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Is Patient-Specific IMRT QA Needed for TomoTherapy 3D Conformal Radiation Therapy (3D-CRT) Treatment Plans?

A Besemer

A Besemer*, J Smilowitz , M Geurts , Department of Human Oncology, University of Wisconsin, Madison, WI


SU-I-GPD-T-287 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: TomoTherapy 3D conformal radiation therapy (3D-CRT) is a relatively new modality that is inherently forward planned but still uses beamlet optimization and intensity modulation to deliver a uniform dose to the target with the unflattened beam, binary MLCs, and translating couch. This work details our institutions experience performing and accessing the need for IMRT QA for TomoTherapy 3D-CRT plans.

Methods: IMRT QA was performed for 27 TomoTherapy 3D-CRT plans for a variety of disease sites. There were 21 direct and 6 helical plans. 22 plans used a 5cm jaw while 5 used a 2.5cm jaw. The pitch was set to 1/10th the jaw width for direct plans and ranged from 0.215-0.43 for the helical plans. The normal tissue uniformity was turned on and high compensation was used for 24 plans. IMRT QA was delivered on a TomoHDA machine and measured with a ScandiDos Delta4 diode array. Plans were analyzed with a 5%/3mm and 3%/3mm dose-difference and distance-to-agreement (DTA) tolerance. The passing criterion required that ≥95% of the diodes have a γ<1.

Results: All but one plan passed well above a 95% (3%/3mm) acceptance level. The average and standard deviation of the 3%/3mm gamma, dose-difference, DTA pass rate was 99.0±2.0%, 92.8±9.8%, and 95.5±3.1%, respectively. One direct plan with a single 8 Gy dose and two posterior beams failed -1.2% cold with a 90.8% gamma pass rate likely due to limits of the couch model for static posterior beams. For a 5%/3mm tolerance, the plan passed with a 99% gamma pass rate.

Conclusion: All plans passed within 5%/3mm which is within acceptable limits for a 3D plan. At our institution, IMRT QA was deemed unnecessary for TomoTherapy 3D-CRT plans with a jaw width ≥2.5 cm and a pitch ≥0.215 for helical plans and ≥2.5 for direct plans regardless of compensation level

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