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The aim of this study was to examine the feasibility of noncoplanar volumetric modulated arc stereotactic radiotherapy (VMAT-SRT) using a 5-mm multileaf collimator (MLC) for multiple brain metastases. We identified 34 multiple-target cases (3 to 19 targets in each case) with a total of 257 of targets and constructed noncoplanar VMAT-SRT plans using 5-mm and 2.5-mm MLCs with 4-arc. The prescribed dose was 36 Gy/6 fr. Plans were evaluated using the Paddick conformity indices (PCI), Paddick gradient index (PGI), and normal brain dose (NBD, equal to the mean brain dose minus gross tumor volume). There were no significant differences in PCI (median [range]: 5 mm, 0.88 [0.78 to 0.94]; 2.5 mm, 0.89 [0.78 to 0.94]; p= 0.691), PGI (median [range]: 5 mm, 3.96 [2.21 to 6.63]; 2.5 mm, 3.96 [2.24 to 6.45]; p= 0.358), or NBD (median [range]: 5 mm, 7.5 Gy [2.5 to 12.4]; 2.5 mm, 7.5 Gy [2.5 to 12.5]; p= 0.675). The performance of the 5-mm MLC was not inferior to the 2.5-mm MLC in applications of noncoplanar VMAT-SRT for multiple brain metastases with regards to dose conformity, gradient, and NBD. This study provides the necessary background for generalizing noncoplanar VMAT-SRT approaches in treating multiple brain lesions.

The aims are (1) to present a clinical brain case where the dose distribution calculated at 2.5-mm grid resolution was significantly different from that of 1.0 mm with Varian HD120 multileaf collimator (MLC; 2.5-mm central leaf width), and (2) to present our analysis results to better understand the influence of dose calculation grid resolution and its spatial alignment with MLC. A simulation using a static MLC pattern with 10 open strips was conducted to demonstrate the possibility that the characteristics of MLC dose profiles can be completely hidden. The effects of the spatial MLC alignment with the dose grid were investigated for grid resolutions from 1.0 mm to 5.0 mm. In the end, 20 recent stereotactic radiotherapy plans (5 brain, 5 lung, 5 spine, and 5 other) were retrospectively selected, and their dose distributions were recalculated at 1.0 mm, 1.5 mm, and 2.5 mm resolutions for comparisons. Film dosimetry results were used as references. At 2.5-mm grid resolution, the MLC open-strip patterns were not visible in the dose distribution when the MLC leaf interspaces were at exact alignment with the dose grid. The effects were smaller at 1.0-mm grid resolution. The dose error at the centers of the open strips varied significantly (7% to 39%) depending on the alignments. Overall, the planning target volume mean dose of 1.5-mm plans was higher than 1.0-mm plans by 1.3 ± 0.3% (mean ± 1SD) with a maximum of 2.6%, while the mean dose of 2.5-mm plans was lower by −0.5 ± 0.5% with a maximum mean dose of −3.7%. In conclusion, the choice of dose calculation grid resolution can affect the plan dose distribution significantly. The dose grid resolution must be smaller than the width of MLC leaf at a minimum. A finder resolution is expected to produce less uncertainty in the calculated dose and influenced less by the MLC-grid spatial alignment.

The Varian Halcyon linear accelerator is designed for faster treatment time and fewer treatment errors than conventional linear accelerators. Its increased gantry speed (4 rpm), dose rate (800 MU/min), and leaf speed (5 cm/s) reduce treatment time. The 2 multileaf collimator banks of 1 cm leaf width could cause planning difficulties, but utilizing many arcs could address this challenge. While 3 or more arcs are often avoided due to the increased treatment time, the Halcyon can deliver 5 arcs in under 3 minutes. For prostate and head-and-neck, we hypothesized plan quality would increase with increasing arcs without compromising treatment time. Fifteen prostate and fifteen head-and-neck previously treated patients were replanned to 78 and 66 Gy, respectively. A total of 150 volumetric modulated arc therapy (VMAT) plans, ranging from 1 to 5 arcs, were created to compare organ-at-risk (OAR) sparing, target coverage, homogeneity, and hotspot volume. Plans were normalized to the prescribed dose of 98% and 95% of the primary target volume for prostate and head-and-neck, respectively. All prostate 4-arc and 5-arc plans met MD Anderson Cancer Center (MDACC) constraints, while 86.7%, 20%, and 6.7% of the 1-arc, 2-arc, and 3-arc plans did not meet constraints, respectively. As the number of arcs increased, all OAR maximum doses decreased. All head-and-neck 5-arc plans met MDACC constraints, while 73.3%, 26.7%, 13.3%, and 6.7% of the 1-arc, 2-arc, 3-arc, and 4-arc plans did not meet constraints, respectively. The contralateral parotid mean dose and the spinal cord maximum dose decreased with increasing arcs. Homogeneity and conformity indices decreased when increasing from 1 to 5 arcs for both prostate and head-and-neck. Overall, homogeneity and conformity indices and OAR doses improved when using three or more arcs. Therefore, using at least 3 arcs for VMAT plans on the Halcyon will increase plan quality without compromising treatment time.

Partial-breast irradiation (PBI) aims to limit the target volume for radiotherapy in women with early breast cancer after partial mastectomy to the region at highest risk of local recurrence, the tumor bed. Multileaf collimators are used to achieve conformal radiation beam portals required for PBI. Narrower leaf widths are generally assumed to allow more conformal shaping of beam portals around irregularly shaped target volumes. The aim was to compare 5-mm and 10-mm leaf widths for patients previously treated using PBI and assess subsequent planning target volume (PTV) coverage and organ at risk (OAR) doses for 16 patients. Several plans (5-mm leaf width or 10-mm leaf width) were generated for each patient using the original treated plan as the basis for attempts at further optimization. Alternating between different leaf widths found no significant difference in terms of overall PTV coverage and OAR doses between treatment plans. Optimization of the original treated plan allowed a small decrease in ipsilateral breast dose, which was offset by a lower PTV minimum. No significant dosimetric difference was found to support an advantage of 5-mm over 10-mm leaf width in this setting.

We compared the impacts of multileaf collimator (MLC) widths (standard MLC width of 10 mm [SMLC] and micro-MLC width of 4 mm [MMLC]) on intensity-modulated radiotherapy (IMRT) planning for nasopharyngeal carcinoma (NPC). Ten patients with NPC were recruited in this study. In each patient's case, plans were generated with the same machine setup parameter and optimizing methods in a treatment planning system according to 2 commercial Elekta MLC devices. All of the parameters were collected from dose-volume histograms of paired plans and evaluated. The average conformity index (CI) and homogeneous index (HI) for the planning gross target volume in IMRT plans with MMLC were 0.790 ± 0.036 and 1.062 ± 0.011, respectively. Data in plans with SMLC were 0.754 ± 0.038 and 1.070 ± 0.010, respectively. The differences were statistically significant (p < 0.05). Compared with CI and HI for planning target volume in paired plans, data with MMLC obviously were better than those with SMLC (CI: 0.858 ± 0.026 vs. 0.850 ± 0.021, p < 0.05; and HI: 1.185 ± 0.011 vs. 1.195 ± 0.011, p < 0.05). However, there was no statistical significance between evaluated parameters (Dmean, Dmax, D5, gEUD, or NTCP) for organs at risk (OARs) in the 2 paired IMRT plans. According to these two kinds of Elekta MLC devices, IMRT plans with the MMLC have significant advantages in dose coverage for the targets, with more efficiency in treatment for NPC but fail to improve dose sparing of the OARs.

We studied the effect of MLC (multileaf collimator) leaf width and PTV (planning target volume) margin on treatment planning of intensity modulated stereotactic radiosurgery (IMSRS) or radiotherapy (IMSRT). Twelve patients previously treated with IMSRS/IMSRT were retrospectively planned with 5- and 3-mm MLC leaf widths and 3- and 2-mm PTV margins using the already contoured clinical target volume and critical structures. The same beam arrangement, planning parameters, and optimization method were used in each of the 4 plans for a given patient. Each plan was normalized so that the prescription dose covered at least 99% of the PTV. Plan indices--D(mean) (mean dose), conformity index (CI), V(70) (volume receiving >or= 70% of the prescription dose), and V(50) (volume receiving >or= 50% of the prescription dose)--were calculated from the dose-volume histograms (DVHs) of the PTV, normal tissue, and organs at risk (OARs). Hypothesis testing was performed on the mean ratios of plan indices to determine the statistical significance of the relative differences. The PTV was well covered for all plans, as no significant differences were observed for D(95), V(95), D(max), D(min), and D(mean) of the PTV. The irradiated volume was approximately 23% smaller when 2-mm instead of 3-mm PTV margin was used, but it was only reduced by approximately 6% when the MLC leaf width was reduced from 5 mm to 3 mm. For normal tissue and brainstem, V(70), V(50), and D(mean) were reduced more effectively by a decrease in MLC width, while D(mean) of optic nerve and chiasm were more sensitive to a change in PTV margin. The DVH statistics for the PTV and normal structures from the treatment plan with 5-mm MLC and 2-mm PTV margin were equal to those with 3-mm MLC and 3-mm PTV margin. PTV margin reduction is more effective in sparing the normal tissue and OARs than a reduction in MLC leaf width. For IMSRS, where highly accurate setup and small PTV margins are routinely employed, the use of 5-mm MLC is therefore less desirable.

We investigated the differences between 3-dimensional conformal radiotherapy (3DCRT) and intensity modulated radiotherapy (IMRT), and the impact of collimator leaf-width on IMRT plans for the treatment of nonspherical brain tumors. Eight patients treated by 3DCRT with Novalis were selected. We developed 3 IMRT plans with different multileaf collimators (Novalis m3, Varian MLC-120, and Varian MLC-80) with the same treatment margins, number of beams, and gantry positions as in the 3DCRT treatment plans. Treatment planning utilized the BrainLAB treatment planning system. For each patient, the dose constraints and optimization parameters remained identical for all plans. The heterogeneity index, the percentage target coverage, critical structures, and normal tissue volumes receiving 50% of the prescription dose were calculated to compare the dosimetric difference. Equivalent uniform dose (EUD) and tumor control probability (TCP) were also introduced to evaluate the radiobiological effect for different plans. We found that IMRT significantly improved the target dose homogeneity compared to the 3DCRT. However, IMRT showed the same radiobiological effect as 3DCRT. For the brain tumors adjacent to (or partially overlapping with) critical structures, IMRT dramatically spared the volume of the critical structures to be irradiated. In IMRT plans, the smaller collimator leaf width could reduce the volume of critical structures irradiated to the 50% level for those partially overlapping with the brain tumors. For relatively large and spherical brain tumors, the smaller collimator leaf widths give no significant benefit.