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Can CyberKife Plans with Small/Narrow MLC Segments Be Effectively Assessed by a Diode Array?

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T Neupane

C Shang* , J Cole* , T Neupane**, A Schramm* , G Gibbard** , M Rahman** , U Dona** , *Boca Raton Regional Hospital, Boca Raton, FL ** Florida Atlantic University, Boca Raton, FL


SU-I-GPD-T-289 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: With more implementations of InCiseᵀᴹ MLC based Cyberknifeᵀᴹ systems, the patient specific QA's for highly intensity-modulated MLC plans became a new challenge due to their none-isocentric FFF beam projections and composition of many discrete small/narrow segments. This investigation is to evaluate the effectiveness of a diode array (MapCheck®) for such tasks.

Methods: Required by imaging alignment, gold Fiducials were embedded to the MapCheck. Two slabs of 5cm solid-water were added for buildup and backscatter. Its calibration was based on the field size dependence tests against an Edge detector and RTPS data. Patient specific plan measurements by MapCheck during past 8 months were reevaluated using standard calibration for QAs at the anterior nominal angles and, in selected cases measured at actual treatment beam projectiles. For highly dose-intensity modulated plans, MLC < 8 mm wide segments were selected and re-measured with an offset calibration.

Results: Mapcheck measurements displayed highly (±0.5%) accurate doses with MLC fields of ≥30 mm wide, 4.1% over response with 7.6mm x7.7mm field, and about 1.1% under response in between sizes. All 90 patient QA's passed (>95%) 2.0%/2.0mm, 28 of which showed pass-rate of 98.5±1.9% at 2.0%/1.5mm. 6 plans delivered at actual treatment angles showed a pass-rate of 95.3±2.3% at 2.4%/1.5mm. In 16 highly modulated plans, tests with <8mm wide MLC segments showed a pass-rate of 98.0±2.1% at 2.0%/1.2mm when measured at a (-4.0%) calibration correction.

Conclusion: This investigation suggests that Mapcheck can be effectively employed in routine patient specific QA with MLC based CyberKnife plans delivered at the nominal angle or real treatment angles. Due to its variable dose responses for the small fields, MapCheck yielded high pass-rates in this group. However, for a highly intensity-modulated MLC plan, small MLC segments of <8mm wide shall be further analyzed with a modified (-4%) calibration factor as presented.

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