Photon skyshine from medical linear accelerators.

Area of Science:

• Medical Physics
• Radiation Shielding

Background:

• Existing photon skyshine prediction formulas show significant inaccuracies compared to measurements.
• These formulas fail to predict observed dependencies on field size and the existence of a local maximum in skyshine dose rates.

Purpose of the Study:

• To develop a more accurate and physically insightful formula for predicting photon skyshine.
• To address the limitations of current models regarding field size dependence and dose rate maxima.

Main Methods:

• Derivation of a new scaling formula with a single fitting parameter.
• Analysis of factors influencing the location of maximum skyshine dose rate, including roof height and room dimensions.
• Comparison of predicted skyshine dose rates with experimental data, considering field area and energy levels (6 MV vs. 18 MV).

Main Results:

• The new formula accurately predicts skyshine properties, including dose rate maxima and field size dependence.
• Skyshine dose rate is found to be proportional to field area, not the solid angle (Ω^1.3).
• For lightly shielded roofs, 6 MV photon skyshine can exceed that of 18 MV, and dose rates decline as one over distance at intermediate ranges.

Conclusions:

• The derived scaling formula offers improved accuracy and physical insight into photon skyshine.
• Accurate prediction of skyshine dose rates necessitates precise knowledge of the roof transmission factor.
• Adequate roof shielding renders photon skyshine negligible, preventing designation as a high radiation area.