the high dose rate ( hdr ) brachytherapy is a technique where small iridium-192 source ( active length 3.5 mm and diameter 0.6 mm ) can be programmed to dwell for a given time at any point in the treatment catheters .
the success of this treatment is strongly dependent on accurate knowledge of the source strength , dwell positions , and dwell times . since the complete treatment may be given in just a few minutes with hdr remote after loading equipment , it is extremely important to perform adequate quality control ( qc ) checks on the equipment before use . for high - activity source
most of these methods , however , depend on visual inspection of either a radiographic image or a closed circuit television image to quantify the accuracy of hdr unit source positioning .
this can be achieved if the verification method is capable of resolving minute differences in source position .
most of the currently used quality assurance ( qa ) tests concentrate on the verification of source position and dwell time .
rickey et al , 2010 and song et al , 2009 verified the source position and dwell time with their own designed equipment .
however , uses of most of the equipments other than film are limited only to brachytherapy .
furthermore , designing equipment for brachytherapy qa is not practicable or economically viable to all radiotherapy departments .
thus , equipments used in external radiotherapy qa may be tailored for use in brachytherapy . in this study ,
commercially available detector array imatrixx was used to verify the dwell position , reference isodose length ( length of 100% isodose line in source direction ) , and step size in comparison with treatment planning system ( tps ) .
all the tests included in this study were performed on two - dimensional , monolithic , large area pixel - segmented ionization chamber ( pxc ) commercially available as imatrixx .
this device consists of 1020 vented ion chambers each having active volume 0.08 cc , arranged in 32  32 grids .
the current , which is proportional to the dose rate , is measured and digitalized by a nonmultiplexed 1020 channels current sensitive analog to digital converter .
the signals are analyzed by the omipro - imrt version 1.7 ( iba , gmbh ) .
first the imatrixx detector array is calibrated to external beam radiation ( 6 mv ) , to get the response of all detectors uniform .
three interstitial flexible catheters were fixed above the imatrixx detector array such that 300thsource position ( distal end ) corresponds with the center of the superior most chamber .
the distance between the center of first and last detector along the catheters was 23.6 cm .
the flexible plastic catheters were placed straight above the flat panel at 5.71 , 0.38 , and 4.95 cm , respectively , from the central axis as shown in figure 1 .
the catheters were placed along the detector line ( row wise / column wise ) to avoid any systematic shift in the catheter placement , hence in the large systematic error .
flexiplan version 2.6 ( isodose control b.v . ) was used to create plans for this fixed geometry .
plans were created to verify ( i ) dwell position , ( ii ) reference isodose length , and ( iii ) step size .
position of catheters over the detector array all plans created for this study used 5 mm step size with 2 s dwell time .
the dwell time was set as 2 s in all the plans with the reason that the detector response was adequate to resolve the peaks in dose profile , which otherwise resulted in a single dark image .
the dose profiles obtained along the source line at 5.71 , 0.38 , and 4.95 cm from central axis were analyzed .
since the dwell positions were set to 2 s , plans created to verify the reference isodose length were not prescribed to any particular distance .
thus a method was adopted in which a point that is 5 mm superior to the first source position ( 300th position ) was selected [ figure 2 ] , and the dose at this point was normalized to 100% . with reference to this point the reference isodose length was measured as maximum length of 100% isodose along catheter axis [ figure 3 ] .
treatment plans with different dwell positions were executed , and measured reference isodose length were compared with the tps calculated values , as tabulated in tables 2 and 3 .
the step size was tested for 2 , 3 , 4 , and 5 cm step sizes .
comparison of planned and measured dwell positions the position of the point where the dose was considered as 100% reference isodose length verification reference isodose length verification for same source loading of 7 cm reference isodose length verification for different source loadings it was not possible to calibrate the detector array for ir-192 energy as well as brachytherapy ( 4 ) geometry , thus only relative isodose could be verified .
the maximum dwell position error was noted to be 1.8 mm with a mean 0.451.99 mm 95% confidence interval .
total error divided as systematic or mean error of 0.45 mm and random component error or standard deviation was 1.01 mm [ table 1 ] .
when planned and measured dwell positions were plotted , a linear relationship was obtained [ figure 4 ] . planned versus measured dwell position
the source was loaded for 10 cm from 270th source position to 170th source position .
the second plan was created with source loading length of 7 cm in all catheters from 270 position as tabulated in table 2 .
the center catheter was loaded from 270th position to 200th position for a length of 7 cm .
the other two catheters were loaded only for 3 cm in which the first position started from 170th position , as indicated in table 3 . the reference isodose length was measured for each catheter .
a total of seven reference isodose lengths in three plans were compared with the tps planned lengths [ tables 2 and 3 ] .
less than 1 mm error was found in 57% of the tested reference isodose lengths ( maximum error was 1.5 mm ) .
planned versus measured reference isodose length step sizes less than 1 cm were not able to be resolved as peaks with imatrixx . however , step sizes 2 , 3 , 4 , and 5 cm were verified [ figure 6 ] .
maximum error was 1.2 mm in a plan where the step size was 2 cm .
the maximum dwell position error was noted to be 1.8 mm with a mean 0.451.99 mm 95% confidence interval .
total error divided as systematic or mean error of 0.45 mm and random component error or standard deviation was 1.01 mm [ table 1 ] .
when planned and measured dwell positions were plotted , a linear relationship was obtained [ figure 4 ] . planned versus measured dwell position
the source was loaded for 10 cm from 270th source position to 170th source position .
the second plan was created with source loading length of 7 cm in all catheters from 270 position as tabulated in table 2 .
the center catheter was loaded from 270th position to 200th position for a length of 7 cm .
the other two catheters were loaded only for 3 cm in which the first position started from 170th position , as indicated in table 3 . the reference isodose length was measured for each catheter .
a total of seven reference isodose lengths in three plans were compared with the tps planned lengths [ tables 2 and 3 ] .
less than 1 mm error was found in 57% of the tested reference isodose lengths ( maximum error was 1.5 mm ) .
step sizes less than 1 cm were not able to be resolved as peaks with imatrixx . however , step sizes 2 , 3 , 4 , and 5 cm were verified [ figure 6 ] .
maximum error was 1.2 mm in a plan where the step size was 2 cm .
pxc was designed for use in external beam dosimetry and to perform two - dimensional verifications of fields with large gradients and complex shapes .
one of the limiting factors for this instrument is volume averaging effect due to finite size of the ion chamber , measured to have full width at half maximum of about 5.8 mm , when a gaussian response function was used .
if the volume of the beam data collecting chamber is comparable with that of the imatrixx chambers then due to similar volume averaging effect , imatrixx measured values shows a very high degree of match with tps - calculated values .
the literatures can be found for intensity - modulated radiotherapy ( imrt ) dose verification by this instrument .
generally , the imrt calculation grid resolution ( voxel size ) is 2  2  2 mm and dose distribution is verified with 3% dose difference 3 mm distance to agreement ( dta ) . under this scenario comparable results were obtained between imatrixx and film .
the use of pxc for brachytherapy dosimetry was intended due to two reasons , first its use in dosimetry of high - gradient fields ( imrt dose profile ) verification , which is encountered in brachytherapy . secondly , due to its high charge collection efficiency ( 0.985 ) for a dose rate of 3.5 gy / min of 6 mv photon beam , this dose rate falls under hdr brachytherapy range as indicated by task group report 56 ( dose rate at 1 cm for 10 ci source is 6.85 gy / min ) .
no published literature could be found for ir-192 energy and brachytherapy geometry for use of pxc . in this study , all tests performed for dwell position verification resulted with errors < 1.8 mm . in order to investigate the spatial resolving efficiency of the detector array , two plans were created with 1 mm dwell position differences ; the dwell position in one plan was related with other plan with only 1 mm difference in their value ( 270th dwell position in one plan corresponds to 269th position in another plan ) .
the omnipro software was able to differentiate the peaks in the dose profile of both plans [ figure 7 ] .
reference isodose length verification was used to indirectly verify these step sizes to the extent of their agreement with the planned reference isodose lengths in tps .
the mean dwell position variation measured by imatrixx was smaller than the value 0.7 mm that had been reported by song et al .
it is clear from the graph that peaks were differentiated with each other for ir-192 hdr treatments , source position and step sizes are usually verified in clinic by visual inspection by autoradiograph .
this autoradiograph is taken as a film by attaching two flexible implant tubes , one having the dummy source and other having the true source as programmed .
the other disadvantage of film is blurring , as reported by rickey noneet al ; the blurring effect was more pronounced with metal applicators than with plastic catheters .
the accuracy of dwell position evaluation in a blur film is subjective and depends on the investigator 's and instrument 's ability to resolve the peak .
this problem was not encountered in imatrixx since peaks obtained in the dose profile provided the spatial information , which in turn was used to find the dwell positions .
the reproducibility of dwell position measurement using imatrixx was appreciable ; ( it measured the dwell position accuracy 0.45  1.99 mm with 95% confidence interval ) .
imatrixx was able to measure the reference isodose lengths with 2 mm accuracy against tps calculated values .
the reference isodose length of the central catheter was expected to be more than that of other catheters as given in tps .
this is because dose distribution against central catheter will have a contribution from each of the lateral catheters , whereas the lateral two catheters will have contribution only from the central catheter because of the large distance between them ( ~10.5 cm ) .
imatrixx was able to quantify the same nature of isodoses and mimic the tps values .
the mean dwell positional accuracy measured was 0.45 mm , with a standard deviation of 1.01 mm ( maximum error of 1.8 mm ) .
this is clearly below the tolerance limit of 2 mm . until the completion of this study
, only one other report describing brachytherapy qa using a prototype flat panel detector with gafchromic film was found .
the commercially available flat panel detector imatrixx proved to be an effective tool in measuring important parameters in brachytherapy .
these promising results build a foundation on which further investigations in brachytherapy routine qa can be achieved in future .
 for high dose rate ( hdr ) brachytherapy , independent treatment verification is needed to ensure that the treatment is performed as per prescription . 
 this study demonstrates dosimetric quality assurance of the hdr brachytherapy using a commercially available two - dimensional ion chamber array called imatrixx , which has a detector separation of 0.7619 cm . 
 the reference isodose length , step size , and source dwell positional accuracy were verified . 
 a total of 24 dwell positions , which were verified for positional accuracy gave a total error ( systematic and random ) of 0.45 mm , with a standard deviation of 1.01 mm and maximum error of 1.8 mm . using a step size of 5 
 mm , reference isodose length ( the length of 100% isodose line ) was verified for single and multiple catheters of same and different source loadings . 
 an error 1 mm was measured in 57% of tests analyzed . 
 step size verification for 2 , 3 , 4 , and 5 cm was performed and 70% of the step size errors were below 1 mm , with maximum of 1.2 mm . 
 the step size 1 cm could not be verified by the imatrixx as it could not resolve the peaks in dose profile . 
Introduction
Materials and Methods
Results
Dwell position tests
Reference isodose length verification
Step size verification
Discussion
Conclusions