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Three‐dimensional portal image‐based dose reconstruction in a virtual phantom for rapid evaluation of IMRT plans
Author(s) -
Ansbacher W.
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2241997
Subject(s) - imaging phantom , isocenter , image guided radiation therapy , dosimetry , medical imaging , iterative reconstruction , nuclear medicine , computer science , mathematics , physics , optics , artificial intelligence , medicine
A new method for rapid evaluation of intensity modulated radiation therapy (IMRT) plans has been developed, using portal images for reconstruction of the dose delivered to a virtual three‐dimensional (3D) phantom. This technique can replace an array of less complete but more time‐consuming measurements. A reference dose calculation is first created by transferring an IMRT plan to a cylindrical phantom, retaining the treatment gantry angles. The isocenter of the fields is placed on or near the phantom axis. This geometry preserves the relative locations of high and low dose regions and has the required symmetry for the dose reconstruction. An electronic portal image (EPI) is acquired for each field, representing the dose in the midplane of a virtual phantom. The image is convolved with a kernel to correct for the lack of scatter, replicating the effect of the cylindrical phantom surrounding the dose plane. This avoids the need to calculate fluence. Images are calibrated to a reference field that delivers a known dose to the isocenter of this phantom. The 3D dose matrix is reconstructed by attenuation and divergence corrections and summed to create a dose matrix (PI‐dose) on the same grid spacing as the reference calculation. Comparison of the two distributions is performed with a gradient‐weighted 3D dose difference based on dose and position tolerances. Because of its inherent simplicity, the technique is optimally suited for detecting clinically significant variances from a planned dose distribution, rather than for use in the validation of IMRT algorithms. An analysis of differences between PI‐dose and calculation,δ PI , compared to differences between conventional quality assurance (QA) and calculation,δ CQ , was performed retrospectively for 20 clinical IMRT cases. PI‐dose differences at the isocenter were in good agreement with ionization chamber differences (meanδ PI = − 0.8 % , standard deviation σ = 1.5 % , againstδ CQ = 0.3 % , σ = 1.0 % , respectively). PI‐dose plane differences had significantly less variance than film plane differences ( σ = 1.1 and 2.1%, respectively). Twenty‐two further cases were evaluated using 3D EPI‐dosimetry alone. The mean difference δ over volumes with doses above 80% of the isocenter value was δ = − 0.3 % ,σ δ = 0.7 % , and standard deviations of the distributions ranged from 1.0 to 2.0%. Verification time per plan, from initial calculation, delivery, dose reconstruction to evaluation, takes less than 1.5 h and is more than four times faster than conventional QA.