Impact of Jaw Position Accuracy on Dose Variation in Volumetric Modulated Arc Therapy Using Jaw Tracking

The technique of jaw tracking can be used in volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT). In this technique, jaw tracks apertures of the multi leaf collimator (MLC) during irradiation. It is reported that dose variation is occurred by the changing accuracy of MLC position in VMAT and IMRT. Though jaw moves dynamically in the irradiation by using this technique, the influence of the jaw position accuracy on dose variation is not examined. The purpose of this study is to verify the influence of jaw position accuracy on dose variation in VMAT using jaw tracking. We appended intentional errors on jaw position in digital image communication in medicine-radiation therapy (DICOM-RT) plans created using jaw tracking technique. These plans were irradiated on the phantom that was inserted ion chamber, and we measured dose variation by changing the intentional error. The dose variation in planning target volume (PTV) was enlarged by increasing the error, and agreed with the variation of a collimator scatter factor within 0.03%. In clinical case of oropharyngeal cancer, the maximum dose variations in parotid gland were 0.179% and 1.23% when the errors were 1 mm and 10 mm, respectively. Dose variation in parotid gland was larger than the variation in PTV and spinal cord because of increasing MLC transmission. The dose variation caused by jaw position error was smaller than it caused by MLC position error. So, we can keep the dose error slightly that is related to jaw position error in VMAT by maintaining jaw position accuracy correctly.