Inspectors were called into the Edinburgh Cancer Centre, at the Western General Hospital, after the patient received double the recommended dose of radiotherapy between September 14 and 18 last year.
The incident created “a significant possibility of serious harm” for the unnamed patient, said Dr Arthur M Johnston, an inspector appointed by Scottish Ministers.
This is only the second time inspectors have been called in during the last 10 years, after the high-profile case of a teenage cancer patient who suffered an overdose of radiation.
Lisa Norris, 16, from Girvan, Ayrshire, died months after when she received 19 times the correct dosage during treatment for a brain tumour at the Beatson Oncology Centre in Glasgow in 2006.
Radiotherapy uses targeted levels of radiation to destroy tumours, which are carefully calculated to avoid harming healthy tissue.
Writing in the report, Dr Johnston said: “In this case, the treatment delivered at the Edinburgh Cancer Centre (ECC) was a palliative radiotherapy treatment for alleviation of pain and existing disability in an older patient, and the dose received was 100 per cent greater than intended dose of 20 Grays.
“In both instances, the extent of the overexposure was such that there was a significant possibility of serious harm to the patient.”
He said the incident arose due to a “combination of errors made by individuals” operating within the system.
The incident was discovered 11 days after the patients treatment ended. Their condition has not been made public but the ECC is providing ongoing support for potential consequences of the overexposure.
Dr Johnston ordered a string of reviews into practice at the centre but acknowledged that efforts had already been made to improve the service.
Dr David Farquharson, Medical Director at NHS Lothian, said: “We offered our most sincere apologies to the patient and their family following this very unusual and deeply distressing incident. Since then, we have ensured that they have been kept informed throughout the full and thorough investigation and reporting stages of the process.
“Cases such as these are thankfully very rare, but as soon as it was identified, we implemented a series of measures to minimise the risk of a similar incident. We carried out a robust internal investigation and immediately informed the external inspector.
“In the report the inspector has expressed his confidence in the dedication of the commitment of Edinburgh Cancer Centre staff to the safety of patients in their care and acknowledges the many thousands of life-saving radiotherapy treatments that are successfully prescribed, planned and delivered at the Edinburgh Cancer Centre every year.
“We fully accept the findings of the report and an action team has been created to ensure that each point will be implemented as a matter of urgency, if it has not already been identified during our own investigations.”
A Scottish Government spokesperson said: “We extend our thoughts and sympathies to the patient and their family, who have been affected by this incident.
“While these incidents are extremely rare, it can be very serious if any patient is overexposed to radiation and so it is right that procedures at the Edinburgh Cancer Centre were fully, and independently, investigated.
“NHS Lothian and the Edinburgh Cancer Centre have taken a number of steps to change practice and minimise the risk of an incident like this happening again. We expect the health board to take forward all of the recommendations in the report and progress will be monitored closely by the Inspector.”