Primary cutaneous B-cell lymphoma originates in the skin, whereas a metastasis is from an internal primary cancer; they can be clinically identical and are distinguished by staging.
A KA is a rapidly growing, crater-like nodule with a central keratin plug, a specific morphology not typical for a metastatic nodule.
A PG is a benign, friable, vascular tumor that bleeds easily, unlike the typically firm, non-friable nature of a skin metastasis.
A primary cutaneous SCC arises on sun-exposed skin, often from a precursor AK, whereas a metastatic SCC comes from another site (e.g., lung, oral cavity).