A prospective observational study evaluating two patient immobilisation methods in lung stereotactic radiotherapy

Abstract

Purpose

The main objective of this study was to assess inter- and intrafraction errors for two patient immobilisation devices in the context of lung stereotactic body radiation therapy: a vacuum cushion and a simple arm support.

Materials and methods

Twenty patients who were treated with lung stereotactic body radiation therapy in supine position with arms above their head were included in the study. Ten patients were setup in a vacuum cushion (Bluebag™, Elekta) and ten other patients with a simple arm support (Posirest™, Civco). A pretreatment four-dimensional cone-beam computed tomography and a post-treatment three-dimensional cone-beam computed tomography were acquired to compare positioning and immobilisation accuracy. Based on a rigid registration with the planning computed tomography on the spine at the target level, translational and rotational errors were reported.

Results

The median number of fractions per treatment was 5 (range: 3–10). Mean interfraction errors based on 112 four-dimensional cone-beam computed tomographies were similar for both setups with deviations less than or equal to 1.3 mm in lateral and vertical direction and 1.2° in roll and yaw. For longitudinal translational errors, mean interfraction errors were 0.7 mm with vacuum cushion and –3.9 mm with arm support. Based on 111 three-dimensional cone-beam computed tomographies, mean lateral, longitudinal and vertical intrafraction errors were –0.1 mm, –0.2 mm and 0.0 mm respectively (SD: 1.0, 1.2 and 1.0 mm respectively) for the patients setup with vacuum cushion, and mean vertical, longitudinal and lateral intrafraction errors were –0.3 mm, –0.7 mm and 0.1 mm respectively (SD: 2.3, 1.8 and 1.4 mm respectively) for the patients setup with arm support. Intrafraction errors means were not statistically different between both positions but standard deviations were statistically larger with arm support.

Conclusion

The results of our study showed similar inter and intrafraction mean deviations between both positioning but a large variability in intrafraction observed with arm support suggested a more accurate immobilization with vacuum cushion.