Radiotherapy uses radiation to target and kill cancerous cells in a specific part of the body, and can be used for many types of cancer.
Healthy cells can recover from this damage, while cancer cells cannot.
The machine at Mulago Hospital was second hand when it was donated in 1995 and has been repaired several times in the past.
But efforts to fix it this time have failed, Ms Namulindwa said.
The cancer unit is currently talking to the ministry of health to get more funds and speed up the building of a special bunker that is needed for a new radioactive machine.
‘Travel to Kenya’
The ministry’s director of cancer treatment, Dr Jackson Oryem, told BBC Focus on Africa radio that a new machine should be delivered within the next year.
He said the need to build the new special facilities was causing a delay.
In the meantime, patients are still able to get other treatments, such as chemotherapy and surgery, but if they need radiotherapy, and they can afford it, they will have to travel to neighbouring Kenya.
Dr Oryem said the government may be able to fund a trip abroad for some of the most urgent treatable cases.
The incidence of cancer is on the rise in Africa overall as life expectancy increases.