A comparative study of canine and feline meningioma classification based on WHO histological classification system in humans

Meningioma is the most common primary central nervous system (CNS) neoplasm affecting brain and spinal cord of dogs, and brain of cats. This tumour share striking similarities to human meningioma in gross and histological appearance, and biological behavior. The current domestic animal WHO histological classification system of meningiomas categorized them into 2 major groups: benign, slow-growing tumours of eight subtypes (meningothelial, fibroblastic, transitional, psammomatous, angiomatous, papillary, granular, myxoid), and anaplastic tumours. The microcystic and chordoid subtypes have been later identified independently. In humans, a more detailed classification system has been applied by the WHO, allowing consistent correlation of histological findings with biological behaviour and outcome prediction. A major component of this classification system is the grouping of tumours into 1 of 3 histological grades (benign, atypical, anaplastic). The most recent human WHO classification (2007) recognizes histological variants other than clear cell, chordoid, papillary, and rhabdoid, with a mitotic index < 4 mitoses/10 HPF, as benign meningiomas (grade I). Diagnostic criteria for atypical meningiomas (grade II) are mitotic index (≥ 4 mitoses/10 HPF); or ≥3 of the following criteria: patternless sheets, increased cellularity, high N/C ratio, macronucleoli, spontaneous necrosis; or brain invasion. The cytological features included in anaplastic meningiomas are a high mitotic index (>20 mitoses/10 HPF) and a frank anaplasia (grade III). 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 formalin-fixed-paraffin embedded and H&E stained tissues from 57 canine (40 brain, 13 spinal cord, 4 retrobulbar) and 38 feline tumors (all brain tumors) 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). In the dogs histological subtype of benign tumours consisted of 11/57 meningothelial, 11/57 transitional, 3/57 fibroblastic, 0/57 psammomatous, 1/57 microcystic, 1/57 angiomatous, 8/57 papillary, 2/57 chordoid. A meningioma with lipomatous metaplasia was identified. In the cats, histological subtype of benign tumours consisted of 1/38 meningothelial, 22/38 transitional, 7/38 fibroblastic, 4/38 psammomatous. All canine and feline meningiomas were graded according to the criteria of the latest human WHO international histological classification of SNC 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. Of the spinal cord meningiomas, six were graded as grade I, five as grade II and two as grade III. Two canine meningiomas were classified as chordoid type and graded as grade I; both of them were localized in the spinal cord. Eight canine meningiomas were classified as papillary; six of them were graded as grade I, the remaining two cases were graded as grade II. Four canine meningiomas were localized in the retrobulbar region; two of them were graded as grade I, the remaining two cases as grade II. In cats histological grading identified 27/38 benign (grade I) (71.05%) and 11/38 atypical (grade II) (28.9%) tumours. No anaplastic meningiomas were identified. 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. ...