Hundreds of cancer patients at West Midlands hospitals have had scans cancelled due to equipment problems.
The PET scans are given to patients with advanced stages of the disease including prostate, lung and bowel cancers to see how far it has spread.
Alliance Medical, a company contracted to supply isotopes needed to perform the scans, has admitted production problems has led to a shortage.
The Royal College of Radiologists said the supply issues were "worrying".
Positron Emission Tomography (PET) scans produce 3D images of the inside of the body.
They need radioactive isotopes, the commonest of which is fludeoxyglucose (FDG), which has a short life and must be used within hours.
Hospitals with scanners in Birmingham, Coventry and Stoke-on-Trent have reported a 10-month shortage of FDG and about 10% of tests have been cancelled since August 2018.
Meanwhile, prostate cancer patients in Birmingham have seen appointments cancelled from a lack of FEC, another isotope.
The trust that runs several hospitals in the city, including the Queen Elizabeth, has cancelled about 25% of these prostate cancer scans in the past 10 months.
One man who has waited seven months and seen his test cancelled four times is being referred to Nottingham as a private patient.
His son, who did not want to be named, said: "My father has another appointment in June, but it's happening all over. This is a national issue."
Alliance Medical, which supplies both isotopes, apologised to patients and said disruption had been caused by the refurbishment of its manufacturing sites.
Since August 2018:The University Hospitals Birmingham cancelled 420 scans The Royal Stoke Hospital cancelled 142 University Hospitals Coventry and Warwickshire cancelled 149
The trust that runs the Royal Stoke said the situation was "managed internally" and it was now receiving enough FDG to meet demand.
University Hospitals Birmingham Trust said it would like to purchase its isotopes from another provider, but could not as it was contracted to Alliance Medical.
While University Hospitals Coventry and Warwickshire said it was working with Alliance Medical to ensure supply disruption continued to be minimised.
Alliance Medical said: "We understand the concern and worry that this can cause and any affected patients are contacted to arrange new appointments within an average of three days."
A spokesperson for NHS England, which awarded Alliance Medical the contract to run 30 scanners in 2015, said: "The provider has confirmed that it is able to source what is needed from alternative suppliers, with all patients offered new appointments."
It has undertaken a second round of procurement and a scanner in Cheltenham, Gloucestershire, has provisionally been contracted to be supplied by Alliance Medical.
Peter Sharpe, chief executive of the charity Cobalt Imaging, which runs the Cheltenham scanner, said it had challenged the decision. over supply fears.
Dr Nicola Strickland, president of The Royal College of Radiologists, said the delays "can cause understandable anxiety for patients".
"We also know local imaging departments will be working hard to communicate with patients, prioritise nuclear medicine workloads and reschedule any affected appointments as soon as possible."