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Phantom 3D Dose Calculation and Anatomy Based DVH Evaluation On VMAT Patient QA Using the Newest Version of Delta4 Dosimetry System

Y Hu

Y Hu1*, C Ruan2, A Nguyen1, W Duggar1, P Mobit1, P Rajaguru1, R He1, C Yang1, (1) University of Mississippi Medical Center, Jackson, MS, (2) University of Oklahoma Health Science Center, Oklahoma City, OK

SU-E-T-44 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To validate the two new patient QA methods on ScandiDos Delta4 system: (1) Phantom 3D dose calculation (2) Anatomy DVH evaluation, compared with conventional gamma analysis criteria.

Methods:Phantom 3D dose calculation and anatomy DVHs evaluation have the capability to perform gamma analysis on critical organ contours and calculate DVHs from Delta4 measurement and calculation on either phantom or patient anatomy. The calculation speed, performance time and algorithm accuracy were investigated on these two novel methods. Elekta Synergy beam PDDs and output factors were characterized by importing plans from Pinnacle (v9.2). VMAT H&N QA plans were performed. The patient CT images and contours were transferred from Pinnacle to Delta4. Three critical structures (PTV, Brainstem and Cord) were analyzed using gamma analysis, phantom 3D dose calculation and anatomy DVH evaluation.

Results:Conventional gamma pass rate is analyzed by combining the dose difference and distance to agreement on each detector. 3D dose calculation is implemented on the homogeneous phantom using ray-tracing algorithm. Patient anatomy dose is calculated using the fluence map and kernel-based pencil beam algorithm. The average calculation time is 1.1, 5.3 and 25.5 minutes for each method. The performance time is 22, 30, and 55 minutes for each patient QA, since anatomy evaluation needs CT images and anatomy structures. Regarding the algorithm accuracy, conventional gamma analysis and phantom based PTV 3D-dose are correlated. The passing rate difference are insignificant (P>0.2, t test). There are discrepancies when the phantom 3D-Dose gamma and DVH are compared with anatomy based evaluation. The passing rate difference are significant (P<0.005, t test).

Conclusion:Conventional gamma analysis has weak correlation to critical DVH errors. Phantom 3D dose and anatomy DVH evaluation on Delta4 are sensitive and specific for VMAT patient QA. Further investigation on accuracy and potential errors of anatomy DVH will be performed.

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