ESTRO 2021 Abstract Book

S1353

ESTRO 2021

and 100 MU was delivered in each beam. The CCPS value was varied in steps of 5 V in the linac control software. For each MVIC exposure the electrometer readings, and pulse countings were registered. The scaling factors and intensities of the MVIC images were extracted to calculate the output, flatness and symmetry parameters of the beam. Results In Fig. 2(a) the deviation in output is shown as function of reference field calibration for different settings of the CCPS. A change of 5 V in CCPS results in approximately 1% decrease in output. This is slightly less than what was observed in Fig. 1 which was around 1.5%. The magnetron feedback software was disabled in the measurements shown in Fig. 2 while it was enabled in Fig. 1. Also, an energy change might be the explanation of the small difference in output compared in Fig. 1 and 2(a) for a 5 V change of CCPS. In Fig. 2(b) the flatness and symmetry properties of the beam are shown for different settings of the CCPS. The CCPS value affect the energy which is seen as a change in flatness. The MVIC panel was able to detect small energy changes. An increase in energy (TPR 20,10 ) results in a decrease in flatness.

Conclusion The MVIC panel of the Elekta Unity linac was found being a valuable, efficient and a reliable tool to detect small changes in output as well as in energy. It is a strength that the MVIC panel is in fixed position all time relative to the linac beam thereby reducing setup uncertainties. Therefore, the MVIC system may detect changes before observed by other external systems such as water phantoms used for routine QA. PO-1632 3D-Printed template to perform in vivo dosimetry in breast high dose rate brachytherapy treatments M. Gutiérrez Ruiz 1 , R. Astudillo Olalla 2 , A.L. Rivero Pérez 2 , J.T. Anchuelo Latorre 2 , J. Alonso Muriedas 2 , S. Ruiz Arrebola 2 , M.T. Pacheco Baldor 2 , J.I. Raba Díez 2 , J.A. Vázquez Rodríguez 3 , V. Cañón García 2 , J. Albendea Roch 2 , P.A. Navarrete Solano 2 , E.E. Arrojo Álvarez 2 , M.P. Galdós Barroso 2 , R. Fabregat Borrás 2 1 Hospital Marqués de Valdecilla, Radiation Oncology, Santander, Spain; 2 Hospital Marqués de Valdecilla , Radiation Oncology, Santander, Spain; 3 Hospital Marqués de Valdecilla , Radiation Oncology, santander, Spain Purpose or Objective The growing trend to hypo fractioned dose schedules in high dose-rate brachytherapy (HDR-BT) treatments increases the clinical impact of the potential errors that could occur. In vivo dosimetry (IVD) permits monitoring the dose administered to patients, enabling an independent verification of it. MOSFET detectors could be suitable for IVD in HDR-BT. However, it is known that their response depends on the temperature and detector-source distance and relative orientation.