in head and neck radiotherapy , patients frequently suffer from severe mucositis adjacent to the teeth with dental alloys . this occurs because of a mucosal dose enhancement caused by backscatter radiation from the high - z metals used in dental alloys [ 14 ] . severe mucositis not only leads to patient discomfort but also decreases the local control rate of the patients owing to the required change in the treatment regimen ( e.g. reduction in the treatment dose and/or extension of the treatment period ) . although this problem can be solved by removing the dental alloys prior to radiotherapy , this approach has several disadvantages , such as the need for time - consuming and costly dental alloy removal procedures . dose enhancement caused by backscatter radiation from the high - z metals can be prevented by using a spacer [ 58 ] . therefore , the use of a spacer is recommended for the prevention of severe mucositis resulting from mucosal dose enhancement caused by backscatter radiation from dental alloys . this study aims to investigate the changes in dose distribution caused by backscatter radiation from a commercially available dental alloy to identify the optimal device materials and device thicknesses for effective prevention of mucositis . palo alto , ca and brainlab ag , feldkirchen , germany ) was employed as the x - ray beam generator ( fig . the photon energy , dose rate , field size , source  axis distance ( sad ) , and source  surface distance ( ssd ) were 6  mv , 100  mu , 10    10  cm , 100  cm and 94  cm , respectively , and the phantom 's dimensions were 40    40    17  cm . a tough water phantom ( kyoto kagaku co. ltd , kyoto , japan ) was used as the water - equivalent phantom . the au  ag  pd dental alloy ( da ; morita kinpara nice 12 , j. morita corporation , osaka , japan ) , the most commonly used da in japan , with a volume of 1  cm was employed in this study . note that 1  cm of da was prepared via a centrifugal casting method after the melting of the da ingots . ethylene vinyl acetate copolymer ( hilite shade up eva sheet , shofu inc . , kyoto , japan ; eva ) , thermoplastic polyolefin elastomer ( capture sheet pro , shofu inc . , kyoto , japan ; tpo ) and polyethylene terephthalate ( capture sheet hard , shofu inc . , kyoto , japan ; pet ) the compositions of dds were taken from their manufacturer 's specifications , and that of da was analyzed using the electron probe micro analyzer ( epma-1610 , shimadzu corporation . , kyoto , japan ; epma ) because of the nonhomogeneous composition distribution caused by the casting process . in addition , the physical density of da was measured as apparent density and calculated as mass divided by volume . table 1.physical characteristics of dental alloy and dental devicesdental alloydental devicesau  ag  pd alloyevatpopetawf 
 au 
 14.6 
 ag 
 48.6 
 pd 
 19.8 
 cu 
 16.1 
 in 
 0.9 
 zn 
 0.9 
 c 
 55.7 
 85.6 
 62.5 
 h 
 7.0 
 14.4 
 4.2 
 o 
 37.3 
 33.3 
 pd 
 10.89 
 0.95 
 0.88 
 1.40 
 zeff 
 48.45 
 6.39 
 5.29 
 6.46 
 awf  =  atomic weight fraction ( % ) , pd  =  physical density ( g / cm ) , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . physical characteristics of dental alloy and dental devices awf  =  atomic weight fraction ( % ) , pd  =  physical density ( g / cm ) , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . the depth dose along the central axis was compared for the following cases : the absence of both da and dd ( no da ) , presence of da without dd ( da without dd ) , and the presence of da with dds ( da with dd ) . the depth dose for the source  chamber distance ( scd ) was analyzed for distances in the 99100  cm range at 1-mm intervals , except for tpo . the depth dose analysis of the scd in tpo was performed for the 99.2100  cm range at 1-mm intervals because a thickness of 1  mm could not be obtained . a plane schwarzenbruck , germany ) was used for dose measurements , and the dose was calculated according to 
 ( 1)dose = ktpnd , wmraw , 
 where ktp , ndw and mraw are the correction factor for the temperature and air pressure differences , absorbed - dose water calibration factor and raw uncorrected ion chamber reading , respectively . parallel ionization chamber was set backward to make contact with the da surface . in this study , we evaluated the following cases : ( i ) relative doses measured ( for no da , da without dd , and da with dds ) with reference to the dose set at the 0-mm point for the no da case and ( ii ) relative doses measured for the da without dd and da with dds cases on the basis of the dose of the corresponding no da case . each of the calculation formulas was as follows . ( i )  the dose relative to the reference dose ( dose at 0 mm in the no da case)(2)=100thedepthdoseineachcasethereferencedose , ( ii )  the dose relative to the no da case 
 ( 3)=100thedepthdoseineachcasethedepthdoseinthenodacase . 
 bonferroni 's post hoc test was used for the statistical analyses . the statistical analyses were conducted using ibm spss statistics 23.0 for windows ( ibm japan ltd , tokyo , japan ) . palo alto , ca and brainlab ag , feldkirchen , germany ) was employed as the x - ray beam generator ( fig . the photon energy , dose rate , field size , source  axis distance ( sad ) , and source  surface distance ( ssd ) were 6  mv , 100  mu , 10    10  cm , 100  cm and 94  cm , respectively , and the phantom 's dimensions were 40    40    17  cm . a tough water phantom ( kyoto kagaku co. ltd , kyoto , japan ) was used as the water - equivalent phantom . the au  ag  pd dental alloy ( da ; morita kinpara nice 12 , j. morita corporation , osaka , japan ) , the most commonly used da in japan , with a volume of 1  cm was employed in this study . note that 1  cm of da was prepared via a centrifugal casting method after the melting of the da ingots . ethylene vinyl acetate copolymer ( hilite shade up eva sheet , shofu inc . , kyoto , japan ; eva ) , thermoplastic polyolefin elastomer ( capture sheet pro , shofu inc . , kyoto , japan ; tpo ) and polyethylene terephthalate ( capture sheet hard , shofu inc . , kyoto , japan ; pet ) the compositions of dds were taken from their manufacturer 's specifications , and that of da was analyzed using the electron probe micro analyzer ( epma-1610 , shimadzu corporation . , kyoto , japan ; epma ) because of the nonhomogeneous composition distribution caused by the casting process . in addition , the physical density of da was measured as apparent density and calculated as mass divided by volume . table 1.physical characteristics of dental alloy and dental devicesdental alloydental devicesau  ag  pd alloyevatpopetawf 
 au 
 14.6 
 ag 
 48.6 
 pd 
 19.8 
 cu 
 16.1 
 in 
 0.9 
 zn 
 0.9 
 c 
 55.7 
 85.6 
 62.5 
 h 
 7.0 
 14.4 
 4.2 
 o 
 37.3 
 33.3 
 pd 
 10.89 
 0.95 
 0.88 
 1.40 
 zeff 
 48.45 
 6.39 
 5.29 
 6.46 
 awf  =  atomic weight fraction ( % ) , pd  =  physical density ( g / cm ) , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . physical characteristics of dental alloy and dental devices awf  =  atomic weight fraction ( % ) , pd  =  physical density ( g / cm ) , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . the depth dose along the central axis was compared for the following cases : the absence of both da and dd ( no da ) , presence of da without dd ( da without dd ) , and the presence of da with dds ( da with dd ) . the depth dose for the source  chamber distance ( scd ) was analyzed for distances in the 99100  cm range at 1-mm intervals , except for tpo . the depth dose analysis of the scd in tpo was performed for the 99.2100  cm range at 1-mm intervals because a thickness of 1  mm could not be obtained . a plane schwarzenbruck , germany ) was used for dose measurements , and the dose was calculated according to 
 ( 1)dose = ktpnd , wmraw , 
 where ktp , ndw and mraw are the correction factor for the temperature and air pressure differences , absorbed - dose water calibration factor and raw uncorrected ion chamber reading , respectively . parallel ionization chamber was set backward to make contact with the da surface . in this study , we evaluated the following cases : ( i ) relative doses measured ( for no da , da without dd , and da with dds ) with reference to the dose set at the 0-mm point for the no da case and ( ii ) relative doses measured for the da without dd and da with dds cases on the basis of the dose of the corresponding no da case . each of the calculation formulas was as follows . ( i )  the dose relative to the reference dose ( dose at 0 mm in the no da case)(2)=100thedepthdoseineachcasethereferencedose , ( ii )  the dose relative to the no da case 
 ( 3)=100thedepthdoseineachcasethedepthdoseinthenodacase . 
 bonferroni 's post hoc test was used for the statistical analyses . the statistical analyses were conducted using ibm spss statistics 23.0 for windows ( ibm japan ltd , tokyo , japan ) . the metal composition analyzed using epma ( au : 14.6% , ag : 48.6% , pd : 19.8% , cu : 16.1% , and others : 1.8% ) was different from the manufacturing specifications ( au : 12.0% , ag : 48.2% , pd : 20.0% , cu : 17.7% , and others : 2.1% ) . therefore , epma data is considered to be closer to the practical data than the manufacturing specifications . figure 2 shows the relative doses measured for no da , da without dd , and da with dds cases with reference to the dose set at the 0-mm point for the no da case . figure  3 shows the dose for each case relative to that of the corresponding no da case . 2.the relative dose curves for a reference dose ( dose at 0  mm of the absence of both dental alloy and dental device ) . backward distance indicates the dental alloy - chamber distance or the thickness of dental device . no da  =  absence of both dental alloy and dental device , da  =  dental alloy , dd  =  dental device , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . backward distance indicates the dental alloy - chamber distance or the thickness of dental device . da  =  dental alloy , dd  =  dental device , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate.the bonferroni 's post hoc test was used for the statistical analyses . * p  <  0.01 the relative dose curves for a reference dose ( dose at 0  mm of the absence of both dental alloy and dental device ) . backward distance indicates the dental alloy - chamber distance or the thickness of dental device . no da  =  absence of both dental alloy and dental device , da  =  dental alloy , dd  =  dental device , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . backward distance indicates the dental alloy - chamber distance or the thickness of dental device . da  =  dental alloy , dd  =  dental device , eva  =  ethylene vinyl acetate copolymer , tpo  =  thermoplastic polyolefin elastomer , pet  =  polyethylene terephthalate . * p  <  0.01 the surface dose for the da showed a 13.17% increase compared with that for the no da case . in previous studies , the values of the surface dose enhancement caused by backscatter radiation from das with compositions close to that of the dental au  ag however , the results obtained in the previous studies show a wide variation that has been attributed to structural differences between the dose measurement tools used in the various studies . relative dose , defined as the actual measured dose divided by the reference dose , decreased with increasing distance from the da surface for distances in the 05  mm range , with the relative dose reaching 100    1% at distances from the da surface 6  mm . thus , dose enhancement due to the da disappeared at distances < 6  mm from the da surface . however , for distances from the da surface of 5  mm , the relative dose gradually increased with the percentage depth dose in the no da case . these results show that both the percentage depth dose and the dose enhancement caused by the backscatter radiation from da should be considered in determining the optimal dd thickness . for da with eva or tpo , the relative dose decreased gradually with increasing dd thickness in the 05  mm range , reaching the lowest value at the 5-mm thickness . when the dd thickness was 5  mm or higher , the relative dose gradually increased while following the same dose curve as the da . conversely , pet showed a slightly atypical dose curve . the pet relative dose decreased with increasing dd thickness for a dd thicknesses of 03  mm , with the lowest value observed at the 3-mm thickness . moreover , when the dd thickness was 4  mm or higher , the relative dose gradually increased and showed significantly higher values compared with the other dose curves at a thickness of 6  mm . the compton scattering probability increases with physical density [ 8 , 10 ] , effective atomic number   and average atomic number . the dose curve differences between pet and the other dds suggest that compton - scattered radiation from the dd is the origin of this effect because pet has the highest effective atomic number and an ~1.5 times higher physical density than the other dds . thus , the obtained results suggest that dd materials with a lower physical density and effective atomic number are more suitable for effective prevention of mucositis caused by backscattering . recently , chang et al .   reported on the ideal material and thickness of dds for the prevention of backscatter from da . they concluded that the use of 3-mm qc-20 as a denture base is recommended for preventing mucosal dose enhancement caused by backscatter radiation from dental alloys . however , the present results can not support their conclusion regarding the dd material because the physical characteristics of qc-20 are similar to those of pet . chang et al . analyzed materials with a maximum thickness of 3  mm . had they analyzed materials with thicknesses > 3  mm , they may have reached the same conclusions regarding more suitable dd materials as those reached in the present study . herein , we investigated the change in dose distribution caused by da and/or a dd for the single - field technique using a plane was designed for measuring the low - energy photon and electron beams that feature steep dose gradients . the effective point of measurement for the chambers was positioned at the center of the frontal surface of the parallel - plate chamber . the size of the ionization chamber does not significantly affect the accuracy of the measurement . however , the dose values obtained in this study are not completely accurate because the values were not corrected for ion recombination , polarity effect , and beam - quality conversion factors . therefore , in future investigations , we will perform a detailed study on the correlation between dose changes and physical density and/or effective atomic number by analyzing dose distribution for other materials using radiochromic film dosimetry and/or monte carlo simulation . additionally , we would like to determine the most suitable dd materials for external beam radiotherapy .
the changes in dose distribution caused by backscatter radiation from a common commercial dental alloy ( au  ag  pd dental alloy ; da ) were investigated to identify the optimal material and thicknesses of a dental device ( dd ) for effective prevention of mucositis . to this end , 1  cm3 of da was irradiated with a 6-mv x - ray beam ( 100  mu ) in a field size of 10    10  cm2 using a novalis tx linear accelerator . 
 ethylene vinyl acetate copolymer , polyolefin elastomer , and polyethylene terephthalate ( pet ) were selected as dd materials . 
 the depth dose along the central axis was determined with respect to the presence / absence of da and dds at thicknesses of 110  mm using a parallel - plate ionization chamber . 
 the dose in the absence of dds showed the lowest value at a distance of 5  mm from the da surface and gradually increased with distance between the measurement point and the da surface for distances of 5  mm . except for pet , no significant difference between the da dose curves for the presence and absence of dds was observed . in the dose curve , 
 pet showed a slightly higher dose for da with dd than for da without dd for thicknesses of 4  mm . 
 the findings herein suggest that the optimal dd material for preventing local dose enhancement of the mucosa caused by da backscatter radiation should have a relatively low atomic number and physical density and that optimal dd thickness should be chosen considering backscatter radiation and percentage depth dose .