Comparison between May Grünwald-Giemsa and rapid stains in fine-needle aspirates of canine mast cell tumor: diagnostic and prognostic implications

Introduction Mast cell tumors (MCTs) are often diagnosed by cytology based on the identification of their characteristic purple intracytoplasmic granules with methanolic Romanowsky stains, including May Grünwald-Giemsa (MGG). In clinical practice, aqueous Romanowsky stains are commonly used because of their rapidity, but the risk exists that mast cell granules may not stain properly. The purpose of this study was to investigate the phenomenon of hypogranularity of MCT with rapid staining (RS) in terms of frequency and correlations with tumor characteristics, along with diagnostic and prognostic implications. Materials and Methods A prospective study was carried out on cytologic preparations of histologically-confirmed canine primary MCTs and metastatic lymph nodes. For each case, good-quality smears were stained with both MGG and RS and comparatively assessed. Results Fifteen out of 60 (25%) primary canine MCTs were hypogranular with RS (P=0.002). These were predominantly cutaneous high-grade tumors and in 7 of them (11.7%) a certain diagnosis could not be made with RS. Accuracy in grading assessment was not significantly different compared with MGG (79% vs 83%). Ten of 25 (40%) metastatic lymph nodes were hypogranular with RS (P=0.002); the negative predictive value in the detection of nodal metastases, assessed with three independent observers, was significantly lower (100% vs 73%; P=0.010). Discussion This study confirms that RS can be ineffective in staining MCT granules, especially in biologically aggressive tumors, and can occasionally prevent the achievement of a correct diagnosis. The use of RS is discouraged for the search of nodal metastases, as the identification of isolated mast cells can be particularly challenging.