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On charged particle equilibrium violation in external photon fields.

Hugo Bouchard1, Jan Seuntjens, Hugo Palmans

  • 1Centre hospitalier de l'Université de Montréal, Montréal, Québec, Canada. hugo.bouchard.chum@ssss.gouv.qc.ca

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A theoretical model for quality correction factors in radiation dosimetry is flawed because it relies on an unachievable condition. Charged particle equilibrium (CPE) is not practically attainable in external beams, invalidating Fano's theorem application and prior models.

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Area of Science:

  • Medical Physics
  • Radiation Dosimetry
  • Radiotherapy Physics

Background:

  • A theoretical model for quality correction factors in plan-class specific reference (PCSR) fields was proposed by Bouchard et al. (2009).
  • This model relied on the definition of PCSR fields by Alfonso et al. (2008), requiring time-averaged charged particle equilibrium (CPE).
  • The derivation used Fano's theorem (1954), which links CPE to dose independence from density variations.

Purpose of the Study:

  • To critically evaluate the practical achievability of CPE in external radiation beams.
  • To identify potential misconceptions regarding the application of Fano's theorem in dosimetry.
  • To reassess the validity of theoretical models and definitions based on CPE.

Main Methods:

  • Detailed treatment of the practical achievability of CPE in external beams.
  • Illustration of CPE non-achievability using intuitive and formal methods.
  • Analysis of Fano's theorem applicability in the context of external beam dosimetry.

Main Results:

  • The condition of charged particle equilibrium (CPE) is not practically achievable in single or composite external beam deliveries.
  • Fano's theorem is not applicable to external beam radiation dosimetry without accounting for attenuation effects.
  • The theoretical model by Bouchard et al. (2009) is not valid in practice due to the unachievable CPE condition.

Conclusions:

  • The definition of PCSR fields and the theoretical model for quality correction factors require revision.
  • The criterion of CPE in the definition of PCSR fields needs modification.
  • Further investigation is needed to confirm if quality correction factors for intensity modulated radiation therapy PCSR fields could be unity under ideal conditions.