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Assessment of Field-Junction Dosimetry: Pinnacle V9.0 Versus Film (EBT2)

A Melancon*, M Gillin, R Tailor, UT MD Anderson Cancer Ctr., Houston, TX

SU-E-T-527 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To investigate dose agreement of Pinnacle V9.0 with measurements across the junction between abutting photon and electron fields.

Methods: Photon-photon, photon-electron, and electron-electron field junctions of energies 6 MV and 9 MeV were evaluated with 0, 5, and 10 mm gaps between the field edges at depths of 2, 15, and 30 mm with Gafchromic film (EBT2). Photon and electron fields (10X10 cm) were set-up to deliver 300 cGy in their field centers to 30 mm depth. Film optical density was measured every millimeter across the field-junction with a densitometer (light beam 2 mm). Optical Density to dose conversion was accomplished using a calibration curve specific to the batch of this film. For comparison, all field combinations were generated in Pinnacle, and doses were calculated with collapsed-cone convolution algorithm employing calculation-grid sizes 2, 3, and 4 mm.

Results: Measured doses at junction are as much as 20% different than Pinnacle calculation. 3 profiles with zero gap, 1 profile with 0.5 cm gap, and 0 profiles with 1.0 cm gap showed a disagreement greater than 12%. Measurement showed a bias towards higher dose in the junction when compared to Pinnacle calculation. With dose-calculation grid reduced below 4 mm, Pinnacle calculations showed improved agreements, but only a few percent at most. Pinnacle versus measured dose disagreements do not show clear trends with field separation or depth.

Conclusions: Doses calculated by Pinnacle V9.0 in the field-junction region may not be accurate for abutting photon and electron fields, particularly when a coarse dose-calculation grid is utilized. Lack of clear trends in dose-agreement with field separation and depth suggests that uncertainties in other factors such as jaw positions and phantom set-ups may be additional contributors. Significant inaccuracies in doses reported by the treatment-planning system in field-junction region should be considered while making clinical decisions.

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