ESTRO 36 Abstract Book

S942 ESTRO 36 _______________________________________________________________________________________________

until doses during CT scans on both scanners were within 5%. Dose profiles were sampled using XR-QA2 model GafChromic TM film strips placed at four sides of the phantom (top, bottom, left, and right) and taped with a paper tape (Fig.1.a). Table 1 lists scanning and image reconstruction parameters used. To further assure unbiased comparison, we have set geometrical image parameters (highlighted) to be the same. First we scanned the phantom on the GE scanner and measured the surface dose. Table 1 reports averaged surface dose values over four film strips. On each film strip we have taken an average dose over a dose profile along 10 cm. Subsequently, we scanned the phantom 3 times on Philips scanner, each time adjusting mAs setting, until the surface dose reached the dose measured on the GE scanner to within 5%. Then, image quality comparison was performed using CATPHAN-504 modules in terms of spatial resolution (Fig.1.b), low contrast detectability (Fig.1.c), image uniformity (Fig.1.d), and contrast to noise ratio (Fig.1.e).

Conclusion The GE CT-simulator demonstrated a slightly better image quality in terms of spatial resolution and low contrast detectability, which was expected due to its smaller bore size, and hence lower impact of scattering on the image quality, while Philips CT produced images with better SNR. In the case of CNR values we have found that the Philips scanner provides images of superior image quality than GE scanner for Pelvis protocol. These findings can be explained by the fact that GE uses harder beam quality ( HVL=7.4 mm Al ) than Philips (HVL=6.9 mm Al) for Pelvis protocol indicating a dependence of image quality parameters on energy spectrum. EP-1718 Application of motion compensation in 4D CT of oesophageal cancer. A. Green 1 , L. Bhatt 2 , R. Goldstraw 3 , H. Sheikh 2 , M. Van Herk 1 , A. McWilliam 1 1 The University of Manchester, Department 58- Radiotherapy Related Research, Manchester, United Kingdom 2 The Christie Hospital NHS Foundation Trust, Consultant Clinical Oncology, Manchester, United Kingdom 3 The Christie Hospital NHS Foundation Trust, Radiotherapy Physics, Manchester, United Kingdom Purpose or Objective The use of 4D CT has become widespread for treatment planning of lung cancers, and motion compensation is known to be useful for the delineation of targets and organs at risk. To our knowledge, however, motion compensation has not yet been used for oesophageal cancer. Here, as in lung cancers, motion due to respiration induces image artefacts that can make delineation difficult leading to lower quality treatments. In this work, we aim to evaluate the potential benefit of motion compensated CT in oesophageal cancers. Material and Methods Using the ADMIRE (Elekta AB, Stockholm, Sweden) auto- segmentation tool, all 10 phases of the 4D CT scan were deformable registered to a reference phase and averaged, excluding 4 phases displaying the greatest motion velocity.

Results The lower part of Table 1 summarizes results of the image quality comparison. In terms of spatial resolution and low contrast detectability, it appears that the GE scanner is slightly better for both Head and Pelvis protocols. On the other hand, while the uniformity of the images obtained with Head protocol are slightly better for the GE scanner, the Philips scanner has better characteristics for the Pelvis protocol. Also, Philips CT images show significantly less noise for both scanning protocols. Finally, with regards to CNR the Philips CT images appear in general to be better than GE except for high Z material (Teflon) for GE Head protocol.