Measurement of Imaging Dose in 2D kV Image Guided Radiotherapy Using Indigenously Developed Portable Free Air Ionization Chamber.

Objective: The objective of this work is to measure the imaging dose in two-dimensional kilovoltage (kV) image guided radiotherapy using in-house developed free air ionization chamber (FAIC). Materials and Methods: In-house developed FAIC and a commercially available shadow-free diagnostic (SFD) detector were used in this work. Reference points of the dosimeters were positioned at 100 cm distance from the focal spot of the x-ray tube of an on-board imager. The blades of x-ray tube were opened to 26.6 cm × 20 cm. The FAIC was operated at 3800 V supplied by an indigenously developed external high-voltage power supply, whereas the SFD was operated at 300 V supplied by the electrometer. The dosimeters were irradiated using head (70 kV, 9.77 mAs), abdomen (100 kV, 25.39 mAs), pelvis (110 kV, 29.30 mAs), and extremity (65 kV, 6.84 mAs) protocols. The air kerma per unit mAs was determined using the collected charge and the values determined in this work were compared with the values reported in the literature. Results: The measured air kerma by FAIC and SFD were in good agreement with each other within the uncertainty of measurement. However, the FAIC and SFD measured values were approximately 42% lesser than the values of air kerma reported in the literature. Conclusions: The default imaging protocols are optimized for image quality and patient dose, further optimization of exposure parameters according to the patient anatomy may help in reducing the imaging dose further.