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Dosimetric Evaluation of a Commercial Proton Spot Scanning Monte Carlo (MC) Dose Calculation for Small Fields

R Regmi*, D Maes , Z Nevitt , E Leuro , T Wong , J Saini , SCCA Proton Therapy Center, Seattle, WA


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: The purpose of this study is to evaluate the commercially available RayStation MC dose calculation algorithm for small proton spot scanning fields less than 4x4 cm by benchmarking its performance against measurements in a water phantom. This study will support clinical implementation of proton MC dose calculation for stereotactic radiosurgery and ocular treatments.

Methods: The validation consisted of comparing pristine Bragg peak depth doses, output factors and lateral beam profiles. Proton spot scanning fields with ranges between 8 and 25 cm were delivered using circular and square brass apertures ranging 1 – 3 cm in field size. Depth dose measurements were performed using a microdiamond detector and were compared against the TPS for range error and point-to-point dose differences. Output factors were measured using microDiamond and pinpoint chambers at a depth of 2 cm for each Bragg peak and compared against TPS-calculated values. Profile measurements were made using Gafchromic film and compared to calculated dose profiles using gamma index analysis.

Results: For pristine Bragg peaks of range 8, 16, and 25 cm, the discrepancy between measured and calculated range (R80) was less than 1.5 mm for circular and square field sizes of 1-3 cm. For 3x3 cm aperture, discrepancy between measured and calculated output factor was within 1.2%. For 1 cm circular aperture, measured output factors were within 4% of the MC calculated dose. Preliminary lateral dose profiles showed >90% match between measured and calculated using 3% dose and 3mm distance-to-agreement tolerances for all combinations.

Conclusion: Initial results show that measured range and output for 3x3 cm field size were in good agreement with MC calculated values. Greater discrepancy between measured and calculated output factor was seen for 1 cm circular field. The proton beam range was within an acceptable tolerance of 1.5 mm for 1-3 cm apertures.


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