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A convolution model to convert transmission dose images to exit dose distributions
R Boellaard1, M van Herk, B J Mijnheer
1The Netherlands Cancer Institute/Antoni van Leeuwenhoek Huis, Radiotherapy Department, Amsterdam, The Netherlands.
Medical Physics
|February 1, 1997
Summary
This study presents a new model using electronic portal imaging device (EPID) data to accurately calculate patient radiation exit doses. The model estimates doses within 2.5% for both homogeneous and inhomogeneous phantoms, improving treatment accuracy.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Dosimetry
Background:
- Accurate calculation of patient radiation exit dose is crucial for effective radiotherapy.
- Electronic Portal Imaging Devices (EPIDs) offer a potential tool for in-vivo dosimetry.
- Existing methods for exit dose calculation often lack precision, especially in the presence of tissue inhomogeneities.
Purpose of the Study:
- To develop and validate a convolution model for computing patient exit dose from EPID transmission measurements.
- To investigate the impact of various parameters (field size, phantom thickness, air gap) on transmission dose.
- To incorporate the effects of inhomogeneities into the exit dose calculation model.
Main Methods:
- Developed a convolution model using primary dose and a scatter kernel to predict exit dose.
- Performed extensive transmission dose measurements using a liquid-filled EPID with various phantoms and an 8 MV beam.
- Separated primary and scattered dose components by analyzing transmission data at different air gaps.
- Modeled the influence of inhomogeneities using a radiological path length approach.
Main Results:
- The convolution model accurately predicts absolute exit doses for homogeneous phantoms within 2% (1 s.d.).
- The model achieves an accuracy of 2.5% for inhomogeneous phantoms by utilizing measured radiological path length.
- Scattered dose distribution was characterized, showing a Gaussian profile that widens with increasing air gap.
Conclusions:
- EPID transmission measurements at large air gaps can be effectively used to estimate absolute exit doses.
- The developed convolution model provides a robust method for accurate exit dose calculation in radiotherapy.
- The model's ability to handle inhomogeneities without CT data enhances its clinical applicability.