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Fixed-Jaw Optimization for Critical Structure Sparing in IMRT Treatment Planning: Beam Modeling Cautions for Non-Routine Use

R Price

R Price*, I Veltchev , G Cherian , C Ma , Fox Chase Cancer Center, Philadelphia, PA


SU-E-T-462 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Multiple publications exist concerning fixed-jaw utilization to avoid linac carriage shifts and reduce intensity modulated radiotherapy (IMRT) treatment times. The purpose of this work is to demonstrate delivery QA discrepancies and illustrate the need for improved treatment planning system (TPS) commissioning for non-routine use.

Methods: A 6cm diameter spherical target was delineated on a virtual phantom containing the Iba Matrixx linear array within the Varian Eclipse TPS. Optimization was performed for target coverage for the following 3 scenarios: a single open, zero degree field where the X and Y jaws completely cover the target; the same field using an asymmetric, fixed-jaw technique where the upper Y jaw does not cover the superior 2cm of the target; and both of the aforementioned directed at the target at 315 and 45 degree gantry angles, respectively. This final orientation was also irradiated on a linac for delivery analysis. A sarcoma patient case was also analyzed where the fixed jaw technique was utilized for kidney sparing.

Results: The open beam results were as predicted but the fixed-jaw results demonstrate a pronounced fluence increase along the asymmetric, upper jaw. Analysis of the delivery of the combined beam plan result in 83% of pixels evaluated passing gamma criteria of 3%, 3mm DTA. Analysis for the sarcoma patient, in the plane of the shielded kidney, indicated 93% passing although the maximum dose discrepancies in this region were approximately 23%.

Conclusion: Optimization within the target is routinely performed using MLC leaf-end characteristics. The fixed-jaw technique forces optimization of target coverage to utilize the penumbra profiles of the associated beam-defining jaw. If the profiles were collected using a common 0.125cc ionization chamber, the resolution may be insufficient resulting in a plan-vs.-delivery mismatch. It is recommended that high-resolution beam characteristics be considered when non-routine planning methods are utilized.

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