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Variable planning margin approach to account for locoregional variations in setup uncertainties
Jinzhong Yang1, Adam S Garden, Yongbin Zhang
1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.
Medical Physics
|August 17, 2012
Summary
This study developed a variable planning margin method for head and neck cancer radiotherapy, improving target coverage by accounting for setup uncertainties. The variable margin approach demonstrated effectiveness compared to conventional constant margins.
Area of Science:
- Radiation Oncology
- Medical Physics
- Cancer Treatment
Background:
- Head and neck cancer radiotherapy requires precise targeting to maximize tumor control and minimize side effects.
- Nonrigid setup uncertainties and locoregional variations can lead to inadequate coverage of the clinical target volume (CTV).
- Conventional constant planning margins may not optimally account for these dynamic uncertainties.
Purpose of the Study:
- To develop and evaluate a novel method for creating variable planning margins around the CTV in head and neck cancer radiotherapy.
- To assess the application of this variable margin approach in accounting for locoregional variations in nonrigid setup uncertainties.
- To compare the efficacy of variable margins versus conventional constant margins in ensuring adequate target coverage.
Main Methods:
- Ten computed tomography (CT) images from head and neck cancer patients were analyzed.
- Variable local margins were calculated based on weighted averages of margins determined at specific anatomical landmark points.
- A Gaussian falloff function was used to weight margins based on distance from landmarks; CTV was expanded to PTV using variable or constant margins.
Main Results:
- The variable margin approach with a 2.5 mm baseline achieved 99.2% CTV coverage within the PTV, outperforming the constant 2.5 mm margin (97.9%).
- At a 2.0 mm baseline, the variable margin approach showed similar CTV coverage (97.8%) but a smaller non-overlapped PTV proportion (32.4% vs. 33.7%) compared to the constant 2.5 mm margin.
- Significantly higher CTV coverage was observed in the lower neck area with the variable margin approach, where setup errors are typically larger.
Conclusions:
- A variable margin approach effectively accounts for locoregional setup uncertainties in head and neck cancer radiotherapy.
- This method demonstrates superior or comparable target coverage compared to conventional global constant margin expansion.
- The variable margin approach holds potential for application in other treatment sites as more data become available.
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