Prosecutors told the Brisbane District Court that 64-year-old Patel, who was the head of surgery at the Bundaberg Hospital in southern Queensland, needlessly removed Ian Rodney Vowles' cancer-free bowel during a surgery in 2004, leaving him impotent.
Crown Prosecutor Peter Davis told the court the surgery, which removed Vowels' bowel and rectum, should not have occurred at all. He said it was an unlawful act that has caused grievous bodily harm to Vowels.
"Vowels has suffered general health problems as a result; fatigue has been one of the things that has affected him and he has had a loss of sexual function."
He said Vowels was initially reluctant to have his bowel removed but consented after Patel told him the growth, while benign, was attached to the bowel's wall and he could not be sure there was no cancer behind it.
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"He told Vowels 'your bowel does not like your body, we will whip it out'. "He assured Vowels that everything would be fine after the operation and that Vowels would be running around like a horse," Davis said.
Earlier, the trial was delayed by an hour after concerns a Seven Network news programme could prejudice the jury.
Justice Terry Martin told jurors they must not seek out a report that aired on Seven's Today Tonight program on Monday, saying he feared it could affect a fair trial.
"The lead into the story was referring to Dr Patel's trial starting yesterday and then goes onto discuss problems with doctors in a very aggressive way," Justice Martin told jurors.
Patel, who was born in Jamnagar in Gujarat, was jailed in 2010 after a jury convicted him of unlawfully killing three patients and causing grievous bodily harm to another while a surgeon at Bundaberg hospital.
The high court quashed his conviction last year and Patel was released.
Patel, who was extradited from the US to face criminal charges, faced a re-trial in February for the manslaughter of 75-year-old patient Mervyn Morris in 2004 and was found not guilty.