22 Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (1) Review article DIAGNOSTIC REFERENCE LEVELS IN DENTAL RADIOLOGY: A SYSTEMATIC REVIEW Diagnostične referenčne ravni v dentalni radiologiji - Sistematični pregled literature Alenka MATJAŠIČ University of Ljubljana, Faculty of health sciences, Medical imaging and radiotherapy department, Zdravstvena pot 5, 1000 Ljubljana, Slovenia * Corresponding author: alenka.matjasic@zf.uni-lj.si Received: 20. 8. 2021 Accepted: 9. 2. 2022 https://doi.org/10.47724/MIRTJ.2021.i02.a003 ABSTRACT Purpose: The purpose of this work was to review published articles in the fi eld of diagnostic reference levels in dental radiology, and to determine which areas have not been covered yet and require further scientifi c studies. The aim was also to determine if there are any dose optimization procedures suggested after DRL establishment. Materials and methods: A systematic review was performed using the Science Direct, PubMed, CINAHL (via EBSCOhost) and Dentistry & Oral Sciences Source (via EBSCOhost) databases, following the Cochrane Network study design guidelines. Articles were analysed and presented by author, year of publication, country of origin, technology (e.g. digital radiography, computed radiography and fi lm-screen), radiographic type (e.g. intraoral, panoramic and CBCT), units of measurement and main conclusions for each study. Results: Thirteen scientifi c articles on dose reference values in dental radiology were evaluated. Full-access articles published between 2001 and 2021 were used, and both reviews and original research articles were included. The studies address the defi nition or analysis of DRLs in intraoral and panoramic dental imaging and in dental CBCT imaging. Many studies report results based on diff erent image-receiving systems (e.g. DR, CR and fi lm-screen). The fi lm-screen system yielded the highest dose values of all three systems. All studies reviewed describe DRLs for the adult population, while only four also describe paediatric DRLs. Conclusion: Most EU countries have not yet set national DRLs for dental radiology. Most studies set or revise DRLs at the national level and compare them with guidelines from literature and from similar studies conducted in other countries. Most of these studies observed DRLs in the adult population. DRLs should also be set in the fi eld of dental CBCT imaging, as the use of this technology is rapidly increasing and the dose levels are incomparably higher than in general dental radiography. Keywords: dental radiography, diagnostic reference levels, intraoral imaging, panoramic dental imaging. IZVLEČEK Namen: Namen tega dela je pregledati objavljene članke s področja diagnostičnih referenčnih ravni v dentalni radiologiji, določiti področja znotraj slednje, ki še niso bila obravnavana in ki zahtevajo nadaljnje raziskave, pa tudi raziskati, ali po vzpostavitvi diagnostičnih referenčnih ravni študije predlagajo katero od oblik optimizacije doze. Materiali in metode: Izvedli smo sistematični pregled literature z uporabo podatkovnih baz Science Direct, PubMed, CINAHL (preko EBSCOhost) ter Dentistry & Oral Sciences (preko EBSCOhost). Pri zasnovi študije smo delno sledili smernicam Cochrane omrežja. Članke smo analizirali in razvrstili glede na avtorje, leto objave, državo nastanka, tehnologijo (digitalna radiografi ja, računalniška radiografi ja, sistem folija-fi lm), vrsto slikanja (intraoralno, panoramsko, CBCT) in uporabljene merske enote, za vsako študijo pa smo zapisali glavne ugotovitve.  Rezultati: Trinajst znanstvenih člankov, ki obravnavajo diagnostične referenčne ravni v dentalni radiolografi ji, smo analizirali in ocenili. Uporabili smo članke s polnim dostopom, objavljene med leti 2001 in 2021. Upoštevali smo tako izvirne kot pregledne znanstvene članke. Raziskave obravnavajo vzpostavitev ali analizo DRR-jev pri intraoralnem, panoramskem in zobnem CBCT slikanju. Velik delež raziskav poroča in ločuje rezultate glede na slikovni sprejemnik (DR, CR, folija-fi lm). Sistem folija-fi lm se je izkazal kot sistem z najvišjimi doznimi vrednostmi. Vse analizirane raziskave obravnavajo odraslo populacijo, le 4 opisujejo tudi DRR-je za pediatrijo.   Zaključek: Večina držav Evropske unije še nima vzpostavljenih DRR-jev na nacionalnih ravneh za področje dentalne radiologije. Večina obravnavanih raziskav vzpostavlja DRR-je na nacionalni ravni in jih primerja s smernicami iz literature ali s podobnimi študijami, izvedenimi v drugih državah. Večina raziskav obravnava odrasle paciente. Pojavlja se pomanjkanje raziskav s področja DRR-jev za dentalno CBCT slikanje, saj je uporaba te tehnologije v strmem porastu, dozne ravni zanjo pa so občutno višje v primerjavi s splošno dentalno radiologijo.  Ključne besede: dentalna radiografi ja, diagnostične referenčne ravni, intraoralno slikanje, panoramsko slikanje Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) 23 Matjašič A. / Diagnostic reference levels in dental radiology: a systematic review Introduction Technological development in dental radiology began after 1919, when adequate electrical insulation made it possible to safely perform intraoral imaging techniques. Panoramic dental imaging was developed and introduced for general use in the 1960s, while computed tomography has been used since the 1970s (1). The newest technology in dental radiology is cone beam computed tomography (CBCT), the use of which is rapidly increasing. It was developed for the maxillofacial region in 1995 and has been available for commercial use since 1999. Its use is popular primarily because it is a low-cost diagnostic technology that enables treatment planning and image- guided surgical and operative procedures (2). Ionizing radiation exposure in dental radiology contributes to approximately 2.5% of the eff ective dose received during medical examinations. The average adult eff ective dose for intraoral radiographs is 0.005 mSv for panoramic radiographs 0.01 mSv, and 0.011 to 1.073 mSv for dental computed tomography (3). According to the European guidelines for radiation protection in dental radiology, 96 to 449 dental radiological examinations are performed per 1,000 inhabitants in the countries of the European Union that have provided such data (4). Because of the large number of professionals performing such procedures and because many examinations in dentistry involve the use of ionizing radiation, certain radiation protection measures must be considered for patients exposed to a certain dose of ionizing radiation during these imaging examinations. One way to ensure optimal performance by a healthcare provider when using ionizing radiation is to determine diagnostic reference levels (DRLs) DRLs are usually easy to measure and are directly related to the radiation dose received by the patient (5). DRLs are the dose levels for ionizing radiation in diagnostic radiologic procedures that should not be exceeded if the procedure is optimized. They are determined using measured dose levels for patients undergoing a specifi c diagnostic examination. It is recommended that they be measured on as many x-ray machines as possible. The DRL is determined by the value of the third quartile of all doses received (6). Diagnostic reference values for radiological procedures in adults have been established for 72% of the 36 European countries. According to the European Commission report, the specifi c DRL values for dental radiology have only been applied at the national level in Finland and France (7). The European guidelines for radiation protection in dental radiology also state that few countries have conducted national or similar studies to determine DRLs and that there are no published DRLs for dental radiography at the European level (4). The establishment of national and local DRLs is proposed by the International Atomic Energy Agency for all medical examinations and procedures, for all clinical indications and for all patient groups (adults and size-dependent groups of children) (8). Because of the aforementioned large number of radiologic procedures performed annually in dentistry, the establishment of DRLs for this profession is of great importance. Specifi cally, for CBCT imaging, there is also a great need to establish DRLs, as the doses of ionizing radiation received in this technology are considerably higher than those received in intraoral or panoramic dental imaging and are comparable to those received by the patient during radiographs of the pelvis or abdomen (7). We use diff erent units of measurement to determine DRL values. In general radiography, air kerma product (KAP or PKA) and entrance surface air kerma (Ke) are commonly used. CTDIvol (computed tomography dose index) or dose length product (DLP) are used in computed tomography, while the received dose is considered in terms of activity delivered to the patient or activity per kilogram of body weight in nuclear medicine. Literature recommends using incident air kerma (Ki) for intraoral dental imaging and PKA for dental panoramic imaging (8). The authors of the articles discussed in this paper also use the unit PED (patient entrance dose) instead of ESD (entrance skin/surface dose). It is defi ned as the absorbed dose in air measured at the end of the spacer 'cone' for typical examinations without backscatter from the patient (9). Aim of the study The aim of this systematic review was to investigate how many countries, health facilities or radiology departments have already established diagnostic reference values for dental radiology. The aim was also to determine which areas of dental radiology (intraoral, panoramic or CBCT imaging) these DRLs cover and whether their establishment has suggested dose optimization for patients. Methods We performed a systematic review of literature. We relied in part on the guidelines of the Cochrane network when designing our study (10). Sources The Science Direct, PubMed in CINAHL (via EBSCOhost) and Dentistry & Oral Sciences Source (via EBSCOhost) scientifi c databases (11–14) were used to perform the search via the University of Ljubljana's and Central Medical Library's remote access. Inclusion and exclusion criteria A search algorithm based on a combination of keywords and logical operators was used in this review and is described in Table 1. No exclusion criteria in the fi rst search (for example the use of logical operator NOT) were applied. In the next step of the process, other conditions were set: full access articles, not older than 10 years (published between 2001 and 2021), and the inclusion of reviews as well as original research articles. After the initial search, which yielded 134 documents, exclusion criteria were applied and, at the end of the process, 13 articles were considered for inclusion in this review. The step-by-step process of document selection is shown in Figure 1. The results of the review were then presented in Table 2. Studies were listed by author, year of publication, country of origin, technology (e.g. digital radiography, computed 24 Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) Matjašič A. / Diagnostic reference levels in dental radiology: a systematic review radiography and fi lm-screen), type of radiography (e.g. intraoral, panoramic and CBCT), units of measurement, and main conclusions for each study. Results By using search terms and exclusion criteria described earlier and after further analysis of titles and abstracts, 13 studies were eligible for inclusion in this systematic review and are presented in Table 2. This systematic review analysed 13 scientifi c articles from 10 diff erent countries that address the area of diagnostic reference values in dental radiology. Most of them deal with the establishment and/or analysis of DRLs in general radiography (intraoral and panoramic dental imaging), while only two studies deal with CBCT imaging (17, 21). The DRLs are considered at the national level, while the authors performed comparisons between institutions and a larger number of radiographic units. Only Izawa et al (20) specify local DRLs and a comparison of three units at an institution with the aim of optimising and standardising the institution's imaging protocols. The authors of studies also frequently reported results on diff erent image-receiving systems (e.g. DR, CR and fi lm- screen). In all studies that made such a comparison, the fi lm- screen system was found to have the highest dose levels of all three systems. All studies reviewed describe DRL values for the adult population, while only four studies (9, 19, 22, 25) also describe paediatric DRL values. The importance of the latter is particularly emphasised in Holroyd's study, as it describes cephalometric imaging and the associated dose burden. Since cephalometric imaging is most commonly used in orthodontics and the patients are mostly children, special attention should be paid to optimal (as low as possible) dose exposure in this type of dental radiology, since children are more sensitive to ionising radiation, which can cause more damage in children than in adults (19). Discussion All articles studied report specifi c DRL values, i.e. the value of the 3rd quartile of measured doses from their data. The values are then compared with literature, with guidelines or, as in the study by Manousaridis et al. (23), with previous studies from the same country. This shows the importance of national DRL facilities everywhere, including Slovenia. Some authors emphasise the legal reasons for conducting these types of studies. For example, Alcaraz et al (15) mention the legal status of mandatory annual DRL reviews as part of the quality assurance programme in Spain. This may serve as a reason for conducting such studies. These reviews are mandatory in most European countries, but not all countries specify the time frame for their implementation. For example, Slovenian legislation does not specify how often a DRL review should be performed, but does states that the institution responsible for radiation protection should set DRL values based on systematic reviews of patient exposure and that it should follow European and other international recommendations in this area (27). Considering the small number of studies performed in CBCT imaging DRLs, this area of radiology seems very suitable for further research. The use of this technology is rapidly increasing, but dose levels can be up to 26 times higher than in dental panoramic imaging (18). Dose optimization for specifi c imaging modalities should always be considered. This applies to exposure parameters for general radiography, as well as FOV and resolution (these two can be controlled by the user) for CBCT imaging. It is especially important to establish and regularly revise DRLs, as they are one of the key factors that guide all parties involved in the process (dentists, radiographers, radiologists, medical physicists and service technicians) toward a high-quality work process that causes the least possible harm to patients. Limitations The fact that there are signifi cantly fewer studies in the fi eld of dental radiology compared to general radiography (X-ray or computed tomography) is the reason why this systematic review has limitations. When the sample is larger, the results are easier to interpret. In our case, we can only compare them in terms of their main results and derive some guidelines for possible further research, for example, the recommendation to extend the research to the fi eld of CBCT imaging and the associated dose burden. Another problem that appears in our review is the problem of comparing the studies correctly because they do not all use the same units of measurement. Some even suggest the use of new units of measurement, although literature recommends using Ki for intraoral and PKA for panoramic images. Table 1: Keywords and logical operators 1st keyword Logical operator 2nd keyword Logical operator 3rd keyword dental OR dentistry OR oral Logical operator: AND x- ray OR radiology OR radiography Logical operator: AND DRL OR diagnostic reference levels Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) 25 Ta bl e 2: R ev ie w o f s tu di es A ut ho r Ye ar Co un tr y Sa m pl e Te ch no lo gy Ra di og ra ph y ty pe U ni ts o f m ea su re m en t M ai n co nc lu si on s A lc ar az e t a l. (1 5) 20 12 Sp ai n 16 17 5 offi c ia l re po rt s, ga th er ed be tw ee n 20 02 an d 20 09 D R, C R, fi l m -s cr ee n In tr ao ra l ES D Ei gh t- ye ar -lo ng o bs er va tio n (2 00 2– 20 09 ). D RL v al ue in 2 00 9 w as 3. 1 m G y (w hi ch is 3 5. 4 % le ss th an in 2 00 2 w he n th e va lu e w as 4 .8 m G y) . E U g ui de lin es fo r i nt ra or al im ag in g in 2 00 4 re co m m en de d a va lu e of 4 m G y; 8 3. 4 % o f i ns tit ut io ns a re b el ow th is v al ue . D RL va lu es a ls o ch an ge w hi ch e ac h sy st em u se d (D R ha s th e lo w es t, w hi le fi lm -s cr ee n ha s th e hi gh es t o ne ). Re gu la r r ev is io ns a re su gg es te d, a t l ea st e ve ry th re e ye ar s (S pa ni sh le gi sl at io n ev en re qu ire s th em e ve ry y ea r) a s pa rt o f t he Q A p ro gr am m e in d en ta l offi c es . L im ita tio n: th er e is n o da ta o n ho w m an y ex po si tio ns w er e re pe at ed . A lc ar az e t a l. (1 6) 20 15 Sp ai n 34 14 3 offi c ia l re po rt s ga th er ed be tw ee n 19 97 an d 20 14 D R, C R, fi l m -s cr ee n In tr ao ra l ES D D RR o bs er va tio n fr om 2 00 2– 20 14 . I n 20 14 , t he v al ue w as 2 .8 m G y (4 1. 7 % le ss th an in 2 00 2 w he n th e va lu e w as 4 .8 m G y) . I n th e la st th re e ye ar s si nc e th ei r l as t s tu dy , D RL v al ue s st ab ili se d. It is a ss um ed th at th is h ap pe ne d be ca us e of th e st ab ili sa tio n in te ch no lo gy s ys te m c ha ng es a nd th e es ta bl is hm en t o f d ig ita l sy st em s. Fo r e ve ry x -r ay m ac hi ne , 1 0 ex po su re s w er e m ad e, m ea su re d in m G y, fo r u pp er s ec on d m ol ar . Ch ris to fi d es et a l. (9 ) 20 16 Cy pr us 20 m ac hi ne s Fi lm -s cr ee n In tr ao ra l D A P Th ey s tr es s th e im po rt an ce o f D RL e st ab lis hm en t f or a ll ag e gr ou ps (t hi s st ud y al so in cl ud ed c hi ld re n) a nd th e ca lc ul at io n of P ED v al ue . D RL s ar e be tw ee n 7. 23 m G y (u pp er m ol ar , a du lts ) a nd 1 .8 8 m G y (lo w er in ci so r, ch ild re n) . T he D RL s ar e sl ig ht ly h ig he r t ha n th os e of EU g ui de lin es , a lth ou gh th os e ar e ex pr es se d as E SD v al ue s, w hi le th e on es fr om th is s tu dy a re in P ED v al ue , w hi ch d oe s no t i nc lu de ba ck sc at te r r ad ia tio n. Th er e ar e si gn ifi ca nt d iff er en ce s be tw ee n th e 20 lo ca tio ns (x -r ay m ac hi ne s) . D os e va lu e st an da rd is at io n an d re du ct io n ar e su pp os ed to b e ac hi ev ed in th e fu tu re , p rim ar ily b y tr an si tio ni ng to di gi ta l r ec ei ve rs . D el eu e t a l. (1 7) 20 20 Sw itz er la nd 22 7 m ac hi ne s D R CB C T P K A , C TD I vo l Be si de s D RL e st ab lis hm en t, th e st ud y al so s ug ge st s th e es ta bl is hm en t o f c er ta in g ui de lin es a nd re co m m en da tio ns o n FO V (fi el d of v ie w ) s iz es , e ve n th ou gh th ei r r es ul ts s ho w ed th at m os tly sm al l s iz e FO V ar e us ed (a ve ra ge a re a 25 c m 2 , w hi ch m ea ns th is as pe ct o f d os e re du ct io n is a lre ad y co ns id er ed a nd in u se ). Th e su gg es te d D RL s in th is s tu dy a re n or m al iz ed to th e FO V di m en si on . H ea d an d ne ck C BC Ts w er e al so c on si de re d in th is s tu dy , n ot o nl y de nt al , a lth ou gh th e in di ca tio ns s til l m os tly re qu ire d en ta l C BC Ts , so th e es ta bl is hm en t o f D RL s in th is a re a is e sp ec ia lly im po rt an t. Matjašič A. / Diagnostic reference levels in dental radiology: a systematic review 26 Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) A ut ho r Ye ar Co un tr y Sa m pl e Te ch no lo gy Ra di og ra ph y ty pe U ni ts o f m ea su re m en t M ai n co nc lu si on s H an e t a l. (1 8) 20 11 So ut h Ko re a 12 9 m ac hi ne s D R In tr ao ra l, pa no ra m ic , ce ph al om et ric , CB C T D A P Th e m ea su re d va lu es fo r i nt ra or al e xa m in at io ns a re 5 5. 5; 4 6 an d 36 .5 m G y* cm 2 ( fo r u pp er m ol ar , p re m ol ar a nd in ci so r, re sp ec tiv el y) an d 12 0. 3; 1 46 a nd 3 ,2 03 m G y* cm 2 f or e xt ra or al e xa m in at io ns (fo r p an or am ic , c ep ha lo m et ric a nd C BC T im ag in g, re sp ec tiv el y) . In in tr ao ra l d en ta l i m ag in g, th e D A P va lu e re la te s to tu be c ur re nt an d ex po su re ti m e pr od uc t ( m A s) , w hi le th e CB C T im ag in g’ s D A P is m or e cl os el y lin ke d to th e FO V. T he re a re s om e di ff e re nc es in th e m ea su re d do se v al ue s be tw ee n pr iv at e cl in ic s an d un iv er si ty ho sp ita ls . H ol ro yd e t a l. (1 9) 20 11 G re at B rit ai n 42 m ac hi ne s D R, C R, fi l m -s cr ee n Ce ph al om et ric D A P A p ha nt om s tu dy . D A P va lu es fo r a du lts : 4 0 m G y* cm 2 f or d ig ita l sy st em s an d 42 m G y* cm 2 f or s cr ee n- fi l m . F or c hi ld re n: 2 5 m G y* cm 2 . Iz aw a et a l. (2 0) 20 17 Ja pa n 3 m ac hi ne s (lo ca l D RL s) Fi lm -s cr ee n In tr ao ra l P K A , P ED Lo ca l D RL s w er e es ta bl is he d ta ki ng in to a cc ou nt a p os si bl e di ff e re nc e be tw ee n ge nd er s. Th e do se v al ue s ar e sl ig ht ly lo w er in w om en , w hi ch is s up po se d to b e a co ns eq ue nc e of th e di ff e re nc e in s iz e, s in ce w om en a e us ua lly s m al le r, so th e op er at or s ho ul d ad ju st e xp os ur e pa ra m et er s. PE D v al ue s ar e 1. 56 ± 0 .2 7, 1 .0 9 ± 0. 31 , 1 .9 2 ± 0. 38 , m G y fo r u pp er in ci so rs , p re m ol ar s an d m ol ar s, re sp ec tiv el y, a nd 1 .2 7 ± 0. 22 , 2 .4 2 ± 0. 33 in 1 .5 9 ± 0. 20 m G y fo r lo w er in ci so rs , p re m ol ar s an d m ol ar s, re sp ec tiv el y. Ki m e t a l. (2 1) 20 12 So ut h Ko re a 12 6 (1 04 co ns id er ed in th e st ud y) m ac hi ne s D R, C R, fi l m -s cr ee n in tr ao ra l D A P, PE D Th e st ud y re co m m en ds D RL v al ue s fo r S ou th K or ea : 3 .1 m G y (P ED ) an d 87 .4 m G y* cm 2 ( D A P) fo r l ow er m ol ar fo r a du lts . T hi s st ud y al so sh ow s lo w er d os e va lu es in d ig ita l s ys te m s co m pa re d to th os e in fi l m -s cr ee n sy st em s. Th ey a ls o co ns id er ed th e in st al la tio n du ra tio n of th e m ac hi ne (< 5 ye ar s in > 6 ye ar s) a nd th e ty pe o f d en ta l x -r ay m ac hi ne (e .g . w al l-m ou nt ed fi xe d ty pe a nd h an d- he ld p or ta bl e ty pe ). Th er e w er e no s ta tis tic al ly s ig ni fi c an t d iff er en ce s w ith re sp ec t t o eq ui pm en t i ns ta lla tio n du ra tio n an d ty pe o f d en ta l X -r ay sy st em . M an ou sa rid is et a l.( 22 ) 20 15 G re ec e 51 9 m ac hi ne s D R, C R fi l m -s cr ee n pa no ra m ic K i Th re e ca te go rie s w er e an al ys ed : c hi ld re n, p et ite a du lts a nd av er ag e ad ul ts . R ec om m en de d D RL s w er e 2. 2; 3 .3 a nd 4 .1 m G y, re sp ec tiv el y. T hr ee s ys te m s w er e co m pa re d as w el l ( D R, C R an d fi l m -s cr ee n) , w ith d os e va lu es a t t he 3 rd q ua rt ile o f 3 .5 ; 4 .2 a nd 3. 7 m G y, re sp ec tiv el y. A s w e ca n se e, th e hi gh es t d os e va lu e is re co rd ed w ith th e CR s ys te m . Matjašič A. / Diagnostic reference levels in dental radiology: a systematic review Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) 27 A ut ho r Ye ar Co un tr y Sa m pl e Te ch no lo gy Ra di og ra ph y ty pe U ni ts o f m ea su re m en t M ai n co nc lu si on s M an ou sa rid is et a l. (2 3) 20 13 G re ec e 52 9 m ac hi ne s D R, C R, fi l m -s cr ee n In tr ao ra l K i D RL v al ue fo r u pp er m ol ar is s et a t 0 .9 5 m G y fo r d ig ita l s ys te m s an d 2. 90 m G y fo r fi lm -s cr ee n, w hi ch is c om pa ra bl e to o r e ve n lo w er th an in th os e co un tr ie s th at a t t he ti m e of th is s tu dy p ro vi de d su ch da ta (S pa in , U SA , R om an ia , G re at B rit ai n an d pr ev io us s tu di es in G re ec e) . Pr as ka lo e t a l. (2 4) 20 20 Bo sn ia a nd H er ze go vi na 41 m ac hi ne s D R, fi l m -s cr ee n In tr ao ra l K i , P KA N ew D RL s fo r t he fi lm -s cr ee n sy st em (3 .5 m G y) a nd fo r d ig ita l re ce iv er s (1 .2 m G y) a re re co m m en de d an d ar e si gn ifi ca nt ly lo w er th an th os e es ta bl is he d un til n ow a t t he n at io na l l ev el w ith a D RL v al ue o f 7 m G y. T hi s va lu e w as ta ke n fr om li te ra tu re a nd n ot es ta bl is he d as a re su lt of a n at io na l s tu dy . T he re a re c on si de ra bl y lo w er d os es fo r d ig ita l r ec ei ve rs . Su lim an , A bd el ga di r (2 5) 20 18 Su da n 14 m ac hi ne s D R, fi l m -s cr ee n In tr ao ra l, pa no ra m ic K i , P KA Th e st ud y re co m m en ds n ew D RL s fo r i nt ra or al im ag in g: 1 .4 5 m G y (D R) , 4 .4 5 m G y (fi lm -s cr ee n) a nd 3 .0 1 m G y (c om bi ne d) . Fo r p an or am ic d en ta l i m ag in g, o nl y av er ag e va lu es a re s ta te d (a nd n ot th e 3r d qu ar til e, w hi ch is s pe ci fi c fo r D RL s) : 7 0. 4 m G y* cm 2 fo r c hi ld re n an d 10 3. 4 m G y* cm 2 f or a du lts . T he s tu dy d es cr ib es si gn ifi ca nt d iff er en ce s be tw ee n ho sp ita ls . T hi s sh ow s th e th er e is a lo t o f r oo m fo r p ro to co l o pt im is at io n. W al ke r e t a l. (2 6) 20 10 Ire la nd 83 m ac hi ne s da ta un av ai la bl e In tr ao ra l, pa no ra m ic ES D , D W P (d os e w id th pr od uc t) Su gg es te d D RL s fr om th is s tu dy a re 2 .4 m G y fo r l ow er m ol ar fo r in tr ao ra l i m ag in g an d 60 m G y m m fo r p an or am ic im ag in g of ad ul ts . T he s tu dy a ls o re co m m en ds th e in tr od uc tio n of a n ew re fe re nc e qu an tit y 1 m G y/ m A s, w hi ch c on si de rs d os e as w el l a s ex po su re ti m e. T he re co m m en de d D RL fo r i nt ra or al im ag in g w ith th is n ew u ni t i s 1. 03 m G y/ m A s. D RL s ar e co m pa ra bl e to li te ra tu re , so m et im es e ve n lo w er d ue to n ew te ch no lo gi es . Matjašič A. / Diagnostic reference levels in dental radiology: a systematic review 28 Medical Imaging and Radiotherapy Journal (MIRTJ) 38 (2) Conclusion As stated in the introduction from the European Commission Guidelines for Radiation Protection in Dental Radiology, most EU countries have not yet established national DRLs for dental radiology. In this systematic review, 13 original research articles on local or national DRLs in dental radiology for the EU and other countries were discussed. Most of these studies focus on intraoral and panoramic dental imaging, with only a few on CBCT imaging. This implies that there is room for further research in this area. Most studies set or revise DRLs at the national level and compare them with guidelines from literature and from similar studies conducted in other countries. Only one study is the result of local DRL establishment with the goal of protocol optimization. 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