I'mCustomerand I would be glad to help with your question since there is not a veterinary oncologist on this forum. I do have a lot of experience with treating mast cell tumors surgically and with chemotherapy.
If there are mast cells in the spleen, it is for one of three reasons:
1) There are normally some mast cells in the spleen
2) There is metastatic disease from the mast cell tumor on the nipple or from another mast cell tumor on the skin or under the skin that hasn't yet been found
3) There is a visceral mast cell tumor (primary site of cancer is the spleen which is very rare
and these tumors
are very aggressive
with median survival of 2 months)
The pathologist looks at the characteristics of the cells (i.e. normal versus unusual) and the number of cells based on the total number of cells in the sample provided from the fine needle aspirate to try and decide if this looks like cancer. Cancer of the spleen, mast cell or otherwise, cannot be ruled out by an aspirate. Splenic aspirates are notoriously false negative.
For a small nodule on the spleen that cannot be diagnosed by aspirate, serial ultrasound measurement would be advised to see if the site changes over time. If it becomes significantly larger, then you can reaspirate to try and learn more or remove the spleen. The reason for not removing the spleen without monitoring the nodule for changes is that a lot of dogs have small nodules in the spleen that are insignificant. While you can live without a spleen, splenectomy is still a major surgery and is not something to be entered into lightly. A small nodule is not a great risk. I've monitored many and never had one lead to hemorrhage yet although some where identified as cancer over time (i.e. note, most do not change or require further attention).
If your dog is determined to have metastatic mast cell disease (i.e. tumor on the nipple that has metastasized to the spleen), then the skin tumor is removed but the spleen is not. The reason is that the tumor moved from the skin to the spleen through the lymphatic and vascular system. Although you may see no evidence elsewhere on imaging, there is undoubtedly microscopic disease present in the lymph nodes and other organs. You won't be helping your dog by removing her spleen but chemotherapy can help slow the disease process. The prognosis would depend a lot of the grading of the tumor removed from the skin (i.e. 1, 2 or 3) and the ability to get clean wide surgical margins.
On that note, if the mast cell tumor removed from your dog's nipple is a grade 1 but they did
see mast cells in her splenic aspirate, then I would hesitate to do chemotherapy unless you get a second opinion from another oncologist and another pathologist. The reason is that grade 1 mast cell tumors almost never metastasize and visceral mast cell tumors are rare in dogs and unlikely to present as a small nodule on the spleen. You would really need to have overwhelming evidence that the mast cells aspirated from the spleen were present in large enough numbers or atypical enough to suggest cancer before I would pursue chemotherapy if a grade 1 mast cell tumor is removed from the nipple. If that situation arises, I would consider that there may be another mast cell tumor elsewhere that is the primary site from which metastasis occured as that is far more likely than a grade 1 mast cell tumor metastasis and/or viseral mast cell disease of the spleen presenting as a small nodule.
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