Mast cell tumors (MCT) are graded histopathologically in two ways; with either the Patnaik or Kiupel grading systems, and often with both. The first system consists of three levels where grade 1 is considered a well-differentiated tumor, grade 2 as intermediate, and grade 3 as an undifferentiated tumor, and thus the most aggressive. Kiupel has only two degrees; low grade and high grade. In one study, pathologists placed 85% of grade 2 tumors in Patnaik’s system as low-grade in the Kiupel system. Another study showed that 30% of grade 2 Kiupel tumors had metastasized to the local lymph node at the time of diagnosis. In other words there is no doubt that Kiupel grade 2 MCT is a grave diagnosis. In the latest edition of the Journal of Small Animal Practice, a research group has published a multi-institutional study where they have looked at the importance of removing the local lymph node when a dog presents with a mast cell tumor grade 2 Kiupel. The study is retrospective, and 49 dogs with metastasis of MCT to a lymph node were included. All had the tumor removed with at least a 2 cm margin and at the same time underwent a protocol consisting of vinblastine, prednisolone and toceranib as well as treatment of any paraneoplastic syndrome. 18/47 dogs did not have the lymph node removed at the same time, and there was a statistically significant difference in survival in these dogs compared to those who had the lymph node removed (29/47); 250 versus 371 days (median values).

Furthermore, the group found that the risk of tumour progression locally and in the lymph node and tumour-related death were several times higher in the group that did not have the lymph node removed. Tumor size was, not surprisingly, correlated with the risk of local recurrence. Even with a retrospective and relatively small study, I think we can safely say that it can be worthwhile to stage MCTs before surgery, and also remove the lymph node while you’re at it. It doesn’t take much longer, and is well worth it if the patient is given a few extra months to live. More information for the curious: https://doi.org/10.1111/jsap.13525