Abstract Book

S1179

ESTRO 37

projections were reconstructed as 3D (re3D-CBCT) and as 4D (re4D-CBCT) by phase binning extracted from the original projections. As the volumes reconstructed from re-projections were motionless, streaks occurring due to undersampling of projections were estimated from their difference (Streaks = re4D-CBCT - re3D-CBCT). Corrected 4D-CBCTs were made by subtracting the streaks estimate from a standard 4D-CBCT reconstruction (Corrected4D- CBCT = 4D-CBCT – Streaks). Total calculation time for a 10 phase reconstruction was below 5 minutes. The expiration phase was used for quantitative analyses. Streak-reduction was measured as the sum of gradient’s magnitudes in the corrected image divided by the sum of gradient’s magnitudes in the uncorrected image, with less than unity indicating a streak reduction. Potential changes in the Hounsfield units (HU) as result of the streak reduction were studied by mapping CBCT images onto CT by deformable image registration. For each CBCT image the average HU error between CT and CBCT was calculated. CBCT values were also studied by dividing each CBCT image into bins of 40 HUs based on the CT values in order to evaluate overall HU changes. Results Visual improvements were clearly observed when applying streak correction for the 4D-CBCTs (see figure). The gradient based measure of streak-reduction also favored the corrected images, having a sample mean (standard deviation) of measured streak-reduction: 0.83 (0.053). No sign of overall degradation of HU values were observed, given that sample mean (standard deviation) of average HU error for corrected CBCT was: 181 (22.1) and for uncorrected CBCT: 193 (24.7). The median of binned CBCT values remained unchanged, and even a small drop in the uncertainties of the binned CBCT values was observed when streak correction was applied.

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 Research, Odense C - DK-5000, Denmark 3 Princess Margaret Cancer Centre, Department of Radiation Oncology, Toronto ON- M5G 2M9, Canada Purpose or Objective 4D-Cone Beam CT (4D-CBCT) images used for image guided radiotherapy (IGRT) are visually hampered by streaking artifacts. A time efficient correction for small field of view (SFOV) 4D-CBCT scans is investigated. Material and Methods This study is based on SFOV 4D-CBCT and 4D-CT scans of 20 non-small-cell lung cancer patients subjected to curatively intended radiotherapy initiated between March 2012 and August 2014. CBCT projections were FDK- reconstructed and forward projected by software in the RTK-package (www.openrtk.org). The applied streak reduction approach resembles elements of the McKinnon- Bates algorithm [IEEE Trans Biomed Eng., 28, 123, 1981]. Initially all CBCT projections were 3D reconstructed and re-projected onto the original projection angles. Re-