Calypso’s array attenuation

Introduction: The Calypso 4D Localization System gives the possibility to track the tumour during treatment, with no additional ionising radiation delivered. To monitor the patient continuously an array is positioned above the patient during the treatment. We intend to study, for various gantry angles, the attenuation effect of the array for 6- and 10 MV and flattening filter free (FFF) 6- and FFF 10 MV photon beams. Materials and methods: Measurements were performed using an ion chamber placed in a slab phantom positioned at the linac isocenter for 6 MV, 10 MV, FFF 6 MV and FFF 10 MV photon beams. Measurements were performed with and without array above the phantom for 0°, 10°, 20°, 40° and 50° beam angle for a True Beam STx linac, for 5×5 and 10×10 and 15×15 cm2 field size beams to evaluate the attenuation of the array. A VMAT treatment plan was measured using an ArcCheck with and without the array in the beam path. Results and discussion: Attenuation measured values were up to 3%. Attenuation values were between 1 and 2% with the exception of the 30°–50° gantry angles which were up to 3.3%. The ratio values calculated in the ArcCheck for relative dose and absolute dose 10 were both 1·00. Conclusion: Attenuation of the treatment beam by the Calypso array is within acceptable limits.