During treatment by brachytherapy, radiation passes beyond the target volume and delivers radiation dose to adjacent tissue such as the lungs and, especially in the case of treatment of the left breast, to the heart. The heart is particularly vulnerable to radiation; to minimise the dose it receives in such circumstances, we propose an apparatus for treatment by brachytherapy comprising an X-ray source sized for insertion into a patient, a respiration state monitor, and a control apparatus adapted to receive respiration state information from the respiration state monitor and control the output of the X-ray source; the control apparatus being arranged to operate the X-ray source at a first output level when the respiration state monitor indicates a degree of lung inflation above a first preset threshold and operate the X-ray source at a second and lower output level when the respiration state monitor indicates a degree of lung inflation below a second preset threshold.