The use of a treatment planning system to investigate the potential for transmission dosimetry in detecting patient breathing during breast 3D CRT

Transmission dosimetry has the potential for identifying dosimetry errors during radiotherapy treatments by detecting changes in effective beam path between the planned and delivered treatment geometry. In the current study, the Pinnacle treatment planning system was used to model transmitted dose in a “virtual” EPID to investigate the possibility of using transmission dosimetry for detecting patient breathing and setup errors in breast conformal radiotherapy treatments. An opposing tangential beams treatment plan was used as a proof-of-principle study for deliberately introducing shifts in the position of the beams and virtual EPID relative to the CT data, to simulate shallow and deep breathing excursions of 2 mm and 11 mm, respectively. In addition, breathing was combined with setup errors of 0 mm and 2.5 mm in a given direction for each beam. Due to spatial limitations in the original CT data, the CT data was modified to include an additional volume of air surrounding the patient to allow for the virtual EPID to be modelled at sufficient distances from the beam focus. Breathing excursions of both 2 mm and 11 mm could be detected in the transmitted dose planes below the patient. Breathing combined with a 2.5 mm set up errors in the superior-inferior direction further accentuated the distribution of the dose errors in the superior-inferior directions. The predicted changes in transmitted dose due to the simulated delivery errors shows promise for using transmitted dosimetry in the clinic.