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Impact of inertia on possible fundamental drawbacks in radiochromic film dosimetry

2019; Elsevier BV; Volume: 66; Linguagem: Inglês

10.1016/j.ejmp.2019.08.019

1724-191X

Slobodan Dević, Saad Aldelaijan, Hamed Bekerat,

Radiation Effects and Dosimetry

From its very inception [[1]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.Radiat Phys Chem. 1990; 35: 767-773Google Scholar] radiochromic films draw an immediate attention for its high spatial resolution and near tissue equivalence in most dosimetry applications [[2]Devic S. Radiochromic film dosimetry: past, present, and future.Phys Med. 2011; 27: 122-134Abstract Full Text Full Text PDF PubMed Scopus (242) Google Scholar]. 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 [[3]Cheung T. Butson M.J. Yu P.K. Post-irradiation colouration of Gafchromic EBT radiochromic film.Phys Med Biol. 2005; 50: N281-N285Crossref PubMed Scopus (165) Google Scholar] with significantly lower (10 times per square inch lower) price and readily accepted radiochromic film protocols (RFPs) using inexpensive flatbed document scanners [[4]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.Med Phys. 2004; 31: 2392-2401Crossref PubMed Scopus (219) Google Scholar]. To date, a number of RFPs have been described, using either transmission single [[5]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.Med Phys. 2005; 32: 2245-2253Crossref PubMed Scopus (465) Google Scholar] or multiple color channels [[6]Micke A. Lewis D.F. Yu X. Multichannel film dosimetry with nonuniformity correction.Med Phys. 2011; 38: 2523-2534Crossref PubMed Scopus (333) Google Scholar], or reflection scanned images [[7]Alva H. Mercado-Uribe H. Rodriguez-Villafuertre M. Brandan M.E. The use of a reflective scanner to study radiochromic film response.Phys Med Biol. 2002; 47: 2925-2933Crossref PubMed Scopus (76) Google Scholar]. 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 [[8]Newton I. The Principia: mathematical principles of natural philosophy. Translated by: Cohen IB, Whitman AM. Berkeley, Calif.; London: University of California Press, 1999.Google Scholar]. It seems that inertia does not only drive the mechanics of the Universe, but the human way of life too. 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 [[9]Devic S. Tomic N. Lewis D. Reference radiochromic film dosimetry: review of technical aspects.Phys Med. 2016; 32: 541-556Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar]. 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 [[10]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.Med Phys. 2012; 39: 4850-4857Crossref PubMed Scopus (59) Google Scholar]. Over the last decade, a few novel functional forms have been suggested that do linearize the response of radiochromic films [10Devic 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.Med Phys. 2012; 39: 4850-4857Crossref PubMed Scopus (59) Google Scholar, 11Aldelaijan S. Devic S. Comparison of dose response functions for EBT3 model GafChromic (TM) film dosimetry system.Phys Med. 2018; 49: 121-126Abstract Full Text Full Text PDF Scopus (20) Google Scholar]. 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. 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 [12Sutherland J.G.H. Rogers D.W.O. Monte Carlo calculated absorbed-dose energy dependence of EBT and EBT2 film.Med Phys. 2010; 37: 1110-1116Crossref PubMed Scopus (129) Google Scholar, 13Chiu-Tsao S. Massillon J.L.G. Domingo-Muñoz I. Chan M. Energy dependence of the new GafChromic-EBT3 film's dose response-curve.Med Phys. 2012; 39: 3724Google Scholar, 14Bekerat 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).Med Phys. 2014; 41022101Crossref PubMed Scopus (63) Google Scholar]. 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 [[15]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.Med Phys. 2007; 34: 4957-4961Crossref PubMed Scopus (22) Google Scholar]. 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.