Abstract
Purpose: The purposes of this study were to estimate accumulated kV X-ray imaging dose throughout dynamic tumor tracking (DTT) irradiation by Vero 4DRT system and to address an analytical skin dose formula for well-balanced kV X-ray imaging conditions between skin dose and image noise. Method: First, skin dose was measured using kV X-ray tube, chamber, and water-equivalent phantoms. Next, imaging dose for six patients in DTT treatment was computed using log files. Subsequently, scattered dose ratio was calculated by amount of ionization in front of flat panel detector (FPD) for fields with size of maximum and the chamber for 0 - 200 mm-thickness phantoms and tube voltage of 60, 80, 100, 120 kV, respectively. Furthermore, image noise was computed from FPD images. Results: The skin dose was greater by a factor of 1.4 - 1.6 than those in Synergy XVI system. The image noise in FPD, was expressed as N = 0.045×(1/QFPDen)0.479, where QFPDen denotes amount of ionization in front of FPD. Then, skin dose, D (N, t, v) was formulated as (0.045/N)(1/0.479)/QFPDen/mAs (t, v) ×D/mAs (v), where QFPDen/mAs (t, v) and D/mAs (v) denote amount of ionization in front of FPD and skin dose per mAs, respectively. Using the formulae, it has been demonstrated that skin dose with 120 kV has become lower than any other tube voltage in this study. Conclusion: Using skin doses for the phantom, the skin dose throughout DTT irradiation was estimated as 0.50 Gy. Furthermore, skin dose by kV X-ray imaging was described as a function of image noise, phantom thickness, and tube voltage, suggesting image noise may be reduced with higher X-ray tube voltage in this phantom study.
Highlights
In dynamic tumor tracking (DTT) irradiation using Vero4DRT (Mitsubishi Heavy Industries, Ltd., Japan, and BrainLAB, Germany), four dimensional correlation model (4D model) between positions of 3 - 4 gold markers implanted closely to a tumor and displacement of abdomen is created just before beam delivery
Our previous study [12] has demonstrated that the skin doses in the Vero4DRT with a 0.1-mm Cu filter attached to the kV X-ray source became 1.1 and 0.99 greater than those in Synergy XVI with the tube voltage of 100 and 120 kV, respectively
We have measured skin dose induced by kV X-ray imaging for image guidance using water-equivalent phantoms and the Vero4DRT system
Summary
In dynamic tumor tracking (DTT) irradiation using Vero4DRT (Mitsubishi Heavy Industries, Ltd., Japan, and BrainLAB, Germany), four dimensional correlation model (4D model) between positions of 3 - 4 gold markers implanted closely to a tumor and displacement of abdomen is created just before beam delivery. 3D positions of gold markers of the diameter of 1.5 mm in the 4D model are calculated by a stereo vision technique using several pairs of 2D positions of each marker in two orthogonal kV X-ray images [3]. It is of great importance of quantifying skin dose during acquisition of kV X-ray images. Detectability of gold markers using image processing techniques [6] is varied due to the image quality of kV X-ray images. Detectability of gold markers in lungs is greatly varied due to difference of water-equivalent path in which the gold markers are moving under mediastinum or diaphragm
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