Optimization of MLC parameters for TPS calculation and dosimetric verification: application to single isocenter radiosurgery of multiple brain lesions using VMAT

Abstract

Linac and MLC-based stereotactic radiosurgery (SRS) using single-isocenter-multiple-target (SIMT) VMAT has become increasingly popular in the management of multi-focal cranial metastases. However, significant geometrical and dosimetric challenges exist due to the typically small target volumes and in most cases, non-isocentric locations. To the best of our knowledge, there hasn't been a study in the optimization of MLC parameters, in the context of SIMT SRS, to ensure TPS calculation accuracy. In this work, we set out to optimize the dosimetric leaf gap (DLG) for the HD MLC installed on dedicated stereotactic Varian STx systems using a diverse group of 21 clinical SRS and SBRT plans. These plans featured a broad range of target sizes and target-to-isocenter distances that are typical of the stereotactic cases treated on these systems. Dose discrepancies between TPS calculations and verification measurements using a previously validated diode array Delta 4 (ScandiDos) were minimized in a balanced manner to accommodate the variety of stereotactic plans. A DLG of 0.6 mm was found to be 'optimal' for the HD MLC and for the 'typical' plans treated on our STx systems. The finding was independently verified using commercially available 3D polymer gel dosimeter CrystalBall TM (MGS Research Inc.). 3D verification for 6 SIMT SRS plans, consisted of 5 to 15 targets, achieved an average gamma score of 97.3% (σ=2.0%) on 3%/2 mm criteria with a cutoff isodose level of 20%. We further examined the practice of routine dosimetric verifications including the selection of appropriate detectors and optimal gamma parameters. We found that the commonly used standard 3%/3 mm criteria would have resulted in all but 4 (out of 2840) clinical plans achieving a gamma score of 95% or better, and therefore, losing sensitivity to detect potential dosimetric discrepancies. Based on the characteristics of stereotactic plans, a more stringent distance-toagreement parameter is needed.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non‐equilibrium conditions

“…It is also observed that there is a strong dependence on the PTV size with the largest differences for higher DLG. 278 It follows that validation of the TPS interpretation of the collimator geometry is pertinent 51 but may not be sufficient due to the approximations. If several layers of collimators are aligned, the resulting source occlusion will be sensitive also to alignment errors.…”

“…A workaround for such TPSs has been suggested 277 through use of values optimized to minimize deviation over ranges of field sizes rather than use the directly measured dosimetric gap. The measured DLG for IMRT is unidirectional (either left to right or right to left), whereas it is bidirectional for VMAT which imposes challenges in beam modeling 278,279 . More accurate modeling can be achieved by combining explicit leaf tip projection corrections with a modulated transition region which may be beneficial for clinical use 280 .…”

Hydrogels for Three-Dimensional Ionizing-Radiation Dosimetry

“…Various studies have analyzed the MRI response of polymer gels for possible application in radiotherapy dose mapping ( [58,63] and references therein). In particular, polymer gels have been investigated for dosimetry in conformal therapy, IMRT, IMAT, IGRT, VMAT and tomotherapy [55,70,83,88,102,, small photon fields [136][137][138], stereotactic radiosurgery [55,[139][140][141][142], 60-cobalt source [143,144], brachytherapy [55,85,92,[145][146][147][148][149], cyberknife [150], gamma-knife [151], low-energy X-rays [152,153], high-LET and proton therapy [55,154,155], boron capture neutron therapy [55,[156][157][158][159], for interface dosimetry in the presence of tissue inhomogeneities [55,160,161], for X-ray tubes [162] and for diagnostic nuclear medicine [163]. Recently, polyacrilamide gels were proposed for dosimetry and end-to-end test for MRI-linac apparatus [164]…”

“…Various studies have analyzed the MRI response of polymer gels for possible application in radiotherapy dose mapping ([ 58 , 63 ] and references therein). In particular, polymer gels have been investigated for dosimetry in conformal therapy, IMRT, IMAT, IGRT, VMAT and tomotherapy [ 55 , 70 , 83 , 88 , 102 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 , 132 , 133 , 134 , 135 ], small photon fields [ 136 , 137 , 138 ], stereotactic radiosurgery [ 55 , 139 , 140 , 141 , 142 ], 60-cobalt source [ 143 , 144 ], brachytherapy [ 55 , 85 , 92 , 145 , ...…”

Routine pretreatment patient‐specific IMRT QA (PS‐IMRT‐QA) should be discontinued and replaced with a real‐time on‐board beam monitoring system (BMS)

“…One would think that after a decade of research articles, AAPM review lectures and debates PS-IMRT-QA, the first part of the proposition does not need any further evidence. [1][2][3][4][5][6][7][8][9] Detection of major errors by PS-IMRT-QA is extremely rare and minor errors typically measured by PS-IMRT-QA are not easily interpretable without additional lengthy studies. Moreover, the PS-IMRT-QA process is very inefficient and costly.…”