Osteosarcoma diagnosed in a dog using a formalin‐fixed fine‐needle aspirate biopsy

Background Fine‐needle aspirate (FNA) biopsy is considered a quick technique to access and identify the cell types present in a pathologic lesion or make a diagnosis. Often, clinicians want to know if they are dealing with an inflammatory lesion with or without infectious agents or a neoplastic lesion. At times, neoplastic lesions may be confounded by the presence of inflammatory cells. Objectives We aimed to evaluate the application of a formalin‐fixed FNA, designated the cytologic/histopathologic (CytoHisto) technique, to determine the diagnostic quality and ability to arrive at a definitive diagnosis without the use of concentrated cell block or invasive full tissue biopsy procedures during sample collection. Methods A 10‐cc syringe with a 22‐gauge 1‐inch needle attached was used to obtain a sizable FNA biopsy sample from a thigh mass in a dog. The needle was removed from the syringe, and the material was expulsed from the syringe directly into 10% buffered formalin. After 24–48‐h fixation, a strainer was used to facilitate placement of the fixed granular material into a micromesh biopsy processing/embedding cassette, and the sample was processed as a routine histopathology sample. A microtome was used to make thin sections stained with H&E initially. Then, subsequent sections were stained with immunohistochemical (IHC) stains vimentin, MUM1, and CD18. Alkaline phosphatase staining was performed on a previously Wright's‐stained cytology following IHC results. All sections were coverslipped and viewed under a light microscope. Results When unable to perform incisional or excisional biopsies, this CytoHisto, FNA in formalin technique was useful for collection and subsequent processing as a histopathology sample, with sectioning and then staining with H&E and IHC stains. Neoplastic cells were strongly immunoreactive for vimentin but negative for MUM1 and CD18. Scattered leukocytes within the background stained positively with CD18. Conclusions The CytoHisto technique is minimally invasive and allows for sectioning similar to a full‐thickness excisional or incisional biopsy with subsequent H&E and IHC staining, and special stains allow for a definitive diagnosis of osteosarcoma. The CytoHisto technique is a practical diagnostic technique to pursue in clinical practice that minimizes patient invasiveness and maximizes sample collection time, similar to the routine FNA technique.