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New Hand-Held Device for Melanoma Imaging

Instrument turns light vibrations into three-dimensional images

A new hand-held device that uses lasers and sound waves may change the way doctors treat and diagnose melanoma, according to a team of researchers at Washington University in St. Louis.

The instrument, described in a paper published in Optics Letters, is the first that can be used directly on a patient to measure accurately how deep a melanoma tumor extends into the skin, providing valuable information for treatment, diagnosis, or prognosis.

Melanoma is the fifth most common cancer type in the U.S., and incidence rates are rising faster than those of any other cancer. It is also the deadliest form of skin cancer, causing more than 75% of skin-cancer deaths.

The thicker the melanoma tumor, the more likely it will spread and the deadlier it becomes, co-author Dr. Lynn Cornelius explained. Being able to measure the depth of the tumor in vivo enables clinicians to determine the patient’s prognosis more accurately — potentially at the time of the initial evaluation — and to plan treatments and surgeries accordingly.

The problem is that current methods cannot directly measure a patient’s tumor very well. Because skin scatters light, high-resolution optical techniques don’t reach deep enough. “None is really sufficient to provide the two- to four-millimeter penetration that’s at least required for melanoma diagnosis, prognosis, or surgical planning,” said another co-author, Dr. Li-hong Wang.

Because taking a biopsy often involves only the removal of part of a tumor — when it is in a cosmetically sensitive area, for instance — provisional measurements of the tumor depth are not always reliable. If, at the time of excision, the surgeon finds that the tumor extends deeper than initially thought, the patient may need more surgery. Recently, Wang and other researchers have applied an approach called photoacoustic microscopy, which can accurately measure melanoma tumors directly on a patient’s skin.

The technique relies on the photoacoustic effect, in which light is converted into vibrations. In the case of the new device, a laser beam shines into the skin at the site of a tumor. Melanin, the skin pigment that is also in tumors, absorbs the light, whose energy is transferred into high-frequency acoustic waves. Unlike light, acoustic waves don’t scatter as much when traveling through skin. Tumor cells will produce more melanin than the surrounding healthy skin cells, and as a result, the acoustic waves can be used to map the entire tumor with high resolution. The device has a detector that can then turn the acoustic signal into a three-dimensional image on a screen.

Wang, Cornelius, and their colleagues previously built a similar desktop device, which shined the laser directly onto the tumor. But so much light was absorbed that very little penetrated to the tumor’s lower layers. The new version, however, is not only hand-held, but it also delivers light around and below the tumor, which generates a bright image of the tumor’s bottom and an accurate measurement of its depth.

The researchers tested their device both on artificial tumors made of black gelatin and on real ones in live mice, showing that the instrument could accurately measure the depths of tumors in living tissue.

Initially, the tool will be used mainly for improving how physicians plan and prepare for surgeries, Cornelius said. But it can also measure a tumor’s entire volume — something that has never been possible with melanoma. If researchers can determine how the volume relates to cancer outcomes, then this tool could give clinicians a new type of measurement for diagnosis and prognosis, Cornelius added.

The researchers are now conducting further tests in human subjects. According to Wang, the device is essentially ready for commercialization, although it will have to prove effective in clinical trials before it can become available.

Source: Optical Society; August 6, 2014.

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