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Quality Correction Factors of Small-Cavity Ionization Chambers in Small MR-Guided Radiotherapy Beams

Y Cervantes1,2*, I Billas3, S Duane3, H Bouchard1,2,4, (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)Centre hospitalier de l'Universite de Montreal, Departement de radio-oncologie, Montreal, QC

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The goal of this study is to evaluate the suitability of small ionization chambers for small MRgRT beam dosimetry and to determine optimal orientations for measurements.

Methods: Monte Carlo calculations (EGSnrc) are performed to determine sensitive volume dose responses of small-chambers (PTW31010, PTW31021 and PTW31022) in a water phantom with Elekta Unity phase spaces of field width ranging from 1 to 10 cm. Five chamber orientations are studied: perpendicular to the B-field with the beam either anti-parallel (orientation 1) or perpendicular (orientation 2 or 3 with FLorentz towards stem or tip); perpendicular to the beam and parallel or anti-parallel to B-field (orientation 4 or 5). Perturbation factors of stem, central electrode, wall, density and volume are evaluated with and without B-field. Quality correction factors, k(Q(B)), are calculated in each orientation.


Results: In the presence of a B-field, perturbations factors are mostly unchanged with field width larger than 2 cm but not for 1 cm. The B-field may affect the stem perturbation by as much as 6% (orientations 1 and 2). For the density perturbation effect, the B-field effect appears mostly for the 1 cm width, especially for PTW31010 in orientations 4 and 5 (=4.8%). Over all chambers, orientations and field sizes, k(Q(B)) ranges from 0.9804±0.0083 to 1.0501±0.0076, with orientation 2 being the most affected in all field sizes since electrons are deflected towards the stem. The smallest (PTW31022) yields a k(Q(B)) closest to unity for orientations 1, 4, and 5. Despite compensation of perturbations in orientation 1, significant stem perturbation suggests avoiding it.


Conclusion: Orientations where the chamber axis is aligned with B-field present k(Q(B)) closer to unity. The orientation where electrons deflect towards the stem should be avoided. This study also shows that k(Q(B)) factors close to unity can be obtained for commercial chambers in small MRgRT beams.

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Keywords

Magnetic Fields, Perturbation Factor, Small Fields

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined computational dosimetry: Monte Carlo

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