Background: Primary melanoma of the iris, for reasons unknown has a lower metastatic rate compared to primary ciliary body melanoma. We identified 6 histology cases of ciliary body melanoma that had spread onto the iris surface and into the stroma, representing a change in tumour microenvironment from aqueous humour non-exposure (ciliary body component) to aqueous humour exposure (iris surface component).
Method: Conventional light microscopy was performed on stained paraffin sections of the identified cases, followed by immunohistochemistry to cell cycle proteins p27 and Cyclin D1. FISH analysis was conducted on the paraffin sections for changes of chromosomes 3 and 8.
Results: Iris surface melanoma cells were smaller compared to the adjacent deeper iris stromal melanoma cells and to those in the ciliary body. Fewer iris surface melanoma cells expressed Cyclin D1 protein but more expressed p27 protein, compared with the larger iris stromal melanoma cells (Paired Wilcoxon Signed Ranks Test: Cyclin D1 p value=0.028; p27 p value=0.046). With FISH, chromosome 3 and 8 alterations were less common amongst the iris surface melanoma cells than the deeper iris stromal melanoma cells which were consistently characterised by a relative genetic imbalance for chromosomes 3 and 8.
Conclusions: This data suggests that there are tumour modulatory factors in the aqueous humour / anterior chamber environment Furthermore, it may help explain why iris melanomas generally have a less aggressive course than ciliary body and choroidal melanomas.