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Quality Assurance of Multiple Isocentric Volumetric Modulated Arc Therapy for Craniospinal Irradiation

J Zhang*, B Lee , W Lien , J Han , D Kim , K Goldman , S Lu , G Hong , S Lee , J Rahimian , Y Qian , Southern California Permanente Medical Group, Los Angeles, CA


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: Multiple Isocentric Volumetric Modulated Arc Therapy (VMAT) technique has been introduced to Craniospinal Irradiation (CSI) which provides significant treatment volume dose conformity. Depends on the total treatment length, 2~3 isocenters are used for the treatment. The treatment QA and patient set up are critical. The purpose of this study was to compare the QA quality of CSI VMAT plan measured from Sun Nuclear ArcCHECK and MapPHAN detector systems and evaluate the patient set up error margin.

Methods: The index case of this CSI plan has 2 isocenters. This plan was optimized using collapsed-cone-convolution algorithm of Pinnacle3 TPS system. ArcCHECK and MapPHAN were used to measure the QA dose distribution individually. The plans and delivered dose distributions were compared with 3% and 3mm dose-to-agreement criteria and 10% threshold. Pass rates were compared between these two devices. Also, ArcCHECK was used for the treatment delivery set up test of 1º and 2º couch rotation from spinal beams part. The plans and delivered dose distributions were compared with 3%/1mm criteria and 20% threshold.

Results: The plan pass rates for MapPhan and ArcCHECK based on 3%/3mm-10% were 91.5% and 99.4%. Even with 3%/1mm-10% tolerance, the ArcCHECK result was 97.1%. With couch rotation (0º, 1º and 2º) through spinal beam delivery, the pass rates with 3%/1mm-20% tolerance were 95%, 93.4% and 90% respectively, which mostly caused the treatment dose errors from high dose treatment volume.

Conclusion: The CSI VMAT QA process helped identifying various potential errors such as treatment plans and treatment delivery systems. ArcCHECK has better potential to verify the QA quality. Also, it helps to verify the treatment delivery errors from the set up. For multi isocentric CSI treatment, the 1º/1mm setup tolerance is recommended. Careful patient set up and high quality of on board imaging system is highly required.


Quality Assurance, Radiation Therapy, Setup Errors


TH- External beam- photons: Quality Assurance - VMAT

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