Alejandro Aguinaga said the former president had a condition that was eating away his stomach.
Mr Fujimori is serving a 25-year sentence for human rights abuses, but his family says he is too frail to be in prison.
They have called for a presidential pardon on humanitarian grounds.
This is a recurrent theme for either jailed ex-dictators or for ex-dictators facing charges: that they are too old to be charged and jailed (this often came up in regards to Augusto Pinochet, for example). Recently, Argentine dictator Jorge Rafael Videla, died in prison.
Sounds like this might be a ploy as much as anything else:
Correspondents say that the doctor’s diagnosis is likely to add more pressure on President Ollanta Humala to issue a pardon.
Last October, his family asked Mr Humala to commute his sentence.
Under Peruvian law, he can be pardoned only on health grounds.
But opponents argue that Peruvian jails are crowded with prisoners in worse health and for lesser crimes than those for which Alberto Fujimori was convicted.
Ultimately, Fujimori’s story is one of great hubris: he thought he could return from exile in Japan (he fled Peru in 2000) and reenter Peruvian politics to great acclamation. Instead, he was arrest in Chile (which was supposed to be the first stepping stone on his path to restoration) and extradited to Peru to face various charges associated with his time as president.