a). Air measurements : In-air measurements were carried out at a distance of 82cm from the surface of the light beam

Transcription

1 Determination of Backscatter Factors For Diagnostic X-Ray Beams By Experimental And Monte Carlo Methods And Determination Of Air Kerma To Dose Equivalent Conversion Factors For The Calibration Of Personal Monitors INTRODUCTION M. Vijayam, J.B. Shigwan, B.S. Dixit, V.V. Shaha and S.C. Misra Radiation Standards & Instrumentation Division Bhabha Atomic Research Centre Mumbai , INDIA. ICRU (1) and ICRP (2) recommended the quantities H p (10) and H s (0.07) as the quantities to be determined for individual dose equivalent evaluations for penetrating and weakly penetrating radiations. Since these quantities are defined for soft tissue, they are not measurable, as the definition requires. However, H * (10) and H (0.07) the ambient Dose Equivalent and the Directional Dose Equivalent defined for the reference ICRU sphere(1) give a good estimate of Hp(10) and H s (0.07). Grosswendt et.al. (3a) published conversion factors to determine the ICRU dose equivalent quantities for calibrating radiation protection dosemeters. Due to the difficulty in carrying out measurements in the ICRU sphere, a cuboid water phantom(3b) of 30cmx 30cm x30cm dimensions was recommended as a substitute phantom and the conversion factors {H(10)/K a } and {H(0.07)/K a } determined using cubic phantom could be converted to the sphere quantities. For individual monitoring, the measurements carried out by the Swiss team (4) lead to the use of a 30 cm x 30 cm x 15 cm perspex slab phantom which is easy to use. Backscatter measurements are essential for the determination of H'(0.07) both for area monitoring and individual monitoring purposes. However, Back Scatter Factor (BSF) data for the diagnostic machines are not available as much as for the radiotherapy beams. Some of the published data are entirely from calculations for idealised irradiation conditions. Bartlett et.al.(5), Grosswendt (3b, 3c), Bohm and Grosswendt (6) did extensive Monte-Carlo calculations to determine the back scatter factors The Monte Carlo calculations by Groosswendt ( 3b) using kerma approximation and neglecting Raliegh scattering and electron binding effects were carried out for monoenergetic photons of 2keV to 1MeV and for four standard spectra of Seelentag (7). Will (8), Harrison (11), Kramer et.al (12),. Klevenhagen (13) determined B.S.F.-s for low energy x- rays experimentally. IPSM (14) redetermined the B.S.F.-s both by calculation and experiment, with special attention to energies below 1mm Al HVT. MATERIALS AND METHODS A Siemens Polymat 50I diagnostic x-ray machine with a target angle of 16 and a focus size of 1mm x1 mm is used for the investigations. KV, ma and time of exposure of the machine are microprocessorcontrolled, giving a highly stable output. It was already established that the % standard deviation is of the order of 0.1% for exposure times of 4s and 2% for exposure times of 8 ms. No monitor chamber was used. A light beam diaphragm was attached to the x-ray tube and the total inherent filtration was estimated as 2.9mm Al. BSF.-s were determined using 30 cm x 30 cm x 15 cm PMMA phantom and conversion coefficients in terms of Surface dose/air kerma were determined. A 3 c.c. spherical volume Exradin type A2 chamber and a NE graphite walled chamber which were calibrated against low kv primary standard free air chamber were used for PMMA and water phantom measurements respectively. The measuring set-up consisted of locally designed Varactor input operational amplifier in conjunction with a Philips Digital Voltmeter (DVM) and a IAEA supplied 1nF reference capacitance (0.9998nF). Back scatter measurements of PMMA phantom : a). Air measurements : In-air measurements were carried out at a distance of 82cm from the surface of the light beam 1

2 diaphragm of the diagnostic machine. This distance corresponds to approximately 1m from the x-ray tube target. Exposure measurements were carried out for field sizes of 10cm x10cm and 34cm x34cm at the chamber. All the readings were corrected for reference T and P. The C.F.-s in air were used for the calculation of exposure X 1. Measurements were taken at 40, 60, and 81 kvs. A light beam diaphragm was always present during the measurements. The 10cm x10cm field measurements were taken for comparing the data with available published data, to check the method we adopted to derive the BSF.s. b).phantom measurements : The PMMA phantom is made up of matching 30 cm x30 cm area perspex sheets. The top sheet is provided with a hemispherical cut on the centre of the front surface and a groove to hold the spherical Exradin chamber and stem, such that the chamber and stem are half-immersed in the phantom. The thickness of the phantom can be varied by using suitable underlying PMMA sheets. Thicknesses from 15 cm to 20 cm showed the variations to be within ±1% for full field exposure at the source to phantom surface distance of 1m. It was ensured that the surface of the phantom was normal to the beam direction. For all the qualities used for air-measurements, measurements were carried out on the phantom surface both for 10 cm x10 cm field and full field of 34 cm x 34 cm at the chamber. The exposure was calculated as X 2. BSF measurements were repeated later at 172 cm distance with field size of 59.6 cmx 59.6cm and were found to tally with the values determined for 34cm x 34cm field size. BSF Calculations : Initially BSF is approximately determined as the ratio of X 2 to X B.S.F u.c. BSF is defined here as the ratio of kerma to a small mass of ICRU tissue (3c) on the surface of the phantom to the kerma to a small mass of tissue in air. B.S.F. has to be corrected taking into account the effective change in energy caused by the back scattered radiation. The Exradin chamber C.F. and ( u k /p ) ICRU,air value will be slightly modified on the phantom. Approximate B.S. energy was calculated by assuming full broad beam conditions. Effective energy of the total comprising of the primary and B.S. components is calculated giving proper weights to the primary and scatter components. The effective energy including B.S. components (E') is used for deriving the C.F. of the Exradin chamber on the phantom, to derive corrected B.S.F. as B.S.F. (c). X 2,PMMA x (µ k / ρ) ICRU,air,E' x (C.F. of Exradin ) E' x kt B.S.F. c = X 1,air x (µ k / ρ) ICRU,air,E x (C.F. of Exradin ) E ( µ k / ρ ) ICRU,air,E and ( µ k / ρ ) ICRU,air,E are the mass energy transfer coefficients of ICRU tissue to air at effective incident energy E and effective energy of primary and B.S. components E'. At the energies considered here, ( µ k / ρ ) = ( µ en / ρ ) and the values of ( µ en / ρ ) are used. kt is a correction factor which takes into account attenuation and scatter in the volume of PMMA which is replaced by the hemispherical air volume of the chamber. The chamber reading has to be reduced by this value. The correction was taken as exp-( µ eff t eff ) where ( µ eff ) =( µ en ) assuming full broad-beam condition, t eff is the effective chord length of the chamber air volume and is calculated as 4v/s where v is the volume of the chamber air hemisphere (2/3)π r 3 and s is the surface area (2π r 2 +π r 2 ). The corrected B.S.F.c values are tabulated. B.S.F. values for 10 cm x10 cm field are compared with those of Stanton (9) and for large field with P.T.B. in Table 1. Surface Dose calculation : Dose to a small mass of ICRU tissue half immersed on the surface of the PMMA phantom was calculated for a 30 cm x 30 cm x 16.7 cm sized phantom. Surface dose = Chamber output in nc x (Chamber C.F. in R/nC) E' x B.S.F. c x f where f=0.876 x ( µ en / ρ ) ICRU,air,E' cgy/roentgen R assuming W/e as equal to ev/i.p. 2

3 = (µ en / ρ ) ICRU,air,E' ) Gy/roentgen R Conversion Factor : The conversion factor is given in terms of H(Surface Dose Equivalent/Air kerma ) = Output in nc x (Chamber C.F. in R/nC) E 'x x(µ en /ρ) ICRU,air,E' Output in nc x (Chamber C.F. in R/nC) E x x(µ en /ρ) ICRU,air,E x ( µ en / ρ ) ICRU,air,E =B.S.F. x ( µ en /ρ ) ICRU,air,E ( µ en / ρ ) ICRU,air,E were calculated for the reference composition of the ICRU tissue using Hubbell's data (10). Table 2. shows the comparison of the conversion factor with that determined by P.T.B. Back scatter measurements of water phantom : a). Air measurements : Three sets of air measurements were taken. 1. Before filling water air measurements in the phantom were taken for both 10 cm x 10 cm field and for the large field. 2. The beam was made horizontal and at the same distance (93.5cm), measurements were taken with NE chamber alone, for both the fields. 3. Using the same geometry as in 2.measurements were taken with chamber covered with the perspex waterproof tube. The set 3. was used for BSF calculations. Corrections were applied based on method given by Harrison (11). Measurements on water phantom: To determine the BSF for water a 30 cm x 30 cm x 30 cm water phantom with provision to measure depth doses in water was used. A calibrated NE c.c. graphite chamber was inserted in a water proof persrex tube and adjusted to measure the surface dose. The chamber was half immersed in water and the phantom depth was 25cm. The x-ray tube was adjusted vertically above the phantom. The distance from the x-ray target to water surface of the LBD and water surface was 93.5cm which was the maximum attainable. The NE chamber was connected to the Varactor amplifier set- up as before and a 1nF capacitance was used. Measurements were carried out both for 10 cm x 10 cm field and full field 31.5 cm x 32.2 cm at the chamber position. All readings were corrected for reference ambient conditions. First approximate BSF was calculated as the ratio of water reading to air reading (3). As it was calculated for the PMMA case, approximate effective energy of primary +BS radiation was calculated. The NE chamber C.F.-s were interpolated at these effective energies (E'). Then (µ en /ρ) ICRU,air values at energies at E & E' were used to obtain the corrected BSF-s. As it was applied for Exradin chamber, correction was applied for the attenuation and scatter in the chamber air volume replaced by water, using effective attenuation coefficient = ( µ en / ρ ) (for full broad beam condition). After modification, the corrected B.S.F. values for 10cmx 10cm field are compared with IPSM (14) and for large field with PTB. Table 3. shows the comparison of B.S.F.-s. CONVERSION FACTORS Conversion factors were calculated using the B.S.F.-s determined for large field and (µ en /ρ) ICRU,air,E calculated, as B.S.F. x ( µ en / ρ ). Table 4. shows the conversion factors in terms of (surface dose to water/air kerma) instead of at 7mg/cm 2 depth. The depth dose values calculated by Will (8) have shown that the difference is <1% except at energies of <15keV, so that the actual value need not differ by more than 1%. Finally correction factors (3b) are applied to the above to obtain ICRP recommended conversion factors in terms {H'(0.07)/K a }. Table 5. gives the correction factors to convert water cube data to ICRU tissue sphere value and the 3

4 conversion factors determined at our institute BARC. MONTE CARLO CALCULATIONS Spectra of the low energy beams of the Siemens machine were determined by the attenuation analysis method which gave spectra in terms of photon fluence and exposure. These spectral values were used in the present Monte Carlo calculations of back scatter factors for reference water phantom and PMMA slab phantom. Monte Carlo Code used : The general purpose Monte Carlo code MCNP ( Monte Carlo for Neutrons and Photons) version 3.0 developed at the Los Alamos Scientific Laboratory, USA was used for calculating the back scatter factors. The code comes with a powerful geometry package, by which complex geometries can be described by unions, intersections and compliments of cells, which are defined by user defined planes. The cross section library makes use of data of Storm and Israel for photons of low energies. A photon history is terminated when it reaches an energy of 1 kev. In the MCP default treatment used, 1. Photoelectric effect, Compton effect and Pair production are considered, 2. Fluorescence is taken into account, 3. Thomson effect is considered after modifying the cross section with atomic form factor, 4. The Compton cross section is modified by the incoherent scattering cross section which corrects for electron binding effect. The Norsk Data computer ND570 as well as Landmark machine 860 of Wipro were used for the computations. The geometries were verified at major steps of the programme build-up by viewing the geometries from different directions with the help of the MC-plot programme available. 1. Back Scatter for water phantom : Two sets of computations were carried out. set I. In the first case, the experimental geometry was reproduced for in-phantom (A) and in air measurements (B). A. Simulation is carried out for the NE2577 chamber used inserted in the water-proof tube in the water phantom, the incident photon beam being divergent and limited by diaphragm vertically incident on the water surface at a focus to surface distance d. The computations were carried out for the chamber in a water-proof tube adjusted half immersed in phantom at a distance of d=93.6cm from the target. The SRC2 ( point isotropic) source with a 100% directional biasing in the used direction and angle of divergence, limited by a diaphragm with a square aperture (giving the same field size as used in practice) produced the incident radiation beam. B. To determine the back scatter factor, the air values were determined as in the practical case, rotating the source system so that a horizontal beam of the same divergence, exposed the chamber in air at a distance of d=93.6cm. and same field size was used as before. Calculations were carried out for the i).chamber in tube and ii). chamber alone, in order to apply corrections recommended by Harrison(11). Computations were done at the three beam qualities both for chamber in water-proof tube and chamber alone. Table 6. gives the correction factors for the attenuation & scatter in tube, determined by Monte Carlo and by measurements. Both the values agree within 1% and show the same trend. Using the values above and the method given by Harrison (11), measured back scatter factors were corrected and are given below. Set II. The second set of calculations were carried out for the case of small mass of tissue on the water phantom at a depth of zero cm from the water surface, and a parallel beam of radius R produced from a disk source of radius R, incident vertically on the phantom at a distance of d cm from the source front surface. Calculations for the air set were as before carried out for a horizontal parallel beam of radius R at a distance d, the small mass of tissue is a thin disk of radius r and thickness t. The presence of air was not neglected and the calculations were carried out under kerma approximation (secondary electrons were assumed to be absorbed at the point where they were produced). At the low energies considered here, the assumption is justified. Both photon fluence (photons cm -2 ) the F14 tally & dose tally values were calculated for the case of chamber (& small mass of tissue) in air and chamber (& small mass of tissue) on water. Four million histories were sampled in each case and the uncertainty in the total value of the tally is between 0.7% & 2%. In addition, back scatter was also calculated from the computed values of chamber when exposed on water phantom and exposed in air as before but from a plane parallel beam from a disk source (SRC5), at the same distance and field size equal to the field size at the half depth of the phantom. As given by Harrison, 4

5 corrections were applied for the back scatter factors calculated from the chamber simulations. In the second set of computations, back scatter was calculated as the ratio of kerma (dose in this case) to a small mass of tissue in the phantom at zero depth to the kerma to the same mass of tissue in free air. The incident beam was assumed to be parallel coming from a disk source with radius R, r was the half of the beam size ( side of the square ) at the centre of the phantom in the previous case. The "small mass of tissue" is a disk of radius r and thickness t at a depth of zero cm from the water surface. Table 7 gives the results. Table 8 gives the comparison of the back scatter factors determined by both the above simulations, the measured back scatter factors after appropriate corrections, back scatter factors published by Boehm and Grosswendt(3c) for narrow spectral and high air kerma rate spectral series. Back Scatter for PMMA phantom : Similar to the case with water phantom, Two sets of computations were carried out set.i. for divergent beam and Exradin chamber simulating experiment II. Parallel beam and small mass of ICRU tissue. Results are given in table 9. Table 10 gives the comparison of the back scatter factors determined by both the above simulations, the measured back scatter factors after appropriate corrections, back scatter factors published by Boehm and Grosswendt(3c) for narrow spectral and high air kerma rate spectral series. CONCLUSIONS : The results show that the back scatter values determined by measurements and Monte Carlo meathods are in good agreement. As the x-ray spectra are different for different machines, it is advisable to determine the backscatter values, instead of using the published values. The lower measured BSF s at larger field for water phantom may be because of the larger radiation.field non-uniformity Table 1. Back Scatter Values Of PMMA Phantom Thickness d=16.7cm Primary 10cm x 10cm 34cm x34cm beam Stanton PTB kv Eff. E Eff. E' BARC Stanton Eff E ' BARC** PTB kev kev * BARC kev BARC * Reference Stanton (10) : ** Same B.S.F.s otained for 59.6cm x 59.6cm field also Table 2. Conversion Factors {H(Surface/Ka)RSS and {H(0.07)/Ka}PTB for PMMA slab Primary B.S.F. {H(Surface)/Ka} BARC {H(0.07)/Ka}PTB PTB/BARC kv Eff.keV Table 3. Back Scatter Values For Water Phantom Thickness d = 25.7cm 5

6 Primary 10cmx 10cm 34cm x34cm beam Eff.E BARC IPSM IPSM Eff.E BARC PTB PTB kv Eff.E E' E kev kev BARC kev BARC Table 4. Conversion Factors {H(surface/Ka)}RSS & {H(0.07)/Ka}PTB Primary B.S.F. {H(surface)/Ka} BARC {H(0.07)/Ka} PTB PTB/BARC kv Eff.keV Table 5. Evaluation of ICRU quantities { H'(0.07)/Ka } Primary Eff. Energy {H'(0.07)/H(0.07)} H'(0.07/Ka)} kv kev * BARC * Conversion Factors { H'(0.07)/H(0.07)} to convert from water phantom to ICRU sphere quantities Table 6. Corrections for the presence of the water-proof tube kv Ratios of chamber alone to chamber in tube Monte Carlo Measured Water Phantom : d=0.94m R=18.5cm ( radius of parallel beam) 6

7 Table 7. Incident Parallel Beams : Back scatter values for water by Monte Carlo kv Thickness Radius Back Scatter factor from t of disk r of disk Photon fluence Dose in cm in cm * ** * Photon cm -2 s -1 ** ICRU tissue dose rate in cgy h -1 Table 8. Comparison of back factors for water kv BSF from MC simulations : Chamber a). divergent beam b). parallel beam Small mass of issue ( Parallel beam) BSF from Measurements : Comparisons : c). Harrison d). PTB marrow e). PTB high air kerma rate Table 9. Incident Parallel Beams : Back scatter values for PMMA by Monte Carlo kv Thickness Radius Back Scatter factor from t of disk r of disk Photon fluence Dose in cm in cm * ** * Photon cm -2 s -1 ** ICRU tissue dose rate in cgy h -1 7

8 Table 10. Comparison of back factors for PMMA slab phantom kv BSF from MC simulations : Chamber a). Divergent beam b). Parallel beam Small mass of tissue ( parallel beam) BSF from Measurements : Comparisons : d). PTB marrow e). PTB high air kerma rate REFERENCES 1). ICRU Determination of Dose Equivalents Resulting from External Radiation Sources Report 39 (Bethesda,MD:ICRU Publications) ). ICRP Data for use in Protection against External Radiation ICRP Publication 51. Ann ICRP 17 (1987). 3 a). Grosswendt B., Hohlfeld K., Kramer H.M. und Selbach H.J. Konversionsfactoren fur die ICRU- Aquivalentdosisgrossen zur Kalibrierung von Strahlenschutzdosemeter Physikalisch Technische Bundesanstalt Report PTB -DOS- 11 ISSN (1985) b). Grosswendt B. Conversion coefficients for ICRU operational quanties in a cubic water phantom for the individual monitoring of photons IAEA-RC-408 (1989). c). Grosswendt B. Conversion Coefficients for calibrating Individual Photon Dosemeters in Terms of Dose Equivalents Defined in an ICRU Cube and PMMA Slabs. Radiation Protection Dosimetry Vol 32, No.4, (1990). 4). Wernli C. Jossen H. and Valley J.F. Methods of Measurement and Calibration in Personnel Dosimetry for External Irradiation :Presentation of Concept and the Results of a Test Programme in Switzerland. Radiation Protection Dosimetry Vol 28, No.1, (1989). 5). Bartlett D.T. Dimbylow P.J. and Francis T.M. calculated Backscatter from Phantoms for photon dosemeter calibrations. Radiation Protection Dosimetry Vol 32, No.2, (1990). 6). Bohm Jurgen and Grosswendt B. 10 year Intercomparison Measurements of Dosemeter Systems for the Individual Monitoring of Photon and Beta Radiation in Retrospective View. New Data and Perspectives. PTB Mitteilungen 99, 2/89. 7). Seelentag W.W., Panzer W., Drexler G., Plantz L. and Santner F. A Catalogue of Spectra for the Calibration of Dosemeters. GSF Report 560 (Munich GSF) ). Will W. Measurement of Conversion Coefficients for Calibrating Individual Dosemeters with Respect to the Operational Dose Equivalent Quantities on the PMMA Slab Phantom Radiation Protection Dosimetry Vol.37,No.2,PP79-84 (1991). 9). Stanton L.,Brattelli S.D. and Day J>.L. Measurement of Diagnostic X-ray backscatter by a novel ion chamber method Med.Phys. 9(1), , Jan/Feb ). Hubbell J.H. Photon mass Attenuation and Mass Energy Absorption Coefficients fro H,C,NO, Ar and Seven Mixtures from 0.1 kev o 20 MeV Radiat. Res. 70, (1977). 11). Harrison P.M. Back Scatter Factors for Diagnostic Radiation (1-4mm Al HVL) Phys.Med.Biol.,1982 vol.27,no.12 pp ). Kramer H.M.,Grosswendt B. and Hohlfeld K. Experimental Determination of the Back Scatter For Soft X-rays in water and Acrylic glass. Nuclear Instruments and Methods in Physics Research B9 pp (1985). 13). Klevenhagen S.C. The Build-Up of Back Scatter in the Energy Range 1mm Al to 8mm AL HVT Phys. Med. Biol. 27, , (1982). 14). IPSM Report of the IPSM Working Party on Low- and medium energy x-ray dosimetry Phys. Med. Biol. Vol. 36, No.8, pp