high - dose - rate ( hdr ) ir brachytherapy is a widely used technique in the treatment of small skin lesions . specific shielded applicators have been developed to focus the absorbed dose to the tumor volume , thus reducing absorbed dose to the surrounding healthy tissues . this is the case of the valencia skin applicators ( nucletron , an elekta company , elekta ab , stockholm , sweden ) [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ] . they are tungsten cup shaped , in which the source has one single dwell position that incorporates a flattering filter providing a homogenous dose distribution at a given depth ( see figure 1 ) . open view of the valencia skin applicator   currently , there are two valencia applicators commercially available . their diameters are 2 cm ( model vh2 ) and 3 cm ( model vh3 ) . they are designed to be used with the afterloaders microselectron hdr digital v3 and v2 and flexitron ( nucletron ) . one of them ( not used for treatment ) has a raised ring on its external surface and is used only to define the useful beam ( inner applicator diameter ) by pressing over the patient skin to visualize the treatment field . as for any other radiotherapy equipment , several key tests must be performed by the medical physicist prior to its clinical use . those are : acceptance testing following the delivery of the device , thorough commissioning and implementation of a quality assurance ( qa ) program to evaluate periodically their functional performance characteristics , and to guarantee the proper and safe delivery of the intended absorbed dose [ 1 , 13 , 14 , 15 , 16 ] . recommendations on acceptance testing , commissioning , and quality control of brachytherapy equipment has been published by the task group 56 of the american association of physicists in medicine ( aapm ) , and by the groupe europen de curiethrapie ( gec ) and the european society for radiotherapy & oncology ( estro ) . similar tests for commissioning and periodic testing used for another skin treatment device ( esteya electronic brachytherapy system from elekta company . the aim of this educational paper is to summarize in one comprehensive document various tests for commissioning and qa of the valencia applicators that were previously published . based on the aapm and gec - estro guidelines for brachytherapy units as well as our experience , the methodology , equipment , frequency , expected results , and some tolerance levels are presented . we consider that including tolerance levels for all steps is out of the scope of this study , as the measurements in the kev energy range will be strongly dependent on the experimental methods used and on its associated uncertainties . for example , the uncertainties associated with a particular ionization chamber is different from that associated with radiochromic film measurements . the following items are part of the commissioning procedure , which should be performed before the first clinical use . the content of the user manual   should be fully understood , and the required internal protocols regarding the following items on the valencia applicators should be established : intend of use , cleaning and sterilization , geometrical and dosimetric information . each user has the responsibility to verify that all items required for the use of the valencia applicators are accounted for and checked for any defects . this is applicable to both applicators ( 2 cm and 3 cm ) , the plastic caps , and the appropriate transfer tube . the valencia applicators can only be used with a specific transfer tube provided by elekta : the gyn transfer tube no . 111.004 for microselectron hdr digital v3 and v2 afterloader , and the gyn transfer tube no . the length of the transfer tube must be verified , since it determines the source to indexer distance ( sid ) that is used in the treatment planning system ( tps ) ( see next section ) . between uses , the transfer tube must be properly stored to avoid bending it to guarantee its effective length . before evaluating the sid value that will be used clinically , the offset between the check cable and the active source should be established . nominally , there is a 4 mm ( elekta personal communication to authors ) difference between the length of the path of the dummy source and the length of the path of the active source , which typically is too large for the valencia applicators given the limited length of the source channel within the applicator . if not corrected , the user will not be able to take films and readings beyond the proximal distance of the applicator filter and determine the proper sid value for each applicator . errors due to the collision of the dummy source will be generated by the system . to avoid such errors , the user should contact the manufacturer to make these corrections . the valencia treatment plan requires the sid as input to set the source at the vertex of the cone . the current nominal sid values is 1321 mm for the microselectron hdr digital v3 and v2 afterloader and 121 mm for the flexitron afterloader . the value of 121 mm for a flexitron afterloader in combination with valencia applicator is not the sid , but the distance from the source to the connector of the applicator . the distance from the source to the afterloader is 1121 mm for the flexitron , including the length of the transfer tube of 1000 mm . in clinical practice , these values should be measured by the medical physicist for the specific afterloader . one of the suggested methods is using radiochromic films because small sid variations have a clear influence in the symmetry of the dose distribution at the applicator exit surface . for this purpose , lewis et al .   presented a practical radiochromic film dosimetry procedure . the latter have the practical advantage that the absorbed dose reading is obtained in real time and therefore the exit - entry transit dose can be removed . however high resolution could not be achieved due to the resolution and volume averaging effects when using an array of ionization chambers . with either radiochromic films or an array of small detectors placed in direct contact with the applicator exit surface , the dose distribution should be evaluated repeatedly while changing the sid value in 0.5 or 1 mm continuous steps until both the maximum and mean absorbed dose is achieved with dose distributions being symmetric ( see figure 2 ) . in the case of the digital v3 , measurements have to be collected for the 1319 - 1322 mm sid range in steps of 0.5 mm or 1 mm in clinical mode . however , the afterloader only allows entire values ( in mm ) for sid . to overcome this limitation , a combination of sid values skipping the first active position can be used . as an example , to achieve a measurement at dwell point 1320.5 mm , the user will program the unit at a starting point of 1323 mm using a step of 2.5 mm , and using just the second active position , which provides a measurement at 1320.5 mm . similar tests should be programmed in the treatment console station ( tcs ) to achieve measurements for all other dwell points previously mentioned . to evaluate the effect of rotation of the applicator , it is proposed to irradiate a film with the applicator rotated at 180. we have done this test and the dose differences obtained with the applicator rotated 180 are less than 1.5% . in addition , the influence of the transfer tube bending should be evaluated , although in clinical practice it must be kept as straight as possible . the sid that results in a homogenous dose distribution will be the one used clinically as it is shown in figure 2 ( red line ) . differences with respect to the nominal value greater than 1 mm must be investigated prior to clinical use . the inset shows a 2d image of the radiochromic films for source to indexer distances ( sid ) of 1319 mm , 1320 mm , 1321 mm , and 1322 mm . the lines show profiles ( along the dotted line indicated in the inset ) for each one of the sid values . the most homogenous dose distribution is associated with the sid of 1321 mm in this example a library plan can be created on oncentra brachy tps ( nucletron ) . first , one single dwell position is added at e.g. , ( 0 , 0 , 0 ) mm . after that , a dose point ( the desired dose at a specific point ) is defined at a distance from the dwell position and its horizontal axis of 22.39 mm for the 2 cm applicator and 26.07 for the 3 cm applicator . these distances are selected so that the absorbed dose ( or dwell time ) calculated by the tps following the tg-43 formalism , equals the dose calculated at 3 mm depth , using monte carlo calculations of the valencia applicator . the dwell time corresponds to the dose prescribed by radiation oncologist and received at this point . some clinical situations may require the absorbed dose to be prescribed at a depth different from 3 mm . this situation may be taken into account by means of the f factor considered in the oncentra tps . the user may enter the value of the pdd   as the f factor into the tps for proper calculation of the treatment time . for example , if the prescription is done at 4 mm , f must be set to 0.900 ( 0.902 ) for the 2 cm ( 3 cm ) valencia applicators ( pdds at 4 mm compared with the ones at 3 mm ) . to create a patient plan , the library plan created in this step can be used with the only precaution of updating the source strength , prescribed dose , f factor ( prescription depth ) , number of fractions , and starting date . it is convenient to create the patient plans using the library plan , and also in all the tests described below , in order to commission both , the applicator itself and the plan created . the dose distribution in a plane perpendicular to the axis of the radiation beam can be measured using radiochromic films . from those dose distributions , flatness and symmetry this can be performed by irradiating a radiochromic film located at 3 mm depth ( similar to common clinical prescription depth ) within a solid water phantom . the use of other material as pmma or polystyrene is also possible for the evaluation of flatness or symmetry , although for dosimetry the preferred material is solid water a plan using a dose high enough to provide an appreciable reading in the film with low noise and to neglect the possible effect of the transit dose must be created . a dose of about 4 to 7 gy is adequate for these films and is similar to the typical prescription dose . the profile must be symmetric and nearly flat along the 2 or 3 cm circle diameter . the dose map should be compared with the monte carlo derived relative isodose distributions in water that can be found at www.uv.es/braphyqs . in this study , the term output factor ( of ) refers to the dose rate per unit of air kerma strength at 3 mm depth on central axis . the of for the valencia applicators ( in units of cgy h u ) should be evaluated and compared with reference data provided in table 1 , which is taken from [ 2 , 20 ] . output factors for the two valencia applicators and the mhdr - v2 source at 3 mm depth on the applicator central axis . the correspondence factors  cfrev ( for two different well chambers )  used for the indirect verification of the outputs are also listed . uncertainties for output factor ( of ) are for k = 1 according to published studies [ 2 , 20 ] , the output can be verified directly or indirectly , as described next . the direct method should be used for commissioning purposes , whereas the indirect method can be optionally used in the commissioning step in order to obtain baseline values to use for periodic quality assurance . the direct method is performed by using a detector and a solid water phantom , and evaluating the dose rate . however , two main dosimetric challenges arise when commissioning the hdr skin applicators with these ionization chambers . first , the rapid fall off dose with depth ( ~10% per mm depth ) implies a high dose gradient over the measuring volume of the dosimeter , which needs to be small enough to minimize volume averaging effects . second , due to the fact that photons emitted by the ir source have a mean energy notably lower than the ones emitted by the co source one used typically to calibrate the chambers , caution should be exercised with the material composition of the chambers and the energy dependence should be correctly evaluated . consequently , the user might want to measure the of with a suitable detector . calibration and uncertainty issues must be carefully examined while using this method , including the active chamber size with respect to the applicator size . as commented on the literature referenced below , radiochromic film can also be used . characterized dosimetrically the leipzig applicators with a 0.055 cm markus parallel plate ionization chamber ( ptw , freiburg , germany ) , whose active volume is a cylinder 2.7 mm in radius and 2 mm in height . the chamber was calibrated with a co source and no energy correction factor was applied . commissioned experimentally the valencia applicators with a pinpoint 31006 ( ptw ) ionization chamber , small tld-100 chips , and radiochromic ebt films . results were in agreement with those obtained from mc calculations considering the dose uncertainties of each detector . usually , calibration from co can be used without correction factors ( as in granero et al .   and prez - calatayud et al . ) , although the energy dependence of other chambers not tested for ir brachytherapy dosimetry should be evaluated before its application in this field . measurements performed at our center with a new large field valencia applicator without flattening filter showed discrepancies of up to 11% between measurements with the pinpoint 31006 chamber calibrated with a co source and the mc data . given that this chamber has a steel electrode , its use might not be recommended in the ir energy range , although it provides consistent results for the valencia applicator with flattening filter , which have higher mean photon energy . on the contrary , the newer pinpoint 31016 chamber ( ptw ) provides consistent results when compared to mc data for both applicators , with and without the flattening filter , with differences being less than 3.2% for the valencia applicator vh3 when no energy correction factor is applied . while tld chips and films were placed at 3 mm depth in the cited literature , the effective point of the pin point chamber is located at 5 mm . therefore , when using the chamber , the reading must be translated to 3 mm depth by applying the pdd in order to compare the output with the data reported in table 1 . additional information regarding the validity of several solid phantom materials in kv ( up to 280 kvp ) dosimetry can be found in . in granero , polymethyl methacrylate ( pmma ) phantoms ( standard composition and density ) were used . if significant differences exist and are not resolved , further investigation is necessary and the applicators should not be used until there is a resolution . experimental uncertainties are , logically , dependent on the experimental method used . in case of using a small ionization chamber , the energy dependence with respect to the calibration quality and the effective measurement point however , if radiochromic film is preferred , the final uncertainty will depend on the scanning , analysis , and calibration procedures . in the following , for the case of the small pin - point chamber ( model 31016 ) from ptw calibrated in co with the energy dependence and effective measurement point provided by the manufacturer , the output was obtained with an uncertainty of 2.6% ( k = 1 ) . for radiochromic film ( ebt3 ) , calibrated for an energy of 6 mv and scanned using the epson 10000xl scanner , an uncertainty of 4% ( k the output of the applicators can be evaluated indirectly using a well chamber and a specific insert designed to accommodate both of them . this is performed by measuring the corresponding factors ( cf ) [ 20 , 25 ] and comparing them with the reference values listed in table 1 . the cfs listed here ( these correspond to the revisited cfrev , as discussed below ) were obtained for a microselectron - hdr afterloader with the mhdr - v2 source . it is noted that these cfrev are only valid for the exact chamber model they were measured and with the electrometer calibration factor set to 1 . in this test , the insert is placed at the top of the well chamber and the valencia applicator is then fitted to it without the plastic cap ( figure 3 ) . insert and set - up for the corresponding factor measurements the two current well chambers used for these measurements are : hdr-1000-plus from standard imaging ( wisconsin , usa ) and the tm33004 from ptw . with the specific insert from standard imaging , the cfrev can be obtained with the following expression [ 20 , 26 , 27 ] : where r is the reading collected from the well chamber and the electrometer , (p, ) is the correction for temperature and pressure , f is the calibration factor of the well chamber for the hdr source , and sk is the current air kerma strength of the source . further details can be found in [ 20 , 25 ] . in , prez - calatayud et al . did not define properly the cf because f was wrongly located in the denominator in eq . ( 1 ) . in 2008 , arthur   pointed out the fact that f should be in the numerator such that the cfrev would be dimensionless . the cfrev was then reevaluated with the correct expression ( 1 ) , and the differences between cf and cfrev were found to be small . in 2012 , gotts   published also that f should be in the numerator being cfrev dimensionless . in addition , gotts reported that the standard deviation of the calibration coefficients for hdr-1000 plus chambers calibrated at the university of wisconsin adcl was 2.2% . gotts was also critical with the application of the cf methodology questioning the assumption that the well chamber sensitivity for the external geometry ( insert at the well entrance ) can account for the calibration coefficient f , which specifies the sensitivity for the internal calibration geometry of the ionization chamber . in the reply , the cf promotors argued that for the three well chambers tested with the same hdr ir source , the measured cfrev were within 1% . in the reply to gotts , cfrev values for both leipzig and valencia applicators with the correct equation were published , and are listed here for the sake of completeness in table 1 . in the case that the cfrev is available , its evaluation is a convenient consistency check , always added to the direct output verification commented above . if the cfrev values obtained by the user differs more than 5% from the tabulated data , further investigation is needed . in that publication , it was obtained as a combination of statistical , calibration coefficient and climatic conditions correction uncertainties . the pdd for each applicator should be measured and compared with the reference data provided in table 2 ( taken from ) . the procedure is the same as described above for the direct evaluation of the output factors . again , an ionization chamber , tld chips , or radiochromic films can be used . however , in this case , slabs of different thicknesses are inserted between the detector and the applicator exit surface , and the detector reading is obtained as a function of depth in the phantom . percent depth dose ( pdd ) normalized at 3 mm on axis for the valencia applicators vh2 and vh3 differences between pdd values measured during the commissioning and pdd data listed in table 2 should be within uncertainties ( with k = 1 ) ( see e.g. ) . this section describes the tests that we consider appropriate for a periodic qa program . with respect to the periodicity , after such source replacement , the most relevant elements influencing the applicator dosimetric properties are the sk and the positioning of the source at the vertex of the applicator ( sid ) . in principle , both elements are checked in the general test of the hdr afterloader . for the valencia applicator output check periodicity , we suggest a 3 - 4 months period , simultaneously with the source replacement . in this way the sid value should be evaluated and updated after each source exchange , typically every 3 to 4 months . the main aspect that may influence the sid is the source positioning and typically it is evaluated daily for the hdr unit . the same methodology and tolerance levels , as in sub - section  evaluation of the source to indexer distance and reproducibility  , are proposed . if necessary , the sid value of the library plan should be updated . since the sid value might compromise the flatness and symmetry of the dose distribution at the reference depth , after each update of the library plan , the test described in sub - section  evaluation of flatness and symmetry at 3 mm depth  should be repeated . after each source exchange , the output of the system must be verified following the direct or indirect method described in sub - section  output verification  . after the treatment plan has been created with the treatment planning console ( tps ) , and also prior to each treatment session , the tps planned time for that specific session should be calculated independently with , e.g. , a spread sheet . independent calculations can be done with the of and pdd provided in table 1 and table 2 , respectively , and using the actual sk of the source , according to : where d is the prescribed dose by the radiation oncologist ,  is the applicator diameter ( 2 cm or 3 cm ) , and z is the prescription depth . no differences between planned dwell times from the tps and from the independent calculation should be found ( < 1% ) . the existing ring can introduce an air gap of roughly 1 mm between the applicator and the skin lesion , and therefore reduce the prescription dose by 7 - 10% . it is also very important to verify that the cap is in place during treatment . if treatment is performed without such cap , a significant overdose ( 180% ) occurs due to the electron contamination component together with another overdose component due to the 1 mm shorter source to skin distance ( 10% ) . the cap is also used to prevent contamination between patients ; these caps have to be sterilized as recommended by the manufacturer . to avoid this potential problem and considering the advantage of using just one cap for all patients ( being its dosimetry impact always the same ) , at our department , one single non - ring or flat cap is used for all patients and dosimetry tests . to prevent the contamination between patients , the applicator is wrapped with a stressed condom . this verification is important due to the size of the radiation field and the 35% output difference between the two applicators . a useful approach is to paint in different colors each applicator tip to avoid confusion . in addition to those listed previously , the user is also referred to the acceptance , commissioning , and qa recommendations from societies and manufacturer regarding the hdr unit [ 6 , 8 , 9 ] . furthermore , all recommendations ( current and future ) from the manufacturer , as well as state or federal guidelines related to the use of this device / technology should be implemented . although not directly as part of commissioning or qa of the valencia applicators , adequate training should be provided and documented for the brachytherapy team ( current and new members ) . a quality management program should be updated continuously . a commissioning procedure and periodic qa program has been proposed for users of the vh2 and vh3 valencia applicators , based on societal recommendations and our experience with these systems . the proposed methodology , equipment , frequency , and tolerance level ( if applicable ) for each test are presented . the performance of these tests guarantees proper and safe delivery of the radiation dose .
the valencia applicators ( nucletron , an elekta company , elekta ab , stockholm , sweden ) are cup - shaped tungsten applicators with a flattening filter used to collimate the radiation produced by a high - dose - rate ( hdr ) 192ir source , and provide a homogeneous absorbed dose at a given depth . this beam quality provides a good option for the treatment of skin lesions at shallow depth ( 3 - 4 mm ) . 
 the user must perform commissioning and periodic testing of these applicators to guarantee the proper and safe delivery of the intended absorbed dose , as recommended in the standards in radiation oncology.in this study , based on aapm and gec - estro guidelines for brachytherapy units and our experience , a set of tests for the commissioning and periodic testing of the valencia applicators is proposed . 
 these include general considerations , verification of the manufacturer documentation and physical integrity , evaluation of the source - to - indexer distance and reproducibility , setting the library plan in the treatment planning system , evaluation of flatness and symmetry , absolute output and percentage depth dose verification , independent calculation of the treatment time , and visual inspection of the applicator before each treatment . for each test , 
 the proposed methodology , equipment , frequency , expected results , and tolerance levels ( when applicable ) are provided .