ESTRO 37 Abstract book

S1181

ESTRO 37

collective in combination with simulations on a self- developed lung phantom. Material and Methods 4DCT data from 50 patients with lung tumors (diameter 1.1–7.0 cm) were chosen for this study. The tumors merged to the chestwall (n=20), the diaphragm (n=4), the mediastinum (n=16) or were surrounded entirely by lung tissue (n=10). In the lung phantom two spherical structures (diameter: 1 cm and 2 cm) composed by water equivalent synthetic substance (RW3) were embedded in corkboards to simulate a tumor inside lung tissue. Tumor movement of 10 patients were measured and 4DCTs of the phantom were acquired while simulating the movements. For each motion pattern a theoretical target volume was calculated. Internal target volumes (ITVs) were contoured by a single physician in MIP (ITV MIP ) and AIP CTs (ITV AIP ) and subsequently compared to ITVs contoured in all 10 phases of a 4DCT (ITV 10 ) or the calculated values respectively (Fig. 1). Conformation numbers (CN) between ITV 10 and ITV MIP or ITV AIP were calculated according to van’t Riet et al.

Figure 1 shows the developed image processing pipeline. As can be visually appreciated (Fig. 2), the presence of tumor and/or its resection cavity or edema in the T1w- MRI influences the intensity frequencies in the tumor- bearing hemisphere. The initialization of the segmentation using the full-TPM may hence be biased and result in overestimating longitudinal GM volume decrease or even in reporting apparent WM volume increase [3,4]. This can be overcome by using the cl-TPM, which results in a more reliable automatic segmentation. In our cohort, the full-TPM method resulted in a 3.7% higher GM volume decrease than the cl-TPM method, while the latter reduced the false, longitudinal WM volume increase (0.47% vs 0.11%, respectively). The increment in the number of Gaussians had no significant effect on the tissue segmentation (data not shown). Conclusion The developed workflow with cl-TPM-based segmentation has proven to be robust and reduces the bias related to analyzing longitudinal, volume-based MRI data. References [1] Petr J et al., Radiother Oncol, under review [2] Petr J et al. ISMRM 2017 [3] Prust et al. Neurol, 2015, 85(8) [4] Chamberlain MC et al., Neurol, 2016, 86(10) EP-2138 Contouring of moving targets in maximum and average intensity projection. K.J. Borm 1 , M. Wiegandt 2 , A. Hofmeister 2 , M. Oechsner 1 , S.E. Combs 1 , M.N. Duma 1 1 Technical University of Munich TUM, Department of Radiation Oncology, München, Germany 2 Technical University of Munich TUM, Medical School, Munich, Germany Purpose or Objective The use of four-dimensional computed tomography (4DCT) is currently state of the art for radiotherapy planning for lung cancer. A relevant drawback is the time required to contour a target volume in every single phase of the 4DCT. Target delineation in maximum (MIP) and average intensity projection (AIP) CTs allows rapid definition of internal target volumes in a 4DCT. However the current literature is inconclusive on this topic and lacks clinical evidence. This study was performed to assess the error of MIP and AIP contouring with special emphasis on tumor localization analyzing a large patient

Results In the phantom study, ITV 10

overestimated the calculated

target size by 9.5 %, whereas ITV MIP lead to an underestimation of -1.8 % and -11.4 %. The relative volume differences between ITV 10 and ITV MIP or ITV AIP were -10.0 % or -18.7 %. In the CT-scans of the patients, deviations between ITV 10 and ITV MIP / ITV AIP were almost twice as large (ITV MIP : -20.2 %, ITV AIP : -33.7 %) compared to the phantom study. Tumors merging other structures in the chest showed lower conformation numbers (chestwall: CN MIP : 0.71, CN AIP : 0.65; mediastinum: CN MIP : 0.67, CN AIP : 0.59; diaphragm: CN MIP : 0.41, CN AIP : 0.57) as compared to tumors solely surrounded by lung tissue (CN MIP : 0.75, CN AIP : 0.67) (Fig. 2). MIP and AIP contouring showed better conformity with the ITV 10 if a larger target (diameter >3.5 cm) was contoured. and ITV AIP

Conclusion Even though ITV contouring in MIP CTs reflected the calculated values in the phantom study well, relevant underestimation of the target size needs to be expected in clinical practice. This is especially true if the tumor borders the mediastinum, the chest wall or the diaphragm. Neither AIP nor MIP CTs can be unquestionably recommended for target delineation. Whenever MIP is used for contouring, it needs to be ensured that ITV encompasses the target in every phase of the 4D-CT. EP-2139 Simple suppression of streaks in small FOV 4D-CBCT K.R. Jensen 1 , U. Bernchou 2 , O. Hansen 1 , D. Moseley 3 , C. Brink 1 1 University of Southern Denmark, Institute of Clinical Research, Odense C - DK-5000, Denmark 2 University of Southern Denmark-, Institute of Clinical