Purpose or Objective: Our aim was to compare Vac-Lok™ and thermoplastic casts differences in interfraction set-up reproducibility in patients treated with three-dimensional conformal radiotherapy (3DCRT) for soft tissue sarcomas (STS) of extremities. Material and Methods: Retrospective review of patients with STS of extremities treated with 3DCRT was performed. Immobilization was performed for 14 patients (Vac-LOKTM= 7; thermoplastic cast = 7). A total of 307 measurements of set-up errors in each direction (vertical, longitudinal, lateral and rotational) were collected. Random (σ) and systemic errors (Σ) were computed. To evaluate the overall shift from isocenter, total vector error (TVE) was also calculated. To assess absolute mean differences between the immobilization systems, independent samples t-test was performed. Results: Median age of patients was 43.8 years (range: 18-74). Predominant STS site was lower extremity (57.1%). Preoperative RT was given to 5 patients (35.7%) and postoperative RT was given to 9 patients (64.3%). Mean displacements for the Vac-Lok™ and thermoplastic cast, respectively, were as follows: vertical, -0.01±0.06 cm vs. 0.02±0.16 cm (P = 0.63); longitudinal, 0.03±0.04 cm vs. 0.04±0.07cm (P = 0.60); lateral, 0.04±0.17cm vs. 0.00±0.11cm (P = 0.55) and TVE, 0.15±0.10 vs. 0.19±0.07 (P = 0.44). No statistical differences were seen in σ or Σ between Vac-Lok™ and thermoplastic cast. Similarly, no statistical differences were seen in the margins calculated for each axis using the two immobilization
systems by using van Herk’s formula. Conclusion: Based on the findings of our study, we concluded that
both the Vac-Lok™ immobilizer and thermoplastic cast had comparable results, as both systems offered better reproducibility during RT in STS of extremities without significant daily set-up displacement. A prospective study needs to be undertaken on a larger scale to better explore the outcomes of patient immobilization systems.