Anni Dewani's killer suffers from an extremely rare brain cancer with a low survival rate, the Western Cape High Court heard on Monday.
Jeanette Parkes, an oncologist at Groote Schuur hospital, said Xolile Mngeni was diagnosed with a pineoblastoma in May last year and underwent surgery two months later.
"It's a very rare kind of brain tumour. It represents less than 0.1 percent of brain tumours," she told the court.
"It's more common in children and disseminates in the brain. It causes seeding in the brain and around the spinal cord."
She said international studies showed there was a less than 20 percent chance of survival over a five-year period.
Parkes was testifying in mitigation of sentence.
Mngeni was last week convicted of the premeditated murder of the tourist.
He was also found guilty of robbery with aggravating circumstances and the illegal possession of a firearm and ammunition.
Dewani was shot dead in Gugulethu on 13 November 2010, in an allegedly faked hijacking. Her body was found the next day.
Parkes said Mngeni underwent radiotherapy and chemotherapy after his surgery in 2011 and it seemed to have been successful.
A scan last week showed the disease to be clear.
"The tumour is no longer visible, but I know from natural behaviour that once you've had spread [of cells], there's a fairly high chance the tumour will recur," she said.
"If the tumour recurs, there is very little treatment offered. It if recurs, the outcome is nearly always fatal."
Qalisile Dayimani, for Mngeni, appealed to Judge Robert Henney not to impose the minimum prescribed sentence of life, citing the medical condition as a substantial and compelling circumstance.
Henney said he had a very difficult decision ahead, balancing Mngeni's medical condition with the fact he had shown no remorse.