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Dosimetric Measurement and Correction On the Leaf Leakage-Penumbra Effect in IMRT

J Chow

G Grigorov1 , J Chow2*, (1) Grand River Regional Cancer Centre, Kitchener, ON, (2) Princess Margaret Cancer Centre, Toronto, ON


TU-C1-GePD-TT-2 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Theater

Purpose: The aim of this study is to measure and correct the inter-leaf leakage effect in the penumbra region produced by a multileaf collimator (MLC) in step-and-shoot intensity modulated radiotherapy (IMRT).

Methods: From 10 prostate IMRT plans using a 120-leaf MLC, 3-D leaf leakage-penumbra doses were measured using 5 photon beams with angles equal to 0⁰, 72⁰, 144⁰, 216⁰ and 288⁰. For each photon beam in the IMRT plan, the first segment was set equal to the planning target volume having the largest area in the leaf sequencing. The leaf leakage-penumbra doses were determined by subtracting the overshoot and segment positional effects measured using a solid water phantom and radiographic films. From the measurement results, we suggest adjusting the treatment field widths in beam segments to reduce the overlap effects and possibly, to include the leaf leakage-penumbra as a parameter in the inverse planning optimization.

Results: Leaf leakage-penumbra doses between 3% and 5% of the beam dose were measured in all prostate IMRT plans. Moreover, an overdose with peak of 10% of the beam profile was found at the gap between two neighbouring beam segments. The overshoot effect was observed in all photon beams and increased from 1 to 1.3 Gy per rectal intersection treatment. The segment positional error was less than 1 mm (maximum tolerance) under a dose rate of 600 monitor units per minute. The leaf leakage-penumbra effect was corrected using our method based on modification of photon field widths.

Conclusion: It is concluded that the methods for measurement and correction of the leaf leakage-penumbra dose distribution can be used to improve the agreement between the planned and delivered doses for step-and-shoot IMRT delivery.

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