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Calculating radiotherapy margins based on Bayesian modelling of patient specific random errors.
A Herschtal1, L Te Marvelde, K Mengersen
1Department of Biostatistics and Clinical Trials, Peter MacCallum Cancer Centre, Melbourne, Australia. Faculty of Health Arts and Design, Swinburne University of Technology, Melbourne, Australia.
Physics in Medicine and Biology
|February 7, 2015
Summary
Patient variability in radiotherapy displacement requires updated margin calculations. An inverse gamma distribution model for random errors suggests increasing clinical target volume to planning target volume margins by a median of 10% to ensure adequate dose coverage.
Area of Science:
- Radiation Oncology
- Medical Physics
- Biostatistics
Background:
- External beam radiotherapy (EBRT) margin calculations traditionally assume constant patient random errors.
- Real-world data reveal significant inter-patient variability in clinical target volume (CTV) displacement during EBRT.
- This variability challenges the accuracy of existing margin estimation methods.
Purpose of the Study:
- To develop and evaluate statistical models of CTV displacements incorporating patient-specific random error variability.
- To assess the impact of this variability on planning target volume (PTV) margin widths using an inverse gamma (IG) distribution.
- To compare the proposed IG-based margin model against traditional methods across diverse patient cohorts and malignancy sites.
Main Methods:
- Collected real-life CTV displacement data from eight patient cohorts (874 patients, 27,391 sessions) across four institutions.
- Developed statistical models characterizing random errors using an inverse gamma (IG) distribution.
- Analyzed the effect of IG-based margins on CTV coverage under various clinical requirements and penumbral widths.
Main Results:
- The IG-based margin model mandates a median increase of 10% in CTV-to-PTV margins compared to traditional models for a 4mm penumbra.
- This adjustment is necessary to meet the clinical requirement of 95% dose coverage to the CTV for 90% of patients.
- The range of margin adjustments varied from -19% to +35% across the eight cohorts, highlighting significant patient-specific differences.
Conclusions:
- Patient-specific random error variability significantly impacts EBRT margin requirements.
- The proposed IG-based margin model provides a more accurate and robust approach to margin estimation than traditional methods.
- Implementing this model can reduce the proportion of patients receiving insufficient radiation dose due to inadequate margins.
