lendoprothse endobiliaire peut tre utilise substitut  la localisation pancratique dans lutilisation de la tomographie par ordinateur  faisceau conique ( tofc ) en radiothrapie externe .
la prsente tude fait tat du dplacement interfraction de lendoprothse durant la radiothrapie externe pour le cancer du pancras localement avanc ( cpla ) .
six patients porteurs dune endoprothse endobiliaire ayant reu des traitements de radiothrapie externe pour un cpla ont t valus . les mesures pour laspect le plus suprieur de lendoprothse ( sup stent ) et son aspect le plus infrieur ( inf stent ) par rapport  laspect le plus suprieur et infrieur de lapophyse pineuse centrale de la vertbre l1 ont t dtermins  partir des traitements quotidiens de tofc et compars  celles tablies lors de la tomographie de planification
. les changements dans la mesure endoprothse - l1 ont t interprts comme des changements dans la position relative de lendoprothse .
trois patients ont prsent un dplacement interfraction de lendoprothse de 1  cm lorsque les mesures de traitement ont t compares aux mesures de planification .
pour le patient  a , la mesure sup stent montre un dplacement vers la droite ( 2,662,77  cm ) et le bas ( 3,03,12  cm ) , tandis que la mesure inf stent se dplaait vers la droite ( 1,922,02  cm ) , le bas ( 3,233,34  cm ) et larrire ( 1,411,43  cm ) .
la mesure inf stent pour le patient  b montre un dplacement vers le haut ( 2,230,49  cm ) et larrire ( 1,720,59  cm ) .
les mesures sup et inf stent du patient  f montrent un dplacement vers le bas ( 0,980,35  cm et 1,210,38  cm , respectivement ) .
chez les trois autres patients ( c , d et e ) , le dplacement interfraction est infrieur  1  cm .
la migration et la dformation de lendoprothse endobiliaire ont t observes chez un petit sous - ensemble de patients .
dautres tudes seront ncessaires avant de pouvoir confirmer leur utilisation comme substitut  la localisation des cibles cpla durant la radiothrapie externe guide par limage .
the 5-year survival rate in england for men and women diagnosed with pancreatic cancer is 3.9% and 4.4% , respectively . for those with pancreatic tumours
deemed surgically unresectable with no distant metastases , their disease is classified as locally advanced pancreatic cancer ( lapc ) .
it is possible to treat lapc with either conformal radiotherapy or intensity - modulated radiotherapy   though the presence of several critical organs near the pancreas ( eg , spinal cord , kidneys , liver , and bowel ) makes the planning and delivery of radiotherapy complex , especially if nodal volume inclusion creates a larger planning target volume ( ptv ) .
image - guided radiotherapy ( igrt ) can be used to ensure treatment accuracy during radiotherapy treatments and limit toxicity to organs at risk . while igrt using megavolt ( mv ) or kilovolt ( kv ) planar images
can enhance accuracy , these images provide mostly bony anatomy information with minimal soft tissue definition .
using bony anatomy to align organs such as the pancreas has proven to be a poor predictor of pancreatic location .
cone - beam computed tomography ( cbct ) can provide additional soft tissue definition , but delineating the pancreas and pancreatic tumours on cbct remains difficult .
although feng et  al   found that the abdominal wall and diaphragm were unsuitable surrogates for pancreatic tumour position , this has not excluded the use of other internal structures as surrogates .
studies have sought to use fiducial  markers , surgical clips , electromagnetic transponders , and endobiliary stents   as pancreatic tumour surrogates to assist with image matching .
endobiliary stents are commonly implanted in pancreatic cancer patients as a palliative measure to relieve symptoms associated with biliary strictures such as jaundice , thereby aiding chemoradiotherapy tolerability . as the common biliary duct runs through the pancreatic head , using an endobiliary stent as a surrogate for pancreatic localization can serve as a potential alternative to the implantation of fiducial markers , which are placed percutaneously under radiographic guidance , intraoperatively , or using endoscopic ultrasound .
fiducial marker insertion for igrt use would be an additional procedure for patients with endobiliary stents already inserted for palliative purposes .
it is also unknown how fiducial marker visibility may be affected if a stent is in place as well .
endobiliary stents can be visualized on cbct , and it has been suggested that they can be used as a surrogate when determining pancreatic tumour positions .
two studies have reported on cbct - determined interfraction motion of endobiliary stents during external - beam radiotherapy ( ebrt ) and found minimal stent position changes ( < 1  cm ) with no indication of stent migration [ 11 , 13 ] .
in contrast , johanson et  al   reported stent migration rates of  4.9% proximally and 5.9% distally in the general patient population .
the work presented here seeks to report on the stability of endobiliary stents in patients who received chemoradiotherapy at our centre .
the patients included in this retrospective study had previously provided informed consent for treatment on a research ethics board  approved phase ii trial conducted in accordance with the helsinki declaration of 1964 and its later amendments , examining the effects of a novel radiosensitizer nelfinavir in combination with chemoradiotherapy in patients with lapc between october 2010 and september 2012 .
all trial patients with an endobiliary stent implanted at the time of their planning computed tomography ( ct ) scan who underwent daily cbcts during their radiotherapy treatments were included in this analysis .
treatment was planned in the supine position with patients ' hands above their head , arms and head supported by a vacuum bag , and knees supported by a kneefix ( civco medical systems , kalona , ia ) .
planning cts were captured during exhale breath - hold with a 16-slice helical ct scanner ( ge optima kv120 , smart ma , 0.25-cm slices ; ge healthcare , little chalfont , uk ) . at planning
, patients also underwent fluoroscopy using a conventional simulator ( varian medical systems , palo alto , ca ) or a 4-dimensional ct ( 4dct ) using the ct scanner with the stent visualized for breathing motion assessment .
breathing motion data collected during planning was used to establish a stent internal target volume contour for use in image matching .
gross tumour volumes ( gtvs ) and clinical tumour volumes ( ctvs ) were contoured on the planning ct by a radiation oncologist using commercially available radiotherapy contouring software  ( varian eclipse 10.0 ; varian medical systems , palo alto , ca ) .
phase 1 consisted of 50.4  gy prescribed to ptv1 (= ptv_oxford using the oxford contouring method as previously described by fokas et  al ) , and phase 2 consisted of 9  gy prescribed to ptv2 (=  gtv + 1  cm superiorly , 2  cm inferiorly , and 1.5  cm circumferentially ) . for each phase
, 3-dimensional conformal radiotherapy plans were delivered using a combination of 6-mv and 15-mv beam energies .
beam arrangements were individualized depending on tumour location in relation to normal tissue with up to 7 coplanar and noncoplanar beam angles .
daily treatment cbcts ( 21ex clinac 110  kv , 20  ma , 20  ms , 360/min , varian medical systems ) were captured during free breathing to assess patient position and tumour displacement .
the treating radiation therapists compared the daily treatment cbcts with the planning ct online using commercially available radiotherapy imaging software ( varian on - board imager , varian medical systems ) to determine displacement prior to treatment .
as tumour definition was difficult to assess on the treatment cbcts , the endobiliary stent , bony anatomy , and adjacent soft tissue structures ( ie , organs and vasculature ) were used as surrogates for the tumour .
if displacement was greater than 0.3  cm in the right - left , superior - inferior , or anterior - posterior direction or if rotation was greater than 3 , then corrective action was taken and a repeat cbct was acquired . if displacement was less than 0.3  cm and
for this retrospective study , three points were identified on all available planning cts and treatment cbct scans for each patient : the most superior aspect of the stent ( sup stent ) ; the most inferior aspect of the stent ( inf stent ) ; and the most inferior , posterior aspect of the l1 vertebra central spinous process ( l1 ) . using contouring software , a digital marker was placed at these points for each planning ct and treatment cbct ( figure  1 ) .
a specific set of x , y , and z coordinates were associated with each marker ; x corresponded to the right(+)/left( ) direction , y corresponded to the superior(+)/inferior( ) direction , and z corresponded to the anterior(+)/posterior( ) direction .
all observations were made by a single radiation therapist with several years of experience in cbct assessment . to determine intrauser variability ,
the stent and l1 marker placements were repeated on three separate occasions for the first five fractions .
stent length was approximated by subtracting the inf stent y coordinate from the sup stent y coordinate from the planning ct .
virtual markers were placed at the sup stent on the images associated with maximum inhale and exhale , and the distance between these markers were measured in the superior - inferior direction to estimate the planning stent breathing motion .
daily treatment stent - to - l1 ( stent - l1 ) measurements were determined using the daily cbcts taken immediately before treatment delivery .
the distance between the sup stent and the l1 vertebra was calculated by subtracting the l1 coordinates from the sup stent coordinates ( sup - l1 ) .
the distance between the inf stent and the l1 vertebra was calculated by subtracting the l1 coordinates from the inf stent coordinates ( inf - l1 ) .
limited soft tissue definition on cbct necessitated the comparison of surrogate motion with a bony match as in previous studies [ 7 , 13 ] .
once the sup - l1 and inf - l1 measurements were obtained for each cbct and planning ct , the planning stent - l1 measurements were then subtracted from the treatment stent - l1 measurements to determine the interfraction stent position relative to the baseline planning ct for both the sup stent and the inf stent .
the mean , median , maximum , minimum , and interquartile range of interfraction stent motion was calculated using commercially available statistics software ( ibm spss statistics 20.0 ; ibm corp , armonk , ny ) .
point - based analysis was performed to facilitate comparison with previous studies that examined interfraction stent position change using motion analysis of a single point ( single end of stent   or centre of mass ) . because two points at opposing ends of the stent were used to determine position change for this study , any rotational error was captured as oppositional movement of the two stent ends . also , since endobiliary stents are flexible structures making assessment of rotation alone difficult , using two points of measurement accounted for both potential rotation and potential deformation .
the patients included in this retrospective study had previously provided informed consent for treatment on a research ethics board  approved phase ii trial conducted in accordance with the helsinki declaration of 1964 and its later amendments , examining the effects of a novel radiosensitizer nelfinavir in combination with chemoradiotherapy in patients with lapc between october 2010 and september 2012 .
all trial patients with an endobiliary stent implanted at the time of their planning computed tomography ( ct ) scan who underwent daily cbcts during their radiotherapy treatments were included in this analysis .
treatment was planned in the supine position with patients ' hands above their head , arms and head supported by a vacuum bag , and knees supported by a kneefix ( civco medical systems , kalona , ia ) .
planning cts were captured during exhale breath - hold with a 16-slice helical ct scanner ( ge optima kv120 , smart ma , 0.25-cm slices ; ge healthcare , little chalfont , uk ) . at planning
, patients also underwent fluoroscopy using a conventional simulator ( varian medical systems , palo alto , ca ) or a 4-dimensional ct ( 4dct ) using the ct scanner with the stent visualized for breathing motion assessment .
breathing motion data collected during planning was used to establish a stent internal target volume contour for use in image matching .
gross tumour volumes ( gtvs ) and clinical tumour volumes ( ctvs ) were contoured on the planning ct by a radiation oncologist using commercially available radiotherapy contouring software  ( varian eclipse 10.0 ; varian medical systems , palo alto , ca ) .
phase 1 consisted of 50.4  gy prescribed to ptv1 (= ptv_oxford using the oxford contouring method as previously described by fokas et  al ) , and phase 2 consisted of 9  gy prescribed to ptv2 (=  gtv + 1  cm superiorly , 2  cm inferiorly , and 1.5  cm circumferentially ) . for each phase ,
3-dimensional conformal radiotherapy plans were delivered using a combination of 6-mv and 15-mv beam energies .
beam arrangements were individualized depending on tumour location in relation to normal tissue with up to 7 coplanar and noncoplanar beam angles .
daily treatment cbcts ( 21ex clinac 110  kv , 20  ma , 20  ms , 360/min , varian medical systems ) were captured during free breathing to assess patient position and tumour displacement .
the treating radiation therapists compared the daily treatment cbcts with the planning ct online using commercially available radiotherapy imaging software ( varian on - board imager , varian medical systems ) to determine displacement prior to treatment .
as tumour definition was difficult to assess on the treatment cbcts , the endobiliary stent , bony anatomy , and adjacent soft tissue structures ( ie , organs and vasculature ) were used as surrogates for the tumour .
if displacement was greater than 0.3  cm in the right - left , superior - inferior , or anterior - posterior direction or if rotation was greater than 3 , then corrective action was taken and a repeat cbct was acquired . if displacement was less than 0.3  cm and
for this retrospective study , three points were identified on all available planning cts and treatment cbct scans for each patient : the most superior aspect of the stent ( sup stent ) ; the most inferior aspect of the stent ( inf stent ) ; and the most inferior , posterior aspect of the l1 vertebra central spinous process ( l1 ) . using contouring software , a digital marker was placed at these points for each planning ct and treatment cbct ( figure  1 ) .
a specific set of x , y , and z coordinates were associated with each marker ; x corresponded to the right(+)/left( ) direction , y corresponded to the superior(+)/inferior( ) direction , and z corresponded to the anterior(+)/posterior( ) direction .
all observations were made by a single radiation therapist with several years of experience in cbct assessment . to determine intrauser variability ,
the stent and l1 marker placements were repeated on three separate occasions for the first five fractions .
stent length was approximated by subtracting the inf stent y coordinate from the sup stent y coordinate from the planning ct .
virtual markers were placed at the sup stent on the images associated with maximum inhale and exhale , and the distance between these markers were measured in the superior - inferior direction to estimate the planning stent breathing motion .
daily treatment stent - to - l1 ( stent - l1 ) measurements were determined using the daily cbcts taken immediately before treatment delivery .
the distance between the sup stent and the l1 vertebra was calculated by subtracting the l1 coordinates from the sup stent coordinates ( sup - l1 ) .
the distance between the inf stent and the l1 vertebra was calculated by subtracting the l1 coordinates from the inf stent coordinates ( inf - l1 ) .
limited soft tissue definition on cbct necessitated the comparison of surrogate motion with a bony match as in previous studies [ 7 , 13 ] .
once the sup - l1 and inf - l1 measurements were obtained for each cbct and planning ct , the planning stent - l1 measurements were then subtracted from the treatment stent - l1 measurements to determine the interfraction stent position relative to the baseline planning ct for both the sup stent and the inf stent .
the mean , median , maximum , minimum , and interquartile range of interfraction stent motion was calculated using commercially available statistics software ( ibm spss statistics 20.0 ; ibm corp , armonk , ny ) .
point - based analysis was performed to facilitate comparison with previous studies that examined interfraction stent position change using motion analysis of a single point ( single end of stent   or centre of mass ) . because two points at opposing ends of the stent were used to determine position change for this study , any rotational error was captured as oppositional movement of the two stent ends . also , since endobiliary stents are flexible structures making assessment of rotation alone difficult , using two points of measurement accounted for both potential rotation and potential deformation .
six patients accrued from october 2010 to september 2012 were included in this study ( a , b , c , d , e , and f ) with a mean age of 63 years ( range , 6065 years ) and a median gtv of 19.33  cm ( range , 6.45  cm41.10  cm ) .
cbcts were acquired for all patients for all 33 radiotherapy fractions with 198 cbcts acquired immediately prior to treatment delivery .
three cbcts were excluded because of software issues and a stent reinsertion , leaving 195 cbcts suitable for stent position analysis . for patient
a , cbcts acquired on fractions 9 and 19 could not be analysed using the contouring software , leaving 31 fractions available for analysis .
six patients were included in the stent position analysis but assessed as seven ( a , b , c , d , e.1 , e.2 , and f ) because one patient required a stent reinsertion .
patient e required a stent reinsertion on fraction 15 as stent patency failure was suspected .
therefore , fraction 15 was omitted from analysis and fractions 114 ( patient e.1 ) were analysed separately from 1633 ( patient e.2 ) .
the initial planning ct provided the baseline stent information for patient e.1 , and a repeat ct scan taken at fraction 15 after stent reinsertion provided the baseline stent information for patient e.2 ( table  1 ) .
of the six patients assessed , three patients ( a , b , and f ) exhibited pronounced and sustained stent position changes as determined by examining the individual mean position change for each patient in each direction ( table  2 ) and by visualizing position change graphically over time ( figure  2 ) .
patient a showed sustained stent position changes with the sup stent moving to the right ( 2.66  2.77  cm ) and inferiorly ( 3.0   3.12  cm ) and the inf stent moving to the right ( 1.92   2.02  cm ) inferiorly ( 3.233.34  cm ) and posteriorly ( 1.41  1.43  cm ) .
patient b showed sustained stent position changes with the inf stent moving superiorly ( 2.23   0.49  cm ) and posteriorly ( 1.72  0.59  cm ) .
patient f showed sustained stent position changes with both the sup and inf stent moving inferiorly ( 0.98  0.35  cm and 1.21   0.38  cm , respectively ) . the sustained position change was apparent from the start of treatment for patients b and f but did not become apparent in patient a until after fraction 5 ( figure  2 ) .
the free - breathing superior - inferior stent motion determined from planning for patients a , b , and f were 1.15  cm , 1.87  cm , and 0.53  cm , respectively ( table  1 ) . although the sup and inf stent for patient a and f moved similarly during treatment , only the inf stent for patient b showed sustained position changes ( figures  2 and 3 ) . the population mean position change ( x , y , z )
( ie , the mean of all patients ' individual means of position change ) for the sup stent was x=  0.44  1.09  cm , y = 0.45   1.26  cm , and z  =  0.09  0.26  cm .
the population mean position change for the inf stent for all patients was x=  0.33   0.84  cm , y=  0.43  1.65  cm , and z = 0.31  0.95  cm .
the standard deviation of position change (  ) exhibited by the inf stent was greater than that exhibited by the sup stent ( table  2 ) .
for the y component of the stent - l1 distance , six patients ( b , c , d , e.1 , e.2 , and f ) had inf stent > sup stent . for the z component of the stent - l1 distance , five patients ( a , b , c , e.1 , and f ) had inf stent > sup stent .
of the six patients assessed , three patients ( a , b , and f ) exhibited pronounced and sustained stent position changes as determined by examining the individual mean position change for each patient in each direction ( table  2 ) and by visualizing position change graphically over time ( figure  2 ) .
patient a showed sustained stent position changes with the sup stent moving to the right ( 2.66  2.77  cm ) and inferiorly ( 3.0   3.12  cm ) and the inf stent moving to the right ( 1.92   2.02  cm ) inferiorly ( 3.233.34  cm ) and posteriorly ( 1.41  1.43  cm ) .
patient b showed sustained stent position changes with the inf stent moving superiorly ( 2.23   0.49  cm ) and posteriorly ( 1.72  0.59  cm ) .
patient f showed sustained stent position changes with both the sup and inf stent moving inferiorly ( 0.98  0.35  cm and 1.21   0.38  cm , respectively ) .
the sustained position change was apparent from the start of treatment for patients b and f but did not become apparent in patient a until after fraction 5 ( figure  2 ) .
the free - breathing superior - inferior stent motion determined from planning for patients a , b , and f were 1.15  cm , 1.87  cm , and 0.53  cm , respectively ( table  1 ) . although the sup and inf stent for patient a and f moved similarly during treatment , only the inf stent for patient b showed sustained position changes ( figures  2 and 3 ) . the population mean position change ( x , y , z ) ( ie , the mean of all patients ' individual means of position change ) for the sup stent was x=  0.44  1.09  cm , y = 0.45   1.26  cm , and z  =  0.09  0.26  cm .
the population mean position change for the inf stent for all patients was x=  0.33   0.84  cm , y=  0.43  1.65  cm , and z = 0.31  0.95  cm .
the standard deviation of position change (  ) exhibited by the inf stent was greater than that exhibited by the sup stent ( table  2 ) .
for the y component of the stent - l1 distance , six patients ( b , c , d , e.1 , e.2 , and f ) had inf stent > sup stent .
for the z component of the stent - l1 distance , five patients ( a , b , c , e.1 , and f ) had inf stent > sup stent .
this study shows that the position of endobiliary stents can vary greatly over the course of ebrt for lapc in six patients . utilizing endobiliary stents as imaging surrogates for pancreatic tumours
should be done cautiously as these stents may lack positional stability during ebrt . in this study , three patients exhibited sustained stent position changes from their planning ct .
we saw a greater rate of stent migration than previously published by johanson et  al , which may be evidence of distal stent migration only .
the migration rate differential may be related to study population selection as johanson 's sample consisted of a generalized patient population that received endobiliary stents , whereas this study had pancreatic cancer patients that received stents prior to chemoradiotherapy .
it is unknown how the presence of malignancy , chemotherapy , or ebrt affected stent migration incidence in this study , but the development of radiotherapy - induced pancreatic fibrosis may play a role . while malignancy and chemotherapy may have influenced stent migration , lofts et  al   found that cancer patients on a chemo - regime of epirubicin , cisplatin , and 5-fluorouracil did not have a significant increase in stent blockage or shortening of stent patency duration .
significant interfraction deformation of the pancreatic head , duodenum , and stomach can occur during pancreatic head irradiation , and these interfraction anatomic changes may affect stent migration . stent migration could be attributed to pancreatic irradiation ; however , other studies have found that endobiliary stents were stable during radiotherapy [ 13 , 19 ] .
zhu et  al   found no incidence of stent migration when they treated biliary malignancy with intraluminal radiotherapy , but this may be due to their use of a stent - in - stent configuration .
engineer et  al   concluded that endobiliary stents were stable tumour surrogates during ebrt after they found population mean position changes of less than 0.4  cm in all directions during ebrt .
we found similar population mean position changes when patients were analysed together ( < 0.5  cm ) , but when patients were examined individually , three patients exhibited individual mean interfraction motions of greater than 1  cm in at least one direction , indicating stent migration occurrence .
this contradicts the findings of whitfield et  al   who reported that their stent patients had individual mean interfraction stent motions of less than 1  cm for all directions during ebrt .
previous studies that examined cbct - determined interfraction stent motion during ebrt had small samples ( three   and five patients ) . perhaps with larger study sizes , more cases of stent migration would have also presented themselves in these studies .
the more inferior stent position during treatment cbcts may be due to the planning cts being captured during exhale breath - hold and the daily cbcts being captured during free - breathing ; the stents appeared inferior on cbcts because inhalation caused inferior pressure on the diaphragm and abdominal contents . however , not all patients showed more inferiorly positioned stents .
the influence of variable inhale and exhale lung volumes on stent position as seen on free - breathing cbcts is unknown .
the majority of the free - breathing cycle is spent in exhale breath position , which suggests that stent appearance on free - breathing cbct roughly correlates with its appearance on exhale breath - hold ct . the inferior stent position change exhibited by patients a and f was larger than the sup - to - inf breathing motion established during planning .
inhalation may not be the only contributing factor to an inferior position change ; distal stent migration may have occurred .
this can not be stated conclusively because 4dct - determined pancreatic tumour motion may not have good correlation with the range of motion demonstrated during radiotherapy [ 21 , 22 ] .
other nonrespiratory physiological processes may impact on stent position . to distinguish between interfraction motion and intrafraction motion , more detailed treatment imaging such as 4-dimensional cbct could be used to determine stent positions on respiratory - correlated cbct projections .
exhale breath - hold cbcts could also be used to minimize the respiratory component of intrafraction motion and facilitate correspondence with the exhale breath - hold planning ct .
ideally , stent position would be compared to the pancreas to monitor migration occurrence , but limited soft tissue definition on cbct necessitates the comparison of surrogate motion with a bony match as in previous studies [ 7 , 13 ] .
more accurate soft tissue imaging and further study would be required to determine whether change in stent position is representative of position change of the pancreatic head .
most patients had greater standard deviation of position change exhibited by the inf stent than the sup stent .
greater freedom of movement was associated with the inf stent in the superior - inferior and anterior - posterior planes than the sup stent .
sup stent motion was likely restricted by the concentration , size , and rigidity of internal organs in the  upper abdomen .
the magnitude and direction of position change did not correspond for the sup stent and inf stent .
the cause of this is unknown but may be a result of bowel filling changes from planning to treatment or weight loss during treatment .
the incidence of stent deformation reinforces the method of selecting two position points on the stent .
previous studies selected a single stent position point such as a single end of stent or centre of mass [ 11 , 13 ] to assess interfraction stent motion and did not report on stent deformation .
as two points at opposing ends of the stent were used to determine position change for this study , any rotational error was captured as oppositional movement of the two stent ends .
endobiliary stents are flexible structures , making the assessment of rotation alone difficult . by using two points of measurement , both potential rotation and potential deformation
could be accounted for . because patients were referred from a number of centres , the patients had a variety of both metal and plastic stents in place .
although the variation in stent type did not affect visualization of the stent on cbct nor did it affect the results , a greater number of patients with both metal and plastic stents are required to determine this statistically .
a limitation of this study was the small sample size , which did not allow for more robust statistical analysis .
there was considerable diversity between the patients ' results as shown by the large standard deviation of all patients ' means . without more statistical tests ,
it is difficult to say whether this diversity would also be found in a larger group of patients .
additional study patients would allow for the identification of possible trends with regards to stent migration as well as enable better extrapolation to the wider irradiated lapc population
. other limitations of this study were that the method and timing of stent insertion were not controlled nor was the stent type .
with patients being referred for radiotherapy after stent insertion , controlling the previously mentioned variables will require forethought and coordination .
careful , consistent stent selection and  insertion could reduce or eliminate stent migration . shortening the time frame from stent insertion to the start  and completion of radiotherapy
may also reduce  stent  migration during treatment . for those receiving  short - course radiotherapy for pancreatic cancer [ 2426 ] , the position of the endobiliary stent may be stable  during igrt because of the shorter time span of treatment . for those patients who may undergo long - course radiotherapy with a pre - existing stent , it is worth investigating whether these patients may require an additional procedure to  exchange for a more stable endobiliary stent or to insert  additional fiducials to facilitate image guidance during ebrt .
through examination of daily treatment images , large sustained endobiliary stent position changes were observed in a small subset of patients during ebrt for lapc , indicating the occurrence of stent migration and deformation .
further study is needed to determine whether change in stent position is representative of position change of the pancreatic head and tumour , and whether stents are appropriate for use as tumour surrogates .
 purposeendobiliary stents can be used as surrogates for pancreatic localization when using cone - beam computed tomography ( cbct ) during external - beam radiotherapy ( ebrt ) . 
 this work reports on interfraction stent position changes during ebrt for locally advanced pancreatic cancer ( lapc).materials and methodssix patients with endobiliary stents who underwent ebrt for lapc were assessed . 
 measurements from the most superior aspect of the stent ( sup stent ) and the most inferior aspect of the stent ( inf stent ) to the most inferior , posterior aspect of the l1 vertebra central spinous process were determined from daily treatment cbcts and compared with those determined from  the planning computed tomography ( ct ) scan . 
 changes in stent - l1 measurements were interpreted as changes in relative stent position.resultsthree patients showed mean interfraction stent position changes of 1  cm when treatment measurements were compared  with planning measurements . 
 the sup stent for patient a moved to the right ( 2.66  2.77  cm ) and inferiorly ( 3.0  3.12  cm ) , and the  inf stent moved to the right ( 1.92  2.02  cm ) inferiorly  ( 3.23    3.34  cm ) and posteriorly ( 1.41  1.43  cm ) . the  inf stent for patient b 
 moved superiorly ( 2.23  0.49  cm ) and posteriorly ( 1.72    0.59  cm ) . 
 the sup and inf stent for patient f moved inferiorly ( 0.98  0.35  cm and 1.21  0.38  cm , respectively ) . 
 the remaining  three patients c , d , and e showed interfraction position changes of < 1  cm.conclusionendobiliary stent migration and deformation were observed in a small subset of patients . 
 further investigation is required before confirming their use as surrogates for lapc target localization during image - guided ebrt . 
But
Matriel et mthodologie
Rsultats
Conclusion
Introduction
Patients and Methods
Patients
Radiotherapy Planning
Daily Image Guidance
Definition of Stent Characteristics
Results
Changes in Stent Position
Discussion
Conclusion