The correlation among different immunostaining evaluation methods for the assessment of proliferative activity in uveal melanoma

Purpose. Proliferative index of uveal melanoma cells serves as a prognostic factor. However, different methods are being used to determine proliferative index using immunostaining for proliferative markers. The major differences among assessment methods are whether the mean proliferative activity of all tumor cells in a section or for areas of rapidly proliferating cells is determined, and whether 10 or 20 fields are being evaluated. We aimed to assess the correlation among proliferative indexes obtained by different immunostaining evaluation methods. Methods. Sections from 60 formalin-fixed paraffin-embedded uveal melanomas were immunostained with MIB-1 antibody. Immunostaining was assessed by counting immunoreactive cells in semi-randomly selected fields (non-selective method) and in areas with maximal immunoreactivity (selective method). Proliferative activity indexes according to the two methods in 10 and 20 high power fields (one high power filed = 0.785mm²) were calculated and compared. Results. The mean positive cell counts per mm² (MPCC/mm²) according to the selective and the non-selective methods were 29.8 ± 8.1 and 10.7 ± 2.4 respectively (p = 0.004, paired t-test). Despite these different values, there was a good correlation between the MPCC/mm² obtained by the non-selective and the selective methods for each tumor (r = 0.737, p < 0.0001, Pearson correlation). In addition, according to both methods, the readings of the first 10 fields and those of fields 1-20 correlated well. Conclusions. Mean proliferative activity of uveal melanoma cells correlates with the proliferative activity in localized areas of the tumor with rapidly proliferating cells. Therefore, uveal melanomas are classified similarly by the selective and non-selective methods of immunostaining evaluation. However, the two methods yield different proliferative index values for the same tumors, a fact that should be taken into account when comparing results of studies in which different techniques were used for immunostaining evaluation.