ESTRO 36 Abstract Book

S786 ESTRO 36 2017 _______________________________________________________________________________________________

correlation between the metrics is shown in figure 2.

undersurface of the (lowest) bar was used. The bars were positioned above each other with parallel longitudinal axes at 95 mm distance between the axes to mimic the lay-out of the positioning device. The set-up with bars was scanned using a large-bore CT scanner (Aquilion, Toshiba Medical Systems, Tokyo, Japan). Measurements were performed at 0, 1, 2, 16, 30 and 50 mm depth in the phantom. Calculations of attenuation were performed using the Pinnacle convolution-superposition algorithm which is used in clinical practice.

Conclusion Converted Aperture Metric and Edge Area Metric have successfully been further developed, with retained correlations as previous study, to account for clinical aspects of VMAT treatment plans and provide information about complexity on a control point level. References 1 Götstedt J, Hauer A K and Bäck A. Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings. Med. Phys. 2015; 42(7): 3911-3921 EP-1487 Dosimetric aspects in the development of a crawl positioning device for prone breast radiotherapy L. Paelinck 1 , B. Boute 2,3 , L. Veldeman 1,2 , C. De Wagter 1,2 , B. Speleers 2 , A. Van Greveling 1 , G. Post 2 , W. De Neve 1,2 1 University Hospital Ghent, Radiotherapy, Ghent, Belgium 2 Ghent University, Radiotherapy and Experimental Cancer Research, Ghent, Belgium 3 Ghent University, Industrial Design Center, Ghent, Belgium Purpose or Objective A new prone patient positioning device for breast cancer radiation therapy was developed to treat patients in crawl position. Prototypes showed excellent patient comfort, stability, set-up precision and beam access to the regional lymph nodes. However, two carbon fiber bars of the external frame may be in beam paths in patients with pendulous breasts. In this study, the influence of these carbon fiber bars on the build-up dose and attenuation was investigated. Material and Methods In figure 1 a picture of the patient positioning device is shown. Beams at gantry angles near 90° or 270° will pass through the 1 or 2 pullwinded carbon fiber bars of 37/40 mm upstream from the breast. The average distance between the breast and the closest surface of the medial bar was estimated 19 cm. Radiochromic film (Gafchromic EBT2, Ashland Specialty Ingredients, USA) placed in a slabbed polystyrene phantom was used to measure the influence of the bars on build-up dose and attenuation. Measurements were performed with a 6 MV photon beam at gantry 0° using a distance of 100 cm between the source and the surface of the phantom. A field size of 10 cm x 10 cm was used. Measurements were performed with or without bars positioned above the phantom. An air gap of 19 cm between the upper surface of the phantom and the

Results For irradiation through the bars loss of build-up at the beam center, calculated by the formula (D-Db)[for depth >16mm]/D[depth=16mm] was measured as -3.6%, -2.0% and -1.6% at depths of 0, 1 and 2 mm, respectively. Hereby, D and Db represent the doses without and with irradiation through the bars, respectively. Attenuation at the beam center, calculated by the formula (D-Db)[for depths >= 16mm]/D[depth=16mm] at depths of 16, 30 and 50 mm depth was measured as 3.6%, 2.7% and 3.2%, respectively. A typical attenuation measurements is shown in figure 2. The attenuation through the bars at the beam center calculated by Pinnacle is 3.7%, 3.1% and 2.8% at 16, 30 and 50 mm depth, respectively.

Conclusion Measurements showed that the carbon fiber frame bars have a clinically irrelevant effect on the build-up dose. Attenuation by the bars as calculated using Pinnacle and measurements were in good agreement. EP-1488 Evaluation of the Efficacy and Accuracy of Customized bolus by using a 3-dimensional printer W.K. Choi 1 , S.G. Ju 1 , J.C. Chum 2 , B.J. Min 3 , S.Y. Park 1 , H.R. Nam 3 , D.H. Lim 1 1 Samsung Medical Center, Radiation Oncology, Seoul, Korea Republic of 2 Kyonggi University, Computer Science, Suwon, Korea Republic of