Inverse method for in vivo dosimetry using mosfet in conformal arctherapy for stereotactic radiosurgery

• Published in: Physica medica Vol. 29; pp. e8 - e9
• Main Authors: Sors, A, Cassol, E, Latorzeff, I, Duthil, P, Sabatier, J, Lotterie, J.A, Redon, A, Berry, I, Franceries, X
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2013
• Subjects: Radiology
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2013.08.031

Introduction The aim of this work is to develop an inverse method of dose reconstruction from surface measurements performed by MOSFET for stereotactic radiosurgery treatments delivered by conformal arctherapy. Materials and methods Measurements were performed on a Novalis 600 N (lMLC m3, BrainLab) with 5 MOSFET sensors (TN-502-RD, BestMedical, Canada). Detectors were positioned at the surface of the spherical phantom Lucy©3D-QA (Standard Imaging). Four MOSFET positions either within the arc or not were studied: – n°1 5 MOSFET at the entrance of the beam, uniformly spread out within the arc and from the start angle, – n°2 5 MOSFET at the entrance of the beam uniformly spread out from the centre of the arc, – n°3 3 MOSFET at the entrance of the beam uniformly spread out on the entire arc and 2 MOSFET at the exit, – n°4 3 MOSFET at the entrance of the beam equidistant from the centre of the arc and 2 at the exit. Dose reconstruction was computed with Matlab (MathWorks) by solving the forward method from different parameters. – isocenter dose: 1 to 7 Gy, – arc amplitude: 90 to 170°, – field size: 18 × 18, 24 × 24, 30 × 30 and 42 × 42 mm2. By solving an inverse problem, readings of each sensor were used as entrance parameters to compute total isocenter dose (DT) during conformal arctherapy.The reproducibility of the MOSFET readings and deviation between theoretical and measured DT were evaluated. Results Isocenter dose attenuation due to MOSFET is negligible. Acceptable MOSFET variability (∼2%) is obtained between 1 and 4 Gy whatever arc amplitude and geometric configuration. Considering all MOSFET response in the DT computation, the inverse method gives satisfying results for each case, with a dose deviation ranging from −2.4% to 0.75% at 30 × 30 mm2 and from −3.7% to −2.5% at18 × 18 mm2. Conclusion Our results are promising and permit to validate the forward model and the inverse method implemented for selective surface measurements by MOSFET in conformal arctherapy, down to 18 × 18 mm2.