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Planar Dosimetry as An Independent Verification of 3D Plans with MLC-Shaped Static Fields: Methodology and Application

C Kim1*, Y Na2, (1) Icahn School of Medicine at Mount Sinai, New York, NY, (2) Mount Sinai Downtown, New York, NY

Presentations

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

Room: AAPM ePoster Library

Purpose: To use portal dosimetry as an independent plan verification of 3D plans generated with static MLC using the sub-field (control point) and fields merging approach.

Methods: For a MLC-shaped static field, a sub-field was generated with a weighting of 0.1% using a minor change of MLC position outside of field-shaping MLCs. Fields merge was followed to generate additional control point, which enabled the creation of a verification plan for portal dosimetry. Twenty-seven 3D fields, 13 field-in-field (FinF) and 14 statics, were tested in this study. Fields were delivered using a perpendicular field-by-field (planned gantry) technique. For gamma analysis, the following parameter set was used: 1% dose threshold to remove background noise with varying doses and DTA to get a gamma score ˜ 95%. In comparison, RadCalc (Ver 6.2, LifeLine Software, Inc.) was used for point-dose calculation.

Results: For the static MLC fields, the dose difference (%) using portal dosimetry was 1.9 ± 0.44 with its corresponding value in RadCalc of 1.3 ± 1.00. For 13 FinF, it was 2.5 ± 0.33 and 3.4 ± 1.88 for Portal dosimetry and RadCalc, respectively. There was a large dose variation found in RadCalc and for two FinF cases, even though the dose difference was less than 2.5% in portal dosimetry, its corresponding value was greater than 5% using RadCalc. It happened when the calculation point was located in a thin chest wall, lacking full scattering.

Conclusion: For conventional 3D plans, a simple dose calculation algorithm has been used to perform an independent monitor unit (MU) or dose verification. A portal dosimetry can be used to include a full dosimetric information of the treating field with help of generating a subfield & merge approach. This study demonstrates that Portal Dosimetry is a viable independent plan verification tool even for MLC-shaped 3D plans.

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