Introduction: Atypical intraepidermal melanocytic proliferation (AIMP) is an early form of lentigo maligna (LM) which itself is a precursor to melanoma. It presents commonly on the head and neck where tissue conserving therapies are attractive. When treating LM with imiquimod, dermatologists treat until a certain level of skin inflammation is achieved. Radiation oncologists treat to a set dose of radiation irrespective of the skin reaction at completion. The dose of radiotherapy for AIMP is unknown and these lesions are currently treated in the same manner as LM.

Case series: Five immunocompetent patients (average age 80 years) with AIMP or early LM (ELM) on the head and neck region were treated with RADICAL2 radiotherapy (RT) protocols. All treatment sites were mapped with in vivo reflectance confocal microscopy (RCM) and measured on average 4.0 cm in diameter (range 2.0–6.0 cm). The median RT dose administered was 50Gy [45-54 Gy] in 1.8-2Gy per fraction to the PTV, usually by megavoltage electrons. All patients completed RT. The peak radiation acute skin toxicity observed at any time in all patients was only dry desquamation, equivalent to a grade 2 acute radiation dermatitis reaction by Common Terminology Criteria for Adverse Events (CTCAE) Version 5.0. At a median of follow up of 10 months, all patients had biopsy proven recurrence of AIMP (n=3) or LM (n=2). All recurrences were within the RT field. Patients were followed for an average total of five years post salvage treatment (range: 26 - 124 months). 

Discussion: This series raises questions. First, what radiation dose is required to cure AIMP and ELM? This series suggests that the same dose, if not higher, used in established in-situ disease, is required. Second, should radiation oncologists treat to a grade 3 skin reaction? It may be then advisable to use standard fractionation (2Gy or less) so that the peak RT reaction coincides with the end of treatment and allows for titration and extra dose to be added.