A report into a Sydney doctor who under-prescribed chemotherapy drugs to cancer patients has criticised the hospital over its response.

A Sydney hospital has apologised to cancer patients given the wrong dose of chemotherapy drugs after a damning investigation into its handling of improper prescribing by one of its doctors.

An interim report into oncologist Dr John Grygiel's under-prescribing of the chemotherapy drug carboplatin to St Vincent's Hospital cancer patients has slammed the hospital's reaction to the incident and warned more patients may be affected.

An inquiry was ordered earlier this year after it emerged Dr Grygiel under-prescribed carboplatin to 78 head and neck cancer patients between 2012 to 2015.

Twenty-three of those patients have since died of cancer, but the inquiry team was unable to link their deaths to under-dosing.

However the inquiry team's interim report found nearly all those affected were only told about the under-dosing after it was revealed in a media report.

It also found the hospital's public statements on the issue contained key omissions and were misleading.

"The response by St Vincent's, when it realised there was an issue, failed to demonstrate an understanding of the distress this issue was likely to cause to patients and their families," said the report, which was publicly released on Tuesday.

The report said St Vincent's showed no urgency to review affected patients, and took too long to responded to concerns about Dr Grygiel once they were escalated.

The authors said the full extent of his prescribing was still unknown, and recommended expanding the inquiry's scope to include patients Dr Grygiel treated as early as 2006.

In a statement St Vincent's said it apologised "deeply and unreservedly" to affected patients and their families.

"We are sorry you've had to go through this, and we are sorry for letting you down in this way," the statement said.

NSW Health Minister Jillian Skinner labelled the report findings concerning, and that a number of its recommendations had been accepted in full.

These included recommendations for the inquiry to investigate whether more patients of Dr Grygiel are affected, as well as actions to safeguard the broader health system.