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Dosimetry audit for megavoltage photon beams applied in non-reference conditions

Open AccessPublished:June 29, 2022DOI:https://doi.org/10.1016/j.ejmp.2022.06.011

      Highlights

      • We conducted for the first time a Malaysian postal dosimetry audit of external beam non- reference conditions.
      • The novel fabricated Ge-doped fibres and TLD-100 powder were used an audit dosimeter.
      • Ten centres were audited, only eight of the centres operated within ± 5% of stated dose.

      Abstract

      Purpose

      We have conducted for the first time a Malaysian postal dosimetry audit of external beam under non-reference conditions by evaluating the output performance while screening for systematic errors within the dosimetry chain. The potential use from the choice of detector were investigated along with the search for other sources of discrepancies.

      Methods

      Ten radiotherapy centres were audited, encompassing 16 megavoltage photon beam arrangements, adopting the IAEA postal dosimetry protocol for non-reference conditions, with a holder modified to accommodate three TLD types: Ge-doped cylindrical silica fibres (CF), Ge-doped flat silica fibres (FF), and TLD-100 powder.

      Results

      Eight of the centres operated within ± 5% of stated dose, one other exceeding tolerance for all measured points, and one did not return any dosimeters for analysis after failing the initial irradiations. Post remedial measures, the mean relative response for CF, FF, and TLD-100 was 1.00, 0.99, and 0.98 respectively, with associated coefficients of variation 6.87%, 6.45%, and 5.06%.

      Conclusion

      High quality radiotherapy clinical practice postal dosimetry audits that are based on sensitive TLDs are seen to be particularly effective in identifying and resolving dose delivery discrepancies.

      Keywords

      Introduction

      Recent years have witnessed increasingly sophisticated practices in radiotherapy, all aimed at improving outcome. Some measures concern better means of delivering dose, technological innovations notably depending on machine engineering and associated control measures. Such introduction has depended on key information arising from outcome, practical radiobiology and attendant clinical trials underpinning developments [
      • Pistenmaa D.A.
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      Changing the global radiation therapy paradigm.
      ,
      • Hussein M.
      • Heijmen B.J.
      • Verellen D.
      • Nisbet A.
      Automation in intensity modulated radiotherapy treatment planning - a review of recent innovations.
      ]. The introduction of electron linear accelerators into clinical practice has led to optimization of dose delivery techniques, such as applications in intensity-modulated radiation therapy (IMRT), image-guided radiation therapy (IGRT), and volumetric modulated arc therapy (VMAT). The non-uniform intensity doses of these techniques place great demand on dose accuracy, requiring comprehensive quality assurance (QA).
      A key step in any dosimetry QA program is the audit, typically developed by recognized independent external bodies [
      • Izewska J.
      • Wesolowska P.
      • Lechner W.
      • Georg D.
      • Tomsej M.
      • Followill D.
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      Development of external dosimetry audits for advanced technology in radiotherapy dose delivery: an IAEA coordinated research project.
      ,
      • De Saint-Hubert M.
      • De Angelis C.
      • Knežević Ž.
      • Michalec B.
      • Reniers B.
      • Pyszka E.
      • et al.
      Characterization of passive dosimeters in proton pencil beam scanning - A EURADOS intercomparison for mailed dosimetry audits in proton therapy centres.
      ,
      • Okamoto H.
      • Minemura T.
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      • Mizuno H.
      • Tohyama N.
      • Nishio T.
      • et al.
      Establishment of postal audit system in intensity-modulated radiotherapy by radiophotoluminescent glass dosimeters and a radiochromic film.
      ]. An example is the dosimetry protocol developed by the International Atomic Energy Agency (IAEA), introduced to enhance the QA of treatment delivery, also to mitigate risks of radiological incidents [

      Holdsworth C, Kukluk J, Molodowitch C, Czerminska M, Hancox C, Cormack RA, et al. Computerized system for safety verification of external beam radiation therapy planning. Int J Radiat Oncol Biol Phys 2017;98(3):691-8. https://doi.org/10.1016/j.ijrobp.2016.06.187.

      ,
      • Huq M.S.
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      • Ibbott G.S.
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      • et al.
      The report of task group 100 of the AAPM: application of risk analysis methods to radiation therapy quality management.
      ]. In 1965 the IAEA initiated a TLD-based postal dosimetry audit program for radiotherapy founded on a simple yet highly effective audit process for reference measurements. Subsequently, in 2001, the IAEA broadened the scope to include measurements relevant to the many current sophisticated irradiation geometries, including non-reference conditions, irregular field set-ups, and small fields, detailed by the IAEA in terms of nine sequential audit steps [

      Izewska J, Bokulic T, Kazantsev P, Wesolowska P. 50 years of the IAEA/WHO postal dose audits for radiotherapy. SSDL Newsletter 2019;70:11-5. https://www-pub.iaea.org/MTCD/Publications/PDF/Newsletters/ ssdl-70.pdf [accessed 13.02.2020].

      ]. In particular, it has been recognized that the postal dosimetry audit program can help determine whether the system of dosimetry has been properly implemented and that final results comply with the commissioning standard. Over time, the IAEA audit program has given rise to significant dosimetry improvements, notably with an increasing fraction of results within the ± 5% tolerance [

      Izewska J. IAEA support to dosimetry audit networks. SSDL Newsletter 2019;70:16-8. https://www-pub.iaea.org/MTCD/Publications/PDF/Newsletters/ ssdl-70.pdf [accessed 13.02.2020].

      ,
      • Abdel-Wahab M.
      • Zubizarreta E.
      • Polo A.
      • Meghzifene A.
      Improving quality and access to radiation therapy - an IAEA perspective.
      ]. This indicator of effectiveness has lead to growth in the number of both new and established radiotherapy centres participating in the scheme [
      • Izewska J.
      • Lechner W.
      • Wesolowska P.
      Global availability of dosimetry audits in radiotherapy: The IAEA dosimetry audit networks database.
      ,
      • Alvarez P.
      • Kry S.F.
      • Stingo F.
      • Followill D.S.
      TLD and OSLD dosimetry systems for remote audits of radiotherapy external beam calibration.
      ]. To fulfill the QA needs of advanced radiotherapy treatment techniques a higher level of radiotherapy postal dosimetry audit is required, moving the IAEA to adapt its TLD postal dose audit to include the non-reference conditions that focus on greater targeted radiation. As herein, some countries have established their own national dose audit, for both reference and non-reference conditions, use being made of various passive dosimeters [
      • He Z.
      • Luo S.
      • Zhu W.
      • Li K.
      TLD comparisons for radiotherapy dosimetry in non-reference conditions.
      ,
      • Schaeken B.
      • Cuypers R.
      • Lelie S.
      • Schroeyers W.
      • Schreurs S.
      • Janssens H.
      • et al.
      Implementation of alanine/EPR as transfer dosimetry system in a radiotherapy audit programme in Belgium.
      ,
      • Mizuno H.
      • Fukumura A.
      • Fukahori M.
      • Sakata S.
      • Yamashita W.
      • Takase N.
      • et al.
      Application of a radiophotoluminescent glass dosimeter to non-reference condition dosimetry in the postal dose audit system.
      ,
      • Santos T.
      • Lopes M.d.C.
      • Gershkevitsh E.
      • Vinagre F.
      • Faria D.
      • Carita L.
      • et al.
      IMRT national audit in Portugal.
      ].
      Malaysia first participated in the IAEA/WHO TLD Postal Audit Program in 1985 [
      • Samat S.B.
      • Evans C.J.
      • Kadni T.
      • Dolah M.T.
      Malaysian participation in the IAEA/WHO postal TLD and postal ionisation chamber intercomparison programmes: analysis of results obtained during 1985–2008.
      ], a time when only five radiotherapy centres were operating in the country. The number has since grown markedly, with in 2018 22 centres participating in the IAEA/WHO reference conditions postal dosimetry audit [
      • Abdullah N.
      • Kadni T.
      • Dolah M.T.
      Malaysian participation in the IAEA/WHO TLD postal dose quality audit service: Data analysis from 2011–2015.
      ]; 72.7% of the radiotherapy centres complied within ± 5% of stated dose. Malaysian centres have also been involved in audits by others; nine participated in a 2002 TLD audit organized by the University of Malaya Medical Centre and University of Wisconsin, Radiation Calibration Laboratory [
      • Rassiah P.
      • Ng K.H.
      • DeWerd L.A.
      • Kunugi K.
      A thermoluminescent dosimetry postal dose inter-comparison of radiation therapy centres in Malaysia.
      ], while in 2017 eight participated in a TLD audit based on the use of optical fibres [
      • M. Noor N.
      • Nisbet A.
      • Hussein M.
      • Chu S S.
      • Kadni T.
      • Abdullah N.
      • et al.
      Dosimetry audits and intercomparisons in radiotherapy: a malaysian profile.
      ], resulting in a passing rate of 87.5%. In each, a small number of centres deviated from stated dose by more than ± 5%, miscalculation of absorbed dose, lack of adherence to irradiation protocol, and errors in reporting absorbed dose being cited. The results suggest a need for more frequent audits, perhaps on an annual basis, extended to include non-reference conditions. Present audit focuses on non-reference conditions, seeking to detect discrepancies and inconsistencies in the radiotherapy dosimetry chain.

      Materials and method

      TLD system

      Three TLD systems have been used, the first two being locally fabricated doped glass, Ge-doped cylindrical silica fibres (CF), diameter 483 µm, and Ge-doped flat silica fibres (FF), 67.5 µm × 273 µm, as well as TLD-100 lithium fluoride powder (LiF:Mg,Ti), grain sizes from 75 µm to 176 µm. Preparation details, annealing, and the read-out processes have been described elsewhere [
      • Noor N.M.
      • Fadzil M.S.A.
      • Ung N.M.
      • Maah M.J.
      • Mahdiraji G.A.
      • Abdul-Rashid H.A.
      • et al.
      Radiotherapy dosimetry and the thermoluminescence characteristics of Ge-doped fibres of differing germanium dopant concentration and outer diameter.
      ,
      • Fadzil M.S.A.
      • Tamchek N.
      • Ung N.M.
      • Ariffin A.
      • Abdullah N.
      • Bradley D.A.
      • et al.
      Assessment of thermoluminescence glow curves and kinetic parameters of fabricated Ge-doped flat fiber for radiotherapy application.
      ]. In use, the dosimeters were retained in watertight colour-coded plastic capsules. Prior to this audit, both the cylindrical and flat fibres have been established their key basic dosimetric characteristics including linearity, reproducibility, fading, energy-, dose rate-, and angular dependency [

      Fadzil MSA. Use of fabricated germanium-doped optical fibres for radiotherapy postal dose audit of megavoltage photon beams under non-reference conditions in selected Malaysian radiotherapy facilities. [dissertation]. Serdang: Universiti Putra Malaysia; 2020. http://psasir.upm.edu.my/id/eprint/85468/1/FPSK%28p%29%202020%206%20ir.pdf.

      ].

      Organization of postal dose audit under non-reference conditions

      The work herein is a collaboration between Universiti Putra Malaysia (UPM) and the national Secondary Standard Dosimetry Laboratory (SSDL) located at the Malaysian Nuclear Agency (MNA). Participant invitations accompanied by an information sheet and reply slip were sent to 36 radiotherapy centres, with 10 positive responses obtained, comprising two government hospitals, one university hospital, and seven private centres. A total of 16 photon beams were checked, ten using a 6 MV photon beam and six a 10 MV photon beam. The mailed dosimetry kit comprised colour-coded capsules, a modified IAEA TLD holder, and an instruction and data booklet, participants being asked to carry out the irradiations in accord with instructions. Completed datasheets and the TLDs were requested to be returned to the audit centre within one month of irradiation, a box being provided for the purpose.

      Irradiation setup for non-reference conditions

      The modified TLD holder weighed at its base by a 40 g mass is directed to be placed in a water tank located on the treatment couch. The hollow tube TLD holder with a capsule hole at 10 cm down from the top, needs to be completely immersed, avoiding air bubbles, the water being filled to the top of the TLD holder. The linac needs to be configured for vertical irradiation, the TLD holder being aligned with the central axis of the beam, the table height adjusted to ensure the water surface is at 100 cm from the source (Fig. 1), all to be rechecked before each irradiation.
      Figure thumbnail gr1
      Fig. 1Irradiation geometry for non-reference conditions using a modified IAEA TLD holder.
      Reference measurements were carried out as part of the same audit of non-reference conditions, allowing evaluation of absorbed dose both on- and off-axis. Two fields are involved in measurement of non-reference conditions; symmetric and asymmetric. For all irradiation fields the CF, FF, and TLD-100 were irradiated to an absorbed dose of 2 Gy [
      • Iżewska J.
      • Georg D.
      • Bera P.
      • Thwaites D.
      • Arib M.
      • Saravi M.
      • et al.
      A methodology for TLD postal dosimetry audit of high-energy radiotherapy photon beams in non-reference conditions.
      ]. Details of the irradiation parameters are presented in Table 1 while detailed summary of the participating centres is provided in Table 2.
      Table 1Irradiation parameters for audit under non-reference conditions.
      FieldAcronymCollimator field size (cm2)AxisPoint of measurement
      Symmetry (S)Reference measurementsSR10 × 10On-axis1
      Open field 1st profile (x-axis)SO1P20 × 20On- and off- axis3
      Open field 2nd profile (y-axis)SO2P20 × 20On- and off- axis3
      Wedge profileSWP20 × 20On- and off -axis3
      Asymmetry

      (A)
      Open fieldAOF10 (7.5 + 2.5) × 10On- and off- axis2
      Wedge fieldAWF10 (7.5 + 2.5) × 10On- and off- axis2
      Half-beam blocked open fieldAHBO10 (10 + 0) × 10Off-axis1
      Half-beam blocked wedge fieldAHBW10 (10 + 0) × 10Off-axis1
      Table 2Summary of the participating centres in Malaysian postal dosimetry audit of external beam under non-reference conditions.
      Radiotherapy centreLinacYear of installationNominal Energy (Quoted in MV)Previous reference auditIonization chamberElectrometerCalibration protocol
      1Elekta Synergy20126YesWellhoffer IC 70Scanditronix Wellhoffer Dose 1TRS 398
      2Varian True Beam STX20156NoPTW 30,010Sun Nuclear PC Electrometer 1014TRS 398
      10
      3Elekta Synergy20106YesPTW 30,013PTW Unidos ETRS 398
      10
      4Siemens Artiste20086YesIBA FC

      65-G
      Scanditronix Wellhoffer Dose 1TRS 277
      10
      5Siemens Onco Impression Plus20056NoPTW 30,013PTW Unidos ETRS 398
      10
      6Varian Trilogy20156YesIBA FC

      65-G
      Supermax Standard ImagingTRS 398
      10
      7Varian Novalis TX20126YesIBA FC

      65-G
      Supermax Standard ImagingTRS 398
      10
      8Varian Clinac 600C19996YesPTW 30,013PTW UnidosTRS 398
      9Siemens Artiste20096NoPTW 30,013PTW Unidos ETRS 398
      10Varian Novalis TX20136YesPTW 30,013PTW Unidos ETRS 398

      Postal dosimetry audit under non-reference conditions report

      Results of the audit are presented in terms of percentage deviation ((ΔD) of measured value (DTLD) from the dose proclaimed by each participating radiotherapy centre (Dparticipant), as follows:
      ΔD=Dparticipant-DTLDDTLD×100


      A tolerance of ± 5% (0.95 ≤ DTLDDparticipant ≤ 1.05) is conventionally applied. Prior to an audit report being transmitted to each participating centre the analysis was first doubly verified, a certificate of completion of the audit under non-reference conditions also accompanying each report. Those centres achieving less than ± 5% deviation were advised that no further investigation would be necessary. Conversely, those recording minor deviation (>5% but less than 10%) or major deviations (>10%) were provided with follow-up dosimetry kits, a request for immediate repeat irradiations being made, specifically for defaulting situations from stated dose. On-site visits were initiated for those cases in which the follow-up evaluations fell short of tolerance.
      Uncertainty in deviation of the absorbed dose ratio.DfibreorDTLDDparticipant
      The uncertainty in the absorbed dose ratio DTLDDparticipant was calculated by combining the determined combined uncertainty in the TLD system dose evaluation and the uncertainty in the participating center's determination of absorbed dose using the following equation:
      uDTLDDparticipant=ucDTLD2+uDparticipant2I


      where ucDTLD is the uncertainty in the absorbed dose measurement by the TLD system (CF, FF or TLD-100 powder). u(Dparticipant) is the uncertainty in the dose stated by the participating centres and I is the number of samples used to determine the ratio. In this case, ten measurements from each TLD system were used to provide more accurate mean values with a lower margin of error.

      Results

      The majority of centres used the TRS-398 calibration protocol, while one used an earlier version, TRS-277. All make use of the SSDL for regular periodic calibration of their ionization chambers and electrometers. Seven of the centres had previously participated in the reference conditions postal audit conducted by the SSDL. In using CF, FF, and TLD-100, the initial audit revealed the mean distribution of ratio of measured to stated absorbed dose to be 1.00, 0.99, and 0.97 respectively with associated coefficient of variation (CV) of 6.51%, 6.80%, and 6.15%. Fig. 2 shows percent deviation from stated dose, 26 of the 172 measurement points (inclusive of both 6- and 10 MV beams) exceeding the ± 5% tolerance. Of the 16 beams involved, seven failed in regard to at least one measurement under non-reference conditions. Two capsules had failed to be irradiated for the asymmetry wedge field (AWF), incomplete measurements requiring follow-up irradiations. One participating radiotherapy centre returned a major deviation involving all measurement evaluations for non-reference conditions, the largest deviation being in excess of 70%.
      Figure thumbnail gr2
      Fig. 2Relative deviation from stated dose for the total of 172 measurement points, for non-reference conditions. Abbreviation: SR1 = Symmetry reference measurements 1; SR2 = Symmetry reference measurements 2; SO1P = Symmetry open field 1st profile (x-axis); SO2P = Symmetry open field 2nd profile (y-axis); SWP = Symmetry wedge profile; AOF = Asymmetry open field; AWF = Asymmetry wedge field; AHBO = Asymmetry half-beam blocked open field; AHBW = Asymmetry half-beam blocked wedge field.

      Follow-up measurements

      Follow-up audit involved five centres comprising seven photon beam irradiations (five 6 MV beams and two 10 MV beams). One centre failed to return any dosimeters for analysis, measurements being required for both 6- and 10 MV photon beams, initial conversation revealing the linac used for initial measurements had since suffered a major breakdown with at the time the problem remaining unresolved. Accordingly, the results obtained from the initial measurements were included in the consolidated results. Pursuant to the follow-up measurements, 19 of the 172 measurement points exceeded ± 5% tolerance (Fig. 3), with 27% of cases resolved, while in other cases discrepancies had become more marked eg several within the field category SWP (wedged) and SO1P (open).
      Figure thumbnail gr3
      Fig. 3Relative deviation from stated values for 172 consolidated measurements points pursuant to follow-up. Abbreviation: SR1 = Symmetry reference measurements 1; SR2 = Symmetry reference measurements 2; SO1P = Symmetry open field 1st profile (x-axis); SO2P = Symmetry open field 2nd profile (y-axis); SWP = Symmetry wedge profile; AOF = Asymmetry open field; AWF = Asymmetry wedge field; AHBO = Asymmetry half-beam blocked open field; AHBW = Asymmetry half-beam blocked wedge field.
      Fig. 4, depicting consolidated data pursuant to follow-up measurements, show the distribution of mean ratios of dose measured using CF, FF, and TLD-100 powder and stated dose claimed by participating centres, obtaining 1.00, 0.99, and 0.98 respectively, with associated CV of 6.87%, 6.45%, and 5.06%. The minimum and maximum ratios are 0.59 and 1.05, being a situation clearly showing a marked bias towards underdosing. After follow-up irradiations, the auditors conducted site visits as referred to earlier, discussions focusing on improving dosimetry practices.
      Figure thumbnail gr4
      Fig. 4Distribution of relative dose: (a) CF, (b) FF, and (c) TLD-100 powder pursuant to follow-up measurements.
      Apparent from the data is that there is no evidence of bias emerging from the choice of dosimetric medium, three distinctly different TL media being chosen for the present work, all producing results strongly in accord with each other.
      ResultsofuncertaintyintheabsorbeddoseratioDTLDDparticipant


      All of the participating radiotherapy centres had their ionization chambers calibrated at the Malaysia Nuclear Agency's Secondary Standard Dosimetry Laboratory (SSDL). The SSDL has a 1.22% uncertainty at a 95% confidence, with all relevant variables traceable to the IAEA and the following traceable to the International Bureau of Weights and Measures (BIPM). For the CF, FF, and TLD-100 powder, the estimated uncertainty of deviation in the absorbed dose ratio is 1.41%, 1.80%, and 0.45%, respectively.

      Discussion

      Participation in the postal dosimetry audits of megavoltage photon beams under non-reference conditions has been voluntary, presumably one of a number of factors behind the disappointing 28% take-up from radiotherapy centres in Malaysia. A rise in cancer incidence in Malaysia in recent years is another [
      • Yahya N.
      • Sukiman N.K.
      • Suhaimi N.A.
      • Azmi N.A.
      • Manan H.A.
      • Trajman A.
      How many roads must a Malaysian walk down? Mapping the accessibility of radiotherapy facilities in Malaysia.
      ], with oncology team members stretched in seeking to handle case numbers, notwithstanding that effective practice demands that dose delivery remains within tolerance [
      • Herring D.F.
      • Compton D.M.J.
      The degree of precision required in radiation dose delivered in cancer radiotherapy.
      ].
      Pursuant to follow-up measurements, the overall picture was one of percentage deviation ranging from −4.95% to an isolated extreme of 70.06%. Negative deviations in this audit indicate centre dose evaluations lower than delivered values (delivered dose remaining within −5% of tolerance) while positive deviations indicate centre evaluations greater than delivered dose, the underdosed regime being associated with an attendant reduced probability for tumour eradication [
      • Abdullah N.
      • Kadni T.
      • Dolah M.T.
      Malaysian participation in the IAEA/WHO TLD postal dose quality audit service: Data analysis from 2011–2015.
      ].
      With one centre failing to comply within tolerance for all measurement points, the auditors initiated a site visit to identify the source of discrepancy, being in line with the practice followed by other radiation authorities [
      • Kry S.F.
      • Peterson C.B.
      • Howell R.M.
      • Izewska J.
      • Lye J.
      • Clark C.H.
      • et al.
      Remote beam output audits: a global assessment of results out of tolerance.
      ]. The deviations were determined to stem from incorrect monitor unit (MU) calculations in respect of reference measurement. This systematic error, which occurred at the earliest stage of reference measurement, affected all subsequent MU calculations for all measurement points under non-reference conditions. The noticeable discrepancies occur across the TLD systems in the symmetry wedge profile (SWP) field as a result of the imprecise adjusted IAEA TLD holder alignment while replacing the capsules for subsequent irradiation.
      An asymmetry half-beam blocked wedge field (AHBW) produced the greatest deviations, with four of sixteen beams beyond the + 5% tolerance limit. This situation was due to a mismatch between the wedge orientation planned in use of the treatment planning system (TPS) and that in installation of the physical wedge on the gantry head prior to irradiation. On another occasion, a medical physicist inadvertently swapped capsule positions along the horizontal arm of the modified TLD holder, resulting in an incorrect dose for that measurement. Such human errors have been cited by the IAEA, urging the need for strict compliance with the dosimetry code of practice. Routine annual audits can help as part of a process that is aimed at overcoming poor attention to detail [
      • Izewska J.
      • Bokulic T.
      • Kazantsev P.
      • Wesolowska P.
      • van der Merwe D.
      50 Years of the IAEA/WHO postal dose audit programme for radiotherapy: what can we learn from 13756 results?.
      ].
      Perhaps surprising was that three of the centres exhibiting deviations beyond + 5% tolerance had previously participated in a phosphor based TLD postal reference dosimetry audit conducted by the SSDL, producing satisfactory results on that previous occasion. Issues of complacency, of inexperience in conduct of the postal dosimetry audit under non-reference conditions, and a lack of advanced dosimetry audit training are potential contributory factors to the failings determined herein [
      • Kry S.F.
      • Peterson C.B.
      • Howell R.M.
      • Izewska J.
      • Lye J.
      • Clark C.H.
      • et al.
      Remote beam output audits: a global assessment of results out of tolerance.
      ,
      • Clark C.H.
      • Jornet N.
      • Muren L.P.
      The role of dosimetry audit in achieving high quality radiotherapy.
      ]. Comprehensive training and constant updates are requirements expected for all medical physicists, as suggested by Noor et al. [
      • M. Noor N.
      • Nisbet A.
      • Hussein M.
      • Chu S S.
      • Kadni T.
      • Abdullah N.
      • et al.
      Dosimetry audits and intercomparisons in radiotherapy: a malaysian profile.
      ]. Moreover, it is important to strengthen quality assurance facilities in radiotherapy departments, enabling in-house dosimetry audits to be conducted following globally recommended guidelines.
      This is the first work of its kind in which three distinct dosimetry systems (CF, FF, and TLD-100 powder) were used in a postal dosimetry audit, particularly under non-reference conditions. All three systems were found to be independently verifiable and had an acceptable level of uncertainty. The results of the remote dosimetry systems evaluated in this study were found to be comparable, with appropriate absorbed dose determination needed for each system.

      Conclusion

      Conduct of the first postal dosimetry audit under non-reference conditions has been carried out for radiotherapy centres in Malaysia. The audits revealed 80% of the participating radiotherapy centres to record deviations within ± 5% tolerance. Several causes of greater deviations have been identified: incorrect monitor unit (MU) calculation, accidental swap of dosimeter capsule position during the irradiation process, and improper wedge orientation. The mishandling issues point to a less careful approach to competency than is to be expected of a medical physicist, leading to questions regarding adequacy of training. This study highlights the importance of a continuous QA program in the dosimetry chain for radiotherapy. Better understanding in implementing the dosimetry code of practice is a crucial component.
      A number of recommendations have been made: i) A national audit and intercomparison dosimetry network should be established [
      • M. Noor N.
      • Nisbet A.
      • Hussein M.
      • Chu S S.
      • Kadni T.
      • Abdullah N.
      • et al.
      Dosimetry audits and intercomparisons in radiotherapy: a malaysian profile.
      ]; ii) The dosimetry audit program needs to be conducted on a regular basis, obligating radiotherapy centers to participate in the dosimetry audit as part of their license renewal requirement; iii) The Malaysian dosimetry audit program should be expanded to include more complex auditing levels [
      • M. Noor N.
      • Nisbet A.
      • Hussein M.
      • Chu S S.
      • Kadni T.
      • Abdullah N.
      • et al.
      Dosimetry audits and intercomparisons in radiotherapy: a malaysian profile.
      ], for instance dose audits for photon beams shaped with multi-leaf collimators and in the presence of heterogeneities to encompass the advanced treatment delivery techniques now coming online in radiotherapy.

      Declaration of Competing Interest

      The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

      Acknowledgements

      The authors are grateful for receipt of an Prototype Research Grant Scheme PRGS/1/2021/SKK07/UPM/02/1 from the Ministry of Higher Education Malaysia. The authors also extend their appreciation to all participating radiotherapy centres, the Secondary Standard Dosimetry Laboratory Malaysian Nuclear Agency and the Department of Radiology, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia for use of their irradiation and research facilities.

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