What is the most likely diagnosis for a firm, mobile subcutaneous mass on the upper back?

A firm, mobile subcutaneous mass on the upper back is most consistent with a lipoma. Lipomas are benign tumors made up of adipose (fat) tissue and are known for their characteristic features: they are typically soft, mobile, and well-defined. These tumors are usually painless and can vary in size. Their location on the upper back aligns with common presentation sites for lipomas.

The other potential diagnoses offer different characteristics that do not align as consistently with the described mass. For instance, epidermoid cysts often present as dome-shaped, may have a central punctum, and are generally situated in the epidermis rather than being a mobile subcutaneous mass. Hemangiomas typically present as vascular lesions and may be more superficial or have a reddish color associated with blood vessels. Keratoacanthoma usually appears as a rapidly growing nodular lesion with a keratinized center, which is more concerning and does not traditionally present as a mobile mass.

In summary, the description of a firm, mobile subcutaneous mass is most consistent with the characteristics of a lipoma, making it the most likely diagnosis.