Abstract Book

S1139

ESTRO 37

phantom (Fig.1.d) compared to the HVL in air of the same beam show the beam softening for HVLs above 6 mm Al and beam hardening for HVLs bellow 6 mm Al. The average differences between tabulated and measured CTDI values (using HVL in air for all CTDI positions) on one side, and using the appropriate calibration curves based on beam quality correction, on the other side, show the improvement on measured CTDIvol values up to 5% for Head (Fig.1.e) and up to 14% for Body CTDI phantom (Fig.1.f).

Results We analyzed the CT numbers in the ROIs. In the fusion images, the maximum was 3108 HU, the minimum was - 976 HU, and the mean was 108.5 ± 357.4 HU. The corresponding figures in the original images were: maximum 3095 HU, minimum -998 HU, and mean 116.4 ± 314.5 HU. In the fusion-normal difference images, the corresponding figures were: maximum 31.9 %, minimum 0.15 %, and mean 12.3 %. Conclusion Computed tomography is an essential tool for planning radiation therapy, but metal artifacts cause many problems, introducing errors into the calculation of the CT number and degrading image quality. In this study, image fusion significantly improved the quality of the image compared with that of the original scan, obviously reduced the metal artifact, and improved the accuracy of the CT value. These results suggest that in CT of the head and neck in the presence of dental implants, the image fusion method will improve image quality and the accuracy of the dose distribution in subsequent radiotherapy. EP-2076 Impact of Beam Quality Changes on Radiochromic Film based CTDI measurements N. Tomic 1 , P. Papaconstadopoulos 1 , J. Seuntjrns 1 , S. Devic 1 1 McGill University, Radiation Oncology, Montreal, Canada Purpose or Objective In this work we investigate beam quality variation during radiochromic film based CTDI measurement within CTDI Head and Body phantoms at five different measurement positions and the impact of the variation on measured CTDI values. Material and Methods Dose profiles were measured with XR-QA2 GAFCHROMIC TM model film strips, sandwiched between acrylic rods cut in half and placed within CTDI phantom (Fig.1.a). Reference dosimetry system was calibrated in terms of air kerma in air (Fig.1.b). Beam quality variations were studied using Monte Carlo (MC) simulations. Photon spectra were generated in-air, using the SpekCalc code, for beam qualities in the range of 3.5 – 8 mm Al (HVL). Spectra were then used as an input to the EGSnrc/cavity MC user code for a CTDI head and body phantom (including couch). Photon spectra were collected in a circular 1 cm radius region in 5 in-phantom positions (center, top, bottom, right and left) and HVL values were re-calculated analytically for each spectrum. A beam quality correction was derived for each phantom type, position and initial beam quality in-air, which we subsequently used to select the appropriate film calibration curve. Obtained dose profiles were averaged over the length of 10 cm to give us dose value used to calculate weighted CTDIvol, to be compared to tabulated values for five CT scanners. Results Beam quality changes for all film positions within 16 cm diameter Head (Fig.1.c), and 32 cm diameter Body CTDI

Conclusion Our results show that for harder beam qualities (in the air) the beam softening is larger within CTDI phantoms and vice versa. Proposed method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom shows better agreement between calculated CTDIvol and tabulated values with maximum difference 11% for Head and 14% for Body phantom, as opposed to 16% for Head and 28% for Body phantom when the beam quality correction is not performed. EP-2077 A critical look at the stoichiometric single- energy CT calibration for proton therapy C. Gomà 1 , I.P. Almeida 2 , F. Verhaegen 2 1 KU Leuven, Department of Oncology- Laboratory of Experimental Radiotherapy, Leuven, Belgium 2 Maastricht Radiation Oncology MAASTRO Clinic, Physics Research, Maastricht, The Netherlands Purpose or Objective Despite extensive research in dual-energy CT imaging, single-energy CT (SECT) is still the standard imaging modality in proton therapy treatment planning. In this context, the stoichiometric calibration is, with more than 500 citations, considered to be the most accurate method to establish a unique relationship between CT numbers and proton stopping power. This work revisits the SECT calibration for proton therapy treatment planning, with a critical look at the stoichiometric method. Material and Methods Three different sets of tissue-substitutes of known elemental composition (Gammex, CIRS and Catphan) were scanned with the same scanning protocol. A