ESTRO 37 Abstract book

S1141

ESTRO 37

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 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).

cancer. However, dental implants cause metal artifacts that degrade CT image quality. Many methods for reducing metal artifacts have been proposed. Here we use a gantry-tilted scan followed by image fusion for this purpose. This study aimed to reduce metal artifacts in CT and improve image quality.

Material and Methods We obtained two computed tomographic images, one with a superior 20° gantry tilt, and one with a normal setup (0° tilt) from each of ten patients with dental implants who had head and neck cancer. The gantry tilt moved the metal artifacts to a superior position. We then reconstructed the gantry-tilted images as normal-scan images. In a second step, image fusion, we combined the normal image excepting the parts containing the metal artifacts with the reconstructed, gantry-tilt image that had a reduced metal artifact. The fusion process was confined to the vicinity of the dental implants. The result was a third image, the fusion image. We selected regions of interest (ROIs) in which to compare the normal-scan image with the fusion image. Then we evaluated the CT number in Hounsfield units (HU) in each of these ROIs to allow quantitative comparison.

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.