Radiochromic Film Dosimetry in Kilovoltage X-­‐ray Beams: A Pre-­‐ Investigation for In Vitro Studies of Bystander Effects

Sammanfattning: Purpose Intensity modulation of radiation is one of several important conditions to evaluate for full understanding of how cells involved in the bystander effect influence each other when cells are exposed to radiation at low and high dose rates. The aim of this study was to develop and apply a useful and practical dosimetric method for measurement of the absorbed dose to cell cultures exposed to intensity modulated radiation in investigations of the bystander effects. Material and methods All irradiations in this study were performed using an orthovoltage x-ray unit at Skåne University Hospital in Lund using 120 kV and 200 kV. Film dosimetry using Gafchromic EBT3 type film was the method of choice for measurement of the absorbed dose. A number of measurements were performed to understand the basic properties of the film with respect to radiation response, including inter- and intra-individual film sheet variations. Furthermore, the effect of different scattering conditions was investigated, including the use of different calibration phantom materials and the amount of nutrition medium in the cell culture flask. Dose calibrations were carried out and to evaluate if the film calibration datasets returned the expected absorbed dose at different dose levels, a depth dose measurement was performed which was compared with ion chamber measurements. For dosimetric analysis of film measurements, three different software tools were evaluated, i.e., (i) the OmniPro software, (ii) a locally developed MATLAB program called FILMGUI and (iii) the FilmQA Pro software. Results Generally, the variations in measured absorbed dose between film sheets, between film pieces within one sheet and between pixels in a ROI was highest for the OmniPro software and the lowest for FilmQA Pro. Evaluation of the inherent properties of EBT3 film showed that the relative deviations under different measurement conditions were ≤ 1 %. In measurement with different calibration phantom materials, i.e., a solid water phantom slab versus a plastic phantom, in which the plastic phantom returned 5 % higher absorbed dose. Finally, the absorbed dose measured under the same conditions as a cell flask exposed to 98 MU (corresponds to 1.0 Gy at the surface) was measured to 1.05 Gy when the flask was fully filled with water and 0.94 Gy when the flask was filled with 5 ml water. Conclusions Film dosimetry is a reliable method with high spatial resolution which is expected to work well for measurement of absorbed dose in cell exposure applications. The basic tests were satisfactory, and the inherent variations in radiation response were modest. The depth dose measurements compared to ion chamber measurements implied that film measurements returned the expected absorbed dose using at 200 kV, and this was validated by Monte Carlo simulations. The use of FilmQA Pro appeared to be optimal in this context, and this program is also recommended for analysis by the EBT3 manufacturer due to the use of accurate multi- channel dosimetry. After ensuring accurate measurement of the absorbed dose the cell irradiation experiments was started.