Simon Duane

Photon absorbed dose standards

In this review the current status of absorbed dose to water standards for high-energy photon beams (60Co?50?MV nominal accelerating potential) is discussed. The review is focused on calorimeter-based absorbed dose standards for photon radiation therapy calibrations with typical dose rates of a few gray per minute. In addition, two alternative types of absorbed dose standards are also discussed. The overall uncertainty on measured dose to water in static reference fields is nowadays on the order of 0.4% to 0.5%. The components contributing to the uncertainty budgets are discussed. The discussed absorbed dose to water standards are expected to continue to have their place not only in the dissemination of absorbed dose to water but also in the determination of beam quality conversion factors essential in reference dosimetry in high-energy photon beams.

Detector dose response in megavoltage small photon beams. I. Theoretical concepts

PURPOSE To explain the reasons for significant quality correction factors in megavoltage small photon fields and clarify the underlying concepts relevant to dosimetry under such conditions. METHODS The validity of cavity theory and the requirement of charged particle equilibrium (CPE) are addressed from a theoretical point of view in the context of nonstandard beams. Perturbation effects are described into four main subeffects, explaining their nature and pointing out their relative importance in small photon fields. RESULTS It is demonstrated that the failure to meet classical cavity theory requirements, such as CPE, is not the reason for significant quality correction factors. On the contrary, it is shown that the lack of CPE alone cannot explain these corrections and that what matters most, apart from volume averaging effects, is the relationship between the lack of CPE in the small field itself and the density of the detector cavity. The density perturbation effect is explained based on Fanos theorem, describing the compensating effect of two main contributions to cavity absorbed dose. Using the same approach, perturbation effects arising from the difference in atomic properties of the cavity medium and the presence of extracameral components are explained. Volume averaging effects are also discussed in detail. CONCLUSIONS Quality correction factors of small megavoltage photon fields are mainly due to differences in electron density between water and the detector medium and to volume averaging over the detector cavity. Other effects, such as the presence of extracameral components and differences in atomic properties of the detection medium with respect to water, can also play an accentuated role in small photon fields compared to standard beams.

A small-body portable graphite calorimeter for dosimetry in low-energy clinical proton beams

Calorimetry has been recommended and performed in proton beams for some time, but never has graphite calorimetry been used as a reference dosimeter in clinical proton beams. Furthermore, only a few calorimetry measurements have been reported in ocular proton beams. In this paper we describe the construction and performance of a small-body portable graphite calorimeter for clinical low-energy proton beams. Perturbation correction factors for the gap effect, volume averaging effect, heat transfer phenomena and impurity effect are calculated and applied in a comparison with ionization chamber dosimetry following IAEA TRS-398. The ratio of absorbed dose to water obtained from the calorimeter measurements and from the ionization measurements varied between 0.983 and 1.019, depending on the beam type and the ionization chamber calibration modality. Standard uncertainties on these values varied between 1.9% and 2.5% including a substantial contribution from the kQ values in IAEA TRS-398. The (Wair/e)p values inferred from these measurements varied between 33.6 J C(-1) and 34.9 J C(-1) with similar standard uncertainties. A number of improvements for the small-body portable graphite calorimeter and the experimental set-up are suggested for potential reduction of the uncertainties.

Ion recombination for ionization chamber dosimetry in a helical tomotherapy unit

PURPOSE Ion recombination for ionization chambers in pulsed high-energy photon beams is a well-studied phenomenon. Despite this, the correction for ion recombination is often determined inaccurately due to the inappropriate combination of using a high polarizing voltage and the simple two-voltage method. An additional complication arises in new treatment modalities such as IMRT and tomotherapy, where the dosimetry of a superposition of many constituting fields becomes more relevant than of single static fields. For these treatment modalities, the irradiation of the ion chamber geometry can be instantaneously inhomogeneous and time dependent. METHODS This article presents a study of ion recombination in ionization chambers used for dosimetry in a helical tomotherapy beam. Models are presented for studying the recombination correction factors in a continuous beam, in pulsed large and small fields, and in helical fields. Measurements using Exradin A1SL, NE2571, and NE2611 type chambers and Monte Carlo simulations usingPENELOPE are performed in support of these models. RESULTS Initial recombination and charge multiplication are found to be the same in C60o and in the pulsed high-energy photon beam for the chambers and operating voltages used in this study. Applying the two-voltage technique for the A1SL chamber at its recommended operating voltage of 300 V leads to an overestimation of the recombination. Operating at a voltage of 100 V yields larger but more accurate values for the recombination correction. The recombination correction measured for this chamber in the TomoTherapy HiArt unit is lower than the 1% applied in the routine dosimetry for this treatment unit. For a helical dose delivery with a small slice width, lateral electron scatter in the cavity makes that the recombination is smaller than for an open beam delivering the same total dose. In a Farmer type chamber, a helical delivery with a 1 cm slice field results in a time and spatially integrated volume recombination of 55% of that with a 2.5 cm slice field. The relative recombination corrections for different slice widths and different field offsets with respect to the chamber center obtained from the developed models are in good agreement with experimental data. CONCLUSIONS Because of the presence of charge multiplication, it is more accurate to determine the recombination correction at lower operating voltages than are often applied using the two-voltage method. Models and experiments for partial irradiation conditions of the ion chamber show that resulting recombination corrections are reduced compared to those for an open field. A model for helical dose deliveries results in recombination corrections that get lower with smaller slice widths. This model could be adapted to any IMRT delivery where the ion chamber is instantaneously partial and/or inhomogeneously irradiated, and could provide a practical procedure to calculate the recombination for complex deliveries for which it is difficult to be measured.

Detector dose response in megavoltage small photon beams. II. Pencil beam perturbation effects

PURPOSE To quantify detector perturbation effects in megavoltage small photon fields and support the theoretical explanation on the nature of quality correction factors in these conditions. METHODS In this second paper, a modern approach to radiation dosimetry is defined for any detector and applied to small photon fields. Fanos theorem is adapted in the form of a cavity theory and applied in the context of nonstandard beams to express four main effects in the form of perturbation factors. The pencil-beam decomposition method is detailed and adapted to the calculation of perturbation factors and quality correction factors. The approach defines a perturbation function which, for a given field size or beam modulation, entirely determines these dosimetric factors. Monte Carlo calculations are performed in different cavity sizes for different detection materials, electron densities, and extracameral components. RESULTS Perturbation effects are detailed with calculated perturbation functions, showing the relative magnitude of the effects as well as the geometrical extent to which collimating or modulating the beam impacts the dosimetric factors. The existence of a perturbation zone around the detector cavity is demonstrated and the approach is discussed and linked to previous approaches in the literature to determine critical field sizes. CONCLUSIONS Monte Carlo simulations are valuable to describe pencil beam perturbation effects and detail the nature of dosimetric factors in megavoltage small photon fields. In practice, it is shown that dosimetric factors could be avoided if the field size remains larger than the detector perturbation zone. However, given a detector and beam quality, a full account for the detector geometry is necessary to determine critical field sizes.

IPEM topical report 1: guidance on implementing flattening filter free (FFF) radiotherapy.

Flattening filter free (FFF) beams are now commonly available with new standard linear accelerators. These beams have recognised clinical advantages in certain circumstances, most notably the reduced beam-on times for high dose per fraction stereotactic treatments. Therefore FFF techniques are quickly being introduced into clinical use. The purpose of this report is to provide practical implementation advice and references for centres implementing FFF beams clinically. In particular UK-specific guidance is given for reference dosimetry and radiation protection.

SU‐FF‐T‐195: Dosimetry Audit for Tomotherapy Using Alanine/EPR

Purpose: To provide an audit of ion chamber‐based dosimetry for IMRT delivered by helical tomotherapy. Method and Materials: Three treatment plans were selected from the commissioning of a TomoTherapy Hi‐Art II machine using a 30 cm diameter cylindrical Virtual Water (“cheese”) phantom. For each plan, measurements were made at 6 points: two in the target volume, two in the steep dose‐gradient region just outside the target volume, and two in the low‐dose region far from the target volume, which was a 6 cm diameter cylinder. Absorbed dose was measured using two independent alanine/EPR dosimetry systems and two Exradin A1SL ion chambers. The planned dose in the target volume was 2 Gy per fraction, and 9 or 10 fractions were delivered to the phantom loaded with alanine dosimeters. The ion chambers had been calibrated in a 60Co beam at Ghent University Dosimetry Laboratory, and correction factors were applied for beam quality and ion recombination as recommended by TomoTherapy Inc. Four or five alanine dosimeter pellets were used per measurement position. The NPL alanine dosimeters were read out using a Bruker EMX spectrometer, and the ZNA‐Middelheim alanine dosimeters were read out using a desktop Bruker EMS‐104 spectrometer. Results: In the target volume, ion chamber and alanine doses agreed to better than 2%. The statistical uncertainty in absorbed dose measured using a single alanine pellet was 0.06 Gy at NPL and 0.3 Gy using the desktop spectrometer. On average, absorbed dose measured using the ZNA‐Middelheim alanine system was 3% higher than the dose measured using the NPL alanine system. Conclusions:Dosimetry audit of IMRT delivered by helical tomotherapy using alanine/EPR is both convenient and independent of the assumptions made in analysing ion chamber measurements.

Evaluation of factors to convert absorbed dose calibrations from graphite to water for the NPL high-energy photon calibration service

The National Physical Laboratory (NPL) provides a high-energy photon calibration service using 4-19 MV x-rays and 60Co gamma-radiation for secondary standard dosemeters in terms of absorbed dose to water. The primary standard used for this service is a graphite calorimeter and so absorbed dose calibrations must be converted from graphite to water. The conversion factors currently in use were determined prior to the launch of this service in 1988. Since then, it has been found that the differences in inherent filtration between the NPL LINAC and typical clinical machines are large enough to affect absorbed dose calibrations and, since 1992, calibrations have been performed in heavily filtered qualities. The conversion factors for heavily filtered qualities were determined by interpolation and extrapolation of lightly filtered results as a function of tissue phantom ratio 20,10 (TPR20,10). This paper aims to evaluate these factors for all mega-voltage photon energies provided by the NPL LINAC for both lightly and heavily filtered qualities and for 60Co y-radiation in two ways. The first method involves the use of the photon fluence-scaling theorem. This states that if two blocks of different material are irradiated by the same photon beam, and if all dimensions are scaled in the inverse ratio of the electron densities of the two media, then, assuming that all photon interactions occur by Compton scatter the photon attenuation and scatter factors at corresponding scaled points of measurement in the phantom will be identical. The second method involves making in-phantom measurements of chamber response at a constant target-chamber distance. Monte Carlo techniques are then used to determine the corresponding dose to the medium in order to determine the chamber calibration factor directly. Values of the ratio of absorbed dose calibration factors in water and in graphite determined in these two ways agree with each other to within 0.2% (1sigma uncertainty). The best fit to both sets of results agrees with values determined in previous work to within 0.3% (1sigma uncertainty). It is found that the conversion factor is not sensitive to beam filtration.

Incorporation of single elastic scattering in the EGS4 Monte Carlo code system: tests of Molière theory

Abstract To avoid prohibitively long computation times, conventional Monte Carlo e − transport algorithms (e.g. EGS4, ETRAN, ITS) employ multiple scattering theories and “condensed history” methods to model e − transport. Although highly successful for many calculations, these techniques do not model backscatter very well, particularly for high- Z materials. In an attempt to correct for this shortcoming, we have extended the EGS4 Monte Carlo code to allow for the simulation of single elastic scattering. The single scattering method also allows quantities to be scored in submicrometer dimension geometries where the Moliere multiple scattering theory fails and the Ooudsmit-Saunderson multiple scattering equations converge very slowly. Two single scattering schemes have been implemented: (i) Screened Rutherford cross sections which form the basis of Molieres multiple scattering theory, (ii) Single scattering cross sections based upon phase-shift data. In this work we describe the implementation of single elastic scattering in the EGS4 Monte Carlo code system and employ it to verify the Moliere multiple scattering theory in its range of validity. We demonstrate that the Moliere multiple scattering formalism provides a good description of multiple scattering despite its use of a relatively crude cross section and that it may be employed with semiquantitative accuracy in the plural scattering regime, where electron step-lengths are so short that only as few as five atoms participate in the angular deflection. However, the remaining differences of the Moliere distributions with the phase-shift data motivate the use of more accurate fundamental data, in particular, for applications involving high- Z elements.

A new method to determine ratios of electron stopping powers to an improved accuracy

A new method is presented to determine the ratio of electron stopping powers which is effective in the transfer of absorbed dose from one medium to another. The method involves an accurate measurement of the electron range in each of the media combined with a full Monte Carlo simulation of each experimental geometry. For the specific case of graphite and water, the uncertainty attainable is estimated to be around +/- 0.5% at the 95% confidence level, which is approximately a factor of three better than the best methods currently in use.