There is provided a device and a method for irradiating vascular tissues. The device generally includes a transfer device having a first chamber and a second chamber and a piston slidably disposed between the chambers. A balloon catheter is provided for positioning within the vascular system and is connected to the transfer device such that an inflation lumen of the balloon catheter is in fluid communication with the second chamber. A proximal end of the balloon catheter is affixed to a mounting block which is configured to receive the transfer device. An inflation device is provided to force fluid into the first chamber such that the piston is driven to force a radioactive fluid contained in the second chamber into the balloon.