Merkel Cell Carcinoma
It's kind of humbling to realize that this cancer was first described in 1972 by Toker. The Merkel Cell is a pluripotential cell that is of neuroendocrine differentiation, and as a malignancy it tends to be rapidly growing, tends to pick elderly patients and often leads to metastasis and death.
Nearly half the time it is on the head and neck, but can be virtually anywhere. The prognosis is worse than that of invasive melanoma, and it has a high rate of local recurrence (one of my patients currently has that) and distant metastases occur up to 75% of the time.
The tough thing about this clinically is that the tumors begin innocuously, and then accelerate so that by the time the patient goes from thinking it is a pimple until the time of metastasis is fairly short.
How best to differentiate this from BCC? If you look at the margin of this tumor, it has a "collarette" of normal skin extending onto the tumor. BCC almost never does. Also, it has none of the pearliness of a large nodular BCC. Nor does it have the telangiectatic surface typical of large BCCs.
How about SCC? This is a much tougher call. It certainly does not fit the keratoacanthoma picture in that there is no central crater, but a large SCC could look like this, but usually it would be more eroded, and instead of the collarette it would be more likely to be wall to wall. This is typical of a Merkel Cell both in patient age, in location and in appearance.