Optimization of electron paramagnetic resonance imaging for visualization of human skin melanoma in various stages of invasion

Malignant melanoma is a tumor characterized by the uncontrolled proliferation of melanocytes, mainly in skin, but also in eyes. Its incidence is rising each year. To improve the diagnosis and treatment of the tumor, it is essential to develop new effective methods to early detect and characterize melanoma. Previously, we demonstrated in a single-shot study that it was possible to map free radicals of melanin pigments using an electron paramagnetic resonance (EPR)-based method. Furthermore, we demonstrated that X-Band (9 GHz) EPR spectrometry was an accurate tool to assess the growth stage of a pigmented tumor. The aim of the present study was to investigate the ability of EPR imaging to detect and localize melanin pigments inside melanin phantoms, B16 melanoma tumor models and resected human melanomas. We show that EPR can provide an accurate image of synthetic samples, both in terms of shape and size, with errors always lower than 10% compared to the real size. Regarding melanoma studies, the ability of EPR imaging to map accurately the melanoma was depending on the concentration of melanin in the sample, which is proportional to the growth stage of the tumor and the consequent signal-to-noise ratio (SNR) provided by the EPR signal intensity. This led us to define an operational concept, considering SNR and interferences with other EPR signals, to determine when EPR imaging was feasible.