Immunohistochemical demonstration of metallothionein in benign and malignant canine mammary tumours

Immunocytochemical demonstration of metallothionein (MT) has been reported as a useful prognostic tool in human breast cancer. The aim of this study w as to determine the immunohistochemical location of MT in canine mammary tu mours and its possible correlation with the morphologic characteristics of these tumours. Surgical specimens from spontaneous malignant (n=20) and ben ign mammary neoplasms (n=20) were processed for routine histological examin ation and immunohistochemical study. An indirect immunoperoxidase technique , using monoclonal antibody E9 against horse MT was employed. Intensity of the stain, the percentage of immunoreactive tumour cells and immunohistoche mical overexpression of MT was estimated for each case. Metallothionein ove r-expression, defined as those cases with more than 10% immunopositive cell s, was detected in both benign and malignant mammary tumours. However, stro ng immunostaining intensity was seen in benign tumours, whereas in malignan t tumours immunopositive cells stained weakly. Positive MT immunostaining o ccurred in neoplastic epithelial cells, and some chondrocytes present in mi xed mammary tumours. However, staining intensity was variable in immunoposi tive cells. Differences in staining intensity between the primary malignant mammary tumour, tumour emboli and metastatic cells within a lymph node wer e also noted. Myoepithelial cells and connective tissue did not stain for M T. We concluded that metallothionein immunostaining cannot be used as a dia gnostic or prognostic tool in canine mammary neoplasms. However, results of this study support the hypothesis that MT has a role in tumour proliferati on and tumour progression.