Impact of MLC leaf width on volumetric‐modulated arc therapy planning for head and neck cancers

Abstract

This dosimetric study investigated the impact of multileaf collimators (MLC) leaf width in volumetric‐modulated arc therapy (VMAT) for head and neck cancers (HNC), either with a “standard” simultaneously integrated boost technique (S‐SIB) or with a “dose painting” SIB technique (DP‐SIB). HNC patients were planned either with an S‐SIB comprising three dose levels, from 56 to 70 Gy (16 patients), or with a DP‐SIB comprising five dose levels, from 56 to 84 Gy (8 patients), in 35 fractions. Two VMAT plans were calculated for each SIB technique using two Elekta MLCs: MLCi2 with 10 mm leaf width and Beam Modulator (BM) with 4 mm leaf width. Dose distributions were evaluated by comparing doses on PTVs, main OARs, and healthy tissue, and by comparing conformation indexes. Treatment efficiencies were evaluated by comparing the number of monitor units and the number of needed arcs. Comparisons of the two MLCs depending on the two SIB techniques showed: i) Regarding PTVs: Dmean and D2% on lower doses PTV decreased respectively by 0.5 Gy false(p=0.01false) and 0.9 Gy false(p=0.01false) with BM than with MLCi2 for S‐SIB; no significant difference was found for DP‐SIB; ii) Regarding OARs: for spinal cord and brainstem, D2% decreased respectively by 1.2 Gy false(p=0.03false) and 4.2 Gy false(p=0.04false) with BM than with MLCi2 for S‐SIB; for controlateral parotid, D50% decreased by 1.5 Gy false(p=0.01false) with BM than with MLCi2 for S‐SIB; iii) Regarding treatment efficiency : the number of monitor units was 44% false(normalp=0.00false) and 51% false(p=0.01false) higher with BM for S‐SIB and DP‐SIB, respectively. Two arcs were more frequently needed with BM to reach an acceptable dose distribution. This study demonstrated that Beam Modulator (4 mm leaf width) and MLCi2 (10 mm leaf width) MLCs from Elekta provided satisfactory dose distributions for treatment delivery with VMAT technique for complex HNC cases with standard and dose painting prescriptions. OAR sparing was better with BM, mainly for brainstem and spinal cord. However, delivery efficiency of VMAT plans was better with MLCi2.PACS numbers: 87.56.N‐, 87.56.nk, 87.55.D‐

Beam complexity and monitor unit efficiency comparison in two different volumetric modulated arc therapy delivery using automated planning

“…Intuitively, a finer leaf width should result in more conformal target shaping. Lafond et al [22] showed that Beam Modulator (4mm leaf width) and MLCi2 (10 mm leaf width) MLCs from Elekta provided satisfactory dose distributions for head and neck cancer VMAT. OAR sparing was better for brainstem and spinal cord in Beam Modulator.…”

Quality of tri-Co-60 MR-IGRT treatment plans in comparison with VMAT treatment plans for spine SABR

“…In addition, the large leaf width of the ViewRay system (1-cm leaf width at 100-cm SSD) likely contributed to the worse reduced target conformity and lower dose gradients near the target volume in the tri-Co-60 IMRT plans than that in the VMAT plans with TrueBeam STx (0.25-cm inner leaf width at 100-cm SSD), as several studies have already shown decreased target conformity when using MLCs with large leaf widths. [21][22][23] Reviewing the results on normal tissue irradiation, the VMAT plans were also superior to the tri-Co-60 IMRT plans because of the lower penetrating power of the photon beams of the ViewRay system in comparison with a linear accelerator. Previous studies have demonstrated that low-energy photon beams increased body integral doses owing to the low penetrating power of the photon beams when treating thicker body parts.…”

“…4 As shown in the results, the beam-on time of the ViewRay system was much longer than that of VMAT because the delivery technique of the ViewRay system is static IMRT, which takes much longer than VMAT. 25,26 The long treatment time was also attributed to the low dose rate of the ViewRay system in comparison with the high dose rate of FFF photon beams (550 vs 2400 cGy min 21 at the depth of dose maximum). 27 Thus, the increased treatment time further indicated that the ViewRay system is inappropriate for spine SABR.…”

Is High Definition MLC Dosimetrically Superior to Standard Definition MLC for SIB-SBRT for Carcinoma Prostate

“…Quite a few studies have investigated the relationship between MLC leaf width and the quality of IMRT plans and have shown that finer leaf widths can result in more conformal dose distributions for target volumes and less dose delivered to normal tissue when the target volumes were small (Rodal et al, 2010;Shang et al, 2013;Hong et al, 2014). Similarly, few studies have shown the dosimetric advantage of using MLCs with narrow leaf width (Lafond et al, 2013;Park et al, 2016). However, there are very little data on the comparison of the dosimetric effect of narrow width MLC on the quality of IMRT and VMAT plans with simultaneous integrated boost techniques in our study setting.…”