Feline intestinal mast cell tumours: clinicopathological characterisation and KIT mutation analysis

Objectives and methods. Feline intestinal mast cell tumours (FIMCTs) are rare and reportedly characterized by poor differentiation, aggressive biologic behaviour and lack of reliable therapeutic aids. KIT proto-oncogene activating mutations have never been investigated in these tumours. This study describes the main clinicopathological and microscopic features observed in 17 FIMCTs. In particular, tumour degree of differentiation, proliferative activity, Kit protein expression and KIT gene mutations were evaluated and correlated with survival to assess their prognostic relevance. Results. Ten tumours were located in the small intestine, two in the ileocieco-colic junction, and five in the large intestine. Survival times ranged from three to 538 days. Fifteen tumours were evaluated histologically, and there were six well-differentiated, six moderately differentiated, and three poorly differentiated FIMCTs. The latter showed a medium to large deposition of collagen tissue (P <0.001), and significantly higher mitotic and Ki67 indexes compared with more differentiated tumours (P = 0.011). On survival analysis, tumour degree of differentiation (P <0.001) and a mitotic index >2 (P = 0.022) were significantly associated with decreased survival times. Twelve cases showed Kit protein immunoexpression. Kit pattern was membranous in five cases (33.3%), focal paranuclear in five (33.3%) and diffuse cytoplasmic in two (13.3%). Cytoplasmic Kit patterns were associated with a lesser differentiation (P = 0.015). Mutation analysis was successfully performed on 12 primary tumours and four lymph node metastases, but no encoding mutation was detected. Conclusion and relevance. Contrary to literature reports, FIMCTs seem to have an extremely variable biologic behaviour. We propose a classification based on tumour degree of differentiation and proliferative activity. These findings need to be confirmed in larger series, and exploration of further genomic regions of KIT is warranted to clarify its role in the development and progression of these neoplasms