By Krystal Harris, DVM
Grading – 3-Tier Patnaik vs. 2-tier Kiupel
For many years, veterinary pathologists have relied on the Patnaik 3-tier grading system for cutaneous MCTs. When this system was published each tumor grade was associated with a distinct survival curve (Figure 1, Patnaik 1984). In recent studies an increased number of tumors are being diagnosed as Grade II, and there is no longer a significant difference in survival between Grade I and 2 MCTs (Figure 2, Sabatinni 2014). The other drawback to current use of this grading scheme is a lack of agreement among pathologist when assigning histologic tumor grades. Only 63-75% agreement exists among veterinary pathologist using the Patnaik system.
Figure 1 (left): Survival curve published by Patnaik et al, 1984; showing significant differences in survival between Grade I, II & III MCT. Figure 2 (right): Survival curve published by Sabattini et al, 2014; showing no significant difference in survival between Grade I & II.
A 2-tier grading system was proposed in 2011 by Kiupel et al to increase consistency among pathologists and improve the prognostic significance of MCT grading. Using the 2-tier system, a tumor becomes high-grade if it meets any 1 of the following objective criteria:
- ≥7 mitotic figures in 10 hpfA
- ≥3 multinucleated cells (multinucleated defines as 3 or more nuclei per cell) in10 hpf
- ≥3 bizarre nuclei (highly atypical) in 10 hpf
- Karyomegaly (nuclear diameters vary by at least 2-fold in ≥10% mast cells)
Agreement between pathologists with the 2-tier system is 97%. This system offers a significant association between high grade tumors and shorter time to recurrence, metastasis, and shorter survival times.
Currently most pathologists are applying both of these grading systems on their reports.
A drawback to the 2-tier system is that it may miss a distinct subgroup of Grade II MCTs, the “Grade II-High grade” which have longer survival than Grade III-High Grade MCTs (Figure 3).
Figure 3 (left): Kaplan-Meier survival curves for mortality due to MCTs in 162 dogs stratified by location (dermal or subcutaneous) and by the combined use of the Patnaik and Kiupel grading systems. Images from Sabattini et al, 2014. When both systems are used, there is a distinct subset of Grade II – High grade MCT that have a statistically significant difference in survival compared to Grade II – low grade or Grade III – High grade.
Subcutaneous MCT are not included in the histologic grading schemes described above for cutaneous MCTs. According to the Patnaik system, these tumors would be classified as Grade II or higher due to their deeper tissue location. The majority of subcutaneous MCTs have a benign course of behavior with metastatic rates as low as 4% and recurrence rates of 8%, even though incomplete surgical excision is common. Mitotic index ˃4 is strongly predictive of survival, local recurrence, and metastasis. Thompson et al, 2011.
MCT Prognostic Panels
MCT prognostic panels are now offered by a variety of laboratories to provide objective information which can improve prognostic certainty over tumor grading alone. These panels offer the most benefit for Grade II and III MCTs to determine if additional therapy is indicated and which type of treatment to recommend.
Proliferation Markers (Ki67, AgNOR)
Ki67 reflects the number of proliferating cells and AgNOR correlates with speed of proliferation. When combined, these two markers aid in predicting risk of metastasis and overall survival. The lab will provide you with a numerical score for Ki67 and AgNOR, as well as clear interpretation guidelines.
KIT Staining & PCR for C-Kit Mutations
The c-kit gene is mutated in 30-40% of canine MCTs. This mutation results in constant activation of the c-kit receptor in the absence of ligand binding. The result is uncontrolled proliferation of the neoplastic cells. Tumors with c-kit mutations are more aggressive, but also more likely to respond to a tyrosine kinase inhibitor (TKI) such as Palladia (toceranib, Zoetis) or Kinavet (masitinib). These medications block activation of the c-kit receptor. A PCR will be performed to look for mutations in the c-kit receptor (Figure 4, 5).
Figure 4 (left): The c-kit receptor in the cell membrane roughly indicating the locations of the most commonly mutated sites, exon 8 and exon 11. Figure 5 (right): The top two images show amplification of exon 11 of c-kit (green) in a normal wild-type cell, where the gene products are the same size on both chromosomes. The lower two images represent a patient with an internal tandem duplication of exon 11 on one of its chromosomes (bottom image), and the wild-type gene on the other chromosome (Images from Anne Avery, PCR BASED DIAGNOSTICS).
About Krystal Harris, DVM
Dr. Krystal Harris completed her undergraduate work at Texas A&M University then earned her Doctorate of Veterinary Medicine from St. George’s University School of Veterinary Medicine in 2010. She completed an oncology internship and residency program at Oregon State University. Dr. Harris is inspired by the strong patient relationships she cultivates. As a veterinary oncologist, she helps her patients through the emotionally difficult decisions associated with cancer. By outlining clear treatment options, she is able to give pets the care they need.
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