ESTRO 38 Abstract book

S915 ESTRO 38

clinically relevant scenarios. Modification of the data readout protocol could prevent saturation of sensor elements at the higher dose rates. EP-1699 Comparing water equivalent phantom materials using CT scans and conformal prostate treatment plans M. Kirby 1 , P. Bridge 1 , J. Callender 1 1 University of Liverpool, Radiotherapy, Liverpool, United Kingdom Purpose or Objective Water is the standard reference material specified in national protocols for radiation dosimetry and QA. However solid phantoms are more versatile and easier to use – but materials must have a high water equivalency and/or equivalence to other human tissues to act as a true surrogate with similar attenuation and scattering characteristics. Good equivalence makes them ideal for QA and technique development in radiotherapy and especially for pre-treatment per-patient QA for both IMRT and VMAT. Typical standard dosimetric characteristics of such materials are often determined by comparing measurements of percentage depth dose curves, build-up and attenuation factors with water. As a precursor to such measurements, we investigated 6 different water equivalent solid phantom materials for their equivalence by comparing analysis made in a treatment planning environment. Material and Methods Six water equivalent materials were investigated; PTW RW3 (PTW, Freiburg, Germany); Gammex SWHE and SW (Sun Nuclear, Middleton, WI, USA); CIRS PWDT and PW (CIRS, Norfolk, VA, USA) and BARTS SW (Phoenix Dosimetry Ltd., Berkshire, UK). All materials were CT scanned using a clinical pelvis scan protocol (120 kV, 260 mA, 55mAs, 2.5 mm slice thickness, 500mm RFOV) on a GE Lightspeed RT16 scanner. Slices were exported to Eclipse TPS (V13.8) for analysis. Ten-mm diameter ROIs were identified in the central portion of each phantom (9 on the central slice, 2 sup and 2 inf) ensuring that each ROI was in the middle of material slabs. ROI properties were used to measure the average HU in each and then compute (from all 13 ROIs) the min, max, mean and SD of the HU for each of the six types of material. Identical scans and measurements were made for real water. The mean HU values were used to override existing values for soft-tissue on a real, three field (anterior and two post- oblique fields) conformal 6MV radical (74Gy/37#) prostate plan. Soft-tissue structures were all those within the patient excluding bony tissue. The plan was then recalculated for each of the six mean HU values from the phantom materials and the DVHs (PTV and rectum ORV) compared with the original clinical plan. Results The Mean (SD) of the measured HUs for each of the water equivalent materials were; PTW RW3 – 14.0 (14.1); Gammex SWHE – 15.1 (11.2); Gammex SW – 26.3 (11.9); CIRS PWDT – -8.5 (10.7); CIRS PW – 66.5 (14.8); BARTS SW – -5.8 (10.6); Water – -1.4 (3.5). The dosimetric impact of applying these mean HUs to the clinical plan is shown in Table 1.

Conclusion Although the range of mean HUs for the six materials is wide (mean -5.8 to 66.5; range (across all ROIs) -41 to 121), there was little dosimetric impact on the three-field plan. All materials were within 0.3% of the target volume dose objectives (D2% and D98%) achieved in the clinical plan; all were within 1% of the ORV constraints (V50 to V70). All therefore would act as suitable equivalent materials to water/soft-tissue in the treatment planning setting. EP-1700 CyberKnife output factors from integral dosimetry A. Kulmala 1 , J. Heikkilä 2 , A. Väänänen 2 , M. Tenhunen 3 1 Clinical Research Institute HUCH, Radiotherapy, Helsinki, Finland ; 2 Cancer Center Kuopio University Hospital, Radiotherapy, Kuopio, Finland ; 3 Helsinki University Hospital- Cancer Center, Radiotherapy, Helsinki, Finland Purpose or Objective The standard approach of determining output factors of small megavoltage beams has been the construction of smaller detectors to fit in the central area of the smallest fields of interest. As an alternative approach it has been shown that a large ionization chamber (IC) can be used for measuring output factor of a small (diameter 4-20 mm) conical collimator, when a radial response of the chamber and high resolution relative dose distribution (off-axis- ratio, OAR) of the field are known. In the current work we will apply this formalism to determine the output factors of the CyberKnife conical collimators. Material and Methods The ‘radial response’ of the PTW34001 (ROOS) parallel plate ionization chamber is deconvoluted from conical collimator OAR sets determined with the IC ‘measured signal’ and Gafchromic EBT2 film ‘true signal’ using a non- parametric super-resolution deconvolution method (Kulmala A and Tenhunen M 2012, Phys. Med. Biol. 57 7075–88). The output factors of the Cyberknife cone beams (diameter 5-30 mm) are estimated based on integral dose measurements with the IC, the deconvoluted radial response of the IC and relative dose distributions measured with the film. Finally, the output factors are compared to a reference set. As a reference method we use a direct measurement with the E60012 p-type