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Validation of a Simple Portal Dose Calculation Model for Plan QA and In-Vivo Dosimetry


W Lu

W Lu*, M Chen, X Mo, D Parnell, G Olivera, D Galmarini, 21 Century Oncology Inc., Madison, WI

MO-D-108-4 Monday 2:00PM - 3:50PM Room: 108

Purpose: To validate a simple portal dose calculator for plan QA and in-vivo dosimetry.
Methods: We model portal dose as a function of the fluence map, patient attenuation, patient scatter and portal response. Fluence maps are reconstructed using control-point sequence in RTPlan. Patient attenuation is calculated via ray-tracing through the patient CT. The effect of patient scatter and portal response is modeled by convolution, where the convolution kernel is derived from the commissioning measurements of different beam energies, different field sizes, different phantom thickness, and different source to image distances (SIDs).
For various IMRT/3D plan, phantom and patient geometry, both in-air and in-transit portals were calculated. The calculations were compared with portal measurements. The Gamma Index of measurements against predicted portals with various dose difference (DD) criteria (1%, 2%, 3%, 4%, 5%, etc) and distance to agreement (DTA) criteria (1 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc) were calculated. The Gamma pass rates of various DD and DTA criteria were evaluated and formed a Gamma table.
Results: For various IMRT beams, the head, body and lung phantoms, the in-air and in-transit portal calculations matched well with portal measurements. The Gamma pass rates for in-air portal are above 97% for 2 mm, 2% criteria and above 99% for 3 mm, 3% criteria. The Gamma pass rates for in-transit portal were above 90% for 2 mm, 2% criteria and above 95% for 3 mm, 3% criteria.
Conclusion: The simple portal dose calculation model is validated via phantom measurements. The model could be used in clinic for in-air and in-transit portal prediction.


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