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Impact of inertia on possible fundamental drawbacks in radiochromic film dosimetry

  • Slobodan Devic
    Correspondence
    Address: Jewish General Hospital, 3755 chemin de la Côte-Sainte-Catherine, Montréal, Québec H3T 1E2, Canada.
    Affiliations
    Medical Physics Unit, McGill University, Montréal, Québec, Canada
    Department of Radiation Oncology, Jewish General Hospital, Montréal, Québec, Canada
    Segal Cancer Centre, Jewish General Hospital, McGill University, Montréal, Québec, Canada
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  • Saad Aldelaijan
    Affiliations
    Medical Physics Unit, McGill University, Canada
    Biological & Biomedical Engineering Department, Montreal Neurological Institute, Montréal, Québec, Canada
    Biomedical Physics Department, King Faisal Specialist Hospital & Research Centre, Riyadh, Saudi Arabia
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  • Hamed Bekerat
    Affiliations
    Medical Physics Unit, McGill University, Montréal, Québec, Canada
    Department of Radiation Oncology, Jewish General Hospital, Canada
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Published:September 26, 2019DOI:https://doi.org/10.1016/j.ejmp.2019.08.019

      History of radiochromic film dosimetry

      From its very inception [
      • Chu R.D.H.
      • VanDyke G.
      • Lewis D.F.
      • O'Hara K.P.J.
      • Buckland B.R.
      • Dinelle F.
      GafChromic dosimetry media: a new high dose rate thin film routine dosimeter and dose mapping tool.
      ] radiochromic films draw an immediate attention for its high spatial resolution and near tissue equivalence in most dosimetry applications [
      • Devic S.
      Radiochromic film dosimetry: past, present, and future.
      ]. The earlier GafChromicTM film models (MD-55, HS) were quite expensive, limiting their use to experimental dosimetry. The renaissance of radiochromic film dosimetry came about in 2005 with the introduction of the EBT film model [
      • Cheung T.
      • Butson M.J.
      • Yu P.K.
      Post-irradiation colouration of Gafchromic EBT radiochromic film.
      ] with significantly lower (10 times per square inch lower) price and readily accepted radiochromic film protocols (RFPs) using inexpensive flatbed document scanners [
      • Devic S.
      • Seuntjens J.
      • Hegyi G.
      • Podgorsak E.B.
      • Soares C.G.
      • Kirov A.S.
      • et al.
      Dosimetric properties of improved GafChromic films for seven different digitizers.
      ]. To date, a number of RFPs have been described, using either transmission single [
      • Devic S.
      • Seuntjens J.
      • Sham E.
      • Podgorsak E.B.
      • Kirov A.S.
      • Schmidtlein R.C.
      • et al.
      Precise radiochromic film dosimetry using a flat-bed document scanner.
      ] or multiple color channels [
      • Micke A.
      • Lewis D.F.
      • Yu X.
      Multichannel film dosimetry with nonuniformity correction.
      ], or reflection scanned images [
      • Alva H.
      • Mercado-Uribe H.
      • Rodriguez-Villafuertre M.
      • Brandan M.E.
      The use of a reflective scanner to study radiochromic film response.
      ].

      Possible drawbacks

      Despite relatively steep development of radiochromic film dosimetry, the authors of this letter came across two significant errors in using radiochromic film dosimetry within not only reviewing phase but in the published articles in some very respectful journals. We sincerely believe the omissions in published papers came due to lack of focus by both authors and reviewers, and our intention is to simply bring to the attention of film users the two issues that may potentially lead to significant errors. The first issue is the use of non-linear response of radiochromic film (netOD, raw PV) and attributing a simple division of such response functions to relative dose. Second problem is the lack of awareness that radiochromic films exhibit a significant energy dependent response when irradiated in low energy photon beams. Such obvious, yet significant omissions could be eventually explained by inertia, as laid down by Newton, more than three centuries ago [

      Newton I. The Principia: mathematical principles of natural philosophy. Translated by: Cohen IB, Whitman AM. Berkeley, Calif.; London: University of California Press, 1999.

      ]. It seems that inertia does not only drive the mechanics of the Universe, but the human way of life too.

      Non-linearity of response function

      Radiographic film dosimetry was around for much longer than radiochromic film based one. At the time, radiographic film response to exposure was commonly expressed in terms of optical density (OD), a quantity derived as logarithm for base 10 of inverse transmission. Rationale for choosing this quantity was the H&D curve, which exhibited linear relation in relatively wide dose range, except at low (toe) and high (saturation) dose values. Quite possibly, the radiochromic film user’s community followed (by inertia) the suit in using the very same quantity despite the fact the response of radiochromic film was non-linear in the whole dose range [
      • Devic S.
      • Tomic N.
      • Lewis D.
      Reference radiochromic film dosimetry: review of technical aspects.
      ]. We can only make a hypothesis that some users tend to use ratio of optical densities (for measuring either PDD curves or profiles) following the practice (inertia) with ion-chambers, who (again in certain ranges of beam qualities) have linear response (measured charge) with dose. When the same methodology is used for (any) dosimeter having non-linear response, such obtained “relative values” in the case of radiochromic films will overestimate the real relative dose values [
      • Devic S.
      • Tomic N.
      • Aldelaijan S.
      • DeBlois F.
      • Seuntjens J.
      • Chan M.F.
      • et al.
      Linearization of dose–response curve of the radi-ochromic film dosimetry system.
      ]. Over the last decade, a few novel functional forms have been suggested that do linearize the response of radiochromic films [
      • Devic S.
      • Tomic N.
      • Aldelaijan S.
      • DeBlois F.
      • Seuntjens J.
      • Chan M.F.
      • et al.
      Linearization of dose–response curve of the radi-ochromic film dosimetry system.
      ,
      • Aldelaijan S.
      • Devic S.
      Comparison of dose response functions for EBT3 model GafChromic (TM) film dosimetry system.
      ]. However, one has to be cautious with using even these new functional forms, as they would provide accurate relative dose values by simple division of responses if and only if the film response for the given beam quality does not change over the plane in which film is used for relative dose measurements.

      Radiochromic film response as a function of beam quality

      The second possible drawback that may lead to inaccurate dose measurements is if one ignores the energy dependent response at low photon energies. It was well established that all the EBT film models exhibit strong energy dependence as the effective beam energy goes bellow 100 keV [
      • Sutherland J.G.H.
      • Rogers D.W.O.
      Monte Carlo calculated absorbed-dose energy dependence of EBT and EBT2 film.
      ,
      • Chiu-Tsao S.
      • Massillon J.L.G.
      • Domingo-Muñoz I.
      • Chan M.
      Energy dependence of the new GafChromic-EBT3 film's dose response-curve.
      ,
      • Bekerat H.
      • Devic S.
      • DeBlois F.
      • Singh K.
      • Sarfehnia A.
      • Seuntjens J.
      • et al.
      Improving the energy response of external beam therapy (EBT) GafChromic™ dosimetry films at low energies (≤100 keV).
      ]. Nevertheless, one may find papers measuring relative dose distributions with films at 50 kVp beam quality and reporting relative dose by completely ignoring this effect by using calibration curve obtained at either different kVp beam quality or sometimes even using calibration at MV beam. We believe this might be yet another case of inertia, following the widely adopted prejudice that film has energy independent response based on the fact that most of the early research with films has been performed in megavoltage photon and electron beams, the range of beam qualities where radiochromic film indeed has response to dose independent of energy [
      • Sarfehnia A.
      • Stewart K.
      • Seuntjens J.
      An absorbed dose to water standard for HDR 192Ir brachytherapy sources based on water calorimetry: numerical and experimental proof-of-principle.
      ].

      How to avoid the two drawbacks when performing radiochromic film dosimetry

      When measuring the relative dose, user could use any function that make the film response linear with dose. This can be easily done once calibration films are scanned – data is there forever to experiment with. If, for whatever reason (existing software, existing protocols), user prefers to use response functions with non-linear response (netOD, raw PV), to obtain relative dose distribution, one has to go through the reference dosimetry protocol, where the calibration curve function will sort out the non-linear response. Once the issue with response function is solved, user has to make sure dose distribution is measured for photon beam qualities with effective energy above 100 keV. For lower photon energies, one has to determine beam quality at every measurement point within the film plane, and then apply the corresponding calibration curve for each measurement point. For this task, user clearly has to go through the reference dosimetry protocol for each measurement point irrespectively of using response function with linear or non-linear response.

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