Abstract
BackgroundMelanomas are rare in cats. The eye is the most commonly involved site, whereas few data are available about feline non-ocular melanomas (NOMs). Ki-67 thresholds with prognostic relevance have been established for canine melanomas, but not in cats. This study was undertaken to investigate the relationship between Ki-67 index, tumour characteristics, and clinical outcome in feline NOMs.Histologic samples were retrospectively reviewed. Amelanotic tumours were admitted upon immunohistochemical positivity for Melan A or S100. Evaluated parameters included morphological diagnosis, histotype, junctional activity, degree of pigmentation, vascular invasion, lymphocytic infiltrate, necrosis, mitotic count (MC) and Ki-67 index. Pigmented tumours were bleached before evaluation. Clinical and follow-up information were retrieved via telephone interviews with the referring veterinarians.ResultsFifty tumours located in skin (n = 33) and mucosae (n = 17) were included. Forty-eight percent and 95% of amelanotic tumours (n = 21) stained positive for Melan A and S100, respectively. Most achromic tumours were mucosal (P < 0.001, Fisher’s exact test) and presented a spindle cell morphology (P = 0.002; Fisher’s exact test). MC and Ki-67 index were significantly correlated (P < 0.001; R = 0.67; Spearman’s rank correlation); median values were 15 (range, 0–153) and 28% (range, 1–78%), respectively. Both were significantly higher in spindle cell melanomas, in tumours lacking junctional activity and in poorly-pigmented tumours. Follow-up information was available for 33 cats (66%). Variables related with a poor clinical outcome included mucosal location, tumour size, spindle, balloon and signet ring cell histotypes, low pigmentation, MC > 5, Ki-67 > 20% and lack of treatment administration. On multivariable analysis, only tumour histotype and treatment retained prognostic significance.ConclusionsAlthough the majority of feline NOMs behave aggressively, Ki-67 index, together with other parameters, may contribute to prognostic assessment. Prospective studies on homogeneous populations are warranted to identify reliable threshold values for this marker.
Highlights
IntroductionThe relative number of tumour cells positive for the nuclear protein Ki-67 (Ki-67 index, tumour growth fraction) is an acknowledged prognostic factor for canine melanoma [7–9]
The most cited parameters associated with a worse outcome include eyelid or mucosal location, achromic phenotype and epithelioid morphology [1, 3, 4, 6] whereas tumours arising on the ear pinna may exhibit a more favourable prognosis [4]
The majority of feline Non-ocular melanomas (NOM) are reported to be malignant, but definitive information about clinical and histologic prognostic factors are lacking, mostly due to the low frequency of these neoplasms. This is the second largest study on feline NOMs and the first investigating the prognostic relevance of Ki-67 in this species
Summary
The relative number of tumour cells positive for the nuclear protein Ki-67 (Ki-67 index, tumour growth fraction) is an acknowledged prognostic factor for canine melanoma [7–9]. In this species, the Ki-67 index has been demonstrated to be significantly different between benign and malignant melanocytic neoplasms, and Sabattini et al BMC Veterinary Research (2018) 14:309 negatively correlated with survival. Thresholds holding a prognostic value have been established for both cutaneous and oral canine melanocytic neoplasms, and the assessment of the growth fraction has become part of the routine histology practice for these tumours [7–9]
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