ESTRO 36 Abstract Book

S940 ESTRO 36 _______________________________________________________________________________________________

Three-dimensional (3D) isotropic volumes were created for each data set and the correspondence between points on the GTV surface of each case was established. To fit the model to an unknown data set texture analysis features were calculated in 5x5 volumes perpendicular to the boundary between the interior and exterior of the GTV surface. This estimates the location of the GTV boundary. At each point a search for the GTV shape was conducted by calculating the texture features and moving within the established shape limits until reaching convergence. Training was performed on 32 randomly selected cases and testing on the remaining 10 cases. The Dice similarity coefficient was used to compare the model results with the clinically defined volumes. Results Figure 1 shows an example of a typical GTV produced by the algorithm in which the seminal vesicles have been included in the apical slice (left). Table 1 shows a summary of the results obtained on the 10 test cases. The mean clinical volume of the test cases was 64.5 cm 3 and calculated by the model was 60.3 cm 3 . The largest difference was observed in the cases with the largest GTV.

Figure 1 : Clinical contour in green, model result in red. Left: apical slice taken form the inferior of the GTV including the seminal vesicles. Middle: central slice of the GTV shape. Right: basal slice from the superior of the GTV.

Conclusion Registration remains accurate even for as little as 10% of projection data, but with significant limitations in visual image quality at 10 phase reconstructions and motion detection at 3 phases. Simulating 25% dose over 10 phases allows for accurate registration without significant loss of image quality or motion detection and is therefore acceptable for 4D CBCT. This result is particularly relevant for patients with a good prognosis as it limits the radiation exposure. EP-1714 Automatic delineation of the gross-tumour volume in prostate cancer using shape models K. Cheng 1 , Y. Feng 1 , D. Montgomery 1 , D.B. McLaren 2 , S. McLaughlin 3 , W. Nailon 1 1 Edinburgh Cancer Centre Western General Hospital, Department of Oncology Physics, Edinburgh, United Kingdom 2 Edinburgh Cancer Centre Western General Hospital, Department of Clinical Oncology, Edinburgh, United Kingdom 3 Heriot Watt University, School of Engineering and Physical Sciences, Edinburgh, United Kingdom Purpose or Objective Digital models of anatomy have potential for assisting in the segmentation of the prostate and organs at risk (OAR) in radiotherapy planning of prostate cancer. However, manual alteration of automatically generated contours is often necessary to produce an accurate gross-tumour volume (GTV). This is generally the case when the tumour extends beyond the prostatic capsule into the bladder or invades the seminal vesicles (>T3a tumours). The aim of this study was to develop a digital model of the GTV in prostate cancer that incorporates the range of shape variability associated with different T-stages. Material and Methods Computerised tomography (CT) images from 42 prostate cancer patients, which contained a range of T-stages and had the prostate GTV and OARs outlined, were selected.

Table 1 : Dice coefficients and volumes obtained on the 10 test cases. Conclusion The proposed model has potential for automatically contouring the GTV when the tumour extends beyond the prostatic capsule into the bladder or invades the seminal vesicles. However, more cases must be included in the model to ensure that the full range of shape variability is represented. EP-1715 Differences in delineation uncertainty using MR images only vs CT-MR in recurrent gynaecological GTV D. Bernstein 1 , A. Taylor 1 , S. Nill 1 , U. Oelfke 1 1 Royal Marsden Hospital Trust & Institute of Cancer Research, Department of Medical Physics, London, United Kingdom Purpose or Objective To build upon previous work [1] to utilise a new contouring concept to quantify the differences in delineation uncertainty when using co-registered CT-MR images vs MRI only for recurrent gynaecological GTVs. Material and Methods A contouring concept was developed in which clinicians draw up to two GTV boundaries per CT slice corresponding to the inner (GTV_i) and outermost (GTV_o) possible boundaries the GTV may have and therefore define a boundary interval for the GTV. Observers contoured centrally recurrent gynaecological GTVs in accordance with this concept first on MRI images