It comes after NHS Tayside’s only breast radiotherapy oncologist retired in January.
The health board then revealed the oncologist who delivers the advanced breast oncology service, who was recruited in August last year, has handed in their notice and plans to leave in June.
In an ongoing shortage of staff, early breast cancer services were already being delivered by NHS Grampian.
NHS Tayside has now extended its mutual aid agreement with other health boards, meaning some patients will need to travel to Aberdeen, Edinburgh and Glasgow.
The health board said “all recruitment opportunities” were being pursued, supported by the Scottish Government, but no posts had yet been filled, amid a nationwide shortage of doctors qualified in this area.
Medical director Professor Peter Stonebridge said NHS Tayside was working to fill the staffing gaps as quickly as possible.
“We will continue to work with Scottish Government to restore the radiotherapy part of the patient pathway in Tayside as soon as is practicable,” he said.
"We remain committed to delivering services locally, as long as it is safe for patients and, in this case, that requires a suitable specialist medical workforce.
“The board has also given an absolute commitment to rebuilding the breast oncology service within Tayside and to that end we are in ongoing discussions with the local oncology team, the medical school at the University of Dundee and Scottish Government to look at all the available options open to us.”