The hospital, also known as the Woodlands, has called in an independent team of experts to examine all bone cancer tests from its pathology laboratory over the past six months.
The error in the diagnosis of the boy, believed to be aged five, came to light on 24 May after his results underwent routine 'quality assurance' checking.
The pathologist responsible is on 'genuine sick leave', according to Gillian Todd, the hospital's chief executive.
No other mistakes have emerged from the double-checking, Dr Todd said. 'This is an extremely difficult area of diagnosis. We will be writing to all patients concerned once the checks have been completed.'
The boy is receiving treatment to help him recover from the effects of the chemotherapy. The alert has left perhaps dozens of patients unsure whether their diagnoses are correct.
At least two patients have had operations postponed for up to a month while the results of biopsies are re-examined.
One man who was told he had a malignant leg tumour said: 'It was bad enough to be told I had a tumour in the first place. At least they said it was contained in one area and that I would be as good as new after the operation. Now we are back to square one. The surgeon said he did not want to build up any hopes that I might not have cancer. Now I don't know if I have got it or not.' The Royal Orthopaedic in Northfield treats bone cancer patients from all over Britain.Reuse content