Small-Cavity Detector Perturbation and Quality Correction Factors in Magnetic Resonance Imaging-Guided Radiotherapy Small Photon Beams
Y Cervantes-Espinosa1, 2*, J Duchaine1, 2, I Billas3,4, S Duane3, H Bouchard1,2,5, (1) Universite de Montreal , Departement de physique, Montreal, QC, CA, (2) Centre de recherche du Centre hospitalier de l'Universite de Montreal, QC, CA (3) National Physical Laboratory Teddington, UK (4) The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK (5) Departement de radio-oncologie, Centre hospitalier de l'Universite de Montreal, QC, CA
Presentations
TH-F-TRACK 3-7 (Thursday, 7/29/2021) 4:30 PM - 5:30 PM [Eastern Time (GMT-4)]
Purpose: Provide physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determine optimal orientations for measurements.
Methods: Small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water are determined with Monte Carlo simulations (EGSnrc), using Elekta Unity 7 MV FFF phase-spaces of field widths between 0.25 and 10 cm, for four detector axis orientations: perpendicular to the B-field (1.5 T) with the beam parallel (orientation 1) or perpendicular (orientation 2 or 3 with FLorentz towards stem or tip); perpendicular to the beam and parallel to B-field (orientation 4). The B-field effect on perturbation factors is evaluated. The overall perturbation factor (P_MC) accounts for the extracameral components, atomic composition, and density. For each orientation, the density (P_ρ) and volume averaging (P_vol) perturbation factors and quality correction factors (k_(Q(B))) are calculated.
Results: For solid-state detectors, P_ρ is unaffected by the B-fiteld and field size. For chambers in B=1.5 T, it increases up to 1.564±0.001 with decreasing field size over all orientations. For chambers, the B-field effect on P_MC is only significant for field widths <1 cm, increasing with decreasing field size in all orientations. For solid-state detectors, the B-field effect on P_MC exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role. A maximal effect of 1.221±0.005 occurs in orientation 2. P_vol remains the most significant perturbation both with and without magnetic fields. In most cases, the B-field effect on P_vol is 1% or less, except for chambers PTW31021 and PTW31022 that over-respond and under-respond in orientations 2 and 3, respectively, at smaller fields.
Conclusion: Solid-state detectors dose-response is strongly affected by the B-field in all orientations. In general, ionization chambers yield a k_(Q(B)) closer to unity, especially in orientations 2 and 4.
Handouts
Keywords
Magnetic Fields, Small Fields, Perturbation Factor
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