"[It] is inoperable because of his age and frailty," she said in a statement.
"Clive [will] stay in hospital for another week. After that, he is scheduled for radiation and chemotherapy from 6 May."
She said an application for medical parole would go ahead next week.
Derby-Lewis was convicted of killing SA Communist Party general secretary Chris Hani.
Hani was shot dead in the driveway of his home on 10 April 1993.
Derby-Lewis, who was sentenced to 25 years behind bars, has already served more than 20 years of his sentence.
The 78-year-old man has twice been denied medical parole.
He has also been stabbed on two different occasions by inmates at the Kgosi Mampuru Central Correctional Centre in Pretoria.