Lichenoid reaction as a potential immune response marker of intratreatment histological response during successful vismodegib treatment for a giant basal cell carcinoma

We report an 83 year-old patient with a 13 × 7.5 cm² basal cell carcinoma (BCC) successfully treated with the combination of vismodegib and minimal surgery. On Day 109, a 0.9 cm papule suspicious for residual BCC was seen centrally within a large pink atrophic plaque. This lesion was excised; pathology confirmed BCC with negative surgical margins. Simultaneously, suspecting noncontiguous histologic response, we performed 21 biopsies at the periphery of the pretreatment tumor location. Seventeen (17/21, 81%) revealed lichenoid dermatitis. No tumor was seen on any. We believe the lichenoid dermatitis observed is a novel finding for two reasons. First, it may be considered a marker of a positive intratreatment response. This may help guide clinicians on the optimal treatment duration of vismodegib to maximize efficacy and mitigate side effects. Second, we think it suggests an additional mechanism of vismodegib action, possibly via local immune effects. Further investigations are warranted.