Background To analyse the expression of melanoma chondroitin sulfate proteoglycan (MCSP) and the preferentially expressed antigen of melanoma (PRAME) in conjunctival melanoma (CoM), lymph node (LN) metastases of cutaneous melanoma (CM) and conjunctival nevi (CoN) by immunohistology.

Methods Immunohistology was performed in 70 samples of CoM, 25 of LN metastases of CM and 12 of CoN, and assessed by an immunoreactive score (0–12 points). Statistical analysis was performed to disclose relevant differences in the expression pattern. The diagnostic value of the markers was tested by receiver operating characteristics (ROC) analysis.

Results MCSP and PRAME were expressed at significantly higher levels in CoM and LN metastases of CM than in CoN (p<0.0001). Within CoM, an MCSP expression <9.0 points meant higher risk for recurrences (Cox HR=3.1) and a shorter recurrence-free survival (p=0.002) than an MCSP expression >9.0 points. ROC analysis showed an area under the curve of 91.3% for MCSP (p=0.0002) and 93.8% for PRAME (p<0.0001).

Conclusions MCSP and PRAME are differentially expressed in conjunctival melanomas and nevi. MCSP might have an impact on the risk for recurrence in being inversely correlated to the event. Both markers have high potential to discriminate CoM from CoN. The results indicate that immunohistological characteristics gain relevance in the assessment of CoM.