A differential method for inhomogeneity correction on dose in a photon beam

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiological Physics

Background:

• Correction factors for uniform inhomogeneities in supervoltage beams are calculable via the Batho equation.
• Calculating dose perturbations from complex inhomogeneities, such as annular shapes, requires advanced methods.

Purpose of the Study:

• To develop and validate a method for calculating the dose effect of annular inhomogeneities in supervoltage radiation beams.
• To compare calculated dose perturbations with experimental measurements for various materials and dimensions.

Main Methods:

• A novel equation for calculating dose at a point on the beam axis below an annular inhomogeneity was derived for supervoltage radiation.
• Measurements were performed using 60Co beams with polystyrene foam, cedar, and aluminum annuli of varying inner diameters.
• Calculated correction values were compared against measured dose perturbations.

Main Results:

• The proposed method showed good agreement between calculated and measured values when the annulus was just submerged in the phantom.
• For general situations, measured scatter perturbation typically fell between two proposed calculation types.
• The technique successfully predicted a sign reversal in scatter perturbation, consistent with prior observations.

Conclusions:

• The developed method provides a reliable way to calculate dose perturbations from annular inhomogeneities in supervoltage beams.
• The findings validate the predictive capability of the technique, including its ability to identify scatter perturbation sign reversals.
• This work contributes to improved accuracy in radiation dosimetry for complex treatment geometries.