystic carcinomas were WDCs or MDCs, and solid carcinomas were mostly MDCs and PDCs.
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Normal mammary glands showed rare MIB-1positive cells. The MIB-1 I in dysplastic and neoplastic mammary tissues is presented in Fig. 5. The MIB-1 I was higher in FMH and invasive carcinomas; the latter had an MIB-1 I significantly higher than that of adenosis (P = 0.005) and noninvasive (in situ) carcinomas (P = 0.006). No significant difference in MIB-1 I was found between invasive carcinomas and FMHs.
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All the queens with FMH and noninvasive (in situ) carcinoma were alive and well at the end of the 2-year trial, without any sign of local or metastatic recurrence. Twenty-one (43.7%) of the 48 queens with invasive carcinoma (group A) were alive and well at the end of the 2-year follow-up, and 27 (58.2%; group B) had died. At the end of the first year of study, 24 of the 27 queens of group B had already died. There was no significant difference in the age at tumor diagnosis between group A (
± SD = 10.3 ± 2.3 years) and group B (10.7 ± 2.8 years) (P = 0.24).
Neoplastic cells of group B queens showed higher MIB-1 I (22.4 ± 10.7) than did those of group A cats (18.7 ± 7.7), but this difference was not significant (P = 0.157). The relationship between MIB-1 I by quartiles and the postsurgery deaths within 2 years is presented in Table 3. Even if a significant association between the survival time and the MIB-1 I was not evident, the queens with an MIB-1 I of >27.2% had a poor prognosis compared with queens of the other quartiles.
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| Discussion |
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Our study confirmed the usefulness of MIB-1 antibody for the assessment of cell kinetics by immunohistochemical localization of the Ki-67 antigen on feline formalin-fixed, paraffin-embedded tissues routinely processed for histochemistry.
Normal mammary gland tissues showed a low proliferative rate (<1%). Mammary tissues with adenosis maintained a proliferative rate similar to that of normal tissues, and noninvasive (in situ) carcinomas had low MIB-1 I. In contrast, MIB-1 I was higher in invasive FMCs and in FMHs.
In spite of these results, a lack of significant prognostic importance of MIB-1 I in mammary carcinomas was found, even though higher mortality was observed in queens with tumors displaying elevated MIB-1 I. As with canine mammary tumors, the prognostic relevance of kinetic parameters for FMCs remains to be determined. Previous studies demonstrated that mitotic index and argyrophilic nucleolar organizer region counts were significantly correlated with the course of mammary carcinomas in cats,3,23 but few studies have investigated MIB-1 as a prognostic factor for these tumors. A previous investigation, carried out on FMCs, demonstrated that the Ki-67 index was a useful tool for identifying tumors with a more aggressive course.4 Similar resutls were obtained in a study on canine mammary tumors: multivariate analysis concerning metastasis, disease-free survival, and overall survival revealed that the Ki-67 index had a prognostic value.19 Notably, however, another study dealing with Ki-67 immunohistochemistry in canine mammary tumors reported a lack of prognostic information of this marker.15 Even though the diagnostic and prognostic value of Ki-67 immunostaining of human tumors has been widely documented and accepted,6,12 some researchers have not found an association between Ki-67 and disease-free survival.14,21 This discrepancy in results has been explained based on the criteria employed in dividing the patients into groups.12 In canine mammary tumors, the contrasting results concerning the prognostic information on the proliferative activity detected by Ki-67 immunohistochemistry were related to the heterogeneity in the prevalence and biologic behavior of the various histologic types.4 In the present study, we used a population similar to that employed in the previous study on feline mammary tumors so that the results of the two studies could be compared.
In the present study, other factors such as cat age, tumor histologic type, and tumor grade were not significantly related to MIB-1 I, but MIB-1 expression had a borderline correlation with presence of lymphatic invasion. This finding is in agreement with those of other studies where no correlations were found between proliferation indexes and morphologic appearance of mammary carcinomas in cats4 and dogs.20
Animal age and tumor histologic type were also not correlated with survival, but tumor grading, determined using the Elston and Ellis method,5 had high predictive value, as shown by survival curves. Comparable results have been found in a previous study,2 especially with respect to grade I and grade III carcinomas.
MIB-1 I measured in paraffin-embedded sections is a useful method for assessing proliferation in a routine setting. However, the results of this and other studies on MIB-1 expression as a prognostic factor for survival in FMC have produced conflicting results, and at this time the data do not support the use of MIB-1 I in clinical decision making. To reach a definitive conclusion on the value of this index, it will be necessary to develop a standardized methodology to compare the results obtained in different studies. More information is needed to determine whether the use of this marker in association with histologic grading will add new prognostic information for the treatment of FMCs.
| Acknowledgments |
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| References |
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