Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife® and linear accelerators equipped with microMLC and circular cones

International audiencePurpose The use of small photon fields is now an established practice in stereotactic radiosurgery and radiotherapy. However, due to a lack of lateral electron equilibrium and high dose gradients, it is difficult to accurately measure the dosimetric quantities required for the commissioning of such systems. Moreover, there is still no metrological dosimetric reference for this kind of beam today. In this context, the first objective of this work was to determine and to compare small fields output factors (OF) measured with different types of active detectors and passive dosimeters for three types of facilities a CyberKnife® system, a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC and circular cones, and an adaptive medical linear accelerator (Clinac 2100) equipped with an additional m3 microMLC. The second one was to determine the kQclin,Q msrfclin,fmsr correction factors introduced in a recently proposed small field dosimetry formalism for different active detectors. Methods Small field sizes were defined either by microMLC down to 6 × 6 mm2 or by circular cones down to 4 mm in diameter. OF measurements were performed with several commercially available active detectors dedicated to measurements in small fields (high 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). Two types of passive dosimeters were used LiF microcubes and EBT2 radiochromic films. Results Significant differences between the results obtained by several dosimetric systems were observed, particularly for the smallest field size for which the difference in the measured OF reaches more than 20%. For passive dosimeters, an excellent agreement was observed (better than 2%) between EBT2 and LiF microcubes for all OF measurements. Moreover, it has been shown that these passive dosimeters do not require correction factors and can then be used as reference dosimeters. Correction factors for the active detectors have then been determined from the mean experimental OF measured by the passive dosimeters. Conclusions Four sets of correction factors needed to apply the new small field dosimetry formalism are provided for several active detectors. A protocol for small photon beams OF determination based on passive dosimeters measurements has been recently proposed to French radiotherapy treatment centers. © 2013 American Association of Physicists in Medicine

De la construction du bunker à la prise en charge du patient : contrôles qualité des techniques modernes de radiothérapie

L'installation et l'utilisation d'un nouvel équipement de radiothérapie requièrent une politique de qualité et de sécurité adaptées. Le processus aboutissant à la mise en service d'un accélérateur suivant la construction d'un bunker comporte, entre autres, l'installation de l'accélérateur, la vérification de la conformité au cahier des charges, la signature du cahier de réception, ainsi que le processus de caractérisation et de modélisation de l'accélérateur avant une utilisation clinique. L'essor des techniques de radiothérapie modernes, telles que la radiothérapie conformationnelle avec modulation d'intensité et la radiothérapie en conditions stéréotaxiques, a abouti à une complexité des différents contrôles qualités. L'objet de cet article est d'expliciter les différentes étapes de la mise en place de techniques de radiothérapie innovantes et de préciser leurs spécificités