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Measurement of MLC Leaf Offset Behavior Using An Electronic Portal Imager

B Barraclough*, D Jacqmin , University of Wisconsin, Madison, WI


(Monday, 7/30/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 7

Purpose: To develop an efficient method to calculate leaf offset tables (LOT) using an electronic portal imaging device (EPID) and use this method to investigate leaf offset for each leaf pair and beam energy.

Methods: Static 4x40 cm2 MLC-defined fields with centers ranging from -17 to +17 cm in 2 cm intervals (1 cm in central ±5 cm) were delivered with a 6 MV beam to an EPID panel at 100 cm SSD. The same fields were calculated in the Pinnacle treatment planning system (TPS) using a null LOT. Field edges were determined for each field position in both calculated and measured data sets for all leaf pairs using a custom MATLAB analysis code. The leaf offset at a given leaf position was the difference in field edge between calculation and measurement. This process was repeated with 6 FFF and 15 MV beams. Dose was recalculated with new LOTs and compared to measurement.

Results: LOTs were successfully created for every leaf pair and beam energy. We observed that leaf offset is dependent on leaf pair with variation up to 0.4 mm between the central and outer leaves. Leaf offset also shows dependence on beam energy. Differences up to 0.7 mm (mean of 0.3 mm) between the 6X and 15X LOTs were found. LOTs for the 6X and 6FFF beams are within 0.3 mm (mean of 0.0 mm). Re-calculated beams with the new LOTs were within 0.2 mm of measurement (mean of 0.1 mm).

Conclusion: An efficient method for generating LOTs using an EPID instead of a scanning water tank has been developed. It has been shown that leaf offsets vary with leaf-pair. Similarly, variation in leaf offset with energy has been observed. A TPS could potentially incorporate LOTs for each energy and each leaf pair to model this behavior.


MLC, Commissioning, Electronic Portal Imaging


TH- External beam- photons: dose computation engines- deterministic

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