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Beam Parameters of Pediatric Plans with a Pencil Beam Scanning Proton System and Their Effects On Patient-Specific Quality Assurance

S Al-ward*, L Zhao, St. Jude Children's Research Hospital, Memphis, TN

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: report beam parameter values of pencil beam scanning (PBS) proton plans for pediatric patients and to study their effects on the passing ratios (PRs) of patient-specific quality assurance (PSQA).


Methods: from 280 pediatric PSQA were analyzed, including 81 craniospinal irradiation (CSI) plans, 71 fourth ventricle plans, 63 suprasellar plans and a variety of other plans. Measurements were performed with a commercial 2D detector array at two depths (d) for each field in a solid water phantom. A total of 2678 measurement points for 1257 fields were included. All fields were optimized using a commercial treatment planning system. A gamma passing ratio of 90% with 3%/3mm criteria was defined as the clinical tolerance. If a measurement failed to pass, it was repeated at different depth(s). Beam parameters included range, modulation, single-field uniform dose (SFUD) or multi-field optimization (MFO), MU, number of spots, range shifter use and number of energy layers.


Results: percent of the fields passed the QA. MFO fields constitute 77% of the fields that did not pass. The low PRs were attributed to the high modulation and complex dose distribution at the measured depths. When the measured doses were compared with the planned doses at d + 2 mm, 70% of them passed QA. Beam parameter values extracted from pediatric proton plans (25?? - 75?? quartiles) were: 10.9-16.7 g/cm² water which correspond to approximately 120-150 MeV, 7.0-12.8 g/cm², 18.2-125.3 MU, 34-54 for range, modulation, MU and energy layers respectively. No strong correlations were observed between any of the beam parameters and the PRs.


Conclusion: work reports PSQA results along with beam parameter values extracted from our unique pediatric proton plans for various disease sites. Most plans passed PSQA, and no strong correlation was found between any of the beam parameters and the PRs.

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Keywords

Protons, Quality Assurance

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – quality assurance

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