histologic examination of the prostate harboring a malignancy often reveals multiple areas with malignant lesions . these malignant lesions differ in size and morphologic appearance and can be divided in dominant lesions and smaller insignificant lesions [ 2 , 3 ] . if radiotherapy is used for treatment it is a common practice to treat the whole prostate . in general , the dose to the prostate is evenly distributed with the same dose applied to malignant lesions as to healthy prostate areas . van lin et al . proposed identifying dominant intraprostatic lesions ( dils ) better in order to treat the entire prostate to a moderate dose and giving a boost on these dils . to apply a boost dose to dils , several techniques are reported as able to identify tumor lesions within the prostate [ 5 , 6 ] . t2-wi can be combined with functional imaging for additional information on tumor presence . for the purpose of functional imaging , dynamic contrast - enhanced mri ( dce - mri ) , diffusion - weighted imaging ( dwi ) , and magnetic spectroscopic imaging ( mrsi ) there is increased evidence that the so - called multiparametric mri ( mpmri ) , in which different sequences and combination of morphological and functional mri are used , will increase the diagnostic accuracy for detection , localization , and characterization of prostate cancer . however , at the european consensus meeting in december 2009 it was clear that there is a large disagreement between experts in the field on criteria to standardize the conduct and interpretation of mpmri . although very promising , for the above - mentioned reasons mpmri still can not be regarded useful for the routine clinical use . combination of pet with computer tomography ( ct ) offers better depiction of tumor areas within the prostate . promising radiotracers are carbon-11 ( c - choline and c - acetate ) and fluoro-18 ( f - fluorocholine and f - fluoroacetate ) . particularly contrast - enhanced ultrasound ( ceus ) imaging has the ability to detect these lesions . this technique visualizes perfusion within the prostate . because of increased microvessel density in tumor areas , discrimination with healthy prostate tissue is obtained . us contrast is provided by small encapsulated intravenously injected gas bubbles , which behave as additional reflectors in the blood stream . in a correlation study with prostatectomy specimens , sedelaar et al . modern contrast imaging technique uses the non - linear behavior of the microbubbles to increase selective imaging of the bubbles for tumor detection . although in the early studies on ceus for prostate cancer detection , low sensitivity and specificity rates were reported , modern ultrasound techniques and quantitative ultrasound imaging are more promising [ 11 , 1315 ] . the recent reports indicating high accuracy for prostate cancer detection enable the ceus technique to be used for prostate cancer radiotherapy [ 16 , 17 ] . brachytherapy is characterized by a heterogeneous dose distribution and it should be possible to treat the whole prostate gland to an elective dose with a boost on macroscopic tumor areas . the aim of this study was to investigate if ceus techniques can support brachytherapy treatment planning . since the value of the ceus technique for routine clinical use has not been determined yet , this study should be considered as a proof of principle for future use . ten patients with prostate cancer who underwent external beam radiotherapy with pulsed dose - rate ( pdr ) brachytherapy were included in this study . details of the treatment were previously reported [ 18 , 19 ] . in brief , patients were treated for the prostate and base of the seminal vesicles with 3-dimensional conformal external beam radiotherapy to deliver a dose of 46 gy in daily 2 gy fractions . subsequently , within 1 week a transperineal implantation was performed with flexible catheters for pdr brachytherapy . definitive treatment planning was done with ct - scan without the rectal us probe to closely simulate the actual treatment geometry . the prescribed brachytherapy dose on the planning target volume ( ptv ) was 28.8 gy in 1.2 gy pulses and a time of 2.0 hours between pulses . prior to start with external beam radiotherapy patients underwent an examination with ceus as an outpatient procedure . the sulphur hexafluoride microbubbles ( sonovue from bracco , milan , italy ) were injected intravenously via the antecubital fossa . us imaging was performed using an iu22 us scanner ( philips healthcare , bothel , usa ) and a c8 - 4v endocavity probe . all imaging sequences were stored in the dicom ( digital imaging and communication in medicine ) format , and transferred to a personal computer for further analysis . this contouring was done in the transverse plane on areas with a fast or increased enhancement as compared to the main enhancement of the peripheral zone . in total , 2 boluses of 2.4 ml of contrast were injected , and the inflow of contrast was monitored . during up to 4 minutes after the injection additional planes were examined using the destruction - replenishment technique . in this manner , the whole prostate was examined for suspected lesions . imaging was performed and stored in the transverse orientation with a continuous grayscale sweep from base to apex , together with one longitudinal maximum cross - section . the midline of each frame in the transverse sweep is also located in the longitudinal cross - section . using correlation techniques we were able to correlate each plane in the transverse sweep to one location in the longitudinal cross - section , and 3-dimensional ( 3d ) reconstruction of the prostate each ceus recording in a transverse plane was searched for in the recorded sweep , and therefore the location of each ceus recording could be reconstructed into the 3d data set . the delineated lesions were drawn in this 3d data set , and the result was used for the fusion with the planning system imaging data set . the us images used for brachytherapy planning were acquired using a 2d side viewing us probe which position was controlled by a stepper . the images had a resolution of 0.4  0.4 mm in - plane with 1 mm slice separation . the brachytherapy planning images were combined into a 3d volume and registered to the 3d reconstructed us scan as described above . besides the reconstructed us scan , a corresponding 3d data set was available in which the ceus detected intraprostatic lesion ( il ) was indicated as binary mask . all 3d data sets were loaded into an in - house software system for image registration and fusion . the software was extended with a rigid registration method suitable for us - us registration similar to the method proposed by roche et al . . first , both us images were manually aligned for 3d translation and rotation ( 6 degrees of freedom ) . during this step both us data sets were visualized in 3 orientations ( transverse , sagital and coronal ) and overlaid in a green - purple color wash or as an interactive cut display . then , the us volumes were processed by an unsharp mask filter to extract gradient information . the final registration was performed by optimizing the correlation ratio metric between both processed us data sets . both for visualization and registration purposes the registration accuracy was mainly limited by differences in deformation between both us data sets because of difference in applied probe pressure . for the visual verification the registration accuracy was estimated to be better than 2 mm . after approval of the registration , the binary volume with the delineated ils was overlaid on the brachytherapy us using the 3 d translation and rotation obtained from the us registration ( fig . 1 ) . registered images of ceus ( a ) and brachytherapy planning us ( b ) with delineated intraprostatic lesion a new treatment plan was created on the fused images . the prostate volume covered by the reference dose ( rd ) should be 95% or more ( v100-p  95% ) , the minimal dose to the 2 ml rectal volume receiving the highest dose should be 0.97 gy / pulse or less ( d2ml - r  0.97 gy/ pulse ) , the maximum urethral dose ( dmax - u ) should be 140% rd ( 1.68 gy / pulse ) . the second treatment plan was created with the above - mentioned constraints and an additional constraint prescribing at least 95% of the il volume covered by the 140% rd ( v140-il  95% ) ( fig . 2 ) . brachytherapy isodose distribution without ( 1a ) and with ( 1b ) knowledge of the position of an intraprostatic lesion in the right peripheral zone . note the coverage of the il by the 140% isodose the brachytherapy treatment plans with and without us contrast were compared to each other . dvh - parameters used for comparison were v100-p , v150-p , d90-p , d2ml - r , dmax - u , and total reference air kerma at 1 m ( trak ) . pairwise comparison of the means of the dvh - parameters was done by the wilcoxon signed rank sum test . all tests were two - sided , and p - values < 0.05 were considered significant . statistical analysis was performed with the predictive analytics software statistics , version 18.0 for mac os x ( pasw 18.0 , chicago , il , usa ) . prior to start with external beam radiotherapy patients underwent an examination with ceus as an outpatient procedure . the sulphur hexafluoride microbubbles ( sonovue from bracco , milan , italy ) were injected intravenously via the antecubital fossa . us imaging was performed using an iu22 us scanner ( philips healthcare , bothel , usa ) and a c8 - 4v endocavity probe . all imaging sequences were stored in the dicom ( digital imaging and communication in medicine ) format , and transferred to a personal computer for further analysis . this contouring was done in the transverse plane on areas with a fast or increased enhancement as compared to the main enhancement of the peripheral zone . in total , 2 boluses of 2.4 ml of contrast were injected , and the inflow of contrast was monitored . during up to 4 minutes after the injection additional planes were examined using the destruction - replenishment technique . in this manner , the whole prostate was examined for suspected lesions . imaging was performed and stored in the transverse orientation with a continuous grayscale sweep from base to apex , together with one longitudinal maximum cross - section . the midline of each frame in the transverse sweep is also located in the longitudinal cross - section . using correlation techniques we were able to correlate each plane in the transverse sweep to one location in the longitudinal cross - section , and 3-dimensional ( 3d ) reconstruction of the prostate each ceus recording in a transverse plane was searched for in the recorded sweep , and therefore the location of each ceus recording could be reconstructed into the 3d data set . the delineated lesions were drawn in this 3d data set , and the result was used for the fusion with the planning system imaging data set . the us images used for brachytherapy planning were acquired using a 2d side viewing us probe which position was controlled by a stepper . the images had a resolution of 0.4  0.4 mm in - plane with 1 mm slice separation . the brachytherapy planning images were combined into a 3d volume and registered to the 3d reconstructed us scan as described above . besides the reconstructed us scan , a corresponding 3d data set was available in which the ceus detected intraprostatic lesion ( il ) was indicated as binary mask . all 3d data sets were loaded into an in - house software system for image registration and fusion . the software was extended with a rigid registration method suitable for us - us registration similar to the method proposed by roche et al . . first , both us images were manually aligned for 3d translation and rotation ( 6 degrees of freedom ) . during this step both us data sets were visualized in 3 orientations ( transverse , sagital and coronal ) and overlaid in a green - purple color wash or as an interactive cut display . then , the us volumes were processed by an unsharp mask filter to extract gradient information . the final registration was performed by optimizing the correlation ratio metric between both processed us data sets . both for visualization and registration purposes the registration accuracy was mainly limited by differences in deformation between both us data sets because of difference in applied probe pressure . for the visual verification after approval of the registration , the binary volume with the delineated ils was overlaid on the brachytherapy us using the 3 d translation and rotation obtained from the us registration ( fig . 1 ) . registered images of ceus ( a ) and brachytherapy planning us ( b ) with delineated intraprostatic lesion a new treatment plan was created on the fused images . the prostate volume covered by the reference dose ( rd ) should be 95% or more ( v100-p  95% ) , the minimal dose to the 2 ml rectal volume receiving the highest dose should be 0.97 gy / pulse or less ( d2ml - r  0.97 gy/ pulse ) , the maximum urethral dose ( dmax - u ) should be 140% rd ( 1.68 gy / pulse ) . the second treatment plan was created with the above - mentioned constraints and an additional constraint prescribing at least 95% of the il volume covered by the 140% rd ( v140-il  95% ) ( fig . 2 ) . brachytherapy isodose distribution without ( 1a ) and with ( 1b ) knowledge of the position of an intraprostatic lesion in the right peripheral zone . note the coverage of the il by the 140% isodose the brachytherapy treatment plans with and without us contrast were compared to each other . dvh - parameters used for comparison were v100-p , v150-p , d90-p , d2ml - r , dmax - u , and total reference air kerma at 1 m ( trak ) . pairwise comparison of the means of the dvh - parameters was done by the wilcoxon signed rank sum test . all tests were two - sided , and p - values < 0.05 were considered significant . statistical analysis was performed with the predictive analytics software statistics , version 18.0 for mac os x ( pasw 18.0 , chicago , il , usa ) . one patient did not show ils on ceus and for another patient fusion of ceus - images on the brachytherapy treatment planning images was not possible due to a large variation in the prostate shape of the two studies . four patients had only 1 il and the other 4 had 2 ils at both sides of the prostate . the largest of both ils within a prostate was coded as il1 and the smallest  as il2 . intraprostatic lesion ( il ) characteristics 
 table 2 shows that the mean v140 of the ils was increased from 66.0 - 67.7% to 92.5 - 95.7% . also , the mean d90 on the ils was increased from 1.49 - 1.57 gy / pulse to 1.76 - 1.81 gy / pulse . in 4 cases , the first treatment planning was performed with 12 catheters and in the other 2 cases , 14 catheters were used . in all cases the same number of catheters was used by repositioning the catheters corresponding to the location of the ils . the adapted treatment plan did not lead to an alteration of the dvh - parameters of the prostate , rectum , and urethra ( table 3 ) . also , a statistically non - significant increase of the trak was observed ( p = 0.09 ) . intraprostatic lesion ( il ) mean dose - volume parameters for a non - adapted and adapted treatment plan mean dose - volume parameters and trak for a non - adapted and adapted treatment plan in this pilot study we have investigated the possibility of introducing ceus data for prostate brachytherapy treatment planning . to our knowledge , this is the first study investigating this technique for brachytherapy . it appeared that with knowledge of the position of ils , a better treatment plan could be generated with improved coverage of the ils in the high dose regions . better coverage of ils was achieved by including the intraprostatic part as well as the extraprostatic extension into the high - dose region . the better coverage of the ils resulted also in a higher dose ( d90 ) in the ils . in this small study what is more important , the adapted plans did not lead to higher doses in the rectum and urethra . repositioning of catheters was done on inspection of the dose distribution in order to cover the ils adequately . the distance for repositioning of a catheter was usually not more than 5-mm compared to the original plan . we have not encountered any need for adding more catheters in our study , but that may be necessary in certain situations . considering repositioning of catheters the catheters can still end up in another position with an implication on the dvh - parameters . however , we belief that with knowledge of localisation of ils it will influence the positioning of catheters to cover them optimally . the trak was calculated to investigate changes in exposure to radioactivity . a statistically non - significant increase in trak careful attention should be paid to trak values to be informed on exposure to radioactivity , which can not be found in the prescribed dose or dose coverage to organs . adaptation of treatment plans can easily lead to large differences in radiation exposure and should be considered in future studies . the question still remains how accurate the ceus technique is for identification of tumor lesions in order to introduce this concept with brachytherapy into the clinic . halpern et al . compared the results of ceus with 12 prostatectomy specimens . in this small study they found a positive predictive value for tumor identification of only 56% and a sensitivity of 42% . these numbers indicate a high false negative rate because tumor lesions are missed . in a similar study , sedelaar et al . found ceus investigation by an experienced investigator to have a detection rate of 61% for prostate tumor lesions . however , they also found a detection rate of 79% for large sized (  5 mm ) lesions . the implication for brachytherapy is that at least the large sized tumor areas , clinically significant lesions , are identified . a new development in the use of ceus for prostate cancer detection is the ultrasound signal analysis and its quantification . the aim is to make the analysis less subjective and to increase the accuracy for tumor detection . at the eindhoven university of technology and academic medical center , a diffusion parameter was investigated and compared to prostatectomy specimens . a high sensitivity ( 81.2% ) and specificity ( 84.6% ) also the roc area ( receiver operating characteristic area ) under the curve was as high as 0.909 . other means of detecting malignant prostatic lesions with high spatial resolution is the use of mri . however , also with the use of mri tumor lesions can be missed or tumor extension can be misdiagnosed . in a systematic review with meta - analysis engelbrecht et al . found , using roc - curves , a test accuracy for discriminating t2 from t3 tumors of 71% . in this study , a classification of t - stage was done on a per - prostate level and probably the accuracy on a per - lesion level would have been lower . at a european consensus meeting , there was a large disagreement between experts in the field on how to use mpmri for prostate cancer imaging . consensus was reached on 67% of items related to imaging parameters for tumor detection and localization , and on only 54% of items related to imaging interpretation and reporting . so , even for the more clinically used imaging technique of mri , as compared to ceus , further investigation should be performed as to its validity . it is expected that both imaging techniques will play a major role in the future for image guided and targeted radiotherapy . because in general prostate brachytherapy , implantations are performed under us guidance , identification of tumor lesions on us is more advantageous than on mri . if mri is used for tumor localization purposes , fusion of images must be used if the us technique is used for implantation bringing in another source of geometric uncertainties . in our study we used rigid matching for contrast - enhanced images and us images for treatment planning . because of shape deformation we encountered some difficulties in fusion of image studies . however , we have shown as a proof of principle that by identification of tumor lesions within the prostate brachytherapy treatment plans can be adapted for better lesion coverage without any increase of dose in the oar . ceus is a promising technique to aid in adaptation of brachytherapy treatment plans . for practical clinical usage a suitable side - viewing probe after analysis of the images , contouring of the prostate and ils is performed for brachytherapy planning . without such a dedicated probe , deformable image fusion techniques are absolutely needed for proper localization and positioning . when the treatment planning is done the implantation can be performed according to the plan and needle placement in the vicinity of the ils . identification of ils in the prostate with ceus can help with adapting brachytherapy treatment planning for an improved coverage of the ils without increasing the dose to the oars . further developments in image fusion and us hard- and software are needed before introduction into the clinic .
purposeto investigate the possibility of localization of intraprostatic lesions ( il ) with contrast - enhanced ultrasound ( ceus ) to support the brachytherapy treatment planning of temporary implants.material and methodstwo brachytherapy treatment plans were generated for 8 patients treated with external beam radiotherapy and pulsed - dose rate brachytherapy boost for prostate cancer . 
 the first and second brachytherapy treatment plan was without and with knowledge of the localization of the ils , respectively . 
 pairwise comparison was performed on prostate , rectum , and urethra dose - volume parameters and total reference air kerma ( trak)-values.resultscoverage of the ils by the 140% isodose was increased from mean 66.0 - 67.7% for the standard plan to mean 92.5 - 95.7% for the adapted plan . 
 the mean d90 of the ils increased from 1.49 - 1.57 gy / pulse to 1.76 - 1.81 gy / pulse . 
 dose - volume parameters for the prostate , rectum , and urethra and the trak did not change.conclusionsceus technique is a promising method for il localization to aid in brachytherapy treatment planning . 
 dose coverage on the il could be improved without any increase of dose in organs at risk .