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The Step-And-Shoot IMRT Overshooting Phenomena: A Novel Method to Mitigate Patient Overdosage


A Pompos

A Pompos*, H Zhen, L Ouyang, Q Bao, S Stojadinovic, UT Southwestern Medical Center, Dallas, TX

SU-E-T-381 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:

To demonstrate a method to mitigate the step-and-shoot (S&S) IMRT overdose phenomenon for set of patients which initially failed IMRT QA process.
Methods:
Five S&S IMRT patients treated on Varian 2100C-EX linacs with larger than +4.5% phantom IC point dose difference relative to planned dose were investigated. For every patient plan, five fractions (Fx) were delivered. Dynalog files were recorded and centiMU pulses from dose integrator board for every control point (CP) were counted using a BK1856D (B&K Precision Corp.) pulse counter (PC). The recorded MUs were imported to Pinnacle9.2 (Philips Healthcare), the 3D dose was recalculated and compared to the planned dose distribution. The initial plans were then modified by adding one starting segment to every beam. This segment consisted of an MLC aperture of 0.5x1cm2 hidden under the jaws with 1MU. For each patient, five Fx of the modified plan were delivered, recorded and the delivered 3D dose recalculated. Individual voxels' dose in high dose region, isocenter point dose, segment MUs, PTV D95 and OAR D2cc were compared between the original plan and the results of both deliveries.

Results:

The PC recorded total number showed the delivered beam MUs exceeded the plan by only ~0.2%. For all initial plans, the first CP on average overshot by 0.6MU. It was determined that this on average translated to an increased dose to isocenter, PTV D95, bladder D2cc and high-dose-region voxels' dose by 2.66% (2.21%-3.12%), 2.06% (1.59%-2.48%), 2.57% (1.97%-3.11%), and 2.19% (1.75%-2.75%) respectively. All modified plans had inconsequential 0.05MU overshoot in first unmodified CP and 0.04%(-0.45% -0.55%), -0.07%(-0.44%-0.48%), 0.08%(-0.28%-0.33%) and -0.07%(-0.31%-0.20%) differences for dosimetric parameters listed above.
Conclusion:
Our proposed method of attaching a special aperture 1MU CP to each beam was experimentally verified by using centiMU pulse counts to successfully eliminate the overdosage.

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