Poster - Thur Eve - 26: Interfraction reproducibility of heart position during breast irradiation using Active...

The moderate deep-inspiration breath hold (mDIBH) technique using the Active Breathing Coordinator (ABC) from Elekta is used in our clinic to lower the heart dose during left breast irradiations. The purpose of this work was to investigate the interfraction reproducibility of the heart to chest distance during these treatments and to evaluate the dosimetric effect of any changes in the heart position. Daily CBCT images were available for 5 patients who had been treated with ABC tangents and a cavity boost. On these images, one-dimensional measurements of the distance between the heart and the chest wall were taken at two anatomical locations corresponding roughly with the location where the radiation field most likely intercepts the heart. The average change in this distance was interpreted as a shift of the heart position. To assess the effect of this shift on the delivered heart dose, the heart contours in the clinical plans of the corresponding patients were shifted towards the treatment field using standard Pinnacle tools. Although the ABC device allows good reproducibility of the volume of air held, this does not warrant reproducibility of heart position for all patients during treatment. The largest average heart shift extracted from CBCT images in this study was 6.2mm. The heart dose reconstructed using this shift for the corresponding patient also showed the largest effect. However, even in the presence of a systematic heart shift of this magnitude, the ABC plan still showed superior heart dose reduction compared to the free-breathing plan.