Quantitative measurements of formalin-induced fluorescence for differential diagnostics of melanomas and lesions of human skin

The usefulness of formaldehyde-induced fluorescence (FIF) for detection of melanoma cells has been suggested by several investigators during the last 40 years. FIF can be easily excited and observed in microscopic sections of formalin-fixed paraffin-embedded skin samples. However, such an approach has never been widely used in melanoma diagnostics for reasons including lack of clear diagnostic criteria, considerable inconsistencies in both the protocols used and qualitatively analysed results reported by different groups. This study aimed at determination of the spectral bands optimum for detecting melanoma cells. The study involved three sets of the excitation and emission bands: λex=366 nm, λem>425 nm; λex=450–480 nm, λem>515 nm; λex=450–480 nm, λem=510–550 nm. Microscopic digital imaging was used to quantitatively determine the fluorescence intensity of 53 primary melanomas and 32 benign lesions. Best classification of melanomas with algorithm based on fluorescence intensity threshold was obtained for λex=450–480 nm, λem=510–550 nm. Receiver operating characteristics (ROC) analysis of the algorithm yielded area under the curve=0.84±0.05 for melanocytic cells present in the stratum corneum. Our results clearly indicate that the FIF emitting molecules (most probably 5-S-cysteinyldopa) are present in melanomas at the concentration significantly higher than in benign lesions. In terms of the ROC analysis, the diagnostic performance of the test based on the FIF intensity is as good as of many other commonly used diagnostic tests.