Dosimetric evaluation of simultaneous multi-energy (2-18 MV) and intensity optimization for IMRT and VMAT

Objective.This study aimed to develop and evaluate a framework for the simultaneous optimization of beam fluence and multiple photon energies beyond dual-energy configurations for intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT), with the goal of improving treatment plan quality and normal tissue sparing.Approach.An Elekta Versa HD linear accelerator (linac) was modeled using the BEAMnrc/EGSnrc Monte Carlo platform to simulate 2 MV, 4 MV, and 6 MV photon beams, complemented with measured beam data for 10 MV and 18 MV. A custom MATLAB-based framework was developed to perform simultaneous optimization of energy and fluence, allowing energy weighting of individual beamlets. A total of 12 various energy configurations including single (SE), dual (DE), triple (TE), and quadruple (QE)-energy combinations for IMRT and VMAT were generated for thirteen prostate cancer patients. Plans were evaluated using dose-volume histograms, homogeneity index, conformity index, and organ-at-risk (OAR) dose metrics.Main results.Gamma analysis confirmed the high degree of agreement between measured and simulated beam data, which validated our Monte Carlo linac model. Target coverage remained comparable across all energy configurations. Compared with 6 SE, multi-energy (ME) IMRT and VMAT achieved superior OAR sparing, with the greatest benefits for bladder and rectum in the 5-40 Gy range. TE-IMRT reduced bladder and rectal mean doses by up to 1.6 Gy, with V10 reductions of ∼8%; on average, mean doses were reduced by 0.9 Gy (p= 0.001) and 1.0 Gy (p= 0.001). For VMAT, the 6&18 DE-VMAT plan yielded the largest sparing, lowering bladder and rectal mean doses by ∼1.0 Gy. Relative to 10 SE, all ME plans achieved equal or lower OAR doses, with TE-IMRT and 6&18 DE-VMAT showing the most consistent benefit, reducing mean doses to the bladder, rectum, right femoral head, and left femoral head by up to 1.26, 0.77, 0.6, and 0.55 Gy, respectively.Significance.This study presents the first assessment of the feasibility and potential clinical value of simultaneous optimization of beam intensity and more than two photon energies for IMRT and VMAT plans, producing optimized ME fluence maps that require subsequent conversion into clinically deliverable treatment plans. The findings demonstrate that ME optimization can enhance OAR sparing without compromising target coverage, supporting its promise for future clinical application.