ESTRO 2023 - Abstract Book

S73

Saturday 13 May

ESTRO 2023

Conclusion ML with LSSM was implemented in RT clinical practice, allowing to reduce outlier complexities of RT plans, reducing the risk of PSQA failure and streamlining the RT workflow.

OC-0113 Extending TRS-483 for small field dosimetry in MR-linacs J. de Pooter 1 , L. de Prez 1 , S. Woodings 2 1 VSL, Ionizing Radiation, Delft, The Netherlands; 2 UMCU, Radiotherapy, Utrecht, The Netherlands

Purpose or Objective IAEA TRS-483 provides a method for small field dosimetry in conventional linac beams. Treatments based on MR-linacs require dosimetry in presence of magnetic fields ( B ), which has a big impact on detector calibration coefficients depending on orientation/magnitude of B not accounted for in TRS-483. This study investigates a new method to extend TRS-483 for small field dosimetry in MR-linacs. Materials and Methods The TRS-483 formalism determines field output factors ( OF ( S )) from the product of ROF (ratio of readings of field with size S and the reference field ( S _msr =10 cm)) and the output correction factor.

The extended formalism for MR-linacs in this study calculates the output correction factor in presence of B, (2nd term rhs of equation) by multiplication of (existing) output correction factors for B =0 with rkB ( S ). To determine rkB ( S ) data for this extended formalism, k _ B factors as a function of S for two detectors with different characteristics (PTW 60019) and PTW 31022) were calculated with Monte Carlo techniques (PENELOPE), using phase space files of square fields ( S =0.5–2.0 cm and S _msr) of an Elekta Unity™ MR-linac (B=1.5 T). For PTW 60019, the axial orientation was applied and for PTW 31022 three orientations (axial, radial (perpendicular and parallel to the B -field)). For each detector/orientation a linear function was fitted to the rkB ( S ), S data set. For the same set of detector/orientation, ROF data measured on the central beam axis of square fields ( S =0.5–2.0 cm) in a clinical Elekta Unity™ MR-linac was corrected according to the extended formalism, yielding OF ( S ) data. The output correction factor in presence of B was calculated from output correction factors for B =0 from [1] and rkB ( S ) (Fig.(a)). S of the measured fields was determined by cross profile measurements.

Results