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interstitial brachytherapy ( ibt ) is a treatment option for a variety of tumor sites including , e.g. , prostate , head and neck , breast , and gynecology .
implantation of the needles / catheters ( without loss of generality , all different types of needles or catheters will be referred to as catheters in this article , unless a differentiation is required ) is patient specific and requires imaging such as computed tomography ( ct ) , magnetic resonance imaging ( mri ) , transrectal ultrasound ( trus ) , or conventional x - rays to determine the track of each implanted catheter , the so - called implant geometry .
often , this procedure is carried out manually , i.e. , by identifying the trace of each catheter in the image data as part of treatment planning . as in any treatment modality , also ibt underlies the risk of errors and uncertainties .
imaging artefacts , limited resolution of the imaging data , and close proximity / overlap of the ( projected ) implant can deteriorate the reconstruction result with discrepancies of up to 5.5 mm [ 5 , 6 ] or even errors in catheter identification .
a number of reports have been published on brachytherapy errors by international bodies including the international commission on radiological protection ( icrp ) , the international atomic energy agency ( iaea ) , and the gec - estro [ 5 , 7 , 8 ] .
in addition , a number of databases exist that collect specific errors or events . among them
is the collection of the nuclear regulatory commission ( nrc ) in the united states ( us ) . during treatment plan optimization ,
dwell positions and dwell times are determined based on the reconstructed implant fulfilling the clinical criteria for target volumes and organs at risk ( oar ) .
the treatment plan is electronically transferred to the afterloader control unit . before treatment administration , patient specific quality assurance steps such as verification of the source position for a number of the channels
treatment is applied as pulsed - dose - rate ( pdr ) or high - dose - rate ( hdr ) ibt with different fractionation schedules . in current clinical practice ,
a single treatment plan may be optimized and used for each treatment fraction , or the same treatment plan may be used for several fractions delivered based on the same implant .
one recently investigated option to detect errors or quantify the uncertainties in ibt is electromagnetic tracking ( emt ) . since many years , emt is used in various medical applications including em navigated punctures [ 10 , 11 ] , cardiac mapping , or external beam radiation therapy .
based on the generation of a precisely defined magnetic field , such systems quantify position and orientation of small dedicated sensors in the field .
recent reviews are provided by franz et al .   for a broad introduction and zhou et al .   with respect to emt application in brachytherapy .
the purpose of this manuscript is to introduce the concept of treatment verification by emt for ibt . due to the novelty of that application , clinical data are scarce and
deviations from the intended treatment outcome can be the results of errors or uncertainties . according to the institute of medicine in the us
, errors are defined as  the failure of planned action to be completed as intended ( i.e. , error of execution ) or the use of a wrong plan to achieve an aim ( i.e. , error of planning )    while uncertainty  is a parameter that characterizes the dispersion of values that can be obtained for a particular measurement when it is performed repeatedly  . in principle , errors can be prevented , while uncertainties can not be eliminated , although their magnitude may be reduced .
errors are mainly human - caused with typical examples being  failure to identify the patient properly 
, wrong specification of source positions , or specifying dwell positions in the reverse order . also , the nrc lists human errors such as wrong indexer length , catheter reconstruction errors , misidentified first dwell positions , and inverted catheter direction as the most frequent errors reported in 2005 - 2013 [ 18 , 19 ] .
other potential errors include swap of catheters during reconstruction or afterloader connection , wrong intersections during implant reconstruction , or usage of wrong transfer tubes .
among them are dosimetric influences such as source strength or limitations in the dose calculation engine ( e.g. , aapm tg-43 [ 20 , 21 ] based algorithms ignoring heterogeneous tissues and applicator materials ) , but also variations in implant geometry due to moving organs , tissue swelling , or shifts of individual catheters or the whole implant . among the intensively discussed options to prevent errors and to minimize uncertainties are in vivo dosimetry ( ivd ) and imaging , ideally performed in real - time during treatment delivery [ 22 , 23 ] .
table 1 lists the most important errors and variations of brachytherapy along with the potential for ivd .
and also tanderup and kertzscher   extended this table to include the potential of real - time imaging .
the main findings of table 2 in ref .   are reproduced in table 1 .
only the combination of ivd and imaging is able to detect all listed errors since patient identification is not feasible by ivd .
imaging is the method of choice for direct determination of organ motion related changes and variations in the implant position or geometry . as reported below
, the motivation of some of the current investigations of emt application in ibt is the potential of emt to act as a complementary technique to ivd and imaging for the detection of errors and variations in ibt .
errors and variations in brachytherapy along with the potential of detectability by real - time in vivo dosimetry ( ivd ) , real - time imaging ( each according to [ 19 , 22 , 23 ] ) , and real - time electromagnetic tracking ( emt ) .
, i.e. only the classification of detectability by real - time emt and the needed emt coordinate systems is original in this manuscript .
, the intrinsic coordinate system of the emt device ;  f  fiducial markers visible in imaging and emt reproducibly placed during each treatment fraction ;  a  an absolute cross - calibration between emt and imaging as , e.g. , reported by bharat et al . for transrectal ultrasound and emt . a schematic drawing of the coordinate system options is shown in figure 2 .
electromagnetic tracking is implemented in various technical options and by several vendors . generally , the systems consist of a field generator ( fg ) that houses a set of coils producing in an alternating manner at least three precisely defined , inhomogeneous magnetic fields .
the fields span a typical tracking volume of 500 x 500 x 500 mm being the specification of the ndi ( waterloo , canada ) planar fg shown in figure 1 , which is used for most of the studies at universittsklinikum ( uk ) erlangen .
the magnetic field can , e.g. , be measured by inductors that determine the gradient of the field at a specific point , i.e. the magnetic flux .
time - resolved measurements at 40 - 250 hz are common for medical emt   in 5 degrees of freedom ( dof ) using a single 5dof sensor ( three degrees for position and two degrees for rotation ) , 6dof sensors are feasible by combining two 5dof sensors typically requiring a larger housing . clinical use of the electromagnetic tracking system aurora ( ndi , waterloo , canada ) .
the field generator spans the cubic measurement volume , in which the position and orientation of small sensors can be determined . in this example , a 5 degree of freedom ( dof ) implant sensor is used to quantify the geometry of an interstitial brachytherapy breast implant .
three 6dof fiducial sensors attached to the chest generate a reference coordinate system ( method  f  in table 1 ) sensors with 5dof are sufficiently small ( 0.9 - 1.3 mm in diameter ) for insertion into typical catheters used for ibt . as reviewed by zhou et al .
, several groups recently started to explore the potential using emt systems from different vendors for brachytherapy .
the systems reach an accuracy of < 0.9 mm if used in environments free of disturbances such as metals that allow eddy currents disturbing the electromagnetic field .
currently , emt is used in feasibility studies based on phantoms typically with the goal of implant definition [ 24 , 25 , 26 ] or quality assurance [ 27 , 28 ] .
focused on prostate treatments and showed that emt can be combined with trus not only  mechanically  with respect to system setup , but also regarding the measurement of coordinate systems .
electromagnetic tracking - based applicator reconstruction could therefore be easily overlaid with trus imaging data .
feasibility of clinical integration is currently showing at uk erlangen where the geometry of breast implants is determined by emt prior the irradiation in each fraction in a study approved by the institutional review board .
the fg is mounted on a mobile stand with a flexible arm that can quickly be positioned next to the patient in the hdr treatment room ( see figure 1 ) .
the catheters of the implant are sequentially tracked by manual insertion of a 5dof sensor .
catheter implants of more than 30 patients have been already measured at uk erlangen throughout their fractionated hdr - ibt treatment including measurements in the ct room .
electromagnetic tracking - determined positions are related to the position of the typically mobile fg ( coordinate system labeled 
in addition , emt only  sees  sensors but does not image the patient s anatomy .
a full quantitative integration of emt with the coordinate systems used in ibt , thus requires cross - calibration of the emt and the coordinate system of the imaging device used for specification of treatment volumes , oar , and dwell positions .
this task can be adequately achieved by identifying at least three landmarks visible in both , emt and imaging , coordinate systems , e.g. , by a reference emt sensor attached to the stepper of the trus in prostate treatments in combination with probing positions with emt - tracked needles that are visible in the trus data . a seamless transition between the two unified coordinate systems ( labeled  a  in table 1 and figure
2 ) is thus feasible , allowing , e.g. , for quick and accurate implant reconstruction and overlay with segmented organs . such co - registrations are not feasible over several days ( e.g. , for the treatment planning ct and emt during each treatment fraction ) unless multiple and stable anatomical landmarks are available at the treatment site
. an option would be fiducial emt sensors placed on the skin ( see figure 1 ) that allow a correlation between ct imaging and each emt measurement .
the accuracy of that method ( labeled  f  in table 1 and figure 2 ) is substantially influenced by the repositioning accuracy of the fiducial sensors and uncertainties due to skin motion .
electromagnetic tracking can either be used standalone (  , method  e  ) , in combination with fiducials visible in electromagnetic tracking and an alternative imaging system (  , method  f  ) , or fully integrated to an imaging modality by an absolute cross - calibration ( method  a  )
table 1 lists the most important errors and variations as reported by tanderup et al . and kertzscher et al . [ 19 , 23 ] .
in addition to error detection by ivd and imaging as proposed by those authors , the table in the current version also lists the potential of error detection by emt for each quality item .
this judgment is not yet proven clinically , but the following paragraphs will motivate why emt could play the indicated role . obviously , emt can not be used for dose assessment as the method determines position and orientation only . in principle , one could couple an emt sensor with the source cable of an afterloader .
then , a fixed connection would be established between source position and emt - determined position . since emt allows high frequent position determination , dwell times
, a reliable and accurate position reading can be expected from the emt system [ 13 , 30 ] . as an alternative solution
, the emt sensor could be mounted to the check - cable of the afterloader .
as emt can not detect dose , decoupling of the source from the cable could not be detected . as reported already by tanderup et al .
, the need for new qa options for that task is of minor importance . as with the previous item ,
most of the potential malfunctions of the afterloader ( mechanical issues , software failures , but not dosimetric properties ) could be detected by emt . in ibt ,
the catheter implant of the patient is individual regarding the implant geometry and the arrangement of dwell positions and times .
electromagnetic tracking applied just before treatment application , either by manual insertion , as performed at uk erlangen for geometry assessments of breast implants , or by an automatic run of the check - cable with integrated emt sensor of an afterloader , can determine the patient - specific implant geometry including dwell positions and dwell times .
a correct dosimetric outcome of the treatment plan can not be measured , but the emt - determined positions could be the basis for daily dose calculations .
movement of individual catheters can be assessed by comparison to the reference implant geometry from treatment planning .
changes of the whole implant , e.g. , due to swelling or other post - implant reactions , can be determined similarly .
relative motion of the implant site , e.g. , motion of the breast / prostate relative to important oar could be tracked relative to emt fiducials ( method  f  ) .
applicator reconstruction can in principle be performed by emt [ 24 , 25 , 26 ] . without matching the coordinate systems , emt
determined implant geometries could be used as applicator / implant libraries   for manual overlay with the imaging data .
if coordinate systems are matched ( method  a  ) as proposed previously [ 24 , 32 ] , the emt determined geometry can be used directly for automatic applicator / implant placement .
potentially , emt - based reconstruction is less prone to errors than manual reconstruction  further research is required . in case of manual reconstruction
, emt is a viable option for error detection as shown by damato et al . with respect to mixing and interchange / swap of catheter traces .
also for this quality item , emt would spot potential errors prior to treatment delivery .
shifts of individual catheters can be determined by emt - only (  e  ) as a relative shift against the remaining catheters of the implant .
multiple individual shifts or global shifts of the whole implant can also be detected by emt , but most likely require fiducial sensors as a reference (  f  ) . in phantom studies , damato et al .
electromagnetic tracking is a promising option for error detection , assessment of uncertainties , and implant definition in ibt .
currently , only data from phantom - based feasibility studies are available , but these indicate that several of the typical incidences reported by irp , iaea , and data bases such as nrc could be detected by emt
. therefore , clinical assessment of emt is urgently needed as the next step , ideally well integrated to the current workflow .
a comparison to ivd and real - time imaging shows that none of these three techniques can address all of the most important error and variability items .
promising combinations will depend on the clinical application , the likelihood of the errors in order to match the appropriate technique , and the level of certainty desired .
several of the potential errors can be caught by emt during the check - cable run , i.e. before dose delivery .
treatment application should then be monitored by ivd , ideally in combination with emt   or an in - room imaging modality , since ivd requires precise localization of the detector .
in addition , ivd and emt integrated into the workflow can act as a trigger for follow - up imaging examinations , e.g. , in case of organ deformation exceeds to be defined thresholds . challenging for all combinations
is the registration of coordinate systems needed for fusion of emt / ivd data with the ( volumetric ) image set used for treatment planning .
for some clinical sites , fluoroscopy , c - arm based cone - beam ct , or ultrasound may be used as an imaging modality in the treatment room , providing sufficient image quality for establishing a relation between emt / ivd coordinates and anatomy , but not necessarily sufficient for adaptive treatments .
electromagnetic tracking ( emt ) is used in several medical fields to determine the position and orientation of dedicated sensors , e.g. , attached to surgical tools . 
 recently , emt has been introduced to brachytherapy for implant reconstruction and error detection . 
 the manuscript briefly summarizes the main issues of emt and error detection in brachytherapy . 
 the potential and complementarity of emt as treatment verification technology will be discussed in relation to in vivo dosimetry and imaging .