Program Information
Dose Calculation Accuracy with a Standard Beam Model for Extended SSD Treatments
K Kisling*, L Court , S Kirsner , C Nelson , UT MD Anderson Cancer Center, Houston, TX
Presentations
SU-E-T-276 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: While most photon treatments are delivered near 100cm SSD or less, a subset of patients may benefit from treatment at SSDs greater than 100cm. A proposed rotating chair for upright treatments would enable isocentric treatments at extended SSDs. The purpose of this study was to assess the accuracy of the Pinnacle³ treatment planning system dose calculation for standard beam geometries delivered at extended SSDs with a beam model commissioned at 100cm SSD.
Methods: Dose to a water phantom at 100, 110, and 120cm SSD was calculated with the Pinnacle³ CC convolve algorithm for 6x beams for 5x5, 10x10, 20x20, and 30x30cm² field sizes (defined at the water surface for each SSD). PDDs and profiles (depths of 1.5, 12.5, and 22cm) were compared to measurements in water with an ionization chamber. Point-by-point agreement was analyzed, as well as agreement in field size defined by the 50% isodose.
Results: The deviations of the calculated PDDs from measurement, analyzed from depth of maximum dose to 23cm, were all within 1.3% for all beam geometries. In particular, the calculated PDDs at 10cm depth were all within 0.7% of measurement. For profiles, the deviations within the central 80% of the field were within 2.2% for all geometries. The field sizes all agreed within 2mm.
Conclusion: The agreement of the PDDs and profiles calculated by Pinnacle3 for extended SSD geometries were within the acceptability criteria defined by Van Dyk (±2% for PDDs and ±3% for profiles). The accuracy of the calculation of more complex beam geometries at extended SSDs will be investigated to further assess the feasibility of using a standard beam model commissioned at 100cm SSD in Pinnacle3 for extended SSD treatments.
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