Dosimetric Impact of the Jaw-Tracking Technique in Volumetric Modulated ArcTherapy

Objective: The jaw-tracking technique has been developed to reduce the radiation beam transmission in the regions blocked by multileaf collimator (MLC). The aim of this study is the dosimetric evaluation of the jaw-tracking technique in Volumetric Modulated Arc Therapy (VMAT). Material and Methods: 31 VMAT cases treated with the jaw-tracking technique were employed and re-planned with fixed jaw to analyze the dosimetric influences of the planning target volume (PTV) and organs at risk (OARs) volume. The treatment sites were liver, lung, and pancreas. All plans were optimized and calculated under jawtracking and fixed jaw conditions to cover the prescription dose to 95% of PTV (D95%) using the treatment planning system. The dosimetric verification of the treatment plans, the uniformity of the target dose distributions, the partial volume doses in OARs, and the low-dose volume were evaluated to verify the dosimetric impact of the jaw-tracking. Results: The jaw-tracking technique appeared to be able to provide some clinical advantages compared to the fixed-jaw technique. The dose uniformities in targets were similar between in jaw-tracking technique and in fixed jaw. It appeared that the jaw-tracking technique could significantly reduce the partial volume dose of OARs, for the kidney (p=0.008) and duodenum (p=0.028) in liver cancer cases, for the esophagus (p=0.015) in the lung cancer cases, and for the normal liver (p=0.005) and kidney (p=0.005) in the pancreatic cancer cases. The low-dose volumes with the jaw tracking technique were calculated to be smaller than those with fixed jaw setup when the effective maximum field dimension were 1.5 cm larger than the equivalent spherical diameter of the PTVs. Conclusions: The partial volume dose of OARs and the low-dose volume could be significantly reduced by application of the jaw-tracking technique without any adverse effect of the dosimetric parameter for targets in VMAT.