Discussion
In penile cancer, Ki-67 expression positivity varies between 36% and 91%, depending on number of cases studied, scoring system and cut-off point. We detected Ki-67 protein in 38.5% (57/148) of PSCC, which is similar to 36% reported by Zhu et al. However, the expression differed significantly between histological subtypes. Sixteen of 17 basaloid tumours showed strong Ki-67 positivity, confirming their more aggressive nature. None of the verrucous cases were Ki-67 positive, which is in agreement with their relatively indolent behaviour. Protzel et al reported an association between strong expression of Ki-67 (cut-off level 50%) and basaloid type of SCC, while verrucous and warty types showed only medium or weak Ki-67 expression though they analysed only three samples of basaloid and two of verrucous tumours. They also found correlation between Ki-67 and HPV infection in 19 penile cancers, which is in agreement with another small study of 11 penile tumours by Gentile et al. We previously assessed HPV status for 102 patients from our series. When we compared high-risk HPV data with Ki-67 immunoexpression we too found strong positive correlation (p<0.0001, Table 2). In cervical carcinogenesis, the proportion of high-risk HPV-positive cases increases with the grade of cervical intraepithelial neoplasia and reaches almost 100% in cervical cancer. Ki-67 immunostaining is predictive for high-risk HPV infection in cervical intraepithelial neoplasia, and it can differentiate reactive lesions from cervical dysplasias. In our cohort of PSCCs, Ki-67 did not show strong predictive value for high-risk HPV infection (sensitivity 61%, specificity 84%). High-risk HPV affects cell cycle control through disruption of RB/p16 and p53/p21 pathways, which will lead to increased proliferation, reflected in increased Ki-67 immunostaining. However, other mechanisms, that is, HPV-independent activation of Akt pathway could also result in increased cell proliferation explaining low sensitivity of Ki-67 for HPV detection, regardless of its strong correlation with the virus.
Ki-67 expression strongly correlated with tumour grade (p<0.0001), which may also reflect tumour aggressiveness as 15 of our 17 Ki-67 positive basaloid tumours were grade 3 while majority of verrucous cases were grade 1. The association of a high Ki-67 labelling index with poorly differentiated tumours was reported earlier by Berdjis et al in a much smaller cohort of 44 patients with only four tumours in grade 3. He also reported the tendency for high Ki-67 labelling index to be associated with advanced local tumour stage, nodal metastasis and clinical disease progression, but with no statistical significance. We did not find Ki-67 correlation with tumour stage (p=0.2193) in our series of PSCCs. It could be explained by intensive growth of localised tumours without metastatic spread, as we registered several tumours with high grade but low histological stage. Furthermore, there was no correlation of Ki-67 with LNM (p=0.7366), similar to the study of Zhu et al on 73 Chinese patients, although, LNM did correlate with tumour grade (p=0001) and stage (p=0.009). Our results may be influenced by a low percentage of patients with LNM in our series (42/148, 28%). Protzel et al found a positive correlation of Ki-67 with LNM in a small series of 28 penile tumour patients. Their tumour samples had similar mean and median Ki-67 expression to our cohort (42.4 vs 42.2 and 46.5 vs 40, respectively), although with twice that many patients with LNM (16/28, 57%). Surprisingly, Guimaraes et al found inverse correlation between Ki-67 expression and LNM in a series of 125 penile tumours (49% with LNM), which was statistically significant in univariate and multivariate analysis. They suggested that this negative correlation could be explained by exophytic rather than vertical growth of the majority of penile tumours. However, in their study, Ki-67 did not show prognostic value for disease-free survival and overall survival, similar to our results (CSS: HR=1.00, 95%, CI 0.99 to 1.02, p=0.54; OS: HR=1.00, 95%, CI 0.99 to 1.02, p=0.45). Zhu et al also reported no prognostic value of Ki-67 for CSS.
In our series, LNM, stage, grade and age were all independent prognostic markers for CSS and OS, similar to Guimaraes et al, who showed urethra infiltration, clinical stage and LNM as independent prognostic markers in disease-free survival; and age, LNM and stage in OS.