A simple equivalent tissue-air ratio method for calculating absorbed dose in a heterogeneous medium

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose calculation in radiation therapy requires accounting for tissue density variations.
• Inhomogeneities like lung and bone significantly impact dose distribution.
• Existing methods for density correction have limitations in accuracy and applicability.

Purpose of the Study:

• To measure density correction factors for 60Co gamma rays and 8-MV x rays in phantoms with simulated inhomogeneities.
• To compare experimental measurements with calculated values from the effective SSD, generalized Batho power law, and SETAR methods.
• To evaluate the accuracy and clinical applicability of different density correction algorithms.

Main Methods:

• Ionization chamber measurements were performed in water-equivalent phantoms containing simulated lung or bone inhomogeneities.
• Beam sizes ranged from 5 x 5 cm to 30 x 30 cm.
• Calculations were performed using the effective SSD method, the generalized Batho power law method, and the simplified equivalent tissue-air ratio (SETAR) method.

Main Results:

• The SETAR method demonstrated the best overall agreement with experimental density correction factors.
• The effective SSD and generalized Batho power law methods showed accuracy only in specific scenarios.
• Discrepancies were observed between measured and calculated values, particularly for larger inhomogeneities.

Conclusions:

• The SETAR method is a more accurate and reliable approach for calculating density correction factors compared to the effective SSD and Batho methods.
• The SETAR method's accuracy makes it suitable for a wide range of clinical radiation therapy applications.
• Further validation in diverse clinical scenarios is recommended to fully establish the SETAR method's utility.