Quality Assurance of MLC Auto–Feathering Junctions for Supine Craniospinal Axis Irradiation (CSI) VMAT On Ring-Gantry Halcyon Linac
Presentations
MO-E115-IePD-F4-5 (Monday, 7/11/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]
Exhibit Hall | Forum 4
Purpose: A 28 cm² MLC field-size at isocenter is a major limitation for jawless Halcyon for VMAT-CSI. To overcome this difficulty, a supine 3-isocenter planning method was developed by implementing MLC junctional auto-feathering during VMAT optimization, enabling larger treatment fields for complex CSI patients. To evaluate the dose homogeneity at auto–feathering junctions, portal dosimetry (PD) quality assurance tool was developed.
Methods: Three adult patients who underwent supine VMAT-CSI treatment on TrueBeam (6MV) for 36Gy in 20 fractions, each requiring 3-junction changes were re-planned with posterior limited-angle 3-isocenter VMAT with one-dimensional superior-inferior shift between the isocenters on Halcyon (6MV-FFF). Both plans used 3-sets (brain, upper/lower spine) of 2-posterior partial arcs with anterior avoidance sectors, ±5°collimator rotations between arcs, 5–8cm of MLC overlap auto–feathering junctions and Acuros-based dose. Two plans were compared via their target coverage, conformity, heterogeneity indices, dose to critical organs including integral dose (V10Gy and V18Gy) and total monitor units. PD-based MATLab script was developed to examine dose homogeneity at MLC auto-feathering junctions for patient-specific QA plans.
Results: For similar target coverage, Halcyon provided more conformal and homogenous dose distribution compared to original TrueBeam plans. Mean doses to critical organs including heart and esophagus were similar. Lower values of V10Gy/V18Gy for Halcyon by 1%/2.5%; 2%/1.4%; 1%/1% for patient 1, 2 and 3–suggesting lower integral dose to patient with Halcyon plans. Although, total deliver monitor units with Halcyon plans increased by a factor of 2–3. Gamma analysis of PD-QA for all 3 patients at MLC auto-feathering junctions resulted with >99.4% pass-rate for 2%/2mm criteria.
Conclusion: If configured, limited field size Halcyon can be used to treat larger and complex fields such as supine CSI-VMAT. Further analyzing MLC auto-feathering junctions of CSI–VMAT via PD-QA tool confirms treatment delivery accuracy while avoiding the manual field-matching and shifting the patient on both platforms.
Keywords
Quality Assurance, MLC, Treatment Techniques
Taxonomy
TH- External Beam- Photons: Development (new technology and techniques)
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