Fine-Tuned Microscopy Offers Breakthrough Resolution of Melanoma

Researchers have refined a classic Raman-based technique and succeeded in imaging the two dominant melanin molecules, which could lead to new understandings and, critically, early detection of melanoma.

People with fair skin have higher concentration of pheomelanin in their skin, and a corresponding higher risk of developing melanoma – particularly the difficult-to-detect subtype amelanotic melanoma. In high concentrations, pheomelanin is responsible for the orange-reddishness in hair, but is essentially invisible in skin. While the brown-black pigment eumelanin found in most melanomas can be easily seen, the light colored pheomelanin is difficult to detect even with current advances in modern microscopy.

Massachusetts General’s chief of dermatology Dr. David Fischer had initially approached Prof. Conor Evans of Harvard University (MA, USA), who took on the pheomelanin imaging challenge with his team. Their research centers on using coherent anti-Stokes Raman Scatterings (CARS) microscopy, a variant Raman spectroscopy. In general, CARS microscopy utilizes two lasers focused on a sample whose energy difference is “tuned” to specific molecular vibrations to generate high-resolution imaging information.

While another optical technology, transient absorption microscopy, does offer possibilities for studying pheomelanin, this method is complex and does not easily lend itself to clinical practice. “We started to look through the Raman literature,” said Prof. Evans, “Raman spectroscopy is a very mature technique that allows you to detect molecules by their unique chemical vibrations, which are themselves derived from the structure of the molecules. CARS microscopy is a coherent Raman tool that is akin to using a tuning fork to specifically detect molecular structures.”

“We started to look at the molecular structure [of pheomelanin] and noticed there was a corresponding unique molecular vibration that might be useful for imaging the pigment with CARS microscopy.” CARS microscopy proved successful for imaging pheomelanin, a breakthrough for detecting and studying this elusive molecule in skin.

Dr. Sam Osseiran and Dr. Tracy Wang, researchers in the group led by Prof. Evans, led the way in developing and refining CARS microscopy for imaging pheomelanin. “The work led by Tracy was really rather novel application of CARS microscopy to target this biomolecule which no one else has tried to do before,” said Dr. Osseiran, “We adjusted our system and aligned and tuned everything so that we could specifically target this one melanin pigment, pheomelanin.”

Serendipitously, while developing their CARS imaging method, the group found a complementary method, sum-frequency absorption (SFA) microscopy, that could be used for the simultaneously detection of eumelanin. SFA makes use of a signal modulation scheme that can detect both species of melanin. This additional imaging tool is important, as most humans produce both species within skin, making mapping the distribution and quantity of both pigments important.

“SFA imaging allows you to visualize where all the melanin absorbers are within tissue,” said Prof. Evans, “As both CARS and SFA can be carried out at the same time, these two techniques can be used together to simultaneously image both melanin pigments.” Drs. Wang and Osseiran believe their CARS and SFA method could be very helpful for future research and treatment of melanoma, including for observing changes that occur with melanin species in different states.

The study’s original motivator, Dr. Fischer, believes that another important benefit of the work might be its potential role in diagnosing cancer: “This may offer a brand-new tool for early diagnosis for some of the most lethal melanomas, possibly at a stage when they might still be curable,” he said, “Time and time again, it is proven that early diagnosis saves lives.”

Dr. Osseiran will present their findings at the OSA Biophotonics Congress: Optics in the Life Sciences meeting (April 2-5; San Diego, CA, USA).