Do “mastophages” hamper the histologic assessment of lymph node metastases in canine mast cell tumor?

Lymph node (LN) metastasis in canine mast cell tumor (MCT) is a negative prognostic factor that affects post-surgical treatment recommendations. The use of metachromatic stains, including toluidine blue (TB) and Giemsa, is recommended for proper histologic evaluation of the number and arrangement of mast cells in LNs, which is required for the assessment of the histologic node (HN) status according to recently proposed criteria. In our experience, the examination of histologic sections of LNs stained with TB for MCT staging frequently leads to the identification of a variable number of cells with metachromatic intracytoplasmic granules in the medullary sinuses, either isolated or arranged in sheets (Fig. 1A). At low magnification, these cells may resemble mast cells, but on closer view they appear larger (20–30 μm), with vesicular, indented nuclei and coarser cytoplasmic granules, uneven in size and color (Fig. 1B). 1127457 VDIXXX10.1177/10406387221127457Letter to the editorSabattini et al. letter2022