Two immobilization techniques in pediatric total marrow and lymphoid irradiation with helical Tomotherapy: Impact on setup errors and PTV margins

Aiming to compare the interfraction setup errors and derived planning target volume (PTV) margins of two immobilization techniques for pediatric total marrow (lymphoid) irradiation (TMI/TMLI), this study evaluated a whole-body vacuum cushion (VC) versus a vacuum cushion combined with a thermoplastic mask system (VC-TM). In this retrospective study, 12 pediatric patients undergoing TMI/TMLI with helical tomotherapy were stratified into VC and VC-TM groups. Daily megavoltage CT (MVCT)-based setup errors in left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were analyzed. Planning target volume (PTV) margins were derived from the observed setup errors. In the VC group, the setup errors were 0.65 [0.30, 1.30] mm (LR), 4.60 ± 3.61 mm (SI), and 1.15 [0.75, 4.28] mm (AP). For the VC-TM group, the setup errors were 0.70 [0.40, 1.40] mm (LR), 2.08 ± 1.36 mm (SI), and 2.06 ± 1.33 mm (AP). With daily MVCT guidance, margins calculated from random errors alone were 2 mm (LR), 4 mm (SI), and 4 mm (AP) for VC group, and 2 mm (LR), 3 mm (SI), and 2 mm (AP) for VC-TM group. Compared with the VC group, the VC-TM group demonstrated superior setup accuracy both at the whole-body level and at local anatomic landmarks, particularly in the SI direction. Both immobilization techniques are feasible for pediatric TMI/TMLI. The VC-TM system provided more stable positioning, especially for the head and neck. Daily MVCT-guided online correction is recommended to utilize this stability and enable a substantial reduction in PTV margins.