Modern linac-based SRS technology is characterised by tremendous flexibility in treatment options. Treatments can be administered by means of circular or multileaf collimator-based forward planning strategies or multileaf collimator-based inverse planning methods, with patients immobilised by frame-based or frameless techniques employing image guidance methodologies [20
]. At present, several treatment planning techniques are available for linac-based SRS, but in an individual case the best choice for one or other of these techniques is not always obvious, in spite of several planning studies that have been published [21
At our institution, the default SRS delivery technique uses the 2.5 mm leaf width Varian/BrainLab high-definition MLC system [14
] to conform to the beams-eye view of the target with the shape changing every arc degree throughout the treatment. Although this results in a highly conformal treatment, location in close proximity of OAR(s) may preclude some tumours from being treated safely with the DCA technique. In such circumstances, including the treatment of irregularly shaped targets, IMRS may provide a superior option [27
], because, being an inverse planning technique, constraints can be set to modulate the intensity of the beam accordingly. Despite this, like many other clinics, our institution is also equipped with the 5 mm leaf width Millennium MLC system, giving us the flexibility to deliver treatment plans originally designed with a 2.5 mm MLC on the 5 mm MLC platform in the event of equipment failure. It is therefore equally important to characterise intracranial tumours into groups that will or will not benefit from being treated by either collimation system.
In the current work, we focused on the dosimetric differences between the 2.5 mm HD-MLC and the 5 mm Millennium MLC systems for the generation of DCA, 3DCRT and IMRS plans. The dosimetric changes from the 2.5 to the 5 mm MLC system were quantified in terms of differences in DVH parameters, target volume conformation, normal tissue avoidance and dose fall-off for patients treated with either of these techniques, categorised in different target volume groups.
The results demonstrated a trend between target conformation, expressed as a conformity index, and target volume, a pattern most exhibited by the DCA technique ( and ). For target volumes defined as small (i.e.
in the current study), conformity index difference between the MLC systems was relatively large, and relatively small for target volumes defined as large (>5 cm3
), favouring the 2.5-mm MLC system with or without collimation optimisation (). Furthermore, the conformity index difference between the MLC systems was smaller for IMRS than for 3DCRT and DCA techniques for three reasons. First, target dose conformation in IMRS is partially contributed by beam modulation around the target boundary. The flexibility of beam modulation in any one dimension (direction of leaf motion, for example) is the same for both MLC systems. Second, highly modulated beams are required to spare the OARs, which could put high dose in areas of normal tissue outside the target and, hence, could significantly influence the conformity index. Because the 2.5 mm MLC may in general have more flexibility to block OARs, hence lending higher doses to areas of normal tissue outside the target, this might not be as significant an issue in the current study since OAR low optimisation as well as a large number of beams were used for IMRS planning. Finally, as presented in the results section, target volumes associated with the IMRS technique were generally relatively larger than those of 3DCRT and DCA techniques [6
In terms of DVH parameters including minimum, mean and maximum doses, the differences between the MLC systems were consistently statistically significant, except for target volumes >5 cm3
(). Nonetheless, it was noticed that collimator optimisation reversed which of the two MLC systems resulted in lower minimum and mean dose values. Furthermore, the minimum dose for the 5 mm MLC system was higher for IMRS than for 3DCRT/DCA in the absence of collimator optimisation (). This could be attributable to variation in the MLC margins (0–2 mm) set around PTVs to account for penumbra [6
]. A more systematic study of the implications of MLC margins on DVH parameters would be needed to validate this assertion. Despite this, absolute differences between the MLC systems were quite small, more so with use of collimation optimisation, attributable in part to uniform target volume coverage as a result of the large number of beams or arc degrees used per treatment plan.
The creation of PRVs for different levels of doses allowed for the quantification of normal tissue sparing. This concept of PRV was adapted from works by Lee et al [30
] and Chern et al [31
]. Unlike the study by Wu et al [11
], in which the authors specifically measured the dose to the brainstem, the PRV was used in the current study to provide a more general framework to evaluate the effect of leaf width on the tissues immediately adjacent to the target, especially since there is little or no consistency in relative PTV critical structure proximity among the cases considered in the current study. Although the clinical importance of the differences between the MLC systems with regards to the radiation tolerance of OARs may be difficult to assess, the 2.5 mm width MLC system demonstrates an advantage in terms of normal tissue avoidance, as confirmed by the mean difference in the volume of normal structure encompassed by the 25%, 50% and 100% isodose levels. Specifically, the overall median differences between the MLC systems were 22.4%, 29.7% and 14.9% with fixed collimation, and 13.4%, 14.3% and 13.4% with optimised collimation (). This difference is attributable, in part, to improvements in the design of the 2.5 mm MLC with reduced inter- and intraleaf leakage and smaller penumbra [22
], although the latter will have less impact on a 3D dose distribution for a multiple beam arrangement when the contributions from the other beams are considered. Nonetheless, this difference in penumbra is expected to principally affect the volume of normal tissue encompassed by lower value isodoses, as is corroborated by results in and . Using the gradient function by Meeks et al [17
] and Wagner et al [18
], it was noted that there was a minimum mean gradient improvement of approximately 6%, corresponding to an approximate 0.4 mm change in gradient, with the 2.5 mm MLC system when collimation was optimised. With a fixed collimation angle, the gradient improved by a minimum mean value of approximately 22%, corresponding to an approximate 1.3 mm change in gradient.
The comparison presented in the current work is purely a computer-based treatment planning study on a single radiotherapy planning platform for two radiotherapy dose delivery systems with no attempt to investigate the isodose distributions delivered in practice by the two systems. 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, optimisation parameters (for IMRS) 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) and source-to-MLC distance are also different and affect dosimetric parameters. Therefore, it is worth noting that 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 [6
]. Finally, the perceived differences in the current study do not address set-up uncertainty and intrafraction motion, which, if not adequately accounted for, will lead to discrepancies between calculated and actually delivered dose.