Textural feature calculated from fluence maps as a modulation index

Texture analysis on fluence maps was performed to evaluate the degree of modulation for volumetric modulated arc therapy (VMAT) plans. A total of 6 textural features including angular second moment (ASM), inverse difference moment (IDM), contrast, variance, correlation and entropy were calculated for fluence maps generated from 20 prostate and 20 head and neck (H&N) VMAT plans. For each of the textural features, particular displacement distances (d) of 1, 5 and 10 were adopted. To investigate the deliverability of each VMAT plan, gamma passing rates of pre-treatment quality assurance (QA), and differences in modulating parameters such as multi-leaf collimator (MLC) positions, gantry angles, and MUs at each control point (CP) between VMAT plans and dynamic log files registered by the linac control system during delivery were acquired. Furthermore, differences between the original VMAT plan, and the plan reconstructed from the dynamic log files were also investigated. To test the performance of the textural features as indicators for the modulation degree of VMAT plans, Spearman¿s rank correlation coefficients (rs) with the plan deliverability were calculated. For were calculated.