Abstract Book

S934

ESTRO 37

Material and Methods The Stealth chamber (Stealth) from IBA is designed for use as a reference detector for scanning. The detector is attached to the collimator head and covers the entire area up to the largest field opening (40x40 cm 2 ). One benefit of the Stealth is not having to reposition the reference detector for different field size measurements as must be done when using standard reference chambers. The Stealth is designed to be “beam invisible” with good reproducible reference signal due to a larger volume. A comparison test between the CC13 and the Stealth when used as a reference detector for measuring scanning data was performed using the IBA BluePhantom2. PDDs and profiles for various field sizes from 4x4 to 40x40 cm 2 were acquired for 6MV on a TrueBeam. Scanned data measurements were performed as follows: (1) CC13 used as reference with no Stealth, (2) CC13 used as reference with Stealth in collimator head but not used as reference, and (3) Stealth used as reference. The measurements were performed utilizing these two devices sequentially for each field size, thus avoiding the introduction of setup variations into the comparison. Most measurements were repeated on a second TrueBeam to verify the results. Results Measurements of PDD data resulted in good agreement up to 20x20 cm 2 . Table 1 shows PDD data comparison for different field sizes at various depths. Deviations between the Stealth and CC13 were observed for 30x30 and 40x40 cm 2 . Changes in the PDD curve at shallow depths (i.e., shift in buildup region) resulted in differences for the PDD values at depth for larger field sizes. Profiles measurements showed good agreement up to 20x20 cm 2 with some minor differences observed in the d max region for field sizes ≥20x20 cm 2 (Fig.1). The 40x40 cm 2 profiles show an irregular shape in the shoulder region when compared to those acquired using CC13 as a reference detector. This irregularity seems to be the result of the screws on the Stealth being mounted close to the edge of the 40x40 cm 2 field along the central axis. Profiles and PDD measurements performed with the Stealth in the beam but not actively used yielded the same results as those obtained with the Stealth as a reference detector indicating that any observed changes were the result of placing the Stealth in the field.

Conclusion The Stealth design makes it easy to use as a reference detector and it does not need repositioning with changing field sizes. The Stealth does not show any noticeable variation in the beam characteristics up to 20x20 cm 2 . Some variations are observed in PDDs and profiles for larger field sizes. For larger field sizes the Stealth can be used for relative measurements for QA purposes but its use for beam data acquisition or beam data validation is best limited up to 20x20 cm 2 for PDDs and 30x30 cm 2 for profiles. EP-1744 The EMPIR RTNORM Research Project contribution to the update of the IAEA TRS-398 Code of Practice M. Pinto 1 , C.E. Andersen 2 , F. Delaunay 3 , L. De Prez 4 , M. Donois 3 , S. Duane 5 , C. Gomà 6 , A. Kosunen 7 , J. Ojala 7 , M. Pimpinella 1 , B. Rapp 3 , T. Siiskonen 7 , L. Sommier 3 , P. Teles 8 , K. Zinc 9 1 ENEA-INMRI, Radiation Dosimetry, Santa Maria di Galeria Roma, Italy 2 DTU, Radiation Dosimetry, Roskilde, Denmark 3 LNE-LNHB, Radiation Dosimetry, Gif-Sur-Yvette, France 4 VSL, Ionizing Radiation Standards, Delft, The Netherlands 5 NPL, Radiation Dosimetry, Teddington, United Kingdom 6 KU Leuven, Department of Oncology - Laboratory of Experimental Radiotherapy, Leuven, Belgium 7 STUK, Radiation Dosimetry, Helsinki, Finland 8 IST-ID, Instituto Tecnológico e Nuclear, Lisboa, Portugal 9 Technische Hochschule Mittelhessen, Institute of Medical Physics, Giessen, Germany Purpose or Objective The IAEA TRS-398 ‘Absorbed Dose Determination in External Beam Radiotherapy’, the leading Code of Practice (CoP) in Europe for absorbed dose determination in external beam radiotherapy, is now being updated to incorporate the latest developments in currently available ionization chambers, treatment modalities, and their associated data. The objective of the EMPIR 16NRM03 RTNORM project is to contribute towards this update by both measuring and calculating k Q,Q0 factors for medium energy x-rays, conventional filtered (cFF) and flattening filter free (FFF) MV photons, and scanned proton beam modalities.