ESTRO 2021 Abstract Book

S1324

ESTRO 2021

Figure 1 : Mean energy and R -1 values as a function of the distance to the axis for a 10x10 cm 2 6MV photon field

Conclusion As expected, at the border of the field, where the mean energy is ̴1 MeV, there is no need for corrections to the calibration factor (i.e., R -1 =1). At 5 cm from the field edge, R -1 becomes 5% for the three linacs studied. At larger distances, the correction becomes more linac-dependent. The largest correction ( ̴ 9%) was found for the Siemens Primus at about 10 cm from the border of the field. References

1. Duggan et al. Phys. Med. Biol. 49: 3831–3845, 2004 and corrigendum 2. Sanchez-Nieto et al. Med. Phys. Med. Phys. 47 (9): 4616-4625, 2020 3. Irazola et al. Applied Radiation and Isotopes 123: 32–35, 2017 4. Scarboro et al. (Med. Phys.) 38 (5):2619-2628, 2011

PO-1602 Dosimetric impact of liquid Helium top-up for a 1.5 T MR Linac M. Jameson 1,2 , U. Jelen 1 , Z. Moutrie 1 , C. Pagulayan 1 , N. Dunkerly 1 1 GenesisCare, Medical Physics, Sydney, Australia; 2 University of New South Wales, Faculty of Medicine, Sydney, Australia Purpose or Objective Liquid Helium is used to cool the superconducting magnet in MR Linacs. As the beam must traverse the cryostat for the Elekta Unity the level of Helium determines the amount of attenuation at specific beam angles. This is characterised at commissioning for the level of Helium present at install. The purpose of this study was to assess the dosimetric impact of performing a liquid Helium top up on a 1.5 T MR Linac. Materials and Methods The liquid Helium level at install was 70.4%. Elekta has a cut off at 66% whereby the radiation beam will be interlocked. Due to intermittent chiller issues a decision was taken to top up the Helium level. The impact of the change in Helium level was assessed based on the cryostat characterisation measurement (CCT) which delivers a 10x10 cm 2 field at gantry increments of 2 degrees. Standard TG119 C-Shape IMRT commissioning plan was measured. A new beam model was created in order to compare plan calculated with the original CCT 70.4% and CCT 78.8% . Results A comparison of the CCT from commissioning and post Helium top up can be seen in Figure 1.