ESTRO 37 Abstract book

ESTRO 37

S461

The percentage change of D w,Q per T was found to be - 0.04% and -0.35% for 60 Co and 8 MV, respectively. Figure 1 (top) shows the alanine signal ratio, M Q / M B Q , as a function of B-field for both 60 Co and 8MV beam energies. The uncertainty bars represent the measurement repeatability. These figures suggest a weak dependence of the alanine signal on B-field strength, which is of the order of -0.4% and -0.02% per T, for 60 Co and 8MV respectively, with a standard uncertainty of 0.2%. Figure 1 (bottom) presents the k Q,B,Alanine as a function of B-field for both beam qualities, after allowing for the effect of B-field on D w,Q . The correction at 1.5 T is 0.994 and 0.997 for 60 Co and 8MV, respectively, with a standard uncertainty of 0.2%. The small quality-dependence of the magnetic field correction is within the measurement uncertainty (k=1).

respectively.

Conclusion These results facilitate the implementation of the range verification methodology to passive scattered proton pediatric CSI. An in vivo measurement to determine the WEPL to the inner surface of the esophagus will allow for personalized adjustment of the treatment plan to ensure sparing of the esophagus while confirming target coverage. PO-0875 The suitability of alanine as a reference class detector for MRI-linac dosimetry I. Billas 1 , H. Bouchard 2 , C. Gouldstone 1 , S. Duane 1 1 National Physical Laboratory, Metrology in Medical Physics, Middlesex, United Kingdom 2 Université de Montréal, Département de physique, Montréal, Canada Purpose or Objective A state-of-the-art MRI-guided radiotherapy treatment uses non-ionising radiation to acquire high quality images of soft tissues and guide the delivery to tumour volumes of therapeutic doses of ionising radiation. Reference dosimetry in conventional radiotherapy machines is usually performed with ionisation chambers (IC). In the presence of a magnetic field (B-field) the charged particles will be influenced by the Lorenz force in the air cavity, which strongly modifies the dose response of the IC. Recent works have shown that the uncertainty of IC-based dosimetry may be significantly increased by the presence of strong B-fields. In this work we assess the suitability of alanine/EPR as a reference dosimeter by investigating the effect on its dose response of a B-field. Material and Methods Alanine pellets were placed in an F-type holder (shaped like a Farmer-type IC) in an electromagnet and irradiated by a 60 Co and by a conventional 8MV Elekta linac beam qualities, Q. The dosimeters were oriented perpendicular to both photon beam and B-field and irradiated at five magnetic strengths (0 T – 2 T). In 60 Co the dosimeters were irradiated in a Perspex phantom, and in the linac beam in water tank. The effect of the B-field on the alanine/EPR signal, M Q , was determined through measurements, while the effect of the B-field on dose to water, D w,Q , was calculated for both energies using MC simulations. The correction factor for the effect of the B- field on alanine, k Q,B,Alanine , was obtained as:

Conclusion The strong magnetic field in an MRI-linac has a measureable but small effect (0.5% at 1.5 T) on the sensitivity of alanine in terms of absorbed dose to water. Independent determinations of the correction for this effect, k Q,B,Alanine , were made at different B-fields for two beam qualities, and was found to be in a range of 0.1% to 0.6%. We conclude that alanine is a suitable dosimeter for reference dosimetry in MRI-linac and for the transfer of absorbed dose standards from zero magnetic field conditions to MRI-linac. PO-0876 Accuracy of the recombination correction factor (ks) in FFF beams for three ion chamber types G. Martin-Martin 1 , P.B. Aguilar 2 , B. Barbés 2 , J.D. Azcona 2 1 Hospital Universitario de Fuenlabrada, Medical Physics Service, Fuenlabrada- Madrid, Spain Purpose or Objective The aim of this work was to investigate the k s for three types of ion chambers in high dose per pulse (DPP) FFF beams. The study also aims to compare different methods for the determination of k s to assess the reliability of the generally used two-voltage analysis (TVA). Also, an analysis of depth-dependent ion recombination effect was performed to assess the 2 Clínica Universidad de Navarra, Department of Radiation Physics, Pamplona- Navarra, Spain

Results