One of the most compelling studies to assess the impact of MLCs on dose distributions was performed by Bortfeld et al. . The authors show that the theoretically calculated optimal leaf width for a 6 MV photon beam is in the range of 1.5–2 mm. Of all the practical studies that have been conducted, there is utter agreement that by changing MLC widths from 10 mm to 5–3 mm the results are both statistically and clinically significant [12, 13, 17–21]. Dosimetric improvements reported by such studies, if applied to the SBRT process, may reduce chronic normal/critical structure injuries as the percentage volume of these structures receiving all ranges of dose is in effect reduced. Furthermore, for the PTV, increased maximum dose and improved dose conformity may benefit SBRT as an ablative process. Nevertheless, the quantitation of any advantage obtained by smaller leaf width MLC systems over the 5 mm leaf width MLC has remained controversial [13, 14, 16, 19, 20, 23].
In the present study, the potential clinical benefit of a novel 2.5 mm leaf width MLC system over a clinically available 5 mm leaf width MLC system was explored for different SBRT treatment planning techniques of lung and liver lesions. A variety of target dose parameters were considered, including mean, minimum and maximum PTV doses; conformity and heterogeneity indices; and normal tissue sparing. Wu et al. , in a similar study on a subset of five liver cancer patients, showed that the HD120 MLC system has no significant impact on Dmin, Dmax, or Dmean values relative to the M120 MLC system. These results were in agreement with findings in the current study. Nonetheless, unlike results in Additional file 1 of the current study, Wu et al.  reported significantly reduced Dmax values for the liver patient subgroup (p < 0.01) with use of IMRT and the HD120 MLC system, albeit small (<2%) compared with the M120 MLC system.
Regarding dose distribution conformity, results in Additional file 2 demonstrated an improvement in conformity index with target volume for all assessed planning techniques. The IMRT technique showed the best PTV coverage of either MLC system, except for large targets (defined in the current study as ITV ≥ 64 cm3). As indicated in Tables 2 and 3, in 51.7% of the IMRT cases, use of the HD120 MLC improved the conformality of the original plans by a mean value of 3.9% and up to a maximum value of 18.5%. In 62.1% and 55.2% of the 3DCRT and DCA cases, respectively, use of the HD120 MLC also resulted in improved PTV dose conformality. The mean and maximum improvements were 2.5% and 9.5% for the 3DCRT technique, and 2.4% and of 8.1% for the DCA technique, respectively. Nevertheless, the conformity index difference between the MLC systems is quite small, regardless of the treatment planning technique (see Additional file 2), attributable in part to the number of beams used for treatment planning.
Normal tissue sparing effect of the MLC systems was assessed, by considering normal tissue outside the PTV but in the dose volume space as a virtual structure. Similar to findings by Wu et al. , a reduction in normal tissue dose was observed with the HD120 MLC system, with at least 19 of the 29 cases per treatment planning technique having lower volumes exposed to the 90%, 50% and 25% dose levels. To be specific, at least 65.5%, 72.4%, and 75.9% cases per planning technique had lower normal tissue volumes exposed to the VHS, VIS, and VLS, respectively (see Table 2). The mean dose reduction attributable to the HD120 MLC was between 1 – 4% for the 3DCRT and DCA techniques, and between 2 – 6% for the IMRT technique. Thus, in terms of dose reduction, the IMRT plans were apparently better than either 3DCRT or DCA plans. However, the quantitative normal tissue volumes exposed to the 90%, 50% and 25% dose levels were smallest for the DCA technique, irrespective of MLC system.
Regarding treatment planning efficiency, while the 3DCRT and DCA techniques showed little difference in treatment monitor units between MLC systems, results in the current study indicated an increase in monitor units, albeit statistically insignificant, with the HD120 MLC system for the IMRT technique. This was attributable to an increase in the number of MLC segments needed to deliver the prescribed dose [12, 20].
On a final note, the current work is purely a treatment planning study on a single treatment planning platform with no dosimetric verification. The dosimetric differences reported here are believed to be solely due to the different leaf widths used in the treatment planning, since our comparisons were performed on the same treatment planning system for two treatment platforms with similar open-field beam characteristics, using the same beam configurations, optimization parameters (for IMRT), and dose constraints. Nevertheless, it should be pointed out that leaf-width is not the only parameter that is different between these MLC systems. Factors such as the leaf transmission and leakage (a function of leaf height, material constituent, and tongue-and-groove), source-to-MLC distance, are also different and affect dosimetric parameters. Therefore, the current planning study is not a simple comparison for different MLC leaf-widths, but rather a complex comparison of two dose delivery systems with different leaf-width MLCs .