Interfraction Treatment Setup Accuracy with Different Immobilisation Devices in VMAT for Oral Cancer Patients

Norfadilah Mat Nor, Rozilawati Ahmad, Heng Siew Ping


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.




Treatment setup accuracy; imobilisation device; oral cancer; VMAT; radiotherapy

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