Considering the large dose fractions and steep gradients used in SBRT, accurate and reproducible set up between fractions is essential, particularly in lung cancer where critical organs and structures are located in close proximity to the target. In the present study, we have demonstrated that thermoplastic masks offers better and easier reproducibility and present significantly less interfractional set up displacement than vacuum cushions. Moreover, radiotherapy technologists find the TMP system to be more “user friendly”. Taken together, these two findings suggest that TMP may be preferable to VCS.
Determining the optimal immobilization system is a complex task given the wide variety of systems currently in use, some of which are custom-made. However, with the growth of SBRT and other high-dose radiotherapy modalities, the importance of accurate and reproducible patient set and immobilization is more important than ever. This is especially true in lung cancer due to organ motion and the presence of critical structures located adjacent to the target volume.
Various groups have published the results of their experience in terms of local control and toxicity using commercially available immobilization systems for SBRT [4, 6, 8]. Treatment guidelines for SBRT, including the EORTC recommendations for planning and delivery of high-dose, high-precision radiotherapy for lung cancer , include general recommendations about immobilization systems. The EORTC recommendations state that such systems should be both reproducible and safe, which is especially important for SBRT. These same guidelines also state that some studies have found that no immobilization system is necessary. Given this ambiguity, until more definitive guidelines or randomized controlled trials are published, each centre needs to select an approach based on the best-available evidence and the characteristic (resources, staff preferences, patient type) of the centre.
Conventional or specific immobilization
In a cohort study of SBRT for pulmonary metastases, Siva et al. found that in addition to reducing tumour excursion and intrafraction error, the vacuum immobilisation facilitates reproducible positioning. However, an important limitation of that study was the small sample size (only 19 treatments) . In contrast, Nielsen et al.  compared a standard fixation system to a custom-made system. These authors concluded that systematic and random setup uncertainties were the same, regardless of the different fixation equipment used. As a result, they conclude that margins cannot be reduced by changing fixating equipment. For these authors, the imaging protocol is a more important factor.
Sonke et al.  published their experience in 65 patients with small peripheral lung lesions treated without body frame to 54Gy in three fractions. They used an imaging protocol involving three 4D-CBCTs. One image was taken prior to treatment, the second was performed after corrections, with the final one at the end of the treatment. They concluded than SBRT treatment can be safely administered without a specific immobilization device when a three 4D-CBCT imaging protocol is used. Similarly, Dahele and colleagues  concluded that rigid external immobilization devices are not necessary in most cases of patients undergoing lung SBRT.
These data suggest that the choice of a specific immobilization system may not be essential because the imaging protocol may actually be more important. However this question remains unresolved. Some SBRT guidelines, such as those developed by Task Group 101 , state that although an imaging protocol can reduce the need for a proper immobilization system, it cannot eliminate it. Moreover, a drawback of relying on imaging is that such protocols are resource intense and require increased machine time. In addition, because many centres may not possess a 4D-CBCT, immobilization systems are crucial for such centres.
Utility of abdominal compression in lung SBRT
Bouilhol et al.  performed a 4D-CT and dosimetric lobe-dependent study to determine the usefulness of abdominal compression in lung SBRT as a function of lobe tumor location. Those authors found that abdominal compression had the most significant impact on outcomes in patients with lower lobe tumors. In contrast, minor or negative effects were reported for patients with lesions located in other areas of the lung, and lung sparing was not substantially improved. Bengua and colleagues  reported a similar findings with regard to the benefits of abdominal compression in lower lung lesions. Finally, Richmond et al.  reported that abdominal compression led to a greater variation in set-up errors and changes in the mean value.
Based on these data, we can conclude that lesions located in the lower lobe are most likely to benefit the most from abdominal compression. At our institute, patients with upper or middle lobe lesions are now systematically treated without compression while the usefulness of compression for lower lobe tumors is considered on an individual basis.
Differences between immobilization systems
The William Beaumont group  published a study of intrafraction variation (IFV) of mean tumour position during image-guided hypofractionated SBRT for lung cancer. The authors found that prolonged delivery times during daily CBCT-guided lung SBRT led to higher IFV of the mean target position (MTP). Significant differences in IFV-MTP were seen between immobilization devices. The stereotactic frame immobilization device was found to be significantly less likely to have an IFV-MTP vector > 2 mm compared to the alpha cradle, BodyFIX, and hybrid immobilization devices. The results of that study suggest that each immobilization system should be tested to determine setup errors and IFV. Although our study did not evaluate IFV, immobilization systems should also be tested to assess IFV, and a study to evaluate this setting is ongoing in our center.
Differences in the Body-Fix and abdominal compression plate (ACP) have been reported by Han et al. . Those authors found no differences between the systems in IFV, but ACP was more comfortable, faster to set up, and presented lower superior-inferior shifts and less overall respiratory tumour motion than the Body-Fix.
Satisfaction of radiotherapy technologists with the various immobilization systems
To our knowledge, none of the studies carried out to date to compare immobilization systems in lung cancer has attempted to determine the level of satisfaction of technologists. We believe this is an important and overlooked aspect of patient set up. The work of the technologist is an essential part of achieving correct patient positioning and, thereby, a lower systematic error. However, we must stress that while satisfaction of the technologists is important in choosing an immobilization system, it is less important than the system’s ability to deliver reliable, reproducible, and accurate patient set-up. Nevertheless, we believe that technologists’ preferences could have an impact on set-up accuracy. In addition, the fact that technologists prefer the TMP system, which we have shown to be significantly more accurate than the VC system in terms of displacement, adds additional support to help in choosing one system over another. The technologists’ preferences may, in part, be related to the patients’ performance status. Most of the patients included in this study were performance status 1 (PS = 1), and, as Li et al reported , such patients are more likely to drift out of position during SBRT treatment than PS 0 patients. Moreover, such patients are also less able to cooperate fully with technicians when using vacuum cushions. Similarly, the technologists’ perceived easier learning curve for thermoplastic masks may be because the masks used at our centre are customized versions of the head and neck masks which the technologists are already familiar with. As a result, the learning curve for the VC system was steeper because it was a new method.
Although we did not evaluate patient satisfaction and preferences in this study, this is obviously another important aspect to consider since patient comfort is important to reduce unwanted movement. This aspect will need to be evaluated in any future studies.
An important limitation of this study is that we evaluated only interfractional displacement, but not interfractional shifts. Another limitation is that we primarily included lung SBRT. There may be important setup differences in different tumour locations, but we did not assess these as nearly all patients treated with SBRT at our centre are lung cancer patients. Finally, the sample size was relatively small (73 treatment fractions), and a larger sample would have provided more robust findings. The same holds true for the satisfaction survey, which involved only 6 technologists, thus making it difficult to reach any definitive conclusions. However this is the first paper to consider the opinion of technologists and for that reason this information is valuable. In addition, we did not survey the patients, even though this would have added valuable information to the study. Based on our experience with the present study, we now routinely ask for and record patient comfort levels.