Dose distribution alignment method based on rigid registration

Abstract

Gamma (γ) analysis is the most important method for dose verification and dose distribution comparison. Because of the inconsistency of the coordinate system definitions and directions of different dose distributions and the inconsistent reference point settings, in γ analysis, the dose distributions should often be aligned for comparison. However, the alignment method of the current commonly used γ analysis software is extremely simple, and there are significant errors in processing the dose distribution of the large radiation field, which seriously affect the accuracy of the dose analysis and comparison. Aligning the two-dimensional dose distribution requires performing spatial transformations such as translation and rotation to achieve the same spatial position of the corresponding dose points on the two dose maps. Because the distance between different points must be constant during this alignment process, in this study, the alignment problem is transformed into a rigid registration problem. A rigid registration method based on normalized cross-correlation is used to register the dose distribution map calculated using a treatment planning system with the actually measured dose distribution map and output the corresponding translation amount after registration. 36 groups of static intensity-modulated quality assurance data designed using the pinnacle3 treatment planning system, whose planning system is consistent with the measured center point, are selected; 18 groups are small irradiation fields, and the other 18 groups are large irradiation fields. PTW VeriSoft software is used to calculate the γ passing rate under 3%/3 mm, 3%/2 mm, and 2%/2 mm pass standards separately. For small irradiation fields, the results of the direct calculation of different standard γ passing rates after alignment with VeriSoft and rigid registration are identical. Among them, under the 3%/3 mm pass standard, the results of VeriSoft alignment and rigid registration are compared with those directly calculated， the average differences in γ passing rate are 0.5% and 0.3%, respectively. For the large irradiation field, compared with the directly calculated γ passing rate results, the average differences of the γ passing rate under 3%/3 mm, 3%/2 mm, and 2%/2 mm pass standards after VeriSoft alignment are 17.1%, 23.3%, and 28.3%, respectively, with ten groups of cases all exhibiting great differences. The average differences of the γ passing rate under the 3%/3 mm, 3%/2 mm, and 2%/2 mm standards after rigid registration and direct calculation are 0.4%, 1.1%, and 2.3%, respectively, and the average difference is small. Rigid registration solves the problem of significant errors in VeriSoft’s handling of large-field dose distributions. It can be used as a method for the alignment of dose distributions in different fields when verifying the dose of radiation treatment plans.

Keywords

• gamma analysis
• normalized cross-correlation
• verisoft
• rigid registration
• dose distribution comparison