This patient had polio as a child. Polio was once a hugely problematic disease globally but due to public health programmes, including vaccination, is now thankfully rare.
In fact, the polio eradication initiative (http://www.polioeradication.org/Keycountries.aspx) declares only Afghanistan and Pakistan as the remaining endemic countries.
Chronically, CT findings illustrate atrophy of affected areas, including bone and muscle, which can be profound as in this case. Similar appearances are observed in other neuromuscular disorders.
This case shows the massive asymmetry of musculature due to polio-related atrophy.