Analysis of the data resulting from this multi-institutional collaboration on the clinical introduction of VMAT for HNC shows that VMAT plans including a double arc generated with Pinnacle3 or Oncentra Masterplan have an improved plan quality compared to IMRT plans with 5–9 beam ports. This observation is valid independently of institution-specific planning strategies, choice of the set of objectives, or preferences for sparing OARs. Based on the effective delivery times reported from four institutes we expect a reduction in the effective delivery times of ~50% with VMAT compared to IMRT. The decreased treatment delivery time obtained with VMAT will improve patient comfort and result in a smaller impact of intra-fraction movements, as described by Hoogeman et al. .
Several single-institution studies comparing VMAT and IMRT for HNC have been reported in literature, most based on RapidArc [9, 10] and on SmartArc [11–15]. The strength of the study presented here bases on its multi-institutional setup and the rather limited regulations regarding the planning environment and procedures, resulting in varying solutions for the same set of patients. All plans were generated using planning environment and procedures that were (or will shortly be) adopted for routine clinical use. As such, the data presented in this study reflects a broad range of clinically achievable and acceptable results and, therefore, allows a planner-independent evaluation of the potential of VMAT vs. IMRT for irradiation of HNC.
IMRT has been the standard for radiation therapy of advanced HNC in all participating institutes for several years by now, and consequently, all institutes had a similar level of experience in IMRT treatment planning when starting this study. All participating institutes performed IMRT and VMAT treatment planning using the same CT data sets including contouring, ensuring excellent comparability of the data between institutes. The TPSs employed in this study, Pinnacle3 and Oncentra, both make use of nearly identical VMAT and IMRT optimization modules developed by RaySearch Laboratories. Therefore, the data retrieved from these two TPSs was pooled and analyzed as one population.
Pair-wise analysis of VMAT and IMRT plans within the same patient and institute showed for almost all plan pairs better sparing of OARs and dose conformity with VMAT (please note that statistical power is low in the stratified comparison). A major reason for this could be the rotational character of VMAT which allows dose delivery from many more directions than with static-beam IMRT with mostly 5–9 beam ports used. With VMAT optimization dose is automatically redistributed along the arc, which means that to a certain extent beam angle optimization is inherent to VMAT. The additional degrees of freedom with VMAT lead to the better dose conformity, which in turn allows for a better sparing of OARs in close proximity to the PTVs.
Further exploration of the underlying dosimetric data yielded that one institute chose to completely sacrifice one of the parotid glands with IMRT for some patients, but was able to spare it quite successfully with VMAT. Already in 1996, Eisbruch et al. reported on a 3D-CRT technique to spare the contralateral parotid gland, while deliberately accepting underdosage in the surrounding target volume with supposedly “lower” risk to contain disease . With the introduction of IMRT, this technique was refined and the contralateral gland could be spared without hazarding underdosage in the target volume; however, often at the price of still sacrificing the ipsilateral gland . It seems that the paradigm “sacrificing one parotid gland to achieve better sparing of the contralateral gland” often applied with advanced HNC can be revisited with the advent of VMAT.
From Table 4 we can see that some institutes have only a few dosimetric parameters with significant statistical difference, whereas others have the majority of parameters showing significant differences between VMAT and IMRT. A reason for this may be that only limited efforts were made to homogenize the IMRT techniques among institutes before starting the study. The dosimetric gain of VMAT compared to each of the individual IMRT techniques may therefore differ depending on how complex/well-designed an IMRT technique of a specific institute was compared to those of other institutes. Keeping these “heterogeneous” sources in mind, the conclusions of the presented study with VMAT leading to better dosimetric results are even more striking.
Limitations of the here presented study include the small sample size in the stratified comparisons, heterogeneity in TPS and equipment of the participating institutes, and limitation to a specific IMRT and VMAT implementation combined with specific linear accelerator equipment. For the comparisons stratified by institutes or patients the sample size of only five institutes and five patients is small, resulting in a low statistical power in the stratified comparisons of VMAT and IMRT. However, the pooled data with n = 25 allows drawing firm conclusions since for each pairing we can assume statistical independency as different treatment planners are involved. Regarding the heterogeneity of equipment, the two different TPSs employed in this study may yield slightly different results for both IMRT and VMAT plans, although the optimization modules in both TPSs are of the same evolutionary origin. Regarding the different MLCs used for treatment planning, the newer generation beam modulator with narrower leaves of 4 mm width also allowing interdigitation may lead to better dosimetric results for both IMRT and VMAT, especially regarding dose conformity, due to increased degrees of freedom in leave motion. However, any differences in boundary conditions will be balanced since only pairs of IMRT and VMAT plans achieved under the same conditions are compared. The superior results for VMAT obtained in this study may be valid only for the specific combination of step-and-shoot IMRT and SmartArc/Oncentra VMAT delivered on Elekta linear accelerators equipped with the specified MLCs, and a similar comparison with sliding-window IMRT could lead to different results.
The data presented in this planning study comparing VMAT and static-beam IMRT for HNC resulted from a collaboration of five institutes in the Netherlands with similar equipment, aiming at a safe and fast clinical introduction of VMAT for HNC. Discussion of the planning results and exchange of ideas and information regarding VMAT treatment planning parameters and objectives between participating institutes during the collaboration resulted in noticeable improvement of the VMAT plans. We would like to stress that collaboration between institutes with similar equipment and treatment planning software on the clinical introduction of a new treatment modality can help to efficiently steepen the learning curve and to achieve a high quality of treatment planning within a short time. Problems and questions arising can be solved during the discussions within the group, and as a result all institutes require less time and effort for the clinical introduction of the advanced treatment modality VMAT for HNC.