Influence of surface shape on tissue-air ratio for cobalt-50

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• The shape of the surface of incidence can influence radiation scatter and alter effective tissue-air ratio (TAR) values.
• Body curvature is a significant factor in large radiation beams used for total body irradiation (TBI).

Purpose of the Study:

• To investigate the impact of surface curvature on effective TAR values during TBI.
• To quantify the differences between effective TAR and standard flat surface TAR due to body shape.

Main Methods:

• Utilized a computational approach to sum differential scatter air ratios from elemental beams.
• Simulated radiation scatter effects considering varying surface geometries.

Main Results:

• The difference between effective TAR and standard flat surface TAR increased with depth, reaching a maximum of approximately 2% at 8-11 cm depth.
• This effect diminished at greater depths, becoming negligible beyond 18 cm.
• Surface shape had a measurable impact on dosimetry within the initial 18 cm of tissue depth.

Conclusions:

• Surface curvature is a relevant factor in TBI dosimetry, particularly at shallower depths.
• Accurate dosimetry for TBI requires accounting for patient surface geometry.
• The observed deviations suggest potential adjustments needed for dose calculations in curved regions.