Evaluation of a novel mask balloon immobilization system for reducing intra-fractional setup errors in spinal stereotactic body radiation therapy.

Background: Accurate immobilization is vital in spinal stereotactic body radiation therapy (SBRT) to minimize intra-fractional setup errors (IntraSE) and optimize therapeutic outcomes. Traditional methods, such as evacuated cushions, may lack sufficient stability, highlighting the need for improved systems. This study evaluates the accuracy and efficacy of a mask-balloon immobilization system, combining a body mask and a specialized balloon, for spinal SBRT.

Methods: Seventy-five patients undergoing spinal SBRT for thoracic or lumbar metastases were analyzed. Of these, 40 patients were immobilized using an evacuated cushion, while 35 used the mask balloon system. Cone-beam computed tomography (CBCT) scans were acquired three times during treatment, and the bony anatomy registration measured translational setup errors in anterior-posterior (AP), superior-inferior (SI), and right-left (RL) directions.

Result: For the evacuated cushion, the mean ± standard deviation of absolute IntraSE post-first arc was 0.4 ± 0.7 mm (AP), 0.5 ± 0.7 mm (SI), and 0.5 ± 0.6 mm (RL). For the mask-balloon system, the corresponding values were 0.2 ± 0.2 mm, 0.3 ± 0.3 mm, and 0.3 ± 0.3 mm. After treatment completion, the IntraSE values were 0.7 ± 0.9 mm, 0.8 ± 0.9 mm, and 0.9 ± 0.8 mm for the evacuated cushion and 0.4 ± 0.3 mm, 0.4 ± 0.4 mm, and 0.6 ± 0.4 mm for the mask-balloon system. In all three translational directions, the mask-balloon system had a significantly smaller IntraSE than the evacuated cushion (p < 0.0001).

Conclusion: The mask-balloon system improves setup accuracy and is a promising immobilization system for spinal SBRT.