I find
amastia is listed by Dictionary.com and Wiktionary as well as, naturally, various medical dictionaries. These references differ as to whether the word applies only to a congenital condition, or could be applied also to the results of surgery.
The closest I could find to a usage example from a non-specialist source was in
a 2016 blog post by a medical intern in Ohio:
On my new favorite show, Botched by Nature, I learned about the difference between amastia and amazia. Amastia is the absence of all breast tissue, areola, and nipple. Amazia is just the absence of breast tissue, with intact nipple and areola. Both are can only be fixed by surgery. See, reality television is educational!
Many of the instances I found of the word
amastia did also refer to the related condition of
amazia, so I'll add both of them as rare words.