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A model-based algorithm to correct for the loss of backscatter in superficial X-ray radiation therapy

Published:September 05, 2019DOI:https://doi.org/10.1016/j.ejmp.2019.08.018

      Highlights

      • Insufficient scattering material results in published Bw values that overestimate scatter.
      • An algorithm was developed for calculating backscatter in situations with reduced scattering medium.
      • Model validation by comparison with published data, Monte Carlo simulations and film measurements.

      Abstract

      Dosimetry protocols for superficial X-rays prescribe the determination of kerma on the surface of a phantom through the use of a backscatter factor (Bw) that accounts for the effect of phantom scatter. Bw values corresponding to full-scatter phantoms are provided by these protocols. In practice, clinical situations arise wherein there is insufficient scattering material downstream, resulting in published Bw values that overestimate the amount of occurring scatter.
      To provide an accurate dose calculation the backscatter values need to be corrected for any reduction in scattered radiation. Estimating the change of Bw in situations with incomplete backscatter has previously been achieved by direct measurements or Monte Carlo modelling. For increasing the accuracy of clinical dosimetries, we developed a physical model to deduce an algorithm for calculating backscatter factors in situations with reduced downstream scattering medium. The predictions of the model were validated by comparison with published data, Monte Carlo simulations and film-based measurements for beams with a half-value layer of 0.8, 2 and 4 mm Al.
      Our algorithm accurately predicts the effect of partial scatter conditions with suitable precision. Its reliability, combined with the simplicity of calculation, makes this methodology suitable to be incorporated into routine clinical dosimetry. The algorithm’s underlying physical model provides an intuitive understanding of the effects of field size and beam energy on backscatter reduction, permitting a rational management of this effect.

      Keywords

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      References

        • Aukett R.J.
        • Burns J.E.
        • Greener A.G.
        • Harrison R.M.
        • Moretti C.
        • Nahum A.E.
        • et al.
        Addendum to the IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL).
        Phys Med Biol. 2005; 50: 2739-2748https://doi.org/10.1088/0031-9155/50/12/001
        • Klevenhagen S.C.
        • Aukett R.J.
        • Harrison R.M.
        • Nahum A.E.
        • Rosser K.E.
        The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0. 035 mm Al – 4 mm Cu HVL; 10–300 kV generating potential).
        Phys Med Biol. 1996; 41: 2605https://doi.org/10.1088/0031-9155/41/12/002
        • Ma C.-M.
        • Coffey C.W.
        • DeWerd L.A.
        • Liu C.
        • Nath R.
        • Seltzer S.M.
        • et al.
        AAPM protocol for 40–300 kV x-ray beam dosimetry in radiotherapy and radiobiology.
        Med Phys. 2001; 28: 868-893https://doi.org/10.1118/1.1374247
      1. British Institute of Radiology/Institute of Physics and Engineering in Medicine and Biology. Central axis depth dose data for use in radiotherapy, 1996 : a survey of depth doses and related data measured in water or equivalent media. British Institute of Radiology; 1996.

        • Hill R.
        • Healy B.
        • Butler D.
        • Odgers D.
        • Gill S.
        • Lye J.
        • et al.
        Australasian recommendations for quality assurance in kilovoltage radiation therapy from the Kilovoltage Dosimetry Working Group of the Australasian College of Physical Scientists and Engineers in Medicine.
        Aust Phys Eng Sci Med. 2018; https://doi.org/10.1007/s13246-018-0692-1
        • Baines J.
        • Zawlodzka S.
        • Markwell T.
        • Chan M.
        Measured and Monte Carlo simulated surface dose reduction for superficial X-rays incident on tissue with underlying air or bone.
        Med Phys. 2018; 45: 926-933https://doi.org/10.1002/mp.12725
        • Kim J.
        • Hill R.
        • Claridge Mackonis E.
        • Kuncic Z.
        An investigation of backscatter factors for kilovoltage x-rays: a comparison between Monte Carlo simulations and Gafchromic EBT film measurements.
        Phys Med Biol. 2010; 55: 783-797https://doi.org/10.1088/0031-9155/55/3/016
        • Eaton D.J.
        • Doolan P.J.
        Review of backscatter measurement in kilovoltage radiotherapy using novel detectors and reduction from lack of underlying scattering material.
        J Appl Clin Med Phys. 2013; 14: 5-17https://doi.org/10.1120/jacmp.v14i6.4358
        • Hill R.
        • Healy B.
        • Holloway L.
        • Kuncic Z.
        • Thwaites D.
        • Baldock C.
        Advances in kilovoltage x-ray beam dosimetry.
        Phys Med Biol. 2014; 59https://doi.org/10.1088/0031-9155/59/6/R183
        • Healy B.J.
        • Sylvander S.
        • Nitschke K.N.
        Dose reduction from loss of backscatter in superficial x-ray radiation therapy with the Pantak SXT 150 unit.
        Aust Phys Eng Sci Med. 2008; 31: 49-55https://doi.org/10.1007/BF03178453
        • Klevenhagen S.C.
        The build-up of backscatter in the energy range 1 mm Al to 8 mm Al HVT (radiotherapy beams).
        Phys Med Biol. 1982; 27: 1035-1043https://doi.org/10.1088/0031-9155/27/8/005
        • Radiation Oncology Tripartite Committee
        Planning for the best: tripartite national strategic plan for radiation oncology 2012–2022.
        R Aust New Zeal Coll Radiol. 2012;
        • The Royal College of Radiologists
        A review of the use of radiotherapy in the UK for the treatment of benign clinical conditions and benign tumours.
        Clin Oncol. 2015; https://doi.org/10.1108/978-1-78714-831-420171008
        • Borras J.M.
        • Lievens Y.
        • Grau C.
        The need for radiotherapy in Europe in 2020: not only data but also a cancer plan.
        Acta Oncol (Madrid). 2015; 54: 1268-1274https://doi.org/10.3109/0284186X.2015.1062139
        • McGregor S.
        • Minni J.
        • Herold D.
        Superficial radiation therapy for the treatment of nonmelanoma skin cancers.
        J Clin Aesthet Dermatol. 2015; 8: 12-14
      2. The range in liquid water of 300 keV electrons is 0.08421 g/cm2, calculated using the continuous slowing down approximation. Source: NIST – ESTAR database, https://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html; n.d.

        • Paelinck L.
        • De Neve W.
        • De Wagter C.
        Precautions and strategies in using a commercial flatbed scanner for radiochromic film dosimetry.
        Phys Med Biol. 2007; 52: 231-242https://doi.org/10.1088/0031-9155/52/1/015
        • Méndez I.
        • Peterlin P.
        • Hudej R.
        • Strojnik A.
        • Casar B.
        On multichannel film dosimetry with channel-independent perturbations.
        Med Phys. 2014; 41011705https://doi.org/10.1118/1.4845095
        • Mayer R.R.
        • Ma F.
        • Chen Y.
        • Miller R.I.
        • Belard A.
        • McDonough J.
        • et al.
        Enhanced dosimetry procedures and assessment for EBT2 radiochromic film.
        Med Phys. 2012; 39: 2147https://doi.org/10.1118/1.3694100
        • Rogers D.W.O.
        • Kawrakow I.
        • Seuntjens J.P.
        • Walters B.R.B.
        • Mainegra-Hing E.
        NRC user codes for EGSnrc. NRCC Rep PIRS-702.
        Rev B. 2013; 702: 1-92
      3. Kawrakow I, Rogers DWO. The EGSnrc code system. NRC Rep PIRS-701, NRC, Ottawa; 2000.

        • Hernandez A.M.
        • Boone J.M.
        Tungsten anode spectral model using interpolating cubic splines: unfiltered x-ray spectra from 20 kV to 640 kV.
        Med Phys. 2014; 41042101https://doi.org/10.1118/1.4866216
        • Lanzon P.J.
        • Sorell G.C.
        The effect of lead underlying water on the backscatter of X-rays of beam qualities 0.5 mm to 8 mm Al HVT.
        Phys Med Biol. 1993; 38: 1137-1144https://doi.org/10.1088/0031-9155/38/8/012
        • Khan F.M.
        The Physics of Radiation Therapy.
        Williams & Wilkins, 1994
      4. We extracted the values from a pdf version of Healy’s paper using the free, web-based tool “WebPlotDigitizer” (http://arohatgi.info/WebPlotDigitizer/app3_12/) created by Ankit Rohatgi; n.d.

      5. Dr. John Baines kindly shared with us the measured PDD of his 150 kV beam. We used his published HVL value, together with the PDD data, as input for our model; n.d.

        • Das I.J.
        • Chopra K.L.
        Backscatter dose perturbation in kilovoltage photon beams at high atomic number interfaces.
        Med Phys. 1995; 22: 767-773https://doi.org/10.1118/1.597594
        • Mayles P.
        • Nahum A.
        • Rosenwald J.C.
        Handbook of radiotherapy physics: theory and practice.
        Taylor & Francis, 2010