@rfannin: Thank you for coming to Mayo Clinic Connect to explore your concerns, which are very understandable given the circumstances. I also had amelanotic melanoma that my dermatologist at the time did not suspect to be melanoma. I was subsequently started on Keytruda some years later when I was found to have metastatic melanoma. My first adverse reaction was a small rash which I reported immediately and was readily treated, but it was mild in nature. Individuals react very differently and at different times throughout the treatment cycle to potential adverse events which can occur at any time. While immunotherapy is generally limited to 2 years as no additional benefit has been found beyond that time period, adverse events may continue therefore one must be alert for them.

Rashes can present with varying severity - in your mother's case, it sounded particularly severe so there may be cause to be more cautious when administering and monitoring this PD-1 checkpoint inhibitor. If this is a new medical provider who will be overseeing your mother's care, be sure they're aware of her previous reaction so they're aware of the risk, which may increase the possibilty of it occuring but is not necessarily guaranteed.

Have you discussed this concern directly with her current medical provider?