The original reports of Miescher and the subsequent reports using his technique involved very low energy x-ray teletherapy (10–20 kV, also known as Grenz rays). With Grenz ray therapy, the majority of the radiation dose is absorbed in the most superficial millimeter of skin, with little to no radiation absorbed beyond 2 mm. Soft or superficial x-rays (20–150 kV) are absorbed over 1–20 mm of skin and subcutaneous tissues. An example of this is presented in Fig. 12.1. Even higher energy x-rays (orthovoltage, 150–400 kV) deposit the majority of radiation dose over 50–80 mm of skin and subcutaneous tissues. The aforementioned forms of radiation are preferable for neoplasms originating in the skin, but they are not widely available because most radiation oncology departments use linear accelerators to produce megavoltage electrons and photons which intentionally spare the skin surface. Through specific manipulations, megavoltage electron radiation can deliver radiation to the skin surface, and unlike x-rays have an advantage of delivering radiation to a limited, finite range of skin and subcutaneous tissue, ranging from 10 to 200 mm deep to the skin surface. Examples of this are presented in Figs. 12.2 and 12.3.

Brachytherapy has been less often used and reported on, but should not be overlooked as a valuable component of the radiotherapeutic armamentarium for LM and LMM. The primary advantage of brachytherapy is the ability of this modality to conform to irregular skin surfaces (i.e., those that are not relatively flat), and deliver a uniform and homogenous dose of radiation across the area. A notable example of this is a case of extensive, unresectable LMM affecting the entire scalp of a patient. As the curved, large surface of the scalp could not be adequately targeted with a superficial teletherapy technique, investigators performed skin surface brachytherapy using catheters afterloaded with Ir-192, with a satisfactory result [11]. Other investigators used interstitial brachytherapy to treat 3 patients with LMM that were deemed inappropriate for surgery. With a median follow-up of 4 years, these patients had good control of the LMM, with no significant side effects [12]. Brachytherapy has not enjoyed a significant amount of usage in the treatment of LM and LMM, probably owing to the technical demands and resources required to perform this successfully. Moreover, custom skin surface brachytherapy techniques generally require access to radioisotopes, which are generally not at the disposal of dermatologists who collectively treat the vast majority of patients with LM and LMM.

Regardless of the specific treatment technique used, defining the target for radiotherapy is paramount to successful treatment. The grossly evident “tumor” or gross tumor volume, (GTV) is typically delineated as the pigmented lesion on the skin surface. In some instances, there is sharp demarcation between the pigmented lesion and the surrounding normal tissue, but this is not always the case. Adjunctive imaging techniques may help identify the target and include Wood’s lamp (ultraviolet light) which requires visualization in a dark room [13]. Conventional radiographic imaging (CT, MRI, PET) is unlikely to be informative given the superficial nature of LM and LMM, but other modalities such as reflectance confocal microscopy, high frequency ultrasonography and optical coherence tomography may be valuable [14]. Once the GTV has been identified and marked on the skin surface, one must determine the subclinical extent of disease, often referred to as the clinical target volume (CTV). A study of 1,120 patients with LM excised using variable margins indicated that 6–9 mm of normal appearing skin radially around the GTV, will yield a negative margin in 86–99 % of cases [15]. Extrapolation of this suggests that a CTV of 6–9 mm radial margin is appropriate. While the depth of invasion of LM and LMM is expected to be limited, however, perifollicular extension is common, and some have reported this extending to 3–5 mm from the surface of the epidermis. Finally, to account for movement, setup uncertainty, and other factors, a planning target volume (PTV) must be generated, which will depend on the specific parameters of the treatment setup. Once the PTV has been created, determination of the appropriate radiation application parameters (including shielding) is carried out. Taking all of this into account, investigators have employed margins of 5–20 mm from the edge of the pigmented skin lesion to the edge of the treatment field, although the specific parameters of the treatment technique will ultimately dictate the margin width necessary.