Purpose: All present dosimetry protocols
recommend well-guarded parallelplate ion chambers for electron dosimetry. For
the guard-less Markus chamber, an energy dependent fluence perturbation
correction pcav is given. This
perturbation correction was experimentally determined by van der Plaetsen by
comparison of the read-out of a Markus and a NACP chamber, which was assumed to
be “perturbation-free”. Aim of the present study is a Monte Carlo based
reiteration of this experiment. Methods: Detailed models of four parallel-plate
chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte
Carlo code EGSnrc and placed in a water phantom. For all chambers, the dose to
the active volume filled with low density water was calculated for 13 clinical
electron spectra (E0 = 6 - 21 MeV)
and three energies of an Electra linear accelerator at the depth of maximum and
at the reference depth under reference conditions. In all cases, the chamber’s
reference point was positioned at the depth of measurement. Moreover, the dose
to water DW was calculated in a small
water voxel positioned at the same depth. Results: The calculated dose ratio
DNACP/DMarkus,
which according to van der Plaetsen reflects the fluence perturbation
correction of the Markus chamber, deviates less from unity than the values
given by van der Plaetsen, but exhibits similar energy dependence. The same
holds for the dose ratios of the other well-guarded chambers. But, in
comparison to water, the Markus chamber reveals the smallest overall
perturbation correction which is nearly energy independent at both investigated
depths. Conclusion: The simulations principally confirm the energy dependence
of the dose ratio
DNACP/DMarkus
as published by van der Plaetsen. But, as shown by our simulations of the ratio
DW/DMarkus,
the conclusion drawn in all dosimetry protocols is questionable: in contrast to
all well-guarded chambers, the guard-less Markus chamber reveals the smallest
overall perturbation correction and also the smallest energy dependence.
References
[1]
Almond, P.R., Biggs, P.J., Coursey, B.M., Hanson, W.F., et al. (1999) AAPM’s TG-51 Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Medical Physics, 26, 1847-1870.
[2]
Andreo, P., Burns, D.T., Hohlfeld, K., Huq, M.S., et al. (2000) Absorbed Dose Determination in External Beam Radiotherapy. An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water. Technical Reports Series TRS-398, International Atomic Energy Agency, Vienna.
[3]
Thwaites, D.I., DuSautoy, A.R., Jordan, T., McEwen, M.R., et al. (2003) The IPEM Code of Practice for Electron Dosimetry for Radiotherapy Beams of Initial Energy from 4 to 25 MeV Based on an Absorbed Dose to Water Calibration. Physics in Medicine and Biology, 48, 2929-2970. https://doi.org/10.1088/0031-9155/48/18/301
[4]
DIN6800-2 (2008) Procedures of Dosimetry with Probe-Type Detectors for Photon and Electron Radiation—Part 2: Ionization Chamber Dosimetry of High Energy Photon and Electron Radiation.
[5]
Ibbott, G.S. (1985) Radiation Dosimetry: Electron Beams with Energies between 1 and 50 MeV (ICRU Report No. 35). Medical Physics, 12, 813. https://doi.org/10.1118/1.595780
[6]
Zink, K., Czarnecki, D., Looe, H.K., von Voigts-Rhetz, P., et al. (2014) Monte Carlo Study of the Depth-Dependent Fluence Perturbation in Parallel-Plate Ionization Chambers in Electron Beams. Medical Physics, 41, Article ID: 111707. https://doi.org/10.1118/1.4897389
[7]
van der Plaetsen, A., Seuntjens, J., Thierens, H. and Vynckier, S. (1994) Verification of Absorbed Doses Determined with Thimble and Parallel-Plate Ionization Chambers in Clinical Electron Beams Using Ferrous Sulphate Dosimetry. Medical Physics, 21, 37-44. https://doi.org/10.1118/1.597362
Kawrakow, I. (2000) Accurate Condensed History Monte Carlo Simulation of Electron Transport. I. EGSnrc, the New EGS4 Version. Medical Physics, 27, 485-498. https://doi.org/10.1118/1.598917
[10]
Kawrakow, I. (2000) Accurate Condensed History Monte Carlo Simulation of Electron Transport. II. Application to Ion Chamber Response Simulations. Medical Physics, 27, 499-513. https://doi.org/10.1118/1.598918
[11]
Kawrakow, I., Mainegra-Hing, E., Rogers, D.W.O., Tessier, F., et al. (2013) The EGSnrc Code System: Monte Carlo Simulation Of Electron and Photon Transport. National Research Council of Canada, Report No. PIRS-701.
[12]
Smilowitz, J.B., Das, I.J., Feygelman, V., Fraass, B.A., et al. (2015) AAPM Medical Physics Practice Guideline 5.a.: Commissioning and QA of Treatment Planning Dose Calculations—Megavoltage Photon and Electron Beams. Journal of Applied Clinical Medical Physics, 16, 14-34. https://doi.org/10.1120/jacmp.v16i5.5768
[13]
Kawrakow, I., Mainegra-Hing, E., Tessier, F. and Walter, B.R.B. (2009) The EGSnrc C++ Class Library. NRC Report PIRS-898 (rev A), Ottawa.
[14]
Verhaegen, F., Zakikhani, R., Dusautoy, A., Palmans, H., et al. (2006) Perturbation Correction Factors for the NACP-02 Plane-Parallel Ionization Chamber in Water in High-Energy Electron Beams. Physics in Medicine and Biology, 51, 1221-1235. https://doi.org/10.1088/0031-9155/51/5/012
[15]
Chin, E., Shipley, D., Bailey, M., Seuntjens, J., et al. (2008) Validation of a Monte Carlo Model of a NACP-02 Plane-Parallel Ionization Chamber Model Using Electron Backscatter Experiments. Physics in Medicine and Biology, 53, N119-N126. https://doi.org/10.1088/0031-9155/53/8/n01
[16]
Chin, E., Palmans, H., Shipley, D., Bailey, M., et al. (2009) Analysis of Dose Perturbation Factors of a NACP-02 Ionization Chamber in Clinical Electron Beams. Physics in Medicine and Biology, 54, 307-326. https://doi.org/10.1088/0031-9155/54/2/009
[17]
Zink, K. and Wulff, J. (2011) On the Wall Perturbation Correction for a Parallel-Plate NACP-02 Chamber in Clinical Electron Beams. ..Medical Physics, 38, 1045-1054. https://doi.org/10.1118/1.3544660
[18]
Zink, K. and Wulff, J. (2012) Beam Quality Corrections for Parallel-Plate Ion Chambers in Electron Reference Dosimetry. Physics in Medicine and Biology, 57, 1831-1854. https://doi.org/10.1088/0031-9155/57/7/1831
[19]
von Voigts-Rhetz, P., Czarnecki, D. and Zink, K. (2014) Effective Point of Measurement for Parallel Plate and Cylindrical Ion Chambers in Megavoltage Electron Beams. Zeitschrift für Medizinische Physik, 24, 216-223. https://doi.org/10.1016/j.zemedi.2013.12.001
[20]
Ding, G.X., Rogers, D.W.O. and Mackie, T.R. (1995) Calculation of Stopping-Power Ratios Using Realistic Clinical Electron Beams. Medical Physics, 22, 489-501. https://doi.org/10.1118/1.597581
[21]
Rogers, D.W.O., Faddegon, B.A., Ding, G.X., Ma, C., et al. (1995) BEAM: A Monte Carlo Code to Simulate Radiotherapy Treatment Units. Medical Physics, 22, 503-524. https://doi.org/10.1118/1.597552
[22]
Wulff, J., Zink, K. and Kawrakow, I. (2008) Efficiency Improvements for Ion Chamber Calculations in High Energy Photon Beams. Medical Physics, 35, 1328-1336. https://doi.org/10.1118/1.2874554
[23]
Rogers, D.W.O., Walters, B. and Kawrakow, I. (2013) BEAMnrc Users Manual. National Research Council of Canada, Report PIRS-509(a) revL.
[24]
Wang, L.L.W. and Rogers, D.W.O. (2008) Calculation of the Replacement Correction Factors for Ion Chambers in Megavoltage Beams by Monte Carlo Simulation. Medical Physics, 35, 1747-1755. https://doi.org/10.1118/1.2898139
[25]
Mattsson, L.O., Johansson, K.A. and Svensson, H. (1981) Calibration and Use of Plane-Parallel Ionization Chambers for the Determination of Absorbed Dose in Electron Beams. Acta radiologica. Oncology, 20, 385-399. https://doi.org/10.3109/02841868109130228
[26]
Nilsson, B., Montelius, A. and Andreo, P. (1996) Wall Effects in Plane-Parallel Ionization Chambers. Physics in Medicine and Biology, 41, 609-623. https://doi.org/10.1088/0031-9155/41/4/003
[27]
Cross, P. and Freeman, N. (1996) The Calibration of Plane Parallel Ionization Chambers for the Measurement of Absorbed Dose in Electron Beams of Low to Medium Energies. Part 1: The Nacp Chamber. Australasian Physical and Engineering Sciences in Medicine, 19, 197-200.
[28]
Cross, P. and Freeman, N. (1997) The Calibration of Plane Parallel Ionization Chambers for the Measurement of Absorbed Dose in Electron Beams of Low to Medium Energies. Part 2: The PTW/MARKUS Chamber. Australasian Physical and Engineering Sciences in Medicine, 20, 117-120.
[29]
Renaud, J., Sarfehnia, A., Marchant, K., McEwen, M., Ross, C. and Seuntjens, J. (2015) Direct Measurement of Electron Beam Quality Conversion Factors Using Water Calorimetry. Medical Physics, 42, 6357-6368. https://doi.org/10.1118/1.4931970
[30]
Buckley, L.A. and Rogers, D.W.O. (2006) Wall Correction Factors, Pwall, for Parallel-Plate Ionization Chambers. Medical Physics, 33, 1788-1796. https://doi.org/10.1118/1.2199988
[31]
Zink, K. and Wulff, J. (2008) Monte Carlo Calculations of Beam Quality Correction Factors kQ for Electron Dosimetry with a Parallel-Plate Roos Chamber. Physics in Medicine and Biology, 53, 1595-1607. https://doi.org/10.1088/0031-9155/53/6/006
[32]
Zink, K. and Wulff, J. (2009) Positioning of a Plane-Parallel Ionization Chamber in Clinical Electron Beams and the Impact on Perturbation Factors. Physics in Medicine and Biology, 54, 2421-2435. https://doi.org/10.1088/0031-9155/54/8/011
[33]
Kuchnir, F.T. and Reft, C.S. (1992) Experimental Values for Pwall,x and repl,E for Five Parallel-Plate, Ion Chambers—A New Analysis of Previously Published Data. Medical Physics, 19, 367. https://doi.org/10.1118/1.596910
[34]
Kuchnir, F.T. and Reft, C.S. (1993) Experimental Determination of Fluence Perturbation Factors for Five Parallel-Plate Ionization Chambers. Medical Physics, 20, 331-335. https://doi.org/10.1118/1.597073
[35]
McEwen, M., Palmans, H. and Williams, A. (2006) An Empirical Method for the Determination of Wall Perturbation Factors for Parallel-Plate Chambers in High-Energy Electron Beams. Physics in Medicine and Biology, 51, 5167-5181. https://doi.org/10.1088/0031-9155/51/20/006
[36]
Araki, F. (2008) Monte Carlo Calculations of Correction Factors for Plane-Parallel Ionization Chambers in Clinical Electron Dosimetry. Medical Physics, 35, 4033-4040. https://doi.org/10.1118/1.2968102
[37]
Wang, L.L.W. and Rogers, D.W.O. (2010) Replacement Correction Factors for Plane-Parallel Ion Chambers in Electron Beams. Medical Physics, 37, 461-465. https://doi.org/10.1118/1.3276735
