Sorry for your loss. Unlike squamous cell and basal cell there isn’t a strong correlation between sun exposure and melanoma. Melanoma can appear on skin that barely even sees the sun or as in your wife’s case, may not even be on the visible surface. My guess as to why your wife didn’t respond to checkpoint inhibitor therapy is since it wasn’t a solar induced melanoma it had low ‘mutational burden’. Checkpoint drugs tend to do well only when there are a lot of mutations in the cancer cells that make them clearly visible to the immune system. Same reason why heavy smokers who get lung cancer do better than non smokers getting lung cancer.
She had a history of both squamous and basal growths. In HS and college she was at Jones Beach as much as at home. Because of this she had semi annual dermatolgy exams. In fact she had one three weeks before her first manifestation which was thought to be breast cancer as it appeared in her left axillary. When the biopsy came back the docs were shocked. She went for ENT, GYN, and Opthamolagic exams which found nothing. She had similar manifestations that Jimmy Carter displayed a year later. She had radiation, yervoy, and then temodar to hopefully give some pause to allow Yervoy time to work. Of course nothing worked. Just last week I watched an ad for NY Presbyterian announcing their success in treating stage 4 melanoma. I really shouldnt be but I was angry watching. As my beloved said often in the hospice, And so it goes.