. 2022 Jul 4;20(4):739–751. doi: 10.1111/vco.12827

TABLE 4.

Summary of recommendations for diagnosis and histopathologic prognostication of canine melanocytic neoplasms

Summary of recommendations for diagnosis and histopathologic prognostication of canine melanocytic neoplasms
Confirm melanocytic origin by identifying classic histologic features of melanocytic neoplasms or by demonstrating labelling of neoplastic cells in amelanotic neoplasms for melanocytic specific immunohistochemical (IHC) markers Classic histologic features: intracytoplasmic melanin, variable cell morphology in the same tumour, junctional activity, pagetoid growth, presence of neoplastic cells at the mucosal‐submucosal (epidermal‐dermal) junction even in the absence of junctional activity, and finely stippled to vesiculate nuclei with a prominent central nucleolus (“owl's eye”). If amelanotic or otherwise poorly differentiated, demonstrate IHC labelling for Melan‐A, PNL2, TRP‐1 or TRP‐2. Determine prognosis of cutaneous and lip/oral melanocytic neoplasms by evaluating the specific histologic and molecular parameters described in Table 3. For heavily pigmented neoplasms, bleaching may be necessary in order to accurately evaluate nuclear features and mitotic count (MC). To evaluate Ki67 labelling in heavily pigmented neoplasms, bleaching should be performed AFTER IHC labelling for Ki67. For lip/oral melanocytic neoplasms: If there is marked nuclear atypia (≥30% atypical nuclei), a high MC ^a (≥4/2.37 mm²), or evidence of vascular invasion or metastasis, the neoplasm is diagnosed as a malignant melanoma and evaluation of the Ki67 count is offered for further prognostication (pathologists may indicate that Ki67 count is not necessary for neoplasms that greatly surpass ^b the nuclear atypia and MC thresholds and/or show evidence of vascular invasion or metastasis). If the histologic parameters give mixed results, are at or near the threshold values, or if all the histologic parameters indicate a favourable prognosis, evaluate the Ki67 count for further prognostication. Ki67 count is the most useful parameter for prognostication due to high specificity and objectivity. Provide the results of all parameters examined in the pathology report. For cutaneous melanocytic neoplasms: If there is marked nuclear atypia (≥20% atypical nuclei), a high MC ^a (≥3/2.37 mm²), a tumour thickness >0.95 cm, or evidence of vascular invasion or metastasis, the neoplasm is diagnosed as a malignant melanoma and evaluation of the Ki67 index is offered for further prognostication (pathologists may indicate that Ki67 index is not necessary for neoplasms that greatly surpass ^b the nuclear atypia and MC thresholds and/or show evidence of vascular invasion or metastasis). If the histologic parameters give mixed results, are at or near the threshold values, or if all the histologic parameters indicate a favourable prognosis, evaluate the Ki67 index for further prognostication. Ki67 index is the most useful parameter for prognostication due to high specificity and objectivity. Provide the results of all parameters examined in the pathology report. Some general rules for prognostication of melanocytic neoplasms: It is impractical to accurately predict, on an individual basis, the biological behaviour of melanocytic neoplasms. More than one parameter should be used to classify melanocytic neoplasms histologically as benign, of low malignant potential, or malignant because of the inherent subjectivity in histological evaluation. If histologic prognostic factors conflict with one another, a neoplasm should be diagnosed as a melanocytic neoplasm and prognostic factors should be discussed. Evaluation of nuclear atypia and MC in combination with Ki67 expression level and clinical features will maximize the correctly classified neoplasms. ¹

Confirm melanocytic origin by identifying classic histologic features of melanocytic neoplasms or by demonstrating labelling of neoplastic cells in amelanotic neoplasms for melanocytic specific immunohistochemical (IHC) markers
- Classic histologic features: intracytoplasmic melanin, variable cell morphology in the same tumour, junctional activity, pagetoid growth, presence of neoplastic cells at the mucosal‐submucosal (epidermal‐dermal) junction even in the absence of junctional activity, and finely stippled to vesiculate nuclei with a prominent central nucleolus (“owl's eye”).
- If amelanotic or otherwise poorly differentiated, demonstrate IHC labelling for Melan‐A, PNL2, TRP‐1 or TRP‐2.
Determine prognosis of cutaneous and lip/oral melanocytic neoplasms by evaluating the specific histologic and molecular parameters described in Table 3. For heavily pigmented neoplasms, bleaching may be necessary in order to accurately evaluate nuclear features and mitotic count (MC). To evaluate Ki67 labelling in heavily pigmented neoplasms, bleaching should be performed AFTER IHC labelling for Ki67.
- For lip/oral melanocytic neoplasms:
  - If there is marked nuclear atypia (≥30% atypical nuclei), a high MC ^a (≥4/2.37 mm²), or evidence of vascular invasion or metastasis, the neoplasm is diagnosed as a malignant melanoma and evaluation of the Ki67 count is offered for further prognostication (pathologists may indicate that Ki67 count is not necessary for neoplasms that greatly surpass ^b the nuclear atypia and MC thresholds and/or show evidence of vascular invasion or metastasis).
  - If the histologic parameters give mixed results, are at or near the threshold values, or if all the histologic parameters indicate a favourable prognosis, evaluate the Ki67 count for further prognostication.
  - Ki67 count is the most useful parameter for prognostication due to high specificity and objectivity.
  - Provide the results of all parameters examined in the pathology report.
- For cutaneous melanocytic neoplasms:
  - If there is marked nuclear atypia (≥20% atypical nuclei), a high MC ^a (≥3/2.37 mm²), a tumour thickness >0.95 cm, or evidence of vascular invasion or metastasis, the neoplasm is diagnosed as a malignant melanoma and evaluation of the Ki67 index is offered for further prognostication (pathologists may indicate that Ki67 index is not necessary for neoplasms that greatly surpass ^b the nuclear atypia and MC thresholds and/or show evidence of vascular invasion or metastasis).
  - If the histologic parameters give mixed results, are at or near the threshold values, or if all the histologic parameters indicate a favourable prognosis, evaluate the Ki67 index for further prognostication.
  - Ki67 index is the most useful parameter for prognostication due to high specificity and objectivity.
  - Provide the results of all parameters examined in the pathology report.
Some general rules for prognostication of melanocytic neoplasms:
- It is impractical to accurately predict, on an individual basis, the biological behaviour of melanocytic neoplasms.
- More than one parameter should be used to classify melanocytic neoplasms histologically as benign, of low malignant potential, or malignant because of the inherent subjectivity in histological evaluation.
- If histologic prognostic factors conflict with one another, a neoplasm should be diagnosed as a melanocytic neoplasm and prognostic factors should be discussed.
- Evaluation of nuclear atypia and MC in combination with Ki67 expression level and clinical features will maximize the correctly classified neoplasms. ¹

^{^a}

For this consensus, the mitotic count (reported as mitotic index, in the literature reviewed) is obtained by counting the absolute number of mitoses in 10 high‐power fields (400× magnification/40× objective) in the region with highest mitotic activity, as determined initially on a low power scan (100× magnification/10× objective) of the specimen. Future studies which evaluate mitotic count as a prognostic parameter should also adopt this methodology, while also defining and standardizing the microscopic area evaluated to 2.37 mm².

^{^b}

There are no established values for “greatly surpassing” the cut‐offs, but pathologists will use their experience to subjectively make this recommendation. In general, the subgroup agrees that Ki67 count is not necessary for tumours with ≥50% atypical nuclei or oral/lip tumours with an MC ≥40/2.37 mm² and cutaneous tumours with an MC ≥30/2.37 mm², which are 10 times higher than the respective threshold values.