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A Monte Carlo study to investigate the dosimetric accuracy in the small field regime Stradiotto, Marco


Stereotactic radiosurgery (SRS) is a technique where an external photon beam is used to deliver a high dose of ionizing radiation to a small, localized brain lesion. The treatment fields used in this technique can be of the order of 1 cm for a circular collimated field. Accurate dosimetry of small-field photon beams such as those used in stereotactic radiosurgery is difficult because of the presence of steep dose gradients and lateral electronic disequilibrium (LED). Conventional measurement devices and treatment planning systems (TPS) have difficulty in determining dose accurately for small treatment fields where LED is present. Further, the pencil beam algorithm in certain TPS has been known to have certain limitations in regards to dose calculation in inhomogeneous media where LED is present. The use of Monte Carlo simulations in radiotherapy has been established as an accurate way of assessing dose distributions in inhomogeneous media regardless of LED, field size and for beam obliquity. In this work a Varian iX linear accelerator equipped with a micro-multileaf collimator was modeled using the Monte Carlo code BEAMnrc and dose calculations were performed with the Monte Carlo code DOSXYZnrc to accurately predict relative dose factors (RDF) and inhomogeneity correction factors (ICF) in inhomogeneous bone and air phantoms. The results were compared with ion chamber measurements and calculations with the TPS BrainSCAN (Brainlab, Germany). After benchmarking our BEAMnrc linear accelerator with measured data and obtaining good agreement, further calculations to determine RDF and ICF were performed with DOSXYZnrc. The RDF values obtained with Monte Carlo simulation agree to within 1% with their corresponding measured values and the ICF values obtained with Monte Carlo simulation agree to within 1% with their corresponding values obtained from BrainSCAN. Based on our observations and data we conclude that Monte Carlo is an effective tool for calculating RDFs for small radiation fields where LED is a problem and for quantifying the possible limitations of a dose calculation algorithm in the region of LED. In general, we also conclude that the TPS BrainSCAN inaccurately predicts dose in the presence of lower density inhomogeneities and this inaccuracy becomes worse for smaller field sizes.

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