Post-mortem examination results on a girl who received radiation overdoses have made no reference to the possible effects of the error, her family said.
Lisa died of brain cancer, the post-mortem examination said
Lisa Norris, 16, died two weeks ago at her home in Ayrshire, nine months after a dose of radiation which was 58% higher than she should have had.
An official report, which said Lisa died of brain cancer, has been met with surprise by her parents.
A lawyer for her family said they would be seeking a further investigation.
They said the report put the cause of death as recurrent pinealblastoma - a brain cancer.
Lisa, from Girvan, had undergone treatment at the Beatson Oncology Centre in Glasgow.
However, an official report into the teenager's treatment pointed to a "critical error", which led to her receiving 19 overdoses of radiation.
In a statement issued through a solicitor, the Norris family said: "We are relieved that that post-mortem has been completed but are surprised that there is no reference on it to excessive radiation as a cause of death.
"We have instructed our solicitor to take this further."
Cameron Fyfe, of Ross Harper Solicitors, said he would commission a medical expert to investigate the case.
Lisa was 15 when she received the overdoses, having originally been diagnosed with a brain tumour in October last year.
She was left with severe burns on the back of her neck and head.
The report into her treatment was compiled by Dr Arthur Johnston, an inspector appointed by Scottish ministers.
He said the error came to light because the same treatment planner made the same mistake for a different patient.
It was picked up by a colleague and an internal investigation was ordered which revealed the error in Lisa's case. Dr Johnston concluded that most of the responsibility and "hence any blame" could be attributed to the principal planner.
It was announced that immediate inspections of Scotland's five cancer radiotherapy centres would take place in the wake of the report.
In the weeks before her death on 18 October, Lisa had been recovering from surgery to remove fluid from her brain
Her funeral is due to take place in Girvan on Friday.