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Surface buildup dose dependence on photon field delivery technique for IMRT
Shigeru Yokoyama1, Peter L Roberson, Dale W Litzenberg
1Department of Radiation Oncology, University of Michigan Medical Center, Ann Arbor, Michigan 48109-0010, USA. Shigyoko@sarmc.org
Journal of Applied Clinical Medical Physics
|March 2, 2005
Summary
Intensity-modulated radiotherapy (IMRT) delivery techniques significantly alter surface buildup dose. Beam segmentation in IMRT reduces buildup dose compared to open fields, influenced by multileaf collimator transmission.
Area of Science:
- Medical Physics
- Radiation Oncology
Background:
- Intensity-modulated radiotherapy (IMRT) utilizes complex inverse planning, altering the dose distribution near the surface.
- Surface buildup dose is primarily influenced by electron contamination from the flattening filter and secondary sources.
Purpose of the Study:
- To evaluate the impact of beam segmentation on surface buildup dose in IMRT.
- To compare buildup dose variations across different static and dynamic IMRT delivery techniques.
Main Methods:
- Measurements of depth dose at 2 mm were performed using parallel plate chambers, stereotactic diode, and TLDs for a 6 MV X-ray beam.
- Comparisons were made between a 10x10 cm2 open field and segmented fields (3, 5, 10 static strips; 1.1 dynamic delivery).
- Measurements were conducted in water and Solid Water phantoms.
Main Results:
- Segmented IMRT fields showed reduced depth doses at 2 mm by 6-11% compared to the open field.
- Multileaf collimator (MLC) transmission was identified as a key factor influencing these dose differences.
- An example IMRT field demonstrated weak dependence of buildup dose on MLC delivery technique (static vs. dynamic).
Conclusions:
- Beam segmentation in IMRT significantly reduces surface buildup dose.
- MLC transmission plays a crucial role in the observed buildup dose variations.
- The delivery technique has a minor impact on buildup dose for efficient IMRT fields.