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Analysing collimator structure effects in head-scatter calculations for IMRT class fields using scatter raytracing.

S A Naqvi1, M Sarfaraz, T Holmes

  • 1Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, USA. snaqvi@sun2.ummc.umaryland.edu

Physics in Medicine and Biology
|July 28, 2001
PubMed
Summary

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Intensity modulated radiation therapy (IMRT) requires accurate modeling of scatter photons. This study presents a dual-source model and raytracing algorithm to precisely calculate scatter, improving IMRT accuracy.

Area of Science:

  • Medical Physics
  • Radiation Oncology

Background:

  • Intensity modulated radiation therapy (IMRT) involves frequent blocking of irradiated volumes, increasing the significance of head-scattered photons compared to conventional therapy.
  • Collimator configurations can block scatter photons ('anomalous blocking'), making primary beams unreliable indicators of scatter fluence.
  • High monitor unit (MU)-to-centigray (cGy) ratios in IMRT amplify head-scatter uncertainties, necessitating accurate modeling of scatter sources and collimator effects.

Purpose of the Study:

  • To develop and validate a method for accurately modeling photon scatter in IMRT, accounting for collimator geometry.
  • To improve the precision of fluence calculations in IMRT by addressing head-scatter uncertainties.

Main Methods:

  • A dual-source model was developed using a Taylor series expansion to derive the effective scatter source distribution from measured data for an Elekta SL20 linac with a multi-leaf collimator (MLC).

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  • A raytracing algorithm was employed to calculate scatter ray transmission from the effective scatter source plane to the patient plane.
  • The method accounts for 'anomalous blocking' of scatter by MLC leaves and backup diaphragms.

    • Main Results:

    • Calculations using the proposed method demonstrated agreement with measurements to an accuracy of 0.002 psi10x10 (total photon fluence for a 10x10 cm2 open field).
    • The model effectively accounts for the impact of collimator structure on scatter reaching the irradiated volume.

    Conclusions:

    • The developed dual-source model and raytracing algorithm provide accurate scatter fluence calculations in IMRT.
    • This method can prevent potential cumulative errors of a few percent in fluence calculations, enhancing treatment accuracy.
    • Accurate modeling of scatter is crucial for IMRT, especially given the influence of field shapes and collimator configurations.