Impact of immobilization devices on the inter-fractional setup error and planning target volume for pelvic radiation therapy

ABSTRACT Objectives: To assess inter-fractional setup errors for patients undergoing pelvic radiation therapy and to determine the optimal immobilization devices, we compared the margins between the clinical target volume and planning target volume while using full immobilization devices, partial immobilization devices, and no immobilization devices in terms of systematic and random errors. Materials and Methods: The study was retrospectively conducted at a tertiary care facility and involved three groups of patients. Group A did not use immobilization devices but instead used a knee rest and a U-shaped pillow for patient comfort during the simulation. Group B used a partial-immobilization configuration with an all-in-board (AIO) base plate, a neutral neck rest (NNR), and a knee rest during the simulation. The final group, Group C, used immobilization devices including an AIO baseplate and Vac-lok with NNR during the simulation. All three groups of patients were positioned in a head-first supine position with arms up. The departmental IGRT protocol was followed, with kV CBCT acquired for the first three consecutive days and twice-weekly kV-CBCT imaging for the remaining fractions. Translational displacements, systematic errors, and random errors were calculated for all patients. The CTV to PTV margins were then calculated using the well-known margin recipe Van-Herk formula. Results: The study included 125 patients receiving pelvic radiation therapy, and 1297 kV-CBCT images were analyzed. The estimated CTV to PTV margin was lower in Group C patients for all three directions, AP, ML, and CC. However, Group B setup errors were observed slightly more in all three directions than in Group C patients. The largest CTV to PTV margin was found to be 7.1 mm in the ML direction for Group A patients. Conclusion: To improve setup reproducibility and patient comfort while minimizing errors in all three directions, we recommend using a partial immobilization setup for patients receiving pelvic radiation therapy.