Pelvic Radiotherapy Immobilization: A Narrative Review of Vac-Lok and Thermoplastic Systems

A BSTRACT Accurate patient positioning is critical in radiotherapy to ensure precise dose delivery and minimize setup errors. Immobilization devices such as thermoplastic molds, blue bags, and Vac-Lok systems are widely employed to achieve this. This review compares Vac-Lok and thermoplastic molds, the most commonly utilized immobilization systems in pelvic radiotherapy. A targeted literature search was conducted using PubMed and Google Scholar with keywords including “Thermoplastic Mould,” “Pelvic Irradiation,” “Pelvic Radiotherapy,” “Vacuum Cushion,” and “Vac-Lok.” Three studies were identified – two from India and one from Japan – focusing on setup accuracy, reproducibility, and precision. The Japanese study analyzed 72 patients, while the Indian studies included 30 and 65 patients, respectively. The Indian investigations aimed to determine optimal positioning techniques for pelvic malignancies, whereas the Japanese study evaluated setup errors. Both Vac-Lok and thermoplastic molds significantly reduced setup errors, with no notable differences in systematic and random errors across most directions. However, Vac-Lok exhibited higher random errors in the yaw axis. Thermoplastic molds demonstrated superior reproducibility and consistent performance throughout treatment. Vac-Lok systems showed minor air leakage over extended use. Iso-centric deviations were comparable between the two modalities. Despite the elevated yaw-axis errors in Vac-Lok, both systems were clinically effective. Thermoplastic molds offered greater reproducibility, making them preferable for longer treatment courses. Ultimately, the choice of immobilization should be guided by clinical objectives, patient-specific factors, and treatment duration.