Determination of output factors of a cyberknife system using active and passive dosemeters and a Monte Carlo method

SFPM Annual Meeting 2011 resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber and PTW 60003 natural diamond) and in a solid water phantom with passive dosemeters (TLD microcubes, EBT2 Gafchromic films). Small field sizes were defined either by microMLC down to 6x6 mm2 or by circular cones down to 4 mm in diameter. Results: Significant differences between the results obtained by the different detectors are observed, particularly for the smallest field size for which the difference of the measured output factors can reach 20%. Results obtained with both passive dosemeters present a good agreement (better than 2%). Some of these differences can be explained with the detector characteristics (detector size, tissue equivalence .).

S21 Conclusion: This study provides data on the behavior of different detectors in terms of dose measurements for small beams. Results obtained with the passive dosemeters in this study as well as in another study performed on a Cyberknife system (Huet et al., 2011) are promising. The perspective of this study is to propose a methodology for the determination of correction factors for the Novalis system. Reference: Huet et al., abstract submitted to SFPM 2011. Keywords: Small beams, OF, Novalis

35 Determination of output factors of a cyberknife system using active and passive dosemeters and a Monte Carlo method C. HUET1, C. BASSINET1, S. DERREUMAUX1, M. BAUMANN1, T. LACORNERIE2, F. TROMPIER1, P. ROCH1 and I. CLAIRAND1 1

Institut de Radioprotection et de Suˆrete´ Nucle´aire ; Fontenay-aux-Roses, France, 2Centre Oscar Lambret, Lille, France

Purpose: Small photon beams used in radiotherapy are characterised by high dose gradients and a lack of lateral electron equilibrium. Thus, the measurement of output factors (OF) of these beams is problematic and OF measurements with different detectors for similar irradiation conditions can lead to a deviation up to 30% (Derreumaux et al., 2010). The aim of this study is to accurately determine the OF of a Cyberknife system thanks to measurements and Monte Carlo data in order to propose, to French radiotherapy treatment centers, a protocol for OF measurements of small photon beams. Methods: OF were measured on a Cyberknife for the 5, 7.5, 10, 20 and 60 mm diameter field sizes using active detectors (high resolution diodes (IBA SFD, PTW 60016, PTW 60017, Sun Nuclear EDGE), PTW 31014 Pinpoint chamber and PTW 60003 natural diamond) and passive dosemeters (EBT2 radiochromic films and TLD micro-cubes). Correction factors for diodes (EDGE, PTW 60016, PTW 60017) and for the PTW 31014 chamber were calculated thanks to a recent methodology based on Monte Carlo data (Francescon et al., 2008 and 2009). Results: For the smallest field size, a deviation of nearly 20% was observed between the values of the OF measured with the different detectors. The agreement between the passive dosemeters was better than 2%. After correction for diodes and for

the chamber, the OF obtained with these active detectors agreed within 2% with the passive OF measurements. Conclusion: This study is encouraging concerning the determination of the OF of the Cyberknife system. For the active detectors, the use of correction factors improves the accuracy in the OF measurements. The OF measured with the passive dosimeters are very close to the corrected OF evaluated with the method of Francescon. The results obtained with the passive dosemeters in this study as well as in another study performed on a Novalis system (Bassinet et al., 2011) are promising. If the OF values are confirmed by the Monte Carlo calculations in progress in our laboratory, passive dosemeters could be used as “reference” dosimeters for this application. References: Bassinet et al., abstract submitted to SFPM 2011. Derreumaux et al., IDOS Vienna, 9-12 November 2010, Paper n 312. Francescon et al., Med. Phys., 35, 504-513, 2008. Francescon et al., J. App. Clin. Med. Phys., 10, 147-152, 2009. Keywords: ouptut factor, Monte Carlo, Cyberknife

36 Search of a small volume ionization chamber for the establishment of LNHB primary standards in a 2x2 cm2 M. LE ROY1, L. DE CARLAN1, F. DELAUNAY1, M. DONOIS1, P. FOURNIER1, A. OSTROWSKY1, A. VOUILLAUME1, J.-M. BORDY1 and S. HACHEM2 1

CEA, LIST, Laboratoire National Henri Becquerel, 91191 Gif-Sur-Yvette, France, 2Universite´ Nice Sophia-Antipolis, Parc Valrose, 06008 Nice, France

The development of new treatment modalities such as IMRT and stereotactic radiotherapy requires developing suitable dosimetric standards. LNE-LNHB is involved in a European project aiming at establishing dosimetric standards for radiation fields down to 2x2 cm2. This requires the use of ionization chambers with appropriate size. Therefore, LNHB carried out a detailed study of small volume chambers available on the market. 24 ionization chambers of 8 different types with volume ranging from 0.007 to 0.057cm3 were tested in a 60Co beam. For each chamber,

two major characteristics were investigated: (1) the stability of the measured current in function of the irradiation time under continuous irradiation; (2) the variation of the ionization current in function of applied polarization voltage and polarity. The criteria defined by LNE-LNHB for that application are as follow: (1) a maximum variation of the current of  0.1 % in comparison with its first value (over a 16 h irradiation time); (2) a saturation curve matching the ionization chamber regime, for which the absolute