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A Dynamic 3-D Dosimetry Phantom for Commissioning and QA of Gated RapidArc Radiotherapy Treatment

K Cheung

K Cheung*, W Lam, H Geng, T Leung, P Wu, S Yu, Hong Kong Sanatorium & Hospital, Hong Kong

SU-E-T-370 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To develop a dynamic dosimetry phantom suitable for commissioning and QA of gated RapidArc radiotherapy of moving targets.

Methods: A high precision dynamic dosimetry phantom was developed for commissioning and dosimetric verification of gated RapidArc radiotherapy treatments. The phantom consisted of two main parts (a) a high precision 3-D dosimetry phantom and (b) a two-dimensional motion driving mechanism. The 3-D dosimetry phantom, which was made of acrylic, consisted of a cylindrical main body with a several replaceable inserts to facilitate point dose measurement using an ionization chamber and dose distribution measurement in the sagittal and coronal planes using radiochromic films. The dosimetry inserts could be replaced by a geometry verification insert which had a 3mm stainless steel ball located (within 0.05mm) at the effective measurement point of the ion chamber or the centre of the radiochromic films. The 2-D motion of the dosimetry phantom was achieved by means of a driving mechanism that utilized three sets of stepping motor-actuators, i.e. one for generating motion in the superior-posterior axis and two for producing the anterior-posterior motion. The pattern, frequency and amplitude of the 2-D motion could be programmed through a motion controller to simulate different breathing patterns of patients. The phantom was tested for positioning reproducibility before being used to commission the gated RapidArc treatment of a Varian TrueBeam linear accelerator.

Results: The phantom motion had a positioning reproducibility of better than 1 mm. It was successfully used for verification of gated RapidArc treatment of a moving C-shaped target. The dose and dose distribution as measured in different treatment fractions given in different days with different target motion patterns were highly consistent and reproducible.

Conclusions: The dynamic 3-D dosimetry phantom developed is useful for commissioning and regular QA of gated RapidArc treatment of moving targets.

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