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The effects of target size and tissue density on the minimum margin required for random errors
Marnix G Witte1, Joris van der Geer, Christoph Schneider
1Department of Radiotherapy, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
Medical Physics
|December 14, 2004
Summary
This study determined radiotherapy margins for random geometric uncertainties. Margin size depends on target size and tissue density, with larger margins needed for denser tissues and smaller targets.
Area of Science:
- Medical Physics
- Radiation Oncology
Background:
- Accurate radiotherapy requires precise targeting to minimize dose to healthy tissues.
- Geometric uncertainties in treatment delivery necessitate planning margins.
Purpose of the Study:
- To determine minimum margins for spherical Clinical Target Volumes to account for random geometric uncertainties in radiotherapy.
- To establish margin calculation criteria ensuring at least 95% of prescribed dose in 90% of patients.
Main Methods:
- Utilized an analytic model for cumulative dose distribution, incorporating two Gaussians.
- Simulated dose profiles for various target sizes in lung and water densities.
- Calculated margins based on achieving a minimum target dose of 95% of the prescribed dose.
Main Results:
- Random error margins increased with tissue density and decreased with target size.
- Margin requirements were similar for spherical and cylindrical dose distributions.
- Dose distributions with significant out-of-field dose increased margins for larger targets.
Conclusions:
- Target size and tissue density significantly influence required radiotherapy margins.
- Previously proposed margins (0.7 times standard deviation) are generally safe up to 5 mm standard deviation, with exceptions for small targets in dense materials.