Investigation of Well-Balanced kV X-Ray Imaging Conditions between Skin Dose and Image Noise for Dynamic Tumor Tracking Irradiation

Purpose: The purposes of this study were to estimateaccumulated kV X-ray imaging dose throughout dynamic tumor tracking (DTT)irradiation by Vero 4DRT system and to address an analytical skin doseformula for well-balanced kV X-ray imagingconditions 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 usinglog files. Subsequently, scattered dose ratio was calculated by amount of ionization in front of flat paneldetector (FPD) for fields with size of maximum and the chamberfor 0 - 200 mm-thickness phantoms andtube voltage of 60, 80, 100, 120 kV, respectively. Furthermore, image noise was computed fromFPD 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 ionizationin frontof 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 beendemonstrated that skin dose with 120 kV has become lower than any other tube voltage inthis study. Conclusion: Using skin doses for thephantom, 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 thisphantom study.