A finite element method for the determination of the relative response of ionization chambers in MR-linacs: simulation and experimental validation up to 1.5 T.

In magnetic resonance (MR) guided radiotherapy, the magnetic field-dependent change in the dose response of ionization chambers is typically included by means of a correction factor [Formula: see text]. This factor can be determined experimentally or calculated by means of Monte Carlo (MC) simulations. To date, a small number of experimental values for [Formula: see text] at magnetic flux densities above 1.2 T have been available to benchmark these simulations. Furthermore, MC simulations of the dose response of ionization chambers in magnetic fields (where such simulations are based on manufacturer blueprints) have been shown to converge with results that deviate considerably from experimental values for orientations where the magnetic field is perpendicular to the axis of the ionization chamber and the influence of the magnetic field is largest. In this work, [Formula: see text] was simulated for a PTW 30013 Farmer ionization chamber using an approach based on finite element simulations. First, the electrical field inside the ionization chamber was simulated using finite element methods. The collecting volume of the ionization was not defined in terms of the physical dimensions of the detector but in terms of the simulated electrical field lines inside the chamber. Then, an MC simulation of the dose response of a Farmer type chamber (PTW 30013) was performed using EGSnrc with a dedicated package to consider the effect of the magnetic field. In the second part, [Formula: see text] was determined experimentally for two different PTW 30013 ionization chambers for a range of magnetic flux densities between B = 0 and 1.5 T, covering the range of commercially available MR-linacs. In the perpendicular orientation, the maximum difference between the simulated values for [Formula: see text] and the experimental values for [Formula: see text] was 0.31(30)% and the minimum difference was 0.02(24)%. For the PTW 30013 ionization chambers, the experimental values for [Formula: see text] were 0.9679(1) and 0.9681(1) for a magnetic flux density of 1.5 T. The value resulting from the simulation was 0.967(3). The comparison of the correction factors simulated using this new approach with the experimental values determined in this study shows excellent agreement for all magnetic flux densities up to 1.5 T. Integrating the explicit simulation of the collection volume inside the ionization chambers into the MC simulation model significantly improves simulations of the chamber response in magnetic fields. The results presented suggest that intra-type variations for [Formula: see text] may be neglectable for ionization chambers of the PTW 30013 type.

Ionization chamber correction factors for MR-linacs.

Previously, readings of air-filled ionization chambers have been described as being influenced by magnetic fields. To use these chambers for dosimetry in magnetic resonance guided radiotherapy (MRgRT), this effect must be taken into account by introducing a correction factor k B. The purpose of this study is to systematically investigate k B for a typical reference setup for commercially available ionization chambers with different magnetic field strengths. The Monte Carlo simulation tool EGSnrc was used to simulate eight commercially available ionization chambers in magnetic fields whose magnetic flux density was in the range of 0-2.5 T. To validate the simulation, the influence of the magnetic field was experimentally determined for a PTW30013 Farmer-type chamber for magnetic flux densities between 0 and 1.425 T. Changes in the detector response of up to 8% depending on the magnetic flux density, on the chamber geometry and on the chamber orientation were obtained. In the experimental setup, a maximum deviation of less than 2% was observed when comparing measured values with simulated values. Dedicated values for two MR-linac systems (ViewRay MRIdian, ViewRay Inc, Cleveland, United States, 0.35 T/ 6 MV and Elekta Unity, Elekta AB, Stockholm, Sweden, 1.5 T/7 MV) were determined for future use in reference dosimetry. Simulated values for thimble-type chambers are in good agreement with experiments as well as with the results of previous publications. After further experimental validation, the results can be considered for definition of standard protocols for purposes of reference dosimetry in MRgRT.

Craniospinal radiotherapy in an advanced technique.

BACKGROUND AND PURPOSE: Whole craniospinal irradiation cannot be achieved in one field at a normal treatment distance for adults. The aim of this newly developed technique is to minimize problems of matching fields and to maximize precision of craniospinal radiotherapy. PATIENTS AND METHODS: Twelve patients (3-59 years) had craniospinal irradiation in supine position. The head was treated with lateral opposed isocentric fields with collimator rotation and isocentric table rotation. Using an extended source-skin distance of 160 cm only one dorsal field is necessary to cover the whole spinal axis. To avoid systematic under- or overdosage, junction field edges were moved twice by 1.5 cm. Treatment planning was performed based on CT scans. For visual verification of field matching an additional line laser was first adjusted to the caudal edge of one lateral light field and then checked against the light field of the spinal field under the table. RESULTS: Control films show good homogeneity in the junction between lateral and vertical fields. Reproducibility of table movements is acceptable. Total time needed for one fraction is about 15-20 min. CONCLUSION: The described technique is now well established, feasible and leads to less risk of dose uncertainties.

[Does the measured dose change when applying the new DIN 6800-2 (2008) versus the edition from 1997?]. / Andert sich die gemessene Dosis bei Anwendung der neuen DIN 6800-2 (2008) gegenüber der Ausgabe aus dem Jahr 1997?

New edition of DIN 6800-2 (1997) has been published in March 2008. The concept of absorbed dose to water has been retained unchanged. In many points modern data and approaches were adopted to international dosimetry protocols. For the first time values for the pertubation correction factors of plane parallel chambers are given in a dosimetry protocol. This enables the customer based on a Co-60 calibration factor to measure absorbed dose to water without any cross-calibration. In this paper new edition will be presented and compared with the old one. But main focus is set on the question, is there any deviation in the determination of dose when applying both protocols to same measured values. For photon beams and for in Germany common used types of ionization chambers the deviations are not larger than about 0.3% and for other types not larger than 0.5%. However, in electron beams partly larger deviations up to 0.5% and for some types of ionization chambers even more than 1% may occur.

[Monitor tables for electron beams in radiotherapy]. / Monitortabellen für Elektronenstrahlung in der Strahlentherapie.

The application of electron beams in radiotherapy is still based on tables of monitor units, although 3-D treatment planning systems for electron beams are available. This have several reasons: The need for 3-D treatment planning is not recognized; there is no confidence in the calculation algorithm; Monte-Carlo algorithms are too time-consuming; and the effort necessary to measure basic beam data for 3-D planning is considered disproportionate. However, the increasing clinical need for higher dosimetric precision and for more conformal electron beams leads to the requirement for more sophisticated tables of monitor units. The present paper summarizes and discusses the main aspects concerning the preparation of tables of monitor units for electron beams. The measurement equipment and procedures for measuring basic beam data needed for tables of monitor units for electron beams are described for a standard radiation therapy linac. The design of tables of monitor units for standard electron applicators is presented; this design can be extended for individual electron inserts, to variable applicator surface distances, to oblique beam incidence, and the use of bolus material. Typical data of an Elekta linac are presented in various tables.