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Analytical setup margin for spinal stereotactic body radiotherapy based on measured errors
Audrey Copeland1, Addie Barron2, Jonas Fontenot2,3
1Department of Physics and Astronomy, Louisiana State University and Agricultural and Mechanical College, Baton Rouge, LA, USA. audreycopeland19@gmail.com.
Radiation Oncology (London, England)
|December 8, 2021
Summary
A 2.4 mm setup margin in the superior/inferior direction is recommended for spinal stereotactic body radiation therapy (SBRT). This margin ensures at least 90% clinical target volume coverage for most patients while minimizing spinal cord dose.
Area of Science:
- Radiation Oncology
- Medical Physics
- Radiotherapy Planning
Background:
- Spinal stereotactic body radiation therapy (SBRT) lacks consensus on optimal margin size.
- Existing margins address individual errors but not combined effects.
- Accurate margin determination is crucial for effective spinal SBRT.
Purpose of the Study:
- To determine a setup margin for five-fraction spinal SBRT.
- To achieve at least 90% clinical target volume (CTV) coverage with prescription dose for ≥90% of patients.
- To limit spinal cord dose to ≤30 Gy point dose or ≤23 Gy to 10% of the subvolume.
Main Methods:
- Quantified random and systematic errors: intrafraction motion, residual setup error, and end-to-end system accuracy.
- Measured patient surface displacement for intrafraction motion.
- Re-registered daily cone-beam CT setup images for residual setup error.
- Assessed phantom dose profiles for end-to-end system accuracy.
- Validated margin recommendation using 140 simulated patient plans.
Main Results:
- An analytical margin of 2.4 mm was generated.
- Applying this margin in the superior/inferior direction achieved ≥90% CTV coverage for 96% of simulated patients.
- Minimal negative impact on spinal cord dose levels was observed.
Conclusions:
- A 2.4 mm superior/inferior margin is supported for spinal SBRT.
- This margin ensures adequate CTV coverage in the presence of treatment errors.
- Findings are applicable to dual-arc VMAT spinal SBRT with vacuum bag immobilization.
