Diagnostic performances of lymph node cytopathology in predicting lymphoma and its WHO histotype in dogs

Introduction: The World Health Organization (WHO) classification of canine nodal lymphomas is mainly based on histopathology and immunohistochemistry (HIS+IHC). The invasive sampling procedure and the reporting time represent a deterrent to the execution of this test, and many clinicians rely on cytopathology only. The aim of the present study was to assess the diagnostic performances of cytopathology in identifying: 1) canine nodal lymphoma, 2) immunophenotype and grade, 3) WHO histotype. Material & Methods: 161 cytopathological glass slides from enlarged peripheral lymph nodes were blindly submitted to 6 examiners. Final HIS+IHC diagnoses were obtained. The examiners indicated: 1) lymphoma vs non-lymphoma; 2) supposed grade and immunophenotype; 3) supposed WHO histotype. Results: Cytopathology was accurate and precise in identifying lymphoma samples (sensitivity = 93.1%, specificity = 89.5%, Kappa inter-observer agreement = 0.79). Agreement was only moderate in identifying grade and phenotype (k= 0.41) and WHO subtype (k=0.41). Only 48.6% evaluations agreed with final diagnosis according to grade and phenotype and 38.7% according to the WHO subtype. Conclusions: Cytopathology alone may accurately diagnose lymphoma but its accuracy drops when further characterization is needed. When B-cell lymphoma is suspected cytopathologically, the phenotype is likely to be confirmed via HIS+IHC. Caution should be made in reporting a high-grade lymphoma by cytopathology, particularly if B-cell phenotype is suspected. Histopathology and IHC are mandatory for a definitive diagnosis of the lymphoma WHO histotype. The role of further diagnostic techniques should be addressed in future studies.