Tumours don’t read books, especially cytology ones! We are accustomed to seeing lovely pictures of round cell tumours in publications, where it is very easy to distinguish them based on multiple cytological features. However, real-life situations are different, and sometimes it takes me while to establish the type of round cell tumour I am looking at through the microscope. There are times I manage to reach a diagnosis, while others I don’t. I am sure the same happens to you! Test yourself and see if you can identify the names of these two round cell tumours from skin masses in two different dogs. What’s your gut feeling?
Have you made up your mind?
First of all, let me tell you that the first lesion is a cutaneous histiocytoma, and the other one is a plasma cell tumour.
- What are the elements that suggest a cutaneous histiocytoma in the first picture? I would say mainly the lightly basophilic cytoplasm, the sometimes poorly defined cytoplasmic borders, and the mostly round but occasionally indented nuclei. We also have a mitotic figure on the top right, which is not uncommon as histiocytes may display some degree of atypia. What we are missing from the typical histiocytoma appearance are the blue background and the nucleus localisation, which should theoretically be paracentral, but in this case is mainly eccentric (as more typically seen in plasma cells)
- What are the elements that suggest a cutaneous plasma cell tumour in the second picture? First of all, the cytoplasm is deeply blue, with defined borders and infrequent clear perinuclear halo; nuclei are eccentric and round, as typically seen for plasma cells.
In case you are unable to provide a definitive differential diagnosis, I always recommend highlighting your main differentials (either verbally to your colleagues or written in the comment section of the cytology report). Having a histiocytoma and a plasma cell tumour (both benign lesions) in the differential list is less concerning than having a histiocytoma and a lymphoma, the latter being a malignant tumour. If you are unsure, ask for support from colleagues who are more expert than you on the subject (e.g. external diagnostic laboratories). If the diagnosis is still uncertain, there are further investigations that can be conducted, from immunocytochemistry to histopathology, which can help to achieve a final diagnosis.