Absorbed dose distribution in liquid water for a CyberKnife VSI using radiochromic EBT3 film

Abstract

This work investigated the absorbed dose distribution in a CyberKnife VSI using a single Gafchromic EBT3 film piece per beam irradiated directly in liquid water and parallel to the radiation beams. In particular, the total scatterfactor, Ssf, correction factor, kQclin,Qmsrfclin,fmsr, percentage depth dose (PDD), tissue maximum ratio (TMR) and off-axis ratio (OAR) were measured. The calibration of the film was performed under reference conditions in a 6 MV X-ray beam produced by a linear accelerator Varian iX, while the measurements in the Cyberknife were carried out at 70 cm source to surface distance parallel to the beams. The results obtained with the EBT3 film are in agreement, within measurement uncertainties, with that measured with an IBA stereotactic field diode (SFD), excluding the 5 mm field diameter where 4.9% and 11% differences were observed in the Ssf values and PDD between 50 mm and 150 mm water depth, respectively. The higher discrepancy in the PDD data can be associated to a bending effect caused by the water pressure over the film’s surface during the irradiation. Based on this observation, one can conclude that the use of a single film piece parallel to the radiation beams to measure PDD directly in liquid water can only apply for field sizes larger than or equal to 10 mm diameter. For smaller field sizes, perpendicular measurements must be performed. Comparing our data for kQclin,Qmsrfclin,fmsr with that published in the literature and calculated through Monte Carlo simulation, quite good agreement is observed except for the field sizes of 7.5 mm and 5 mm in diameter where variations of 11% and 16 % are obtained, respectively. This suggests that the method proposed to measure the percentage depth dose in a 10 cm × 10 cm square field from a single exposure can accurately apply in small field dosimetry if measurements are performed under strict precautions.