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Poster - Thur Eve - 40: Dynamic arc sliding window tests for checking MLC gap consistency in rapid arc delivery
X Mei1,2, J Konieczny1,2, K Leszczynsky1
1Northeast Cancer Centre of Health Sciences, Sudbury, ON, Canada.
Medical Physics
|May 19, 2017
Summary
Accurate Multileaf Collimator (MLC) gap control is vital for precise radiation therapy, especially in RapidArc treatments. Dynamic arc sliding window tests effectively verify MLC leaf constancy, ensuring dosimetric accuracy in VMAT delivery.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Technology
Background:
- Precise control of Multileaf Collimator (MLC) leaf gaps is crucial for dosimetric accuracy in rotational intensity-modulated radiation therapy (IMRT), such as RapidArc and Volumetric Modulated Arc Therapy (VMAT).
- Even minor variations (1 mm) in MLC gap can significantly alter the equivalent uniform dose (EUD) to the planning target volume (PTV) by 3-4% in head and neck RapidArc plans.
- Robust quality control (QC) procedures are essential for monitoring MLC gap integrity.
Purpose of the Study:
- To develop and validate dynamic arc sliding window (SW) plans as a QC method for assessing MLC leaf constancy during rotational IMRT delivery.
- To compare the dosimetric leaf gap (DLG) and MLC leaf gap stability between dynamic arc SW tests and traditional fixed gantry angle SW fields.
Main Methods:
- Dynamic arc SW plans were created using 6MV photons with fixed MLC slit widths (5, 10, 15, 20 mm) and a fixed length (20 cm) across a single arc (gantry angles 182° to 178°).
- Dose measurements were performed using an ion chamber in a cylindrical phantom at the isocenter to derive DLG values.
- MLC leaf positions were recorded in dynalog files during dynamic arc deliveries to extract and compare MLC leaf gaps with those from fixed gantry SW fields.
Main Results:
- DLG values derived from dynamic arc SW tests showed excellent agreement with those from fixed gantry SW fields, with a difference within 0.02 mm.
- MLC leaf gaps remained consistently stable during dynamic arc deliveries, exhibiting a typical standard deviation of approximately 0.01 mm for all leaf pairs.
- This stability was comparable to that observed in fixed gantry angle SW deliveries.
Conclusions:
- Dynamic arc sliding window tests provide a reliable and effective method for evaluating MLC leaf constancy in RapidArc treatments.
- These tests are highly valuable for ensuring the dosimetric accuracy and quality assurance of VMAT delivery by confirming precise MLC positioning.
- The proposed method offers a practical approach for routine QC procedures in radiotherapy departments utilizing rotational IMRT.
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