EDIT: I didn't consider dielectric heating (https://en.wikipedia.org/wiki/Dielectric_heating). Seeing as how microwave ovens have 500W rating on the low end, I assume heat generated by 10W source can be dissipated fast enough that proteins don't get denatured even if it's directed at a small area of the body.

Most energetic photon in radio frequencies has wavelength of 1mm and energy of 0.00124eV (http://www.photonics.byu.edu/fwnomograph.phtml) I think. Would just pass through a single cell as cell is much smaller than the wavelength. And 0.0012eV is really small amount of energy. "Van der Waals bonds are weak bonds with a dissociation energy of about 0.01 eV" (http://lamp.tu-graz.ac.at/~hadley/ss1/crystalbinding/bonds/b...) I wouldn't expect it to do anything cellular/molecular level.

Not really a biology person so don't know if there's some effect on aggregate of cells on the millimeter level that could be harmful. Human body's electrical resistance inside is high and through skin is higher so I don't expect much effect on that scale either?