Abstract:

Objective To investigate the impact of immobilization devices and treatment couches on the planned dose in stereotactic body radiation therapy （SBRT）. Methods A retrospective study was conducted involving 23 SBRT patients， all of whom underwent CT simulation with foam padding or vacuum bag immobilization. For each patient， two sets of contours were outlined on CT images： one encompassing only the patient′s skin （Body）， and the other including the skin plus immobilization devices （Body_F）. Initially， a reference plan（noFC） meeting clinical requirements was generated based on the Body contour. Without altering the plan（noFC） parameters and field setups， plan calculations were performed separately based on three different contours： Body_F （with immobilization devices only）， Body + C （with treatment couch only）， and Body_F + C （with both immobilization devices and treatment couch）， yielding plan（F）， plan（C）， and plan（FC）， respectively. By comparing the target and skin dose parameters across these four plans， the effects of immobilization devices and treatment couches on the planned dose were evaluated. Results Compared to plans based solely on the patient′s skin contour， plans incorporating immobilization devices showed reduced high?dose， prescription dose coverage， and average dose in the target volume. Notably， the difference in the percentage of the planning target volume （PTV） receiving 105% of the prescribed dose （PTV/V105%p（%）） between plan（FC） and plan（noFC） could reach 61.86%. Conversely， plans with immobilization devices increased both the maximum and average skin doses. Specifically， the dose to 10 cc of skin within 2 mm of the surface （body 2 mm/D 10 cc（Gy）） showed a 21.36% difference between plan（FC） and plan（noFC）. For all target and skin parameters， no statistically significant differences were observed between plan（C） and plan（noFC）. Among plans with immobilization devices， the minimum distance from the target to the skin correlated inversely with skin dose， indicating greater impact on skin dose with closer proximity. Conclusions Immobilization devices in SBRT lead to beam attenuation and altered build?up effects， significantly reducing target dose parameters while increasing skin dose. The closer the target is to the skin， the greater the impact of immobilization devices on skin dose. It is recommended to incorporate immobilization devices into the contour design during radiotherapy planning.

Key words: stereotactic body radiation therapy, immobilization device, treatment table, planning dose