ESTRO 37 Abstract book

ESTRO 37

S462

suitability of these chambers for relative dosimetry in FFF beams Material and Methods The study was performed in two Institutions with linac 1, an Artiste Siemens with 6MV and 7MV FFF beams and linac 2, an Elekta Versa HD with 6MV, 6MV FFF, 10MV, 10MV FFF and 15MV beams. All the measurements were performed at clinical nominal dose rates which are 1200, 2000 and 2400 MU min -1 for the 6MV FFF, 7MV FFF and 10MV FFF beams respectively. Eight ion chambers classified in three groups according to their model and volume (PTW30013 Farmer 0.6 cm 3 , PTW31010 Semiflex 0.125 cm 3 and PTW31016 Pinpoint 0.016 cm 3 ) were selected as they represent commonly used ion chambers in radiotherapy clinics. Three methods were used to determine k s : the TVA, measurements of the readings 1/M versus the inverse chamber voltages 1/U (the Jaffé plots, which are a prerequisite for the application of the TVA) and a more robust method based on a general equation with coefficients γ and δ introduced by Bruggmoser et al .(2007, Phys. Med. Biol . 52 N35). Jaffé plots were acquired for DPP ranging from 0.09 to 1.73 mGy by varying the energy and SSD. The variation of k s with depth that could result in skew of collected PDD data was also determined Results Figure1 shows the variation of DPP versus the k s obtained from Jaffé plots, being the DPP range consistent with that from clinical beams of the linacs used. The parameters γ, δ and the regression coefficient R 2 of these plots are shown in tableI, showing general agreement with values reported in the literature. The k s values calculated with these coefficients for Farmer and Semiflex chambers coincide with those obtained with TVA and Jaffé plots. Figure2 shows the quality of the linear regression fit of the Jaffé plot for the Pinpoint chamber for the 7MV FFF beam. However, based on the poor quality of the linear regression fit of the k s vs DPP for this chamber, determination of the k s following the TVA would not be appropriate. The variation of k s with depth exhibited a minimal dependence in clinical FFF beams (<0.5%): the change of k s varied by 0.4% and 0.2% between d max and 20cm depth for the Semiflex and Pinpoint chambers respectively This study Literature values

Conclusion For absolute dosimetry of FFF beams, the TVA is a reliable and straightforward method for the determination of accurate k s values for PTW30013 Farmer and PTW31010 Semiflex chambers, but not applicable for PTW31016 Pinpoint chamber. Regarding relative dosimetry, choice of the PTW31010 Semiflex and the PTW31016 Pinpoint chambers to minimize the effect of ion recombination in PDD is recommended PO-0877 Multi-detector dosimetry for QA in advanced radiotherapy modalities: a comparative study E.P. Pappas 1 , E. Zoros 1 , G. Anagnostopoulos 2 , G. Antorkas 2 , E. Pantelis 1 , P. Karaiskos 1 1 National and Kapodistrian University of Athens, Medical Physics Laboratory- Medical School, Athens, Greece 2 German Oncology Center, Medical Physics Department, Limassol, Cyprus Purpose or Objective Advanced treatment modalities such as VMAT and SRS/SRT are dosimetrically challenging due to the non- standard fields involved and the delivered steep dose gradients. Therefore, commissioning and periodic quality assurance (QA) tests require more stringent dosimetric methods capable of providing dose results with high accuracy and reproducibility for a wide dose range with fine spatial resolution. The aim of this work is to employ a variety of detector types (point, 2D, 3D, passive, active) in a single QA protocol, utilizing a modular phantom, in order to highlight the challenges involved and discuss advantages and limitations of each dosimetric system. Material and Methods A custom acrylic based phantom was designed t o resemble head geometry (cylinder of 16 cm in diameter, 19 cm in height with rounded top). The phantom was constructed in two halves, allowing a central volume of 9x9x11 cm 3 for accommodating custom exchangeable inserts (figure). One insert was constructed per dosimetric system incorporating all necessary design characteristics to facilitate implementation of the detector (e.g., fiducial markers for spatial registration, reproducible positioning, etc.). A total of five inserts were constructed, housing 59 TLD 1x1x1 mm 3 mirocubes, three EBT3 films, two PRESAGE 3D plastic dosimeters, two ion chambers (one PTW 31010 and one Sun Nuclear

δ/U (mGy^- 1)

δ/U (mGy^- 1) 3.9 and 3.4 2.4 and 2.2

Type

γ/U

R^2

γ/U

0.01 and 0.024 0.38 and 1.23

PTW30013 0.44 2.84

0.997

PTW31010 1.12 1.36

0.991