The agency says reusable devices called rectal paddles have a design flaw that prevents the instrument from being fully sterilized after use.
An executive says that while the agency is only alerting 131 women in British Columbia, it's possible other cancer treatment centres across Canada have been using the same device.
The manufacturer has recalled the devices, which look a bit like a rubber spatula used in cooking.
During radiation therapy for cervical cancer, the device is inserted into the vagina to serve as a shield, reducing the amount of radiation to which the rectum is exposed.
The cancer agency has assessed the infection risk with the help of experts and says it doesn't believe the women are at risk of having contracted communicable diseases from exposure to the instruments.
"We don't think there's any infection risk so we're not recommending extra blood tests or any treatments," says Dr. Ivo Olivotto, vice president for radiation therapy and functional imaging at the BC Cancer Agency.
"We're just letting them know in the interests of full disclosure, basically."
The devices became available in the province in November 2007 and were used until April 2012.
The BC Cancer Agency noticed the problem. It removed the paddles from service and notified the manufacturer.
Olivotto describes a problem that appears mostly theoretical.
Bodily fluids could conceivable seep into the core of the device, which consists of a paddle-shaped piece of plastic on a long metal rod. Sterilization procedures don't get at the core of the instrument.