Abstract Book

S918

ESTRO 37

as the light passes twice through the sensitive layer. It is of note that in transmission mode both single- and double-scan normalized pixel value responses exhibit linear behavior within dose range investigated. All functions were successfully fitted with rational functional form and provided an overall one-sigma uncertainty of 2% for doses above 2 Gy. Use of pre-scanned images to calculate response functions resulted in negligible improvement in dose measurement accuracy. However, transmission scanned images provide response functions that can be used in wider dose range than reflection based response functions, which saturate at lower doses.

central cavity is acquired to calculate the dose distribution. In the case of PD, the distribution is determined by the prediction algorithm implemented in Eclipse. The analysis is performed on treatments of three different pathologies: 5 hypophysis, 5 prostates and 5 H&N. In all cases, the treatment technique is: VMAT, 2 complete arcs, constant gantry rotation and rate 600 UM/min. In addition, we study the impact of the dose, the modulation index and the volume on the dosimetric results. The modulation index is calculated from the information provided by the Dynalogs files according to the expression: For a dose threshold of 10%, the systems show results above 95% for gamma criteria 3%/3 mm and above 90% for 2%/2 mm in most of the cases (figure 1). There is no dependence with the plan complexity. However, for small volumes measured with PD lower results are obtained. A possible contribution is the lack of MLC modelling in the prediction algorithm. In ACD there is also a slight decrease with the lesion size. The results with RFD are dependent on the dose obtaining lower values for lower doses for gamma criteria 2%/2mm. This behaviour can be explained by the higher uncertainty in the lower doses region Where k represents the control point, i the leaf and x the corresponding position in bank A or B. Results

Conclusion Response functions defined by normalized pixel values obtained from both transmission and reflection images appear to be the most suitable response functions for radiochromic film dosimetry system using flatbed document scanners. Reflection mode provides higher sensitivity and could lead to a more widespread use of radiochromic film dosimetry, but with slightly limited dose range and increased uncertainty. Double-scanning technique, either in transmission or reflection mode, should be an option only if high accuracy (better than 2%) is required for a given dosimetry application. EP-1718 Comparative analysis of gamma results obtained using three VMAT treatment verification systems P. Castro 1 , M. Roch 1 , D. Hernández 1 , L. Pérez 1 1 Hospital Universitario La Princesa, Medical Physics, Madrid, Spain Purpose or Objective To compare the dosimetric results of three existing commercial solutions for the VMAT treatment verification. Variability in the gamma index for different pathologies is analysed by studying possible correlations with the absorbed dose, the modulation index and the treatment volume Material and Methods The following systems are considered: • EPID as1000 with Portal Dosimetry (PD): consisting of a 40cmx30cm amorphous silicon flat detector with a detector gap of 0.392mm. The dosimetry includes dark field and flat field corrections

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ArcCheck dosimetry with SNC software (ACD): 1386 diodes helical arranged located three centimetres deep along a PMMA hollow cylindrical phantom. An insert containing radiochromic films is placed in the central cavity Radiochromic film dosimetry EBT3 with FilmQA- Pro (RFD): sheets of 20cmx16cm are placed into the cited insert. Scanning is done with an Epson 11000 XL flatbed scanner at a resolution of 75 dpi. The triple channel dosimetry implemented in FilmQA-Pro is applied for dose determination

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Conclusion All the analysed systems are able to carry out an accurate quality control of the treatment. System responses to the gamma index do not depend on the complexity of the treatment although there seems to be an influence of the treatment volume in both PD and ACD and of the dose in RFD

The dose distributions are measured in a Varian Clinac 2300 iX and compared to those calculated by the Eclipse TPS. The coincidence indicator is the percentage of points where the gamma index is less than unity, using as criteria 3%/3mm and 2%/2mm. In the case of ACD and RFD a CT of the ArcCheck with the film insert in the