S142 ESTRO 33, 2014 Table 1. Dose measurements in phantom using OSLD for eight clinical TBI patients were found to agree with calculated dose to within (- 1.53±2.00)%,and with MOSFET to within (-0.60±3.02)%. The results of in- vivo measurement of 29 patients using OSLDs were illustrated in figure 1(d). The difference between OSLD dose and plan dose averaged over the 29 patients was (1.59±3.49)%. The above results indicated that the responses of OSLDs were similar to MOSFETs. Conclusions: This investigation indicates that individual calibration of OSLDs is necessary to achieve accurate results. The sensitivity of the OSLDs does not change with dose accumulation up to ~3000cGy when annealing/zeroing of the OSLDs is performed using fluorescent light. The results of this study confirm that the use of OSLD is suitable for in-vivo dose verification of TBI patients. EP-1453 Investigation of a bleaching dye for 3D radiochromic dosimetry E. Høye 1 , P.S. Skyt 2 , L.P. Muren 2 , J.B.B. Petersen 2 , P. Balling 1 1 Aarhus University, Department of Physics and Astronomy, Aarhus C, Denmark 2 Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark Purpose/Objective: In 3D radiochromic dosimetry the response to irradiation is often measured by an increase in optical density. However, the dyes used are often not soluble in water which complicates the production of gels. By using a bleaching method, i.e. a decrease in optical density upon irradiation, water soluble dyes could potentially be used. In this project we have therefore explored the use of a bleaching dosimeter for radiochromic 3D dosimetry. Materials and Methods: The dye used for the response to irradiation was Light Green SF Yellowish (LG), which is easily dissolved in water. Two water solutions with dye concentrations 8.40 mg/L and 3.17 mg/L were produced, and the optical density was measured three times during a three day period after irradiation. To obtain a solid dosimeter, gels were produced with gelatin (6% (w/w)) and agar (3% (w/w)) at different LG concentrations. The effect of hydrogen peroxide on gelatin gels with LG was also examined in two concentrations; 1.73 g/L and 3.45 g/L. The solutions or gels were poured into cuvettes and placed in a fridge overnight, before irradiation the next day. Irradiation was performed with 6 MV photon beams using a Varian linac, in a 10 × 10 cm 2 field, with a 6 Gy/min dose rate. The cuvettes were irradiated at doses from 0-60 Gy. The cuvettes were scanned before and after irradiation using a spectrophotometer (Helios Alpha, Thermo Spectronic). The change in optical density at the absorption peak at wavelength 634 nm caused by irradiation was then plotted as a function of dose. The dose response was in all cases found by a linear regression to the data. Results: The change in the optical density over the irradiated dose range for the solution with 8.40 mg/L LG is shown in the figure. The dose response within 0-10 Gy was found to be linear with a response of 8.5 ± 0.3 × 10 -3 cm -1 Gy -1 for this solution. For the solution containing 3.17 mg/L LG, the response was 6.8 ± 0.2 × 10 -3 cm -1 Gy -1 . During the three days after irradiation the response increased by 0.15% per hour for the 8.40 mg/L LG solution and 0.09% per hour for the 3.17 mg/L LG solution. The gelatin solution with LG concentration 5.30 mg/L gave a dose response of 7 ± 3 × 10 -5 cm -1 Gy -1 . The other gelatin solutions resulted in slightly negative responses, and could not be properly fitted in a linear regression. The agar gels were opaque and gave a negative response to the irradiation, and the gelatin gels with hydrogen peroxide gave no response. Conclusions: Compared to existing radiochromic dosimeters based on a radiation-induced optical density increase, the dose response of LG water solutions are slightly better but still of the same order. The response could be increased with higher LG concentrations, but the maximally allowed optical density before irradiation is restricted by a maximum optical density measurable in the optical CT scanner used for readout of larger gels. The stability in time for the water solutions is good, which is a necessary condition for a reliable 3D dosimeter. When LG is mixed into gels, the dose response is very low, currently limiting its use as a 3D dosimeter. EP-1454 Development of a proton beam dosimetry system using optical fiber array for the instant measurement of PDD curves. J.M. Son 1 , M.Y. Kim 1 , D.H. Shin 1 , M.G. Yoon 2 , U.J. Hwang 3 , S.B. Lee 4 , Y.K. Lim 4 1 National Cancer Center, Proton Therapy Center, Goyang-si Gyeonggi- do, Korea Republic of 2 Korea University, Radiology, Seoul, Korea Republic of 3 National Medical Center, Radiation oncology, Seoul, Korea Republic of 4 National Cancer Center, Proton Therapy Center, Goyang-si Gyeonggi- do, Korea Republic of Purpose/Objective: For the efficient quality assurance of proton therapy plans, it is essential to develop dosimetry tools which measure the proton beam ranges and doses precisely and instantly. Recently, we have developed a dosimetry system using Fiber-Optic Cerenkov Radiation Sensor (FOCRS). As a consequent research, we develop a therapeutic proton beam dosimetry system using optical fiber array for the measurement of percent depth dose (PDD) curves without scanning. Materials and Methods: FOCRS measures Cerenkov radiation(C-R) produced in clear plastic optical fibers (cPOF). C-R is generated not by incident protons in itself but by subsequent high energy delta rays because the maximum kinetic energy commonly required for a medical proton accelerator is 230 MeV which is far below the Cerenkov threshold energy of proton in cPOF. Cherenkov radiation is weak but the response is linear to the absorbed dose. We have built a homemade phantom to