Excellent treatment setup accuracy with highly conformal radiation technique will improve oral mucosal sparing by
limiting uninvolved mucosal structures from receiving high dose radiation. Therefore, a study was conducted to identify
the ideal immobilization device for interfraction treatment setup accuracy improvement. A total of twelve oral cancer
patients underwent volumetric modulated arc therapy (VMAT) was categorized into three different group depending on
immobilization device they used for treatment. HFW: headFIX® mouthpiece molded with wax, SYR: 10 cc/ml syringe and
TDW: wooden tongue depressor molded by wax. Each patient underwent image-guided radiotherapy with a total of 292
cone beam computed tomography (CBCT) data sets for position treatment setup errors measurement. The variations in
translational (lateral, longitudinal, vertical) and rotational (pitch, yaw, roll) in each CBCT image were calculated. Patient
positioning errors were analyzed for time trends over the course of radiotherapy. CTV-PTV margins were calculated from
the systematic (Σ) and random (σ) errors. Mean ± SD for absolute treatment setup error was statistically significant
(p < 0.001) lower for all translational errors and yaw direction in HFW. The interfraction 3D vector errors were 1.93 ±
0.66, 3.84 ± 1.34 and 2.79 ± 1.17 mm for the HFW, SYR and TDW respectively. There are positive increments between 3D
vector errors over the treatment fraction for all devices. The calculated CTV-PTV margins were 3.08, 2.22 and 0.81 mm,
3.76, 6.24 and 5.06 mm and 3.06, 3.45 and 4.84 mm in R-L, S-I and A-P directions, respectively. HFW shows smaller errors
in almost all comparison indicating higher accuracy and reproducibility of the immobilization device in maintaining
patient’s position. All margins calculated did not exceed hospital protocol (5 mm) except S-I and A-P directions using
SYR. However, in some special situations, such as re-irradiation or the close proximity of organs at risk and high-dose
regions or lower (i.e., 3 mm) margins could benefit from daily image guidance.