Lung dose corrections for 6- and 15-MV x rays.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Radiation therapy planning relies on accurate dose calculations, particularly in heterogeneous tissues.
• Established methods like tissue-air ratios (TAR) and Batho's scatter correction are validated mainly for cobalt-60 radiation.
• The accuracy of these methods in modern megavoltage X-ray therapy, especially in low-density media, requires further investigation.

Purpose of the Study:

• To evaluate the accuracy of routine dose calculation methods in heterogeneous phantoms using 6- and 15-MV X-rays.
• To identify the conditions under which current dose calculation algorithms may fail.
• To compare computational results with experimental measurements and Monte Carlo simulations.

Main Methods:

• Experimental measurements of radiation dose in simple heterogeneous phantoms containing simulated lung regions.
• Comparison of measured doses with calculations from TAR, Batho, and equivalent TAR methods.
• Validation using Monte Carlo simulations to directly track charged particle transport.

Main Results:

• Significant dose calculation errors were observed within and near simulated low-density lung regions for 6- and 15-MV X-rays.
• Errors were more pronounced with smaller field sizes.
• Failure to achieve electronic equilibrium laterally was identified as a key reason for inaccuracies, violating assumptions of standard methods.

Conclusions:

• Routine dose calculation methods may yield incorrect radiation doses in heterogeneous environments, particularly near low-density tissues.
• The assumption of electronic equilibrium is critical and often violated in these scenarios.
• Monte Carlo simulations offer a more accurate approach for dose calculation in complex radiation therapy planning.