Optimization of Patient-Specific QA Metrics for Detecting Spinal Cord Dmax delivery Deviations In Spine Stereotactic Body Radiotherapy
Abstract
Purpose
We aimed to optimize patient-specific quality assurance (PSQA) evaluation criteria using an electronic portal imaging device (EPID) and myQA SRS (IBA Dosimetry) to detect spinal cord Dmax deviations in spine stereotactic body radiotherapy (SBRT).
Methods
Three patients with spinal bone metastases were included. Volumetric modulated arc therapy plans (2–3 arcs) were generated with RayStation v10A (RaySearch Laboratories) using the collapsed cone convolution algorithm for 10-MV flattening filter–free beams. Baseline plans were error-free. Thirty-nine error plans were created using a custom RayStation script by introducing multileaf collimator (MLC) opening/closing errors of 1–2 mm, MLC positional shifts of ±1 and ±2 mm, and output errors simulated by scaling monitor units ±1% and ±2%. Plans were delivered on a TrueBeam linear accelerator (Varian Medical Systems) with a Millennium 120 MLC and a-Si 1200 EPID. EPID images were analyzed using PerFRACTION. To evaluate orientation dependence, plans were delivered to myQA SRS at 0° (coronal) and 90° (sagittal) intersecting the spinal cord. Two-dimensional gamma analysis used criteria (3%/2 mm, 2%/2 mm, 3%/1 mm, 2%/1 mm, and 1%/1 mm) with a 10% threshold. Spinal cord Dmax is expressed as percentage changes from baseline. Receiver operating characteristic (ROC) analysis defined positives as |ΔDmax|≥2%. Discrimination was assessed using the area under the curve (AUC), and optimal cutoffs were selected from ROC analysis.
Results
The best-performing EPID metric was 3%/2 mm (AUC 0.63; 95% confidence interval [CI], 0.43–0.79; cutoff 91.6%). myQA SRS showed higher discrimination: 2%/2 mm at 0° (AUC 0.84; 95% CI, 0.68–0.93; cutoff 91.9%) and 3%/1 mm at 90° (AUC 0.79; 95% CI, 0.62–0.90; cutoff 87.4%).
Conclusion
MyQA SRS at 0° with 2%/2 mm showed the highest discrimination (cutoff, 91.9%). EPID showed limited discrimination (3%/2 mm; cutoff, 91.6%), underscoring the value of high-resolution semiconductor detectors over EPID dosimetry for spine SBRT PSQA.