160. Kilovoltage rotational radiotherapy of breast cancer with synchrotron radiation: A phantom study with 100 keV and 6 MV VMAT dose delivery

Purpose We proposed a new technique for breast cancer rotational radiotherapy using kilovoltage synchrotron radiation (SR) [1] . Here, we compared 2D dose distributions in a breast phantom at 6 MV in circular orbit Volumetric Arc Therapy (VMAT) with respect to 100 keV irradiation in rotational radiotherapy with SR. Methods The dose distribution was evaluated with calibrated EBT3 radiochromic films inserted at midplane in a 14-cm diameter PMMA cylindrical phantom, simulating the pendant breast. We irradiated a cylindrical target (15 × 20 mm 2 (W × H)) at the centre of phantom. Measurements were performed at the Imaging and Medical Beamline (Australian Synchrotron, Clayton, Australia) at 100 keV, with a SR beam of size 15  ×  20 mm 2 (W  ×  H) continuously irradiating the phantom during a 360 deg rotation around the cylinder axis with a central dose to the target of 7 Gy. Megavoltage measurements were performed at 6 MV using a Varian Clinac iX System Linac, with a target dose of 6 Gy. Results The principle of dose summation along the axis of rotation produced a dose distribution peaked at the centre of rotation for small fields. The target-to-surface dose ratio was 20:1 at 6 MV and 9:1 at 100 keV, for a target at the center of the phantom. Conclusions We performed a feasibility test of kilovoltage rotational radiotherapy dedicated to the breast showing a target-to-surface dose ratio higher than with MV radiotherapy but still acceptable for skin tissue sparing.