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Blocked field effects on collimator scatter factors
P D Higgins1, D N Mihailidis, F M Khan
1University of Minnesota Dept. of Therapeutic Radiology, Minneapolis 55455, USA.
Physics in Medicine and Biology
|January 22, 1998
Summary
This study introduces methods to measure how blocking affects collimator scatter (Sc) in radiation therapy. Findings show blocking reduces scatter, especially at higher energies, impacting monitor unit calculations.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Background:
- Routine dosimetry assumes separable collimator (Sc) and phantom (Sp) scatter components for total scatter factor (Sc,p).
- Blocking in radiation therapy complicates photon fluence by altering head scatter and adding secondary scatter from blocks and trays.
Purpose of the Study:
- To develop and present techniques for directly measuring the aperture effect on collimator scatter (Sc).
- To quantify the impact of blocking on Sc in air and in a full-scatter phantom across different photon beam energies.
Main Methods:
- Direct measurement techniques were employed to assess the aperture effect on Sc.
- A linear fit model was developed using the ratio of projected open-to-blocked equivalent square fields.
- Measurements were conducted for 6, 18, and 24 MV photon beams on Varian 2500 and 2100c accelerators.
Main Results:
- The change in Sc due to blocking was found to be a scalable quantity.
- Collimator scatter contribution progressively decreases with increased field blocking, an effect amplified at higher energies.
- Secondary scatter from blocks and trays significantly impacts Sc only for large fields and short treatment distances (≤80 cm).
Conclusions:
- The study provides a quantifiable method to correct for the aperture effect on collimator scatter.
- Accurate monitor unit calculations can be achieved by applying these measured corrections.
- Understanding and correcting for blocking effects on scatter is crucial for precise radiation dosimetry.