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Dose uncertainties in photon pencil kernel calculations at off-axis positions
Jörgen Olofsson1, Tufve Nyholm, Anders Ahnesjö
1Department of Radiation Sciences, Radiation Physics, Umeå University, SE-901 87 Umeå, Sweden. jorgen.olofsson@vll.se
Medical Physics
|October 7, 2006
Summary
Accurate photon dose calculations require modeling off-axis softening (OAS) and lateral energy variations. Explicitly including these effects improves accuracy by up to 4%, crucial for precise radiation therapy planning.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Background:
- Photon dose calculations present challenges away from the central beam axis due to lateral energy fluence inhomogeneity and spectral variations.
- Off-axis softening (OAS) describes the beam quality shift occurring at increasing distances from the central beam axis, impacting dose accuracy.
Purpose of the Study:
- To investigate the impact of laterally inhomogeneous energy fluence distributions and spectral variations (OAS) on photon dose calculation accuracy.
- To evaluate the dose calculation accuracy when explicitly modeling these off-axis effects compared to simplified methods.
Main Methods:
- Utilized a pencil kernel dose calculation algorithm incorporating laterally varying kernel properties to model OAS.
- Employed a multi-source model to determine lateral energy fluence distribution for total dose output calculations.
- Validated calculations against 264 measured output factors across four megavoltage photon beams at various depths and off-axis distances.
Main Results:
- Explicit modeling of OAS improved dose calculation accuracy by up to 4% compared to calculations ignoring the lateral beam quality shift.
- Simplified head scatter modeling introduced minor errors, with additional inaccuracies near field edges approaching 1%.
- Full physics modeling resulted in deviations with a mean of -0.1% and a standard deviation of 0.7%, reaching a maximum of -2.2%.
Conclusions:
- Explicit modeling of off-axis effects, including OAS, is crucial for accurate photon dose calculations in radiation therapy.
- Off-axis uncertainties increase with distance from the central axis and depth, reaching 1% (1 standard deviation) at 20 cm depth.
- The study quantifies inherent uncertainties in dose calculations when moving away from the central beam axis.
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