ESTRO 36 Abstract Book

S412 ESTRO 36 _______________________________________________________________________________________________

Benderitter 1 , F. Milliat 2 1 Institute for Radiobiological Protection and Nuclear Safety IRSN, Department of Radiobiology and Epidemiology SRBE, Fontenay-aux-Roses- Paris, France 2 Institute for Radiobiological Protection and Nuclear Safety IRSN, Department of Radiobiology and Epidemiology SRBE- Research on Radiobiology and Radiopathology Laboratory L3R, Fontenay-aux-Roses- Paris, France 3 Institute for Radiobiological Protection and Nuclear Safety IRSN, Department of external dosimetry SDE- Ionizing Radiation Dosimetry Laboratory LDRI, Fontenay- aux-Roses- Paris, France Purpose or Objective Since last three decades, the importance of the dosimetry in radiobiology studies and the standardization of the dosimetry protocols have been highlighted. Nevertheless, most of time, it is very difficult to reproduce experiments described on literature due to a lack of details in the description of dosimetry protocols. As the main objective of radiobiology is to establish links between doses and the radiations-induced biological effects, well-defined dosimetry protocols appear to be a crucial point within the determination of experimental protocols. In this context, detailed dosimetry protocols for cells irradiation have been implemented on the Small Animal Radiation Research Platform (SARRP). To support the importance of all parameters described on dosimetry protocols, manual protocol changes were performed by modifying the cell growth medium volume and/or the additional filtration for an irradiation at 80 kV. Impacts of modifications of the physical dose induced by these errors/protocols changes were studied on RBE (Relative Biological Effectiveness) using the survival clonogenic assay. Material and Methods In first, all parameters of the configuration setup (HT, HVL …) have to be defined. Then, measurements of absolute dosimetry with ionization chamber calibrated in air Kerma free in air condition, converted then in water kerma free in air, and relative dosimetry with EBT3 radiochromic films were performed to determine dose rate and evaluate the attenuation due to the cell growth medium in each containers used for cells irr adiation. In order to evaluate the influence of the modific ation of parameters like cell medium volume (1 or 9 mL I nstead of 3 mL as the reference condition) and/or the additional filtration, 6 plate wells containing EBT3 films with water were used to determine the impact on the physical dose at 80 kV. Then, experiments with rigorously the same irradiation conditions were performed by replacing EBT3 films by HUVECs. The biological response of HUVECs was assessed by using clonogenic assay. Results Characterization of the beam quality index in the range of 30 to 220 kV for copper and aluminum filtrations and the homogeneity of the field size have been measured. Then, impact of the cell culture volume and filtration have been evaluated thanks to measurements with EBT3 films and show a variation between 1 to 8% with the copper filtration and 8 to 40% with aluminum filtration compared to each reference condition. HUVECs cells irradiated in the same conditions showed significant differences in cell survival fraction, perfectly corroborating the dosimetric changes observed on physical dose. Conclusion All together these results strongly support the fact that an accurate dosimetry needs to be performed before an experiment but also to cautiously follow all the defined parameters for one condition of irradiation to avoid errors in the dose delivered on the sample and to be able to properly compare and interpret experiments.

PO-0778 New Razor silicon diode for Cyber Knife small beam relative dosimetry: a multi-site evaluation S. Russo 1 , L. Masi 2 , P.R. Dicarolo 3 , R. Doro 2 , E. De Martin 4 , M.L. Fumagalli 4 , A.S. Martinotti 5 , A. Bergantin 5 , E. Rondi 6 , S. Vigorito 6 , P. Mancosu 7 1 Aziend USL Toscana Centro, Fisica Sanitaria, Florence, Italy 2 IFCA, Radiotherapy, Firenze, Italy 3 Medical Physycs, Meyer Children’s University Hospital-, Florence, Italy 4 Istituto Besta, Radiotherapy, Milano, Italy 5 C.D.I., Radiotherapy, Milano, Italy 6 I. E. O., Radiotherapy, Milano, Italy 7 Humanitas Research Hospital, Medical Physics Unit of Radiation Oncology, Milan, Italy Purpose or Objective The aim of this work was to evaluate the suitability of a new unshielded p-type silicon diode (Razor, IBA Dosimetry, Germany) for relative small beams dosimetry over different CyberKnife systems. Material and Methods Output Factors (OFs) measurements with Razor detector were performed by four Italian Radiotherapy Centers equipped with CyberKnife units for field sizes ranging from 5 to 60 mm, defined by fixed circular collimators. Setup conditions were 80 cm source to detector distance and 1.5 cm depth in water. Measurements were repeated by each center with a PTW-60017 diode. Monte Carlo correction factors reported in literature were applied to PTW-60017 measured data and corrected values were considered as a reference. Crossplane and inplane dose profiles ranging from 5-60 cm fixed collimators were measured by Razor detector at a depth of 10 cm in water and SSD 70 cm. The effective field size (EFS), defined as EFS=, where A and B correspond to the in- and cross-line FWHM, were calculated. Penumbra 20%- 80% was also evaluated. This work has been conducted in the framework of the Italian Association of Medical Physics (AIFM) SBRT working group. Results Razor OFs measured for fixed collimators in the four enrolled centers showed a variability (relative range) decreasing from 1.2% to 0.4% for field sizes from 7.5 to 60 mm and equal to 2.2% for the smallest cone. The variability obtained for OF measured by PTW-60017 was analogous: lower than 1 % for field sizes from 7.5 to 60 mm and equal to 3.5% for the smallest diameter. For field sizes down to 7.5 mm Razor measured OFs were lower than PTW-60017 uncorrected measured values. Relative differences between Razor OFs and Monte Carlo corrected PTW-60017 data were below 1% for 60-10 mm cone sizes and within 2 % for 7.5 mm field size over all centers. For the smallest collimator differences ranging from to 2.5% to 6% were observed among centers. Average values and SD of OFs measured by Razor and PTW-60017 diode (MC corrected and not) are shown in figure.

Nominal field size NFS, effective field size EFS and penumbra Razor measurements averaged over the four CyberKnife centers are reported in table. Maximum