The aim of this study was to compare human and domestic animal WHO histological classification system of meningioma to identify possible critical points in applying human WHO classification to canine and feline meningiomas. Selected paraffin embedded tissues from 57 canine and 38 feline tumours recorded as meningiomas were used in this study. Based on the current domestic animal WHO histological classification system they had been achieved as benign (38 canine, 34 feline) and malignant (19 canine, 4 feline). All these meningiomas were graded according to the criteria of the latest human WHO international histological classification of CSN tumours as benign (grade I), atypical (grade II) or anaplastic (grade III). Based on human WHO classification system, histological grading in the dogs indicated 27/57 benign (grade I) (47.3 %), 26/57 atypical (grade II) (45.6 %) and 4/57 anaplastic (grade III) (7.0%) tumours. Eleven tumours recorded as benign meningiomas were graded as grade II, 15 malignant as grade II and 4 malignant as grade III. Two canine meningiomas were classified as chordoid type and graded as grade I. Eight canine meningiomas were classified as papillary; six of them were graded as grade I, the remaining two cases were graded as grade II. In cats histological grading identified 27/38 benign (grade I) (71.05%) and 11/38 atypical (grade II) (28.9%) tumours. Twenty-six tumours classified as benign were graded as grade I while the remaining eight benign-classified tumours as grade II, two malignant as grade I and two malignant as grade II. Based on our results, we believed mitotic index ≥ 4 mitoses/10 HPF and brain invasion are sufficient criteria to identify grade II meningiomas and we suggested patternless sheets alone could be assumed as a criterion to attribute grade II to a meningioma. Interestingly, in the cats, meningiomas of grade III were not detected, confirming the less aggressive behaviour of feline meningioma and suggesting no current grading is applicable to the feline meningioma. Moreover, our results confirmed a higher incidence of canine atypical (grade II) and anaplastic (grade III) meningioma than in humans.
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