Phantom Scatter Correction Factors Research Articles

Commissioning of total body irradiation using plastic bead bags.

The goal of total body irradiation (TBI) is to deliver a dose to the whole body with uniformity within ±10%. The purpose of this study was to establish the technique of TBI using plastic bead bags. A lifting TBI bed, Model ORP-TBI-MN, was used. The space between the patient’s body and the acrylic walls of the bed was filled with polyacetal bead bags. Patients were irradiated by a 10 MV photon beam with a source to mid-plane distance of 400 cm. The monitor unit (MU) was calculated by dose-per-MU, tissue-phantom-ratio and a spoiler factor measured in solid water using an ionization chamber. The phantom-scatter correction factor, off-center ratio and the effective density of the beads were also measured. Diode detectors were used for in vivo dosimetry (IVD). The effective density of the beads was 0.90 ± 0.09. The point doses calculated in an I’mRT phantom with and without heterogeneity material showed good agreement, with measurements within 3%. An end-to-end test was performed using a RANDO phantom. The mean ± SD (range) of the differences between the calculated and IVD-measured mid-plane doses was 1.1 ± 4.8% (−5.9 to 5.0%). The differences between the IVD-measured doses and the doses calculated with Acuros XB of the Eclipse treatment planning system (TPS) were within 5%. For two patients treated with this method, the differences between the calculated and IVD-measured doses were within ±6% when excluding the chest region. We have established the technique of TBI using plastic bead bags. The TPS may be useful to roughly estimate patient dose.

A novel simple phantom for verifying the dose of radiation therapy.

A standard protocol of dosimetric measurements is used by the organizations responsible for verifying that the doses delivered in radiation-therapy institutions are within authorized limits. This study evaluated a self-designed simple auditing phantom for use in verifying the dose of radiation therapy; the phantom design, dose audit system, and clinical tests are described. Thermoluminescent dosimeters (TLDs) were used as postal dosimeters, and mailable phantoms were produced for use in postal audits. Correction factors are important for converting TLD readout values from phantoms into the absorbed dose in water. The phantom scatter correction factor was used to quantify the difference in the scattered dose between a solid water phantom and homemade phantoms; its value ranged from 1.084 to 1.031. The energy-dependence correction factor was used to compare the TLD readout of the unit dose irradiated by audit beam energies with 60Co in the solid water phantom; its value was 0.99 to 1.01. The setup-condition factor was used to correct for differences in dose-output calibration conditions. Clinical tests of the device calibrating the dose output revealed that the dose deviation was within 3%. Therefore, our homemade phantoms and dosimetric system can be applied for accurately verifying the doses applied in radiation-therapy institutions.

Monte Carlo simulation of the effect of miniphantom on in‐air output ratio

The aim of the study was to quantify the effect of miniphantoms on in-air output ratio measurements, i.e., to determine correction factors for in-air output ratio. Monte Carlo (MC) simulations were performed to simulate in-air output ratio measurements by using miniphantoms made of various materials (PMMA, graphite, copper, brass, and lead) and with different longitudinal thicknesses or depths (2-30 g/cm2) in photon beams of 6 and 15 MV, respectively, and with collimator settings ranging from 3 x 3 to 40 x 40 cm2. EGSnrc and BEAMnrc (2007) software packages were used. Photon energy spectra corresponding to the collimator settings were obtained from BEAMnrc code simulations on a linear accelerator and were used to quantify the components of in-air output ratio correction factors, i.e., attenuation, mass energy absorption, and phantom scatter correction factors. In-air output ratio correction factors as functions of miniphantom material, miniphantom longitudinal thickness, and collimator setting were calculated and compared to a previous experimental study. The in-air output ratio correction factors increase with collimator opening and miniphantom longitudinal thickness for all the materials and for both energies. At small longitudinal thicknesses, the in-air output ratio correction factors for PMMA and graphite are close to 1. The maximum magnitudes of the in-air output ratio correction factors occur at the largest collimator setting (40 x 40 cm2) and the largest miniphantom longitudinal thickness (30 g/cm2): 1.008 +/- 0.001 for 6 MV and 1.012 +/- 0.001 for 15 MV, respectively. The MC simulations of the in-air output ratio correction factor confirm the previous experimental study. The study has verified that a correction factor for in-air output ratio can be obtained as a product of attenuation correction factor, mass energy absorption correction factor, and phantom scatter correction factor. The correction factors obtained in the present study can be used in studies involving in-air output ratio measurements using miniphantoms.

Determination of the primary dose component and primary linear attenuation coefficient in a 6 MV photon beam using a small attenuator

It is a common technique in radiotherapy treatment planning systems to simplify the calculations by splitting the radiation beam into two components: namely the primary and scattered components. The contributions of the two components are evaluated separately and then summed to give the dose at the point of interest. Usually the primary dose is obtained experimentally by extrapolating the ionization measured within the medium to zero-field size ( Godden, 1983 ). This approach offers the opportunity to obtain the primary component of dose without the need for a non-linear extrapolation. It is based on a paper by Nizin and Kase from 1988. The primary dose can be obtained from four measurements of ionization in narrow beam geometry and two measurements of ionization in a large beam in a phantom. If these measurements are performed over a range of different depths, the primary linear attenuation coefficient can also be obtained. The value for the primary dose at d max in a 10 cm × 10 cm field obtained in a 6 MV beam using this method is D p ( d max , 10 cm × 10 cm) = 0.925 Gy/100 MU for a 1 cm thick lead attenuator and is D p ( d max , 10 cm × 10 cm) = 0.941 Gy/100 MU for a 2 cm thick lead attenuator. The primary linear attenuation coefficient is μ 0 = 0.0445 ± 0.0007 cm −1 . The obtained values of the primary dose component compare well with the extrapolation of the phantom scatter correction factor to zero-field size from measurements done in the same beam and also to literature ( Rice and Chin, 1990 ). One can thus conclude that this method has the potential to provide an independent measurable verification of calculations of primary dose.

Design of mini phantom and measurement of cobalt-60 beam data parameters

Low cost mini phantoms were fabricated indigenously with different water equivalent material such as polymethyl methacrylate and Bee's wax of different shapes (with dome top surface and flat top surface). The beam parameters of the Co-60 machine, such as head scatter correction factor (Sh), phantom scatter correction factor (SP), total scatter correction factor (SC,P), collimator exchange effect were measured. Output ratio measurements were taken for both mini phantom and water phantom for different square and rectangular field sizes. Normalized output ratios were compared with ESTRO published values and (Storchi and Van Gasteren) S and G data. The percentage of variation between the measured and the literature values is about 0.7%. Collimator exchange effect were measured for water and mini phantom for different field size, were compared with ESTRO value. This was found to be 0.5% and 1.0% respectively. Phantom scatter correction factors were calculated for square and rectangular filed sizes; this was compared with ESTRO values, found to be 0.7% for square and 1.0% for rectangular filed size. It was also noted that there were no appreciable variation observed in ion chamber readings of different materials of mini phantoms for dome and flat surfaces. Mini phantom measurements were done for all types of phantoms and the measured values were compared with the existing data and they were in good agreement with the published values.

Measurement of in-air output ratios using different miniphantom materials

In-air output ratios, Sc, were measured using miniphantoms made of PMMA (thickness 2.4–24 g cm−2), graphite (1.8–26.5 g cm−2), copper (1.6–23.3 g cm−2) and lead (2.3–21.6 g cm−2), for collimator settings of 3 × 3 to 40 × 40 cm2, and x-ray energies of 6 MV and 15 MV, respectively. The effects of the miniphantom on Sc were quantified as correction factors as functions of collimator setting, material types and miniphantom thickness for each photon energy to correct the measured values. For miniphantoms with sufficient thickness to eliminate electron disequilibrium, the total correction factors can be expressed as multiplications of three factors: the attenuation correction factor, the mass energy absorption correction factor and the phantom scatter correction factor. This formalism implies that the collimator setting dependence of the correction factor is mainly caused by the energy spectrum shift. The narrow-beam attenuation coefficients in various phantom materials for different collimator settings were determined in narrow-beam geometries using a specially constructed collimator mounted on the tray holder of the accelerator. We have determined that the maximum total correction factor is approximately 1.01. For miniphantoms made of PMMA, graphite, copper and lead, at the miniphantom thickness of 10 g cm−2, the maximum total correction factors are 1.002, 1.003, 1.005, 1.007, and 1.002, 1.003, 1.008 and 1.009 for 6 MV and 15 MV, respectively.

The separation of the head and phantom scatter components from a phase space description

The formalism based on phantom and collimator scatter factors for high energy photon beams is deduced using a phase space description. The phantom scatter factors (Sp) depend on the field size and shape at the level of the phantom and are generally considered independent of the collimation details used to form the desired field provided the effect of contaminant electrons can be neglected. As demonstrated in this work, this behaviour leads to the applicability of the Clarkson method in irregular fields. However, for a given field formed with a tertiary collimator it is not a priori clear that the variations of extrafocal radiation due to secondary collimator setting do not affect the phantom scatter correction factors. In fact, the extrafocal radiation has a lower mean energy than that of unscattered photons, and this radiation can reach points well outside the radiation field increasing the irradiated phantom volume. Besides, transmission through the blocks contributes to phantom scatter. Therefore, for a given block-defined field, the associated phantom scatter dose, per unit of fluence in air on the central axis, should in principle increase when enlarging the secondary collimator field. To confirm this, isocentric Sp data for 6 MV photons were measured at 10 cm depth in water, reducing with cerrobend blocks several fields formed with the secondary collimators. In particular, when a 30 × 30 cm2 collimator field is reduced with blocks to a 7 × 7 cm2 field, the dose per unit of fluence in air is 1.4% higher than that of the square collimator field equating the given block field. Our calculations indicate that in this case the block transmission accounts for only 0.2% of this increment, showing that the remaining effect is due to extrafocal radiation. As a concluding remark, this work contributes to a better understanding of the classical Clarkson method for irregular fields giving, additionally, a formal interpretation of the commonly used quantities.

On empirical methods to determine scatter factors for irregular MLC shaped beams.

Multileaf collimators (MLCs) are in clinical use for more than a decade and are a well accepted tool in radiotherapy. For almost each MLC design different empirical or semianalytical methods have been presented for calculating output ratios in air for irregularly shaped beams. However, until now no clear recommendations have been given on how to handle irregular fields shaped by multileaf collimators for independent monitor unit (MU) verification. The present article compares different empirical methods, which have been proposed for independent MU verification, to determine (1) output ratios in air (Sc) and (2) phantom scatter factors (Sp) for irregular MLC shaped fields. Ten dedicated field shapes were applied to five different types of MLCs (Elekta, Siemens, Varian, Scanditronix, General Electric). All calculations based on empirical relations were compared with measurements and with calculations performed by a treatment planning system with a fluence based algorithm. For most irregular MLC shaped beams output ratios in air could be adequately modeled with an accuracy of about 1%-1.5% applying a method based on the open field aperture defined by the leaf and jaw setting combined with the equivalent square formula suggested by Vadash and Bjärngard [P. Vadash and B. E. Bjärngard, Med. Phys. 20, 733-734 (1993)]. The accuracy of this approach strongly depends on the inherent head scatter characteristics of the accelerator in use and on the irregular field under consideration. Deviations of up to 3% were obtained for fields where leaves obscure central parts of the flattening filter. Simple equivalent square methods for Sp calculations in irregular fields did not provide acceptable results (deviations mostly >3%). Sp values derived from Clarkson integration, based on published tables of phantom scatter correction factors, showed the same accuracy level as calculations performed using a pencil beam algorithm of a treatment planning system (in a homogeneous media). The separation of head scatter and phantom scatter contributions is strongly recommended for irregular MLC shaped beams as both contributions have different factors of influence. With rather simple methods Sc and Sp can be determined for independent MU calculation with an accuracy better than 1.5% for most clinical situations encountered in conformal radiotherapy.

Monitor unit calculation on the beam axis of open and wedged asymmetric high-energy photon beams

An ESTRO booklet and a report of the Netherlands Commission on Radiation Dosimetry have been published recently describing empirical methods for monitor unit (MU) calculations in symmetrical high-energy photon beams. Both documents support the same basic ideas; firstly the separation of head scatter and volume scatter components and secondly the determination of head scatter quantities in a mini-phantom. Based on these ideas the methods previously described for MU calculations in symmetrical beams are extended to asymmetrical open and wedged beams in isocentric treatment conditions. All required dosimetric parameters (normalized head scatter factors, phantom scatter correction factors, wedge factors, off-axis ratios, quality index, and depth dose parameters) are determined as a function of beam axis position in order to study their off-axis dependence. Measurements are performed for 6 MV and 18 MV photon beams provided by two different dual-energy linear accelerators, a GE Saturne 42 and a Varian 2100 CD linac.

Effect of electron contamination on scatter correction factors for photon beam dosimetry.

Physical quantities for use in megavoltage photon beam dose calculations which are defined at the depth of maximum absorbed dose are sensitive to electron contamination and are difficult to measure and to calculate. Recently, formalisms have therefore been presented to assess the dose using collimator and phantom scatter correction factors, Sc and Sp, defined at a reference depth of 10 cm. The data can be obtained from measurements at that depth in a miniphantom and in a full scatter phantom. Equations are presented that show the relation between these quantities and corresponding quantities obtained from measurements at the depth of the dose maximum. It is shown that conversion of Sc and Sp determined at a 10 cm depth to quantities defined at the dose maximum such as (normalized) peak scatter factor, (normalized) tissue-air ratio, and vice versa is not possible without quantitative knowledge of the electron contamination. The difference in Sc at dmax resulting from this electron contamination compared with Sc values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplication factor, Scel, for a number of photon beams of different accelerator types. It is shown that Scel may vary up to 5%. Because in the new formalisms output factors are defined at a reference depth of 10 cm, they do not require Scel data. The use of Sc and Sp values, defined at a 10 cm depth, combined with relative depth-dose data or tissue-phantom ratios is therefore recommended. For a transition period the use of the equations provided in this article and Scel data might be required, for instance, if treatment planning systems apply Sc data normalized at d(max).

Methods for beam data acquisition offered by a mini-phantom

Mini-phantoms are an important tool for measurement of basic head scatter parameters in high-energy photon beams, and recently they have also been used for beam quality specification. Therefore the feasibility and reliability of basic beam parameter acquisition using only a mini-phantom is checked in 6, 18 and 25 MV photon beams. These parameters include head scatter correction factors, phantom scatter correction factors, total scatter correction factors, wedge factors, off-axis ratios, as well as beam attenuation coefficients and beam hardening coefficients. In order to specify beam quality variations and beam quality modifications by a wedge, two different methods are compared: the first method uses a constant source to chamber distance of 1 m, the second method refers to narrow beam geometry. values derived with two different beam quality specification methods show a systematic deviation. However, relative variations of the attenuation coefficient within the beam and the associated beam quality modifications observed with the two methods show good agreement in open and wedged beams. Phantom scatter correction factors are calculated from measured head scatter correction factors and total scatter correction factors as well as from attenuation coefficients. Measured and calculated phantom scatter correction factors agree within 1% with the values given in literature. For 18 and 25 MV photon beam, wedge factors measured in water or in the mini-phantom agree within 0.5%, but maximum deviations of % are observed at 6 MV for the largest field sizes. It is demonstrated that the determination of several beam data related to full scatter conditions does not necessarily require the availability of a full scatter phantom. The mini-phantom is a reliable but very cheap and simple tool. It offers versatile possibilities to measure, check and verify basic beam parameters in high-energy photon beams.

A consistent formalism for the application of phantom and collimator scatter factors

A coherent system for the use of scatter correction factors, determined at 10 cm depth, is described for dose calculations on the central axis of arbitrarily shaped photon beams. The system is suitable for application in both the fixed source-surface distance (SSD) and in the isocentric treatment set-up. This is in contrast to some other proposals where only one of these approaches forms the basis of the calculation system or where distinct quantities and data sets are needed. In order to derive the relations in the formalism, we introduced a separation of the phenomena related to the energy fluence in air and to the phantom scatter contribution to the dose. Both are used relative to quantities defined for the reference irradiation set-up. It is shown that dose calculations can be performed with only one set of basic beam data, obtained at a reference depth of 10 cm. These data consist for each photon beam quality of measured collimator and phantom scatter correction factors, in combination with a set of (percentage/relative) depth-dose or tissue-phantom ratio values measured along the central axis of the beam. Problems related to measurements performed at the depth of maximum absorbed dose, due to the electron contamination of the beam, are avoided in this way. Collimator scatter correction factors are obtained by using a mini-phantom, while phantom scatter correction factors are derived from measurements in a full scatter phantom in combination with the results of the mini-phantom measurements. For practical reasons the fixed SSD system was chosen to determine the data. Then, dose calculations in a fixed SSD treatment set-up itself are straightforward. Application in the isocentric treatment set-up needs simple conversion steps, while the inverse approach, from isocentric to fixed SSD, is described as well. Differences between the two approaches are discussed and the equations for the conversions are given.

A three-Gaussian fit of phantom scatter correction data

The phantom scatter correction factor of megavoltage photon beams can be accurately described using a three-Gaussian fit. The model leads to six parameters, with which is described as a smooth function of the field radius r for beam qualities in the range from up to 25 MV. The parameters allow values to be calculated at intermediate beam energies and for any field shape. Calculated values for rectangular fields can be subsequently used as reference values to compare with measured values, for example when appraising a new beam.

10MV X線照射における側方電子平衡不成立領域でのファントム散乱補正係数

10MV X線照射における側方電子平衡不成立領域でのファントム散乱補正係数

Is there a need for a revised table of equivalent square fields for the determination of phantom scatter correction factors?

The use of the British Journal of Radiology (BJR) (supplement 17) tables of equivalent square fields for dose calculations is widespread. A revised version of the supplement was published recently, with a more elaborate discussion, but without changes in data given in these tables ( Br. J. Radiol. suppl 25). The tables were generated for use in dose calculations, with relative beam data such as PDD, BSF, PSF, all with as the reference depth. However, the current philosophy in dose calculational methods is based on quantities defined at a reference depth, , on a separation of phantom and head scatter, and on the use of the relative depth - dose or tissue - phantom ratios normalized at . By using these quantities as a starting point, problems at shallow depths related to the influence of contaminating electrons in the beam can be eliminated. Recently, a comprehensive set of phantom scatter factor data with has been published for a set of square field sizes and a wide range of photon beam energies, showing that phantom scatter is a smoothly varying function of field size and quality index. It is not a priori evident that the conventional concept of equivalent squares for rectangular fields is also fully applicable for phantom scatter factors and phantom scatter related quantities at a depth of 10 cm. It was questioned whether or not new tables of equivalent square fields are needed for this purpose. In this paper, new tables have been constructed for four photon beam energies in the range of Co-60 to 25 MV (quality index from 0.572 to 0.783). The small differences between the outcome of these new tables allowed the construction of one averaged table of equivalent square fields. Phantom scatter factors were calculated for rectangular fields based on the use of the BJR table and on the use of the newly constructed tables and the differences were quantified. For Co-60 no improvements could be shown when using the new averaged table, but for beam energies of 6 to 10 MV small improvements of the order of 0.5 to 1.0% were found. For a higher beam energy of 25 MV the improvement is smaller. Deviations resulting from the BJR table are within the limits of accuracy as stated by the authors. Therefore, for clinical use, the continued use of the BJR table of equivalent squares for phantom scatter factors and phantom scatter related quantities of rectangular fields is justified, irrespective of photon beam energy.

191. 10MV X線照射における側方電子平衡不成立領域でのファントム散乱補正係数

191. 10MV X線照射における側方電子平衡不成立領域でのファントム散乱補正係数

10 MV x-ray zero-area phantom scatter correction factors obtained using three extrapolation methods

Three sets of 10 MV x-ray zero-area phantom scatter correction factors have been obtained using three methods. The three methods are all based on the Bjärngard - Petti extrapolation principle. The three sets of data assume lateral CPE (charged particle equilibrium) for the primary absorbed dose. Using the most reliable set of data, a set of 10 MV x-ray SMRs (scatter - maximum ratios) is produced and parameterized. With respect to the zero-area correction factor at a depth of 2.5 cm, the parameterized expression gives and a Rice - Chin Monte Carlo simulation gives . The former is only 0.4% larger than the latter.

Two types of phantom scatter correction factors for high-energy photons

Two types of phantom scatter correction factors are introduced for high-energy photons. One is the traditional correction factor. It is produced using the primary absorbed dose under lateral CPE (charged particle equilibrium). It usually takes a non-zero value at zero-area field. The other is produced using the actual primary absorbed dose, which fails in lateral CPE when the field is small. It takes a value of zero at zero-area field. The two types of correction factors can be connected with the laterally spread primary absorbed dose (LSD).

10 MV x-ray SMRs obtained using zero-area correction factors derived by means of the Bjärngard - Petti method

Based on the Khan concept, a new set of 10 MV x-ray scatter - maximum ratios (SMRs) has been produced. These new SMRs reflect no electron contamination and are parametrized. A method has been developed for determining the minimum field size with lateral CPE (charged particle equilibrium) at the field centre. It is found that for 10 MV x-rays the minimum square field with lateral CPE at large depths on the central axis in water is . The zero-area phantom scatter correction factor is derived using the Bjärngard - Petti method. It is found that the effect of electron contamination is to reduce the zero-area correction factor.

191.10 MV X 線照射における側方電子平衡不成立領域でのファントム散乱補正係数(第24回秋季学術大会 一般研究発表予稿集)

191.10 MV X 線照射における側方電子平衡不成立領域でのファントム散乱補正係数(第24回秋季学術大会 一般研究発表予稿集)