it 's known that 230,000 cases of prostate cancers are reported annually in america , with 27,000 of them are leading to death . in 1960 , brachytherapy is one of the common radiotherapy modalities applied for treatment of cancer . in this method , one of the aims of radiotherapy is to irradiate the tumor with a lethal dose , while minimizing the radiation dose received by the surrounding normal tissues . one option for treatment of prostate cancer is the use of low energy brachytherapy sources such as i and pd . recently , cs radionuclide has been introduced as a brachytherapy source [ 5 , 6 ] . having a uniform dose distribution inside the tumor to minimize the quantity of hot / cold spots is an advantage in prostate brachytherapy using interstitial implants . the doses to other healthy tissues outside the tumor should be decreased to the minimum amounts . the dose to the sensitive organs should not exceed their tolerance levels . in various studies , i , pd , yb , and ir brachytherapy sources were evaluated in prostate brachytherapy [ 2 , 4 , 8 , 9 ] . it should be noted that in addition to i and pd brachytherapy sources , there are also high - dose - rate ( hdr ) radionuclides ( ir ) that are routinely used in prostate brachytherapy . i and cs have the same energy spectrum , and their average energies are 28.37 and 30.45 kev , respectively . this results to lower dose to the organs at risk such as rectum and urethra , on the other hand , increases the risk of underdosage in the prostate . in contrary , i and cs have extension of dose distributions to larger distances , and this results to deliver more dose to the organs at risk for a given prostate dose . however , with these sources , there is a lower probability of cold spots in the prostate . generally , studies have reported that i , pd , and cs radionuclides are used , in addition to prostate , for treatment of tumors in lung , brain , breast , and eye [ 3 , 4 , 5 , 6 , 7 ] . in order to simplify and speed up the calculations , dose distributions can be calculated more accurately by monte carlo codes such as mcnp , egs4 , and geant4 . zhang et al .   simulated a prostate model as a sphere with radius of 1.5 cm , while forty eight i sources were defined within the prostate . then isodose curves were obtained for prescribed doses of 108 , 144 , 145 , and 160 gy on the target volume . their results demonstrated that the variseed treatment planning system ( varian medical systems , inc . , palo alto , ca , usa ) has 29% and 136% larger dose coverage than the monte carlo simulation with 150% and 200% isodose lines , respectively . this was due to the simplification of the seed characteristics using a point source approximation and neglecting inter - seed attenuation in this treatment planning system . 64 of american association of physicists in medicine ( aapm )   suggests that technical innovations , image - based planning , computerized dosimetry analysis , and improved quality assurance have important roles in modern prostate brachytherapy . this task group has been also committed to summarize the knowledge in various aspects in prostate brachytherapy including treatment planning , dose specification , and reporting . it recommends practical guidelines for the clinical medical physicist community and identifies issues for future investigation in this field . the details on physical aspects and the related issues in prostate brachytherapy can be found in this report . heintz et al .   evaluated physical and dosimetric characteristics for various i sources in prostate brachytherapy . their assessment was performed on the differences in designs , construction , and dosimetric characteristics of each source and the results were compared with those of an amersham 6711 source model ( amersham , little chalfont , uk ) . this led to present a simple equation that can be used clinically to convert the standard strength of 6711 source to an equivalent strength of a new brachytherapy source . in a study by meigooni et al . , the effect of the dosimetric differences of i and pd sources were evaluated within the scope of their clinical applications . the quantitative and qualitative evaluations were performed via comparisons of dose distributions and dose volume histograms in prostate implants with various designs of the i and pd sources . the comparisons were with identical implant scheme including the same number of seeds for each source . the results were compared with the amersham 6711 i and the theragenics 200 pd seeds ( theragenics corporation , buford , georgia , usa ) . there was no significant difference between the dose distributions of the new designs of i and pd sources in a typical prostate implant compared to the i and pd reference sources . however , this plan needs to be evaluated for any new design of brachytherapy source . while various studies have been performed on dose distribution evaluation in prostate brachytherapy , to the best of our knowledge , there are not any study on dosimetric assessment and comparison of i , pd , and cs sources with the same geometries in interstitial prostate brachytherapy . the aim of the current study is to compare dose distributions of i , pd , and cs sources in interstitial prostate brachytherapy . additionally , the use of cs radionuclide as a hypothetical source in interstitial prostate brachytherapy is evaluated . in this study , a mills biopharmaceuticals prostaseed i source ( model 125sl ) ( oklahoma city , usa ) has been simulated . the validation of this source simulation was performed in a previous study on this source model . in the present study , all the simulations were performed using a monte carlo n - particle transport code ( mcnpx , version 2.6.0 ) . mcnpx code was developed by los alamos laboratory , and is used worldwide for transport of various types of particles in nuclear physics and medical physics applications . the prostaseed i source ( core oncology , santa barbara , us ) is composed of five silver spheres , on which radioactive i radionuclide was coated superficially . these spheres are placed in a titanium capsule having side walls with thickness of about 0.05 mm . the external length of the capsule is 4.5 mm and its external diameter is 0.8 mm . the titanium capsule has thickness of 0.3 mm on both end welds of the source . the geometry of the source model , including dimensions as well as the compositions of materials is shown in figure 1 , [ 2 , 15 ] . the hypothetical pd and cs sources were simulated and used for prostate brachytherapy simulations in this study with geometries based on the commercially available prostaseed i source ( model 125sl ) . however , in the hypothetical pd and cs sources , the active cores were replaced by pure pd and cs radionuclides , respectively . the photon spectra used in the simulations of these sources are based on previous studies on the sources : rivard et al .   for i , rivard   for pd , and rivard   for cs , which are presented in table 1 . using these spectra the photon yields for the i , pd , and cs are : 1.4757 , 0.7714 , and 0.8138 photons / dis , respectively . probable electron emissions ( auger , conversion , etc . ) by these radionuclides were not considered , since they are absorbed in the source 's capsule due to their low energy and have not considerable effect on the dose in the phantom . 
  a schematic figure of the iai i brachytherapy source indicating dimensions and materials photon energy spectrum for i , pd , and cs brachytherapy sources with the same procedure as the study by ghorbani et al . 
 , monte carlo simulations of the prostate tumor and brachytherapy source implants were performed . the prostate was considered as a sphere with radius of 1.5 cm and forty - eight sources were distributed within the prostate . a schematic geometry indicating the distribution of the sources , prostate and the surrounding phantom is illustrated in figure 2 . the prostate was composed of four - element soft tissue , which was presented by international commission on radiation units and measurement ( icru ) report no . , soft tissue has four elements with weight fractions as follows : 76.2% oxygen , 11.1% carbon , 10.1% hydrogen , and 2.6% nitrogen . the normal tissue was defined as the surrounding medium ( phantom ) and was composed of four - component soft tissue in the form of a sphere with radius of 25.0 cm . a sphere composed of air with radius of 150.0 cm around the soft tissue phantom the forty - eight sources were distributed in the volume of the tumor in such an arrangement that included eight arrays of sources , each array included six sources . the sources arrays were placed symmetrically on a circle with 1.2 cm radius in the prostate ( figure 2 ) . it should be noted that by alignment of six sources having distances between each other , especially on the peripheral regions , some of them are located outside the prostate . this arrangement is optimum to have a suitable dose distribution in the prostate and it is routine in clinical practice of prostate brachytherapy . the arrangement of the sources and tumor was similar to those used in the previous studies on prostate brachytherapy [ 2 , 11 ] . the distributions of the i brachytherapy sources and hypothetical pd and cs sources were similar to each other . a ) transverse cross section , b ) longitudinal cross section type 1 mesh tally ( using  pedep  option ) was used to score the photon doses in this step . the use of this tally type and this option in mcnp equals to application of f6 tally , which scores kerma . kerma can be used as an approximation for absorbed dose calculation in the positions where there is electronic equilibrium . it can be mentioned that electronic equilibrium exist at distances , which are not in the close vicinity of the source . this mesh included 2 mm  2 mm  2 mm voxels , in which the value of tally was scored . energy cut off for photon and electron was 10 kev in the monte carlo input programs . except for the energy cut - off , no other variance reduction technique was used in the above mentioned simulations . in the monte carlo programs , a photon source was defined , while both photons and electrons were transported and the photon tally was scored for the purpose of calculation of photon dose . this leads to transport of various photon interactions and scoring primary and secondary particles including photons and electrons . a number of 2.0  10 , 2.0  10 , and 2.0  10 photons were run in each input file for the i , pd and cs , respectively . the type a statistical uncertainties of all monte carlo calculations were less than ( or equal to in one case ) 2.30% in these programs . absolute and relative doses were calculated for this configuration in various points inside and outside the tumor on the central transverse ( y ) and longitudinal ( z ) axes . the transverse and longitudinal data were extracted from the 2d dose distribution matrix obtained from the above described mesh tally . in relative dose calculations , dose values were normalized on dose on the surface of prostate ( at 1.5 distance ) on the transverse axis . dose profiles were obtained on y and z axes for the sources in terms of cgy/(h.u ) . isodose curves of 10% , 30% , 50% , 100% , and 150% were plotted while the prescribed dose was at the point located at distance of 1.5 cm from the center of the tumor on the y - axis ( on the surface of the prostate ) . the dose at this point was considered as the reference dose and the relative doses were calculated as the ratio of dose relative to the dose at this point . it is obvious that the doses at this point were different for the i , pd , and cs sources , and for each source the dose at this point with the same source was considered as the reference . therefore , the contour of 100% covered the surface of the prostate . finally , the transverse and longitudinal dose profiles , isodose curves for the sources were compared . advantages and disadvantages of the sources were discussed from dose distribution aspects for use in interstitial prostate brachytherapy as well . figure 3 ( part a and b ) illustrate dose profiles ( cgy/(h.u ) ) determined by the cumulative dose distribution from the multi - seed implant simulations for the i , pd , and cs sources in the transverse and longitudinal axes , respectively . dose profiles ( cgy/(h.u ) ) for the i , pd , and cs sources in prostate brachytherapy . ( a ) transverse axis , ( b ) longitudinal axis isodose curves including 10 - 150% contours are plotted in figure 4 . the contours are located inside and outside the prostate tumor , and are plotted for the i , pd , and cs sources . in this figure , the dotted and dashed lines are related to pd , and cs sources , respectively . doses were normalized to the dose on the tumor surface . in order to have better comparisons , each part in this figure illustrates the dose distribution for two sources . from the top to the bottom , the curves are related to the i and pd sources ( part a ) , i and cs sources ( part b ) , and pd and cs sources ( part c ) , respectively . eight circular contours around the sources are isodose contours of 150% . in these simulations , the prostate was defined as a sphere with radius of 1.5 cm and forty - eight sources of each type were distributed in the prostate volume separately . isodose curves including 10% , 30% , 50% , 100% , and 150% contours inside and outside the prostate tumor , for ( a ) i and pd sources , ( b ) i and cs sources , ( c ) pd and cs sources . solid contours represent those of i source , dotted lines represent pd source isodose , and dashed lines represent cs source isodose 
 figure 5 presents percentage differences between the relative dose values for the i , pd , and cs source implants . in this figure , from the top to the bottom , percentages differences are related to the i and pd sources ( part a ) , i and cs sources ( part b ) , and pd and cs sources ( part c ) , respectively . absolute dose differences ( % ) between the three sources.(a ) between the i and pd sources , ( b ) between the i and cs sources , ( c ) between the pd and cs sources . in this study , a prostaseed i source and two hypothetical pd and cs brachytherapy sources were simulated with the same geometry of the prostaseed i source . then comparison of y and z axes dose profiles and dose distributions for the sources inside and outside the tumor was performed in interstitial prostate brachytherapy . for the purpose of comparison of various radionuclides in prostate brachytherapy , not only dosimetric and treatment planning aspects but also zhang et al .   in a systematic review , compared the effectiveness and adverse effects in 1406 patients with prostate cancer treated with i and pd brachytherapy sources . their results have shown that after one or six months following the treatment , there were differences between the adverse effects in the patients treated with i or pd sources . however , it was not found significant difference in adverse effects between the i and pd groups , following 12 months after source implantation . finally , it was reported that the effects of brachytherapy with i and pd sources for low risk prostate radiotherapy are similar . determined normal tissue complication probability for urethra in three prostates patients implanted with i , pd , cs , pd - i , or pd - cs seeds . the large uncertainties in the fitting parameters in the modeling of the urethral normal tissue complication probability , resulted in large uncertainty on this probability value . therefore , it was announced that application of a model for normal tissue complication probability in permanent brachytherapy is possible , but it is necessary to minimize the uncertainties in the parameters in this model . as it can be seen in figure 3 ( dose profiles in terms of cgy/(h.u ) in the transverse and longitudinal axes ) , the results show that the dose distribution in terms of cgy/(h.u ) are quite different with the i , pd , and cs sources . since the same geometries were defined with these sources , this effect is due to the differences in photon energy spectra , and photon yields ( number of photons per disintegration ) of these sources . these differences are incorporated in calculation of air kerma strength ( in terms of u ) and therefore have their effect on dose distribution in terms of cgy/(h.u ) . since tg-43 proposed air kerma strength for characterization of the source strength instead of activity , presentation of dose rate in terms of cgy/(h.u ) are of relevance in dose calculation and optimization in treatment planning in prostate brachytherapy . therefore , based on interpretation of the results in cgy/(h.u ) as the reference results ( based on the recommendations of tg-43 ) , cs , i , and then pd has higher dose rates per u inside and outside the tumor , respectively . this will be accounted as an advantage of cs over the others , however , the higher dose outside the tumor with this source is a disadvantage . in other words , from clinical and radiobiological point of view , a higher dose by cs source will reduce the likelihood of cold spots inside the prostate . on the other hand , this is with the cost of higher dose to organs at risk ( rectum and urethra ) for a given prostate dose . a higher dose to organs at risk will increase the risk of rectal and urethral complications with this source . considering figure 4 ( part a ) , generally speaking , there is no significant difference in hot / cold spots for various sources . when one considers figure 4 , with an accurate assessment of 10% isodose lines , it can be evident that i source presents a more suitable dose relative to cs this is due to the fact that 10% isodose line for i is more internal than that for cs . this manner for i is advantageous than cs in terms of reduction of dose to the around oar . as can be seen in figure 4 ( part b ) , using pd source , dose decreases outside the tumor faster than i source . these isodose lines are more internal for pd than the i ones , which indicates less dose delivery to outside the tumor . this is mentioned in some extent as an advantage of i source . regarding little difference between the 100% isodose lines for these two sources , it can be claimed that using pd is more beneficial than i. as it can be seen in figure 4 ( part c ) explicitly , 10% , 30% , and 50% isodose lines of pd are more internal relative to cs cold spots . additionally , considering 100% isodose lines for these two sources , it can be concluded that the 100% isodose line for cs is more internal . as it is demonstrated in figure 5 , with increase in the distance from the source , the percentages differences between the two - source cases become larger . for distances greater than about 2 cm with i - pd and there is also an increasing trend of dose differences for pd - cs source comparisons with increase in distance from the center of prostate in other words , absolute dose is smaller at larger distances , and when the dose differences are divided to a smaller dose value at larger distances , the relative dose increases . as another effect , as it can be seen from figure 5 , the percentage relative dose differences for i - pd and i - cs are smaller than the pd - cs sources . this can be related to the differences in photon energy spectra , photon yields , and in - phantom photon attenuations for these three sources . since in the current study cs radionuclide in the studied form was proposed as a hypothetical source in brachytherapy , based on assessment of its isodose curves and longitudinal and transverse dose profiles relative to i and pd brachytherapy sources , it can be concluded that cold spots for the hypothetical cs source are more external than the other two sources , which may be disadvantageous for cs . this is due to the organs at risk around the tumor receives higher dose relative to the other two sources . comparing hot spots for hypothetical cs source with pd and i brachytherapy sources , there is no significant difference . therefore , it can be claimed that cs have no significant difference with the other two sources in terms of distribution of hot spots . actually , 100% isodose lines for hypothetical cs source are more internal than the pd and i brachytherapy sources . this effect indicates that cs delivers higher dose to the tumor relative to the other two sources , which can be reported as an advantage of using the cs source in brachytherapy . another consideration for these sources is that they are used as permanent implants in prostate brachytherapy . in this modality , the prescription dose is the criteria and is calculated as the total dose during the presence of the implants inside the tumor . in this case , the total dose is calculated as 1.44  half - life  initial dose rate ( cgy / h.u ) . in this study , the initial dose rates for these sources were obtained from the monte carlo simulations . therefore , while having the number of seeds ( 48 seeds ) , air kerma strength for each source , half - life of each radionuclide , and the initial dose rates , it is possible to calculate the total dose . in the above sections only initial dose rates were as the criteria for comparison of the sources . as examples , initial dose rate per air kerma strength on prostate in the transverse axis for the cs , i , and pd sources are 0.54 cgy/(h.u ) , 0.43 cgy/(h.u ) , and 0.32 cgy/(h.u ) , respectively . the corresponding values for these sources on the longitudinal axis are 0.32 cgy/(h.u ) , 0.25 cgy/(h.u ) , and 0.15 cgy/(h.u ) , respectively . it is obvious that with the same number of seeds and initial air kerma strengths , the half - life will have effect on the total dose . the half - lives of the i , pd , and cs sources are 59.40 days , 16.991 days , and 9.7 days , respectively . with these half - lives , while having the same number of seeds and initial air kerma strengths , i will have a higher total dose due to its longer half - life . in the above discussions , the initial dose rates were the criteria , however , based on the total dose the i source has advantage over the others for example it was considered the prostate shape as a symmetric sphere but we know that it is not the true shape in reality . it is suggested that the evaluation of this study with more real treatment plans in terms of the asymmetric prostate shape and presence of urethra in this organ asymmetrically . in routine brachytherapy practice , a comparison of dose volume histograms by these sources in prostate brachytherapy can be a subject of further research in this field . it is obvious that since all of these sources are not available clinically with the same treatment planning system , this subject can be performed via monte carlo simulation of these sources . this is beneficial to have comparison between these sources with the same conditions and the dose differences are related to the radiation characteristics of these sources . on the other hand , a comparison of these sources with their own commercially available geometries can be a subject of further research in this field . in the current study a number of simplifications were used in definition of the geometry of phantom and sources . additionally , in order to have a precise comparison between various radionuclides , the geometries of pd and cs sources were considered the same as the prostaseed i source . in other words , the comparison of these sources could be completed doing the measurements with real sources in an anthropomorphic phantom , and with this work there may be very interesting parameters for decision about the correct source for application in brachytherapy treatment . implication of these issues on the study design and subsequent source comparisons can diminish the accuracy of the results , and these effects can be subjects for future researches . considering the transverse and longitudinal dose profiles and isodose contours for the i , pd , and cs source in interstitial prostate brachytherapy indicates that with the same geometries , these sources have different dose distributions . therefore , based on the results in cgy/(h.u ) as the reference ( in accordance with tg-43 recommendation ) , a comparison of dose distributions of these three sources indicates that , with the same time duration and air kerma strength , it is possible to deliver a higher dose to the prostate volume with the cs source . however , relative dose rate inside the tumor is higher and outside the tumor is lower for the pd source . pd has more rapid dose falloff outside the tumor , which can be beneficial for the normal tissues outside the tumor . additionally , the results show the dose distribution in terms of cgy/(h.u ) are quite different and this should be considered in clinical applications of prostate brachytherapy . while having the same number of seeds and initial air kerma strengths , i will have a higher total dose due to its longer half - life . therefore , based on the total dose , the i source has advantage over the others .
purposelow energy sources are routinely used in prostate brachytherapy . 
 125i is one of the most commonly used sources . 
 low energy 131cs source was introduced recently as a brachytherapy source . 
 the aim of this study is to compare dose distributions of 125i , 103pd , and 131cs sources in interstitial brachytherapy of prostate.material and methodsprostaseed 125i brachytherapy source was simulated using mcnpx monte carlo code . 
 additionally , two hypothetical sources of 103pd and 131cs were simulated with the same geometry as the prostaseed 125i source , while having their specific emitted gamma spectra . 
 these brachytherapy sources were simulated with distribution of forty - eight seeds in a phantom including prostate . 
 the prostate was considered as a sphere with radius of 1.5 cm . 
 absolute and relative dose rates were obtained in various distances from the source along the transverse and longitudinal axes inside and outside the tumor . 
 furthermore , isodose curves were plotted around the sources.resultsanalyzing the initial dose profiles for various sources indicated that with the same time duration and air kerma strength , 131cs delivers higher dose to tumor . however , relative dose rate inside the tumor is higher and outside the tumor is lower for the 103pd source.conclusionsthe higher initial absolute dose in cgy/(h.u ) of 131cs brachytherapy source is an advantage of this source over the others . 
 the higher relative dose inside the tumor and lower relative dose outside the tumor for the 103pd source are advantages of this later brachytherapy source . 
 based on the total dose the 125i source has advantage over the others due to its longer half - life .