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Lung compensation in total body irradiation: a radiographic method
1Division of Medical Physics, British Columbia Cancer Agency, Vancouver, Canada.
Medical Physics
|March 1, 1996
Summary
This study establishes a method using megavoltage radiography and optical film density to calculate lung tissue deficits for total body irradiation (TBI) compensation. Lead compensators accurately replace the deficit, validated by CT and TLD measurements.
Area of Science:
- Medical Physics
- Radiotherapy Physics
- Diagnostic Imaging
Background:
- Accurate dose compensation is crucial in total body irradiation (TBI) to account for tissue heterogeneity, particularly lung deficits.
- Megavoltage radiography offers a potential tool for assessing tissue deficits in treatment geometry.
- Establishing a reliable relationship between radiographic film density and tissue-equivalent thickness is necessary for dose compensation.
Purpose of the Study:
- To establish an empirical relationship between optical film density and equivalent phantom thickness using megavoltage radiography.
- To determine the feasibility of using this relationship for calculating lung tissue deficits in TBI.
- To validate the accuracy of lead compensator thickness derived from radiographic data.
Main Methods:
- Exit dose measurements were performed using megavoltage radiography (Co-60, 4-MV, 10-MV) and a composite chest phantom.
- An empirical relationship was developed between optical film density and overlying phantom material thickness.
- Lead compensator thickness was calculated based on radiographic optical density to replace lung tissue deficits.
- Validation was conducted using computed tomography (CT) data and in-phantom thermoluminescent dosimetry (TLD).
Main Results:
- A dependency was observed between optical film density and equivalent phantom thickness, influenced by film sensitometry and photon beam quality.
- The method successfully calculated lung tissue deficits for TBI compensation.
- Results showed good agreement with CT data and TLD measurements across different photon energies.
Conclusions:
- Megavoltage radiography combined with optical film density analysis provides a valid method for determining lung tissue deficits in TBI.
- Lead compensators designed using this technique accurately replace the calculated tissue deficits.
- This approach offers a practical and validated method for improving dose accuracy in TBI.