Dosimetric characteristics of the small diameter BrainLab™ cones used for stereotactic radiosurgery

Gocha Khelashvili,James Chu,Julius Turian,Aidnag Diaz

doi:10.1120/jacmp.v13i1.3610

Abstract

The purpose was to study the dosimetric characteristics of the small diameter (≤10.0 mm) BrainLAB cones used for stereotactic radiosurgery (SRS) treatments in conjunction with a Varian Trilogy accelerator. Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields is completely understood. Although a number of investigators have published the dosimetric characteristics of SRS cones, to our knowledge, there is no generally accepted value for the relative output factor (ROF) for the 5.0 mm diameter cone. Therefore, we have investigated the dosimetric properties of the small (≤10.0 mm) diameter BrainLAB SRS cones used in conjunction with the iPlan TPS and a Trilogy linear accelerator with a SRS beam mode. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a SRS diode and verified with Monte Carlo (MC) simulations. The dependence of ROF on detector material response was studied. The dependence of PDD, OAR, and ROF on the alignment of the beam CAX with the detector motion line was also investigated using MC simulations. An agreement of 1% and 1 mm was observed between measurements and MC for PDD and OAR. The calculated ROF for the 5.0 mm diameter cone was 0.692±0.008 — in good agreement with the measured value of 0.683±0.007 after the diode response was corrected. Simulations of the misalignment between the beam axis and detector motion axis for angles between 0.5°–1.0° have shown a deviation > 2% in PDD beyond a certain depth. We have also provided a full set of dosimetric data for BrainLAB SRS cones. Monte Carlo calculated ROF values for cones with diameters less than 10.0 mm agrees with measured values to within 1.8%. Care should be exercised when measuring PDD and OAR for small cones. We recommend the use of MC to confirm the measurement under these conditions.PACS numbers: 87.53.Ly, 87.55.‐x, 87.53.Bn, 87.55.K‐

Highlights

IntroductionThe main source of uncertainties (other than drawbacks of particular detector systems) are volume averaging and exact positioning of the detector.[8,9] The finite size of any detector results in underestimation of measured relative output factor (ROF)[5,7,10] which, in turn, may result in significant overdosage of the small radiosurgery PTV
From the listed factors, the main source of uncertainties are volume averaging and exact positioning of the detector.[8,9] The finite size of any detector results in underestimation of measured relative output factor (ROF)[5,7,10] which, in turn, may result in significant overdosage of the small radiosurgery PTV
One of the problems that we have encountered during commissioning of BrainLAB stereotactic cones of varying diameters was a virtual absence of published dosimetric data related to usage of a particular configuration. (BrainLab cones mounted on a Varian Trilogy linear accelerator (Varian Medical Systems, Palo Alto, CA) equipped with a stereotactic radiosurgery (SRS) mode.) The goal of this study is to describe a simple methodology for obtaining a complete set of dosimetric data for the above-mentioned configuration

Summary

Introduction

The main source of uncertainties (other than drawbacks of particular detector systems) are volume averaging and exact positioning of the detector.[8,9] The finite size of any detector results in underestimation of measured ROF[5,7,10] which, in turn, may result in significant overdosage of the small radiosurgery PTV. Diamond detectors are tissue-equivalent and have a good spatial resolution These properties would make them appropriate detectors for small field measurements, they are expensive and have been shown to have dose-rate dependence.[6] In addition, volume effect correction factors cannot be estimated properly due to the irregular shape of the active volume of diamond crystal.[6]. Because of the small electron range in common diode material (such as silicon), they still represent intermediate-size cavities for typical SRS fields They introduce new issues that are associated with the energy, dose rate, and directional dependence of their responses. For correct interpretation of measurement results, modeling of Burlin’s general cavity theory is required.[2]

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