How flatbed scanners upset accurate film dosimetry.

Film is an excellent dosimeter for verification of dose distributions due to its high spatial resolution. Irradiated film can be digitized with low-cost, transmission, flatbed scanners. However, a disadvantage is their lateral scan effect (LSE): a scanner readout change over its lateral scan axis. Although anisotropic light scattering was presented as the origin of the LSE, this paper presents an alternative cause. Hereto, LSE for two flatbed scanners (Epson 1680 Expression Pro and Epson 10000XL), and Gafchromic film (EBT, EBT2, EBT3) was investigated, focused on three effects: cross talk, optical path length and polarization. Cross talk was examined using triangular sheets of various optical densities. The optical path length effect was studied using absorptive and reflective neutral density filters with well-defined optical characteristics (OD range 0.2-2.0). Linear polarizer sheets were used to investigate light polarization on the CCD signal in absence and presence of (un)irradiated Gafchromic film. Film dose values ranged between 0.2 to 9 Gy, i.e. an optical density range between 0.25 to 1.1. Measurements were performed in the scanner's transmission mode, with red-green-blue channels. LSE was found to depend on scanner construction and film type. Its magnitude depends on dose: for 9 Gy increasing up to 14% at maximum lateral position. Cross talk was only significant in high contrast regions, up to 2% for very small fields. The optical path length effect introduced by film on the scanner causes 3% for pixels in the extreme lateral position. Light polarization due to film and the scanner's optical mirror system is the main contributor, different in magnitude for the red, green and blue channel. We concluded that any Gafchromic EBT type film scanned with a flatbed scanner will face these optical effects. Accurate dosimetry requires correction of LSE, therefore, determination of the LSE per color channel and dose delivered to the film.

Accurate dosimetry with GafChromic EBT film of a 6 MV photon beam in water: what level is achievable?

This paper focuses on the accuracy, in absolute dose measurements, with GafChromicTM EBT film achievable in water for a 6 MV photon beam up to a dose of 2.3 Gy. Motivation is to get an absolute dose detection system to measure up dose distributions in a (water) phantom, to check dose calculations. An Epson 1680 color (red green blue) transmission flatbed scanner has been used as film scanning system, where the response in the red color channel has been extracted and used for the analyses. The influence of the flatbed film scanner on the film based dose detection process was investigated. The scan procedure has been optimized; i.e. for instance a lateral correction curve was derived to correct the scan value, up to 10%, as a function of optical density and lateral position. Sensitometric curves of different film batches were evaluated in portrait and landscape scan mode. Between various batches important variations in sensitometric curve were observed. Energy dependence of the film is negligible, while a slight variation in dose response is observed for very large angles between film surface and incident photon beam. Improved accuracy in absolute dose detection can be obtained by repetition of a film measurement to tackle at least the inherent presence of film inhomogeneous construction. We state that the overall uncertainty is random in absolute EBT film dose detection and of the order of 1.3% (1 SD) under the condition that the film is scanned in a limited centered area on the scanner and at least two films have been applied. At last we advise to check a new film batch on its characteristics compared to available information, before using that batch for absolute dose measurements.

The curvature of sensitometric curves for Kodak XV-2 film irradiated with photon and electron beams.

Sensitometric curves of Kodak XV-2 film, obtained in a time period of ten years with various types of equipment, have been analyzed both for photon and electron beams. The sensitometric slope in the dataset varies more than a factor of 2, which is attributed mainly to variations in developer conditions. In the literature, the single hit equation has been proposed as a model for the sensitometric curve, as with the parameters of the sensitivity and maximum optical density. In this work, the single hit equation has been translated into a polynomial like function as with the parameters of the sensitometric slope and curvature. The model has been applied to fit the sensitometric data. If the dataset is fitted for each single sensitometric curve separately, a large variation is observed for both fit parameters. When sensitometric curves are fitted simultaneously it appears that all curves can be fitted adequately with a sensitometric curvature that is related to the sensitometric slope. When fitting each curve separately, apparently measurement uncertainty hides this relation. This relation appears to be dependent only on the type of densitometer used. No significant differences between beam energies or beam modalities are observed. Using the intrinsic relation between slope and curvature in fitting sensitometric data, e.g., for pretreatment verification of intensity-modulated radiotherapy, will increase the accuracy of the sensitometric curve. A calibration at a single dose point, together with a predetermined densitometer-dependent parameter ODmax will be adequate to find the actual relation between optical density and dose.

Scattered radiation from applicators in clinical electron beams.

In radiotherapy with high-energy (4-25 MeV) electron beams, scattered radiation from the electron applicator influences the dose distribution in the patient. In most currently available treatment planning systems for radiotherapy this component is not explicitly included and handled only by a slight change of the intensity of the primary beam. The scattered radiation from an applicator changes with the field size and distance from the applicator. The amount of scattered radiation is dependent on the applicator design and on the formation of the electron beam in the treatment head. Electron applicators currently applied in most treatment machines are essentially a set of diaphragms, but still do produce scattered radiation. This paper investigates the present level of scattered dose from electron applicators, and as such provides an extensive set of measured data. The data provided could for instance serve as example input data or benchmark data for advanced treatment planning algorithms which employ a parametrized initial phase space to characterize the clinical electron beam. Central axis depth dose curves of the electron beams have been measured with and without applicators in place, for various applicator sizes and energies, for a Siemens Primus, a Varian 2300 C/D and an Elekta SLi accelerator. Scattered radiation generated by the applicator has been found by subtraction of the central axis depth dose curves, obtained with and without applicator. Scattered radiation from Siemens, Varian and Elekta electron applicators is still significant and cannot be neglected in advanced treatment planning. Scattered radiation at the surface of a water phantom can be as high as 12%. Scattered radiation decreases almost linearly with depth. Scattered radiation from Varian applicators shows clear dependence on beam energy. The Elekta applicators produce less scattered radiation than those of Varian and Siemens, but feature a higher effective angular variance. The scattered radiation decreases somewhat with increasing field size and is spread uniformly over the aperture. Experimental results comply with the results of simulations of the treatment head and electron applicator, using the BEAM Monte Carlo code, and Siemens, but feature a higher effective angular variance. The scattered radiation decreases somewhat with increasing field size and is spread uniformly over the aperture. Experimental results comply with the results of simulations of the treatment head and electron applicator, using the BEAM Monte Carlo code.

Conversion of measured percentage depth dose to tissue maximum ratio values in stereotactic radiotherapy.

For many treatment planning systems tissue maximum ratios (TMR) are required as input. These tissue maximum ratios can be measured with a 3D computer-controlled water phantom; however, a TMR measurement option is not always available on such a system. Alternatively TMR values can be measured 'manually' by lowering the detector and raising the water phantom with the same distance, but this makes TMR measurements time consuming. Therefore we have derived TMR values from percentage depth dose (PDD) curves. Existing conversion methods express TMR values in terms of PDD, phantom scatter factor (Sp), and inverse square law. For stereotactic treatments circular fields ranging from 5-50 mm (19 cones) are used with the treatment planning system XKnife (Radionics). The calculation of TMR curves for this range is not possible with existing methods. This is because PDD curves of field sizes smaller than 5 mm (smallest cone size) are needed, but these cones are not provided. Besides, for field sizes smaller than 40 mm, the phantom scatter factor is difficult to determine and will introduce significant errors. To overcome these uncertainties, an alternative method has been developed to obtain TMR values from PDD data, where absolute doses are expressed in terms of PDD, total scatter factor and inverse square law. For each depth, the dose as a function of field size is fitted to a double exponential function. Then the TMR is calculated by taking the ratio of this function at the depth of interest and the reference depth, for the correct field size. For all 19 cones the total scatter factor and PDDs have been measured with a shielded diode in water for a 6 MV photon beam. Calculated TMR curves are compared with TMR values measured with a diode. The agreement is within 2%. Therefore this relatively simple conversion method meets the required accuracy for daily dose calculation in stereotactic radiotherapy. In principle this method could also be applied for other small field sizes such as those formed with a mini multileaf collimator.

A model to determine the initial phase space of a clinical electron beam from measured beam data.

Advanced electron beam dose calculation models for radiation oncology require as input an initial phase space (IPS) that describes a clinical electron beam. The IPS is a distribution in position, energy and direction of electrons and photons in a plane in front of the patient. A method is presented to derive the IPS of a clinical electron beam from a limited set of measured beam data. The electron beam is modelled by a sum of four beam components: a main diverging beam, applicator edge scatter, applicator transmission and a second diverging beam. The two diverging beam components are described by weighted sums of monoenergetic diverging electron and photon beams. The weight factors of these monoenergetic beams are determined by the method of simulated annealing such that a best fit is obtained with depth-dose curves measured for several field sizes at two source-surface distances. The resulting IPSs are applied by the phase-space evolution electron beam dose calculation model to calculate absolute 3D dose distributions. The accuracy of the calculated results is in general within 1.5% or 1.5 mm; worst cases show differences of up to 3% or 3 mm. The method presented here to describe clinical electron beams yields accurate results, requires only a limited set of measurements and might be considered as an alternative to the use of Monte Carlo methods to generate full initial phase spaces.

On the initial angular variances of clinical electron beams.

Electron beam radiotherapy treatment planning systems need to be fed with the characteristics of the high-energy electron beams (4-50 MeV) from the specifically applied accelerator. Beams can be characterized by their mean initial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This information is the only input to pencil beam dose calculation models. Newer calculation models like macro Monte Carlo, voxel Monte Carlo and phase space evolution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam component and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to investigate to what extent standard values can be used both for the effective initial angular variance as input to pencil beam models and for the primary beam initial angular variance. Comprehensive benchmark data were obtained on the initial angular variance of various types of accelerator, for various energies and field sizes. The initial angular variance sigma2theta(x) has been derived from penumbra measurements in air by means of film dosimetry at various distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for sigma2theta(x)/T(E) of around 13 cm where T(E) is the ICRU-35 linear angular scattering power in air. This value can be chosen as standard value for the primary beam initial angular variance, only slightly compromising the dose calculation accuracy. As input to pencil beam models, an effective sigma2theta(x)/T(E) should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-10 cm) an effective sigma2theata(x)/T(E) of 20 cm has been found and is recommended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 50% higher for the effective sigma2theta(x)/T(E).

Calculation of a pencil beam kernel from measured photon beam data.

Usually, pencil beam kernels for photon beam calculations are obtained by Monte Carlo calculations. In this paper, we present a method to derive a pencil beam kernel from measured beam data, i.e. central axis depth doses, phantom scatter factors and off-axis ratios. These data are usually available in a radiotherapy planning system. The differences from other similar works are: (a) the central part of the pencil beam is derived from the measured penumbra of large fields and (b) the dependence of the primary photon fluence on the depth caused by beam hardening in the phantom is taken into account. The calculated pencil beam will evidently be influenced by the methods and instruments used for measurement of the basic data set. This is of particular importance for an accurate prediction of the absorbed dose delivered by small fields. Comparisons with measurements show that the accuracy of the calculated dose distributions fits well in a 2% error interval in the open part of the field, and in a 2 mm isodose shift in the penumbra region.

Film dosimetry in water in a 23 MV therapeutic photon beam.

The field size and water depth dependence of the measured optical density of Kodak XV-2 film, irradiated in a 23 MV photon beam has been investigated. The films were positioned in a water tank in a vertical plane containing the beam axis with the upper film edge parallel to the water surface at a depth of 0.3 mm. The observed field size and water depth dependence of the film sensitivity cannot be fully attributed to the usual variation of the photon spectrum with field size and water depth: measured optical densities do significantly depend on the amount of film material above the point of measurement and on the film orientation. A method for application of film for relative water dose measurements in a plane containing the beam axis in a 23 MV therapeutic photon beam is presented; the observed agreement between film and ionisation chamber measurements is very good: typically within 1% or 2 mm.

Should inhomogeneity corrections be applied during treatment planning of tangential breast irradiation?

Due to the inclusion of lung tissue in the treatment volume, some parts of the breast will get a higher dose during tangential breast irradiation because of the lower lung density. Data on the accuracy of dose calculation algorithms, investigated by phantom measurements, determinations of the geometry and density of the actual lung in the patient and the results of in vivo dose measurements, are presented. From this information it can be concluded that a lung correction varying between about 3% and 7% is needed but its magnitude is slightly overpredicted in a number of commercial treatment planning systems. Because this increase in dose is already in a high dose region, it is recommended that inhomogeneity corrections should be applied during tangential breast irradiation.

Three-dimensional dose distribution of tangential breast treatment: a national dosimetry intercomparison.

From August 1990 to February 1991, a dosimetry intercomparison of breast treatment was performed at all 21 radiotherapy centres in The Netherlands. The absorbed dose was measured in three planes in a breast phantom during tangential breast irradiation, according to a prescribed technique. The beam energy could be chosen by the radiotherapy centre as normally applied for this type of "patient", and varied between 60Co and 8 MV X rays. The dose measured by the visiting team in 22 points inside the phantom was compared with the dose calculated by the institution using their local treatment planning system. In the institutions the mean ratio (the mean value of the ratios of the absolute calculated dose and the measured absolute dose in the 22 points) varied between 0.92 and 1.08 with an overall mean ratio of 1.04. There was no significant difference in this ratio between the three planes in a particular institution. In the isocentre the mean ratio of calculated and measured dose was 1.021 with a SD of 0.028, i.e. the algorithms in the six different commercial treatment planning systems calculate the dose generally somewhat too high. In order to explain the results, a measurement of the output under reference conditions was performed at each treatment unit. The mean ratio of the dose stated by the institution and the dose measured by the visiting team was 1.011 with a SD of 0.015 with a maximum deviation of 0.040. This small deviation explains therefore only part of the variation in the ratio of calculated and measured dose for tangential breast irradiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Film dosimetry of clinical electron beams.

Film dosimetry is used widely to obtain relative dose distributions of clinical electron beams in phantoms. Nevertheless, measurement results obtained with film dosimetry may lack precision and reliability. In this paper well defined and reproducible methods in film dosimetry are discussed. By application of these methods, film dosimetry appears to be adequate in measuring relative dose distributions of clinically applied electron beams, with an accuracy of 1% to 2% of the dose maximum, in water and plastics as well as in heterogeneously composed material.