Not All Abbreviations Are Acronyms
The words acronym and abbreviation are not synonymous.
An abbreviation is a shortened form of a word, like Dr. for doctor or PFS for progression-free survival.
An acronym is a type of abbreviation that’s formed using the first letter of each word. What makes an acronym different than, say, the PFS example above is that it can be pronounced as a word, like CAD for computer-aided design.