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Patient movement

Assessment of secondary patient motion inducedby automated couch movement during on-line 6

dimensional repositioning in prostate cancer treatment

Nadine Linthout*, Dirk Verellen, Koen Tournel, Truus Reynders,Michael Duchateau, Guy Storme

Radiotherapy Department, Oncologisch Centrum, Universitair Ziekenhuis Brussel, Belgium

Abstract

Background and purpose: The purpose of this study is to assess retrospectively secondary patient motion induced by

6D patient setup correction.

Materials and methods: For 104 patients, treated with Novalis, 6D setup correction prior to treatment was performed

by ExacTrac5.0/NovalisBody in combination with the Robotic Tilt ModuleTM mounted underneath the Exact Couch top.

This 6D correction might induce additional setup errors due to patient reaction against the rotations. To evaluate

induced secondary motion, the 6D setup correction is verified and evaluated with respect to the tolerance limits.

Results: The majority of measured secondary motions are found within the tolerance limits. Detected secondary

motions are mostly found in longitudinal shifts and lateral rotations, and mainly found in only 1 dimension during the

same verification.

The verifications indicate that the patient population can be divided into a group that hardly moves and a group that

moves throughout all 6D setup corrections. The patients behavior can be predicted by the evaluation of the first five

fractions as none of the patients demonstrate a learning curve during the treatment.

Conclusions: 6D setup correction does not induce secondary motion for the majority of the patients and can therefore

be applied for all treatment indications.

c 2007 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 83 (2007) 168174.

Keywords: On-line target localization; SBRT; IGRT; 6D correction of setup error; Prostate cancer

With the introduction of innovative treatment techniquessuch as conformal radiation therapy, intensity modulatedradiation therapy, dynamic conformal arc therapy andtomotherapy; it is possible to shape the dose distributionclosely to the defined target volume in a way that neithertarget coverage nor neighboring healthy tissues are sacri-ficed. When these highly conformal dose distributions are

to be delivered to the patient, very precise and accurate pa-tient positioning and target localization is necessary. Thegoal is to have the best possible correlation between the pa-tients position during computed tomography (CT) dataacquisition and the actual patients position on the treat-ment couch in order to have an optimal dosimetric accuracyin the dose delivery as the treatment plan is based on the CTdata set.

Image guidance can help to optimize this correlation.Several publications have shown the benefit of image guidedradiation therapy (IGRT) with respect to patient setup accu-racy[4,9,15,17,19,21]and even the dosimetric benefit thatcomes with the application of IGRT [3,13,16]. Several sys-

tems are available to visualize the actual patients positionprior to treatment and correct for any deviation in 3 dimen-sions (3 translations) with respect to the position in theplanning CT. The volumetric imaging tools such as kilo-volt-age (kV) cone-beam (CB) CT, megavoltage (MV) CBCT, MVCT and kV CT provide a 3 dimensional (3D) image set thatcan easily be fused with the CT data set to identify not only

the translational but also the rotational setup errors of thepatient.

In our clinic, stereoscopic X-ray imaging is available forpatient setup verifications. This system uses two isocentricoblique kV images to perform a 6 degrees of freedom(DOF) registration with digitally reconstructed radiographs(DRR) calculated from the planning CT data set to calculateboth translational and rotational setup errors of the patient.The stereoscopic X-ray imaging device is combined with areal-time infra-red (IR) marker tracking device and a robot-ics system allowing the correction of the setup errors in all 6dimensions (6D). Soete et al have proven that this 6D pa-tient setup correction improves the patient setup accuracy

Radiotherapy and Oncology 83 (2007) 168174www.thegreenjournal.com

0167-8140/$ - see front matter c 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2007.04.015

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[18]. It has been suggested though, that couch correctionsprior to treatment may result in further positioning errors[5,11,20]. A counteract of the patient to the couch move-ments can result in secondary patient motion, especiallywhen large rotations (tilt and roll) are to be corrected.Although the tilt and roll rotations are limited to 3.0 inour department for patient safety reasons, these rotations

create very large movements because the center of rotationis located at the foot end of the treatment couch. For in-stance, a tilt rotation of 2.5results in a vertical movementof the treatment couch of about 4.5 cm at isocenter levelfor a standard prostate cancer patient positioning. With thismovement the head end of the treatment couch, where alsothe head of the patient is positioned in prostate treatments,moves about 7.5 cm vertically. It is imaginable that thesemovements cause some counteraction of the patient.

The first purpose of this study was to assess retrospec-tively the magnitude of this induced secondary patient mo-tion. Additionally, a strategy will be proposed for the use ofthe robotics system, such as using the first 5 treatment frac-tions as a predictor for the patients behavior during the

remaining treatment fractions. This strategy will be appliedfor the prostate treatments and subsequently for other tar-get locations such as thoracic and abdominal treatments.

Materials and methodsPatient characteristics and treatment planning

Between October 2005 and December 2006 104 patientshave been treated for prostate cancer on the Novalis Sys-tem (BrainLAB AG, Feldkirchen, Germany). The patientsages ranged from 48 to 80 year (mean: 66 year). The major-ity of the patients (58%) received a dose of 78 Gy in 2 Gyfractions while 27% of the patients were scheduled for

70 Gy in 2 Gy fractions. A hypofractionation scheme of16 3.5 Gy was applied to 9% of the patients and theremainder of the patient group received only a boost treat-ment of 14 2 Gy with the Novalis System after a conven-tional treatment of 50 Gy to the pelvic region.

Treatment planning for these patients is performed withthe treatment planning system (TPS) BrainSCAN v5.3(BrainLAB AG, Feldkirchen, Germany) on a CT data set thatis acquired with a slice thickness of 2 mm and a slice spacingof 2 mm covering the entire pelvic region. The clinical tar-get volume (CTV), bladder and rectum are defined by thephysician. A CTV to planning target volume (PTV) marginof 6mm leftright (LR), 10 mm anteriorposterior (AP)

and 10 mm superiorinferior (SI) was added.All patients are positioned supine on the treatment couchwith a conventional head support. Arms are positioned atthe thorax and the knees and ankles are supported withthe Combifix (Cablon Medical, Leusden, The Netherlands).

The Novalis SystemThe Novalis System consists of a single energy (6 MV

photons) linear accelerator (linac) with an integrated minimulti-leaf collimator (MLC) [2,22], dedicated for shapedbeam surgery[6]. The linac is part of an integrated system,combined with a TPS and a patient positioning and targetlocalization system.

Patient positioning and target localization for extra-cra-nial treatments is performed by ExacTrac5.0/Novalis Body

(ET/NB) (BrainLAB AG, Feldkirchen, Germany). Verellenet al[21]and Yan et al[23]described the system in detailpreviously. In brief, the patient positioning is based on infrared (IR) reflecting marker detection by ExacTrac (ET)allowing real-time patient set-up monitoring and adjust-

ment. The IR markers are placed on marked spots on the pa-tients skin linked to the treatment isocenter and aredetected by 2 IR cameras mounted to the ceiling of thetreatment room. ET will steer the treatment couch (VarianExact Couch; Varian Medical Systems, Milpitas, CA, USA)based on the location of the markers to match the treat-ment isocenter with the linac isocenter. Novalis Body

(NB) is used for target localization and consists of 2 X-raytubes (MP 801 X-ray generator and comet X-ray tubes; K&SRontgenwerk, Bochum, Germany) placed in holes in thefloor and two Amorphous Silicon (AmSi) detectors (Perkin-Elmer Optoelectronics GmbH, Wiesbaden, Germany)mounted at the treatment room ceiling. NB is mounted ina configuration where the beam axes of both tubes cross

the treatment beam axis isocentrically. This X-ray imagingsystem is fully integrated into the IR tracking system by cal-ibration allowing an on-line computer-assisted control ofthe treatment couch to predefined positions from outsidethe treatment room.

Patient positioning and verificationAfter positioning the patient, the IR markers are

placed on the marked spots on the patients skin previ-ously used during CT data acquisition. When the pre-posi-tioning by ET is finished, two stereoscopic X-ray imagesare acquired and registered with two DRRs using a6DOF-fusion algorithm based on the registration of thebony structures visible in both image pairs, to definethe error between the actual and the expected positionof the patient. The algorithm used for this 6DOF-fusionis based on the gradient correlation between the two im-age sets and optimizes the similarity measure for eachimage pair using bony structures to determine the bestalignment between the two image sets. The used optimi-zation algorithm has a high rate of convergence to makethe 6D registration clinically applicable with respect tocalculation time [10,12].

The output of the 6DOF-fusion algorithm is 6 parameters,3 translations and 3 rotations, that are on-line applied tothe treatment couch. The 3 translations and the vertical

rotation (yaw) are performed by the treatment couch itself.The longitudinal rotation (roll) and the transversal rotation(pitch) are executed by the Robotic Tilt ModuleTM device(BrainLAB AG, Feldkirchen, Germany). The robotics systemis mounted underneath the treatment couch top. The cor-rection of longitudinal and lateral rotations is limited to3.0 degrees for patient safety reasons. The vertical rotationis limited to 10.0 and there is no limitation for the threetranslational corrections.

The accuracy of the 6DOF-fusion algorithm is verifiedwith a Hidden Target Test (HTT) as described by Verellenet al. [21]. The 6D positioning showed sub-millimeteraccuracy for phantom positioning after removal of the

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systematic error. The residual 3D setup deviation vector was0.42 mm with an overall SD of 0.92 mm.

Measurement of induced secondary patient motionThe application of the couch movements, translations as

well as rotations, to correct the actual position of the pa-tient can result in secondary patient motion. To evaluate

this secondary motion, a second pair of X-ray images is ac-quired after the 6D setup correction. These X-ray imagesare again registered with the DRR images using the 6DOF-fu-sion algorithm to give again 6D information of the actual pa-tient position. In the verifications used in this study thesecondary shifts larger than 2.0 mm and secondary rotationslarger than 1.0 are corrected in a second 6D setup correc-tion prior to treatment.

These tolerance limits (2.0 mm and 1.0) are proposed bythe company and are found too stringent for evaluation ofverification measurements. In the literature threshold valuesof 3.0 mm and 2.0are proposed[4,19]. These threshold val-ues are found clinically relevant and therefore the secondarymotions are alsoevaluated against the latter tolerance limits.

Time measurements are performed during the entire set-up procedure to evaluate the workload of the 6D setup cor-rection procedure and the verification procedure including asecond 6D setup correction when necessary.

A possible strategy for the use of the robotics system canbe to use the results of the secondary motion of the first 5treatment fractions as a predictor for the behavior of thepatient during all treatment fractions. Therefore the resultsof the second 6D setup correction of the first 5 fractions areevaluated against the corresponding results of all fractions.

In this evaluation a true negative will be found when thesecondary motion of both the first 5 fractions and theremainder of the fractions does not exceed the tolerance

limits where a true positive indicates both values for thesecondary motion exceeded the tolerance limits. In both sit-uations the behavior of the patient is correctly predicted bythe first 5 fractions. A false negative result is found whenthe first 5 fractions do not show exceeded tolerance limitsin the secondary motion while in the remainder of the frac-tions larger secondary motion is detected. A false positiveon the other hand shows secondary motion violating the lim-it during the first 5 fraction and not during the remainder ofthe treatment fractions.

Results

In 91.7% of the treatment fractions (3184 out of 3472),verifications of the patient position after a 6D patient setupcorrection were performed. In 89.8 % of the fractions (3118out of 3472) the verifications could be used for evaluation.The remainder of the fractions was lost for analysis due tothe fact that the robotics system was not activated during6D patient setup correction and only 3D patient setup cor-rection was performed.

Evaluation per verificationFig. 1a shows the secondary shifts in a spider graph where

the 3 measured shifts of the same verification are plotted onone radial axis and a radial axis is added for all 3118 verifi-

cations consecutively. The consecutive verifications aregrouped per patient.

For the lateral shifts the mean measured value was0.1 mm (SD 1.2 mm; range 7.3 to 8.0 mm). The mean mea-sured longitudinal shift was 0.6 mm (SD 1.6 mm; range16.9 to 9.3 mm) and the mean measured vertical shiftwas 0.3 mm (SD 0.7 mm; range 4.8 to 5.1 mm).

Fig. 1b shows analogously the secondary rotations of allverifications.For the lateral rotations the mean measured value was

0.3 (SD 0.8; range 7.0 to 5.6). The mean measuredlongitudinal rotation was 0.0 (SD 0.4; range 4.1 to2.9) and the mean measured vertical rotation was 0.0(SD 0.4; range 7.7 to 2.5).

For both graphs it is clearly visible that the majority ofthe measured shifts and rotations are within the tolerancerange of 2.0 mm and 1.0that are displayed in the graphsby 2 concentric black circles. The graphs show some groupsof consecutive verifications that are outside the tolerancelimits. This indicates that some patients systematicallyshow one of the 6 dimensions violating the tolerance limits.

Table 1shows the percentages of the verifications thatare within the tolerance limits for both the currently used(2.0 mm/1.0) and the proposed threshold values(3.0 mm/2.0). The tolerance limits are most frequentlyviolated for the longitudinal shift and second most for thetransversal rotation, independent of the used tolerance lim-its. With the currently used tolerance limits only 73.2% ofthe verifications are within the limits for the longitudinalshift and 81.5% for the lateral rotation. With the proposedtolerance values of 3.0 mm and 2.0 these values are93.2% and 94.7%, respectively.

Table 2 displays the amount of verifications with 16dimensions violating the tolerance limits in the same verifi-cation in proportion to the total amount of verifications thatexceeded tolerance limits. The proportions are given forboth the currently used and the proposed tolerance limits.The verifications that exceeded the tolerance limits showmainly only 1 dimension violating the tolerance limit inthe same verification: 76.0% for the currently used toler-ance limits of 2.0 mm and 1.0 and 88.7% for the proposedtolerance limits of 3.0 mm and 2.0.

Evaluation per patientWhen the verifications are evaluated for each patient

individually, it is noticed that some patients show veryfew parameters violating the limits during all verifications

of the entire treatment while other patients have parame-ters that exceeded the limits in nearly all verifications dur-ing their treatment, what is visualized byFig. 1a and b. Thisindicated that the patient population can be divided in pa-tients that move during the setup and patients that do notmove during the setup.

None of the patients where the parameters exceeded thetolerance limit in the majority of the fractions did demon-strate a learning process during the successive fractions ofthe treatment. If the measured shifts and rotations are plot-ted against the fraction numbers, the linear trend line ofthe measurements shows no considerable improvement asa function of the increasing fraction number.

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The measured secondary patient motions are evaluatedper patient by calculating the mean values for each of the6 dimensions per patient. The values are plotted in spiderdiagrams shown in Fig. 2a and b. Fig. 2a shows the mean

shifts where the 3 dimensions are plotted on the same radialaxis for one patient. All patients are added consecutively tothe graph. Fig. 2b shows analogously the mean measuredrotations. The tolerance limits of 2.0 mm and 1.0 arevisualized on the graphs by 2 concentric black circles.

The maximum mean value measured per patient was4.0 mm for the lateral shift (mean 0.0 mm; SD 0.8 mm),3.9 mm for the longitudinal shift (mean 0.6 mm; SD0.9 mm) and 1.8 for the vertical shift (mean 0.3 mm; SD0.4 mm). For the rotations the maximum mean values were3.2 for the lateral rotation (mean 0.4; SD 0.6), 1.0for the longitudinal rotation (mean 0.0; SD 0.2) and0.8 for the vertical rotation (mean 0.0; SD 0.2).

Fig. 2a shows 6 patients with a secondary shift clearlyviolating the limits. The longitudinal shift exceeds the limitsystematically for 4 patients, the lateral shift only for 2 pa-tients.Fig. 2b indicates that the tolerance limits for rota-tions are only exceeded in the transversal direction andfor 10 patients the value is clearly out of the thresholdrange.

This evaluation indicates that 84.6% (88/104) of the pa-tients show no secondary motion after the 6D setup correc-

tion when tolerance limits of 2.0 mm and 1.0 are used.When the threshold values of 3.0 mm and 2.0 are usedfor evaluation this value increases to 98.1% (102/014).

When the graphs ofFigs. 1 and 2are compared with eachother, a correspondence can be found between the positionof the group of verification measurements outside the toler-ance range and the mean value per patient outside the toler-ance range. This originates from the fact that theverifications and the patients are added to the graphs inthe same consecutive sequence. As in each of the graphs ofFig. 1more than 9000 points are included these graphs arenot clear enough for an evaluation per patient. The graphsinFig. 2give therefore clear patient related information.

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Fig. 2. This figure groups (a) the mean secondary shifts (lateral,longitudinal and vertical) and (b) the mean secondary rotations(lateral, longitudinal and vertical) measured for all patients. Thespider graph in Fig. 2a spans a range from 4.0 mm to +4.0 mmwhere 4.0 mm is the center of the graph and +4.0 mm is the outerconcentric circle. The distance between the concentric circlesstands for 1.0 mm. The spider graph in Fig. 2b spans a range from

2.0 degree to +2.0 where 2.0 is the center of the graph and+2.0 corresponds with the outer concentric circle. The distancebetween the concentric circles stands for 1.0. Each of thepatients, including 3 measured dimensions, is plotted on one radialaxis and the patients are added consecutively to the graph. Thetolerance limits are added to both graphs as 2 concentric blackcircles corresponding with 2.0 mm for the shifts and 1.0for therotations.

Fig. 1. This figure groups all (a) secondary shifts (lateral, longitu-dinal and vertical) and (b) secondary rotations (lateral, longitudinaland vertical) measured during all verifications. The spider graph in(a) spans a range from 10.0 mm to +10.0 mm where 10.0 mm isthe center of the graph and +10.0 mm is the outer concentric circle.The distance between the concentric circles stands for 1.0 mm. Thespider graph in (b) spans a range from 5.0to +5.0where 5.0isthe center of the graph and +5.0 corresponds with the outerconcentric circle. The distance between the concentric circlesstands for 1.0. Each of the verifications, including 3 measureddimensions, is plotted on one radial axis. The verifications aregrouped per patient and added consecutively to the graph. Thetolerance limits are added to both graphs as 2 concentric blackcircles corresponding with 2.0 mm for the shifts and 1.0 for therotations.

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Evaluation of the strategyThe evaluation of the secondary motion measurements of

the first five fractions against the measurements of all frac-tions, as a possible strategy for the use of the robotics sys-tem, is also based on the mean values calculated per patientfor all 6 dimensions. This evaluation is only given for theproposed threshold values of 3.0 mm and 2.0. The compar-ison of the mean values showed 96/104 true negatives,4/104 true positives, 1/104 false negative and 3/104 falsepositive results.

Time measurementsThe time that is required to acquire the X-ray image set,

perform the 6DOF-fusion and correct the patient setup in 6dimensions varies between 100 and 150 s with an average of113 s. The verification procedure is performed faster and re-quires between 55 and 100 s with an average of 78 s, includ-ing the second patient setup correction. The totalprocedure of setting up the patient and verify this setuptakes on average 3 min and 11 s.

DiscussionThe introduction of the robotics system in our depart-

ment allows achieving a better correlation between the

position of the patient during CT data acquisition and theactual position of the patient for prostate cancer treatment[18], assuming the patient is to be considered as a rigidbody. The tilt and roll of the robotics system can cause acounteraction of the patient that introduces a secondarypatient motion. In this study the application of stereoscopicX-ray imaging in combination with a 6D setup correction is

evaluated to define the secondary patient motion that canbe induced by the application of the robotics system. Thesecondary motion is studied retrospectively on a group of104 patients treated for prostate cancer.

The currently used tolerance limits for the verificationmeasurements (2.0 mm/1.0) are exceeded in more than 1out of 3 verifications with a majority of verifications show-ing only 1 dimension violating the tolerance limit. The spi-der graphs (Fig. 1a and b) indicate that when the limitsare exceeded the measured values are most frequent closeto the tolerance limits and that the frequency of the largesecondary shifts or rotations is very low. This indicates thatthe large amount of parameters violating the limits is notalarming and that secondary patient motion is hardly in-

duced by the 6D patient setup correction. If the tolerancelimits would be increased to 3.0 mm and 2.0, as proposedby the literature, at least 93% of the measurements arewithin the tolerance limits for all dimensions.

A proposed strategy for the use of the robotics system isto use the secondary motion measurements of the first 5fractions as a predictor for the behavior of the patient dur-ing all treatment fractions. The mean values of the first 5fractions are compared with the mean values of all fractionsper patient to demonstrate the applicability of the strategy.For this evaluation the threshold values of 3.0 mm and 2.0are used as these are found clinically relevant for the veri-fication measurements. The comparison indicates that92.3% of the patients never move considerably due to the6D setup correction what corresponds with the verificationmeasurements inTable 1. In 3.8% of all patients the patientwill move during the 6D setup correction in all treatmentfractions and thus the verification of the first five fractionspredicts the need of a verification of the setup and a second6D setup correction. The one patient with the false negativeresult (0.9% of the patients) where no need for second cor-rection is indicated by the first 5 fractions will not be veri-fied during the remainder of the treatment where in fact itwould have been necessary. The false positive results willlead to unnecessary verifications during all treatment frac-tions of these 3 patients (2.9%), what can be preferred overa false negative result with respect to accurate patient

setup.These results prove that the strategy to use the verifica-

tion results of the first 5 treatment fractions as a predictorof the patient movement during the entire treatment is fea-sible when threshold values of 3.0 mm and 2.0 are used.The latter values are chosen as a tradeoff between work-load related to the verifications and efficacy.

Both sets of tolerance limits of the setup verification aremost frequently exceeded in the longitudinal direction andsecond most in the lateral rotation. The latter is of coursedue to the limited rotation of the robotics system of 3.0 around the lateral axis. Patients with an initial rotational er-ror larger than 3.0 will still have a residual error after the

Table 1The percentage of verifications within the tolerance limits islisted for each of the 6 dimensions for both the currently used(2.0 mm/1.0) as the proposed threshold values (3.0 mm/2.0)

Percentage of verificationswithin tolerance limits

2.0 mm/1.0 3.0 mm/2.0

Lateral shift 89.4 95.9Longitudinal shift 73.2 93.2Vertical shift 95.8 99.2Lateral rotation 81.5 94.7Longitudinal rotation 91.9 99.5Vertical rotation 90.4 99.6

Table 2The amount of verifications with 16 dimensions violating thetolerance limits in the same verification is displayed in propor-tion to the total amount of verifications that exceeded thetolerance limits

# Dimensions violatingthe tolerance limits inthe same verification

# Verifications/total #verifications out of tolerance (%)

2.0 mm/1.0 3.0 mm/2.0

1 866/1139 (76.0) 400/451 (88.7)2 203/1139 (17.8) 41/451 (9.1)3 53/1139 (4.7) 8/451 (1.8)4 15/1139 (1.3) 2/451 (0.4)5 2/1139 (0.2) 0/451 (0.0)6 0/1139 (0.0) 0/451 (0.0)

The proportions are given for both the currently used (2.0 mm/1.0) and the proposed tolerance limits (3.0 mm/2.0).

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robotics setup, which is not related to the counteraction ofthe patient investigated in the study. The secondary motionmeasured in the longitudinal direction is mainly due to thebreathing of the patient. During the breathing the IR track-ing of ET detects a longitudinal movement as the IR markersmove with the lower belly of the patient during breathing.As both X-ray image sets are not necessarily taken at the

same point in the breathing cycle this can introduce a sec-ondary shift measured during the verification of the patientsetup. Therefore the detected longitudinal shifts might beconsidered as not being caused by a secondary motion ofthe patient. To limit these secondary shifts in the longitudi-nal direction attention must be paid to the positioning ofthe IR markers on the patient. Care must be taken to placethe markers on stable places of the pelvic region of the pa-tient, what is not always possible in case of more obesepatients.

These arguments indicate that the majority of the mea-sured secondary motions are not directly linked to a coun-teraction of the patient to the rotational movements ofthe couch top during the 6D setup correction. Results of a

previous study performed in our department can supportthis statement[8]. The study included the setup verificationof patients immobilized for a head and neck treatment,positioned using translational correction only, and showed6D parameters in the same order of magnitude as the 6Dparameters found in this study. The verification measure-ments in the head and neck treatments are taken at theend of the treatment fraction and can be used as verifica-tion of the patient setup as immobilized patients are notsupposed to be able to move considerably during treatment.The overall 3D vector of the resulting shifts has a value of0.5 mm (SD 3.3 mm). For the rotations an overall mean of0.2(SD 1.1) is found. The verifications of the patient setupin this prostate study showed an overall 3D vector of 0.7 mm(SD 2.1 mm) for the measured shifts and 0.3 (SD 1.0) forthe rotations. Considering the fact that the prostate pa-tients are not immobilized, these values are surprisinglyclose to each other. This supports the idea that the patientsare in general not counteracting the rotational correctionsperformed prior to treatment with the robotics system.The detected secondary motions will therefore also occurwhen the 6D correction setup is not applied.

The stereoscopic X-ray imaging system combined withthe 6D setup correction is in several aspects superior toother systems available.

The total procedure of setting up the patient and verifingthis setup with the system described in this study takes on

average 3 min and 11 s. This approach to verify and correctthe patient setup is much faster than the approaches usingCBCT. Letourneau et al. performed the same procedurewith CBCT in a time frame ranging from 23 to 35 min [7].It should be noted that the latter study uses a non-inte-grated system for image acquisition, reconstruction andco-registration of the images and didnt have a remotecouch control available. Thilmann et al. introduced a cor-rection of the patient positioning based on in-line CBCT[19]. This study only evaluated the patient setup with a6DOF registration of the CBCT data with the CT data usingbony structures without performing rotational correctionsand a verification of the patient setup after correction. This

procedure introduced an extra work load of 1012 min. Ourexperience with the Tomotherapy unit (TomoTherapy Inc.,WI, USA) that uses MVCT for patient setup verification showsan extra workload of 710 min for the acquisition and reg-istration of the MVCT-scan and the correction of the patientsetup[1].

The fast approach of stereoscopic X-ray imaging with re-

mote couch control allows using the system for all patientsand performing the verification within an acceptable timeframe without additional workload.

It can be argued that the CBCT has full 3D information ofthe actual position of the patient to be fused with a full 3Ddata set acquired at CT, while stereoscopic imaging onlyuses 2 images. Stereoscopic X-ray imaging gets 3D informa-tion by acquiring oblique X-ray images of the patient thatare registered with oblique DRR images calculated fromthe 3D CT data set. Despite the difference in amount ofinformation used to calculate the patient setup correction,the different approaches have similar accuracies [7,21].

Another advantage of stereoscopic X-ray imaging ap-proach is the dose that is delivered to the patient by the dai-

ly imaging. One X-ray image gives 0.5 mSv extra dose to thepatient. When a verification of the patient setup is includedin the treatment, a total dose of 2.0 mSv is absorbed by thepatient. This is very low compared to the CBCT where dosesof 14.0 mSv (1 mGy 1 mSv for photons) have been re-ported for the acquisition of one CBCT without verificationof the patient setup[19].

As the setup procedure automatically corrects for all 6dimensions simultaneously, it is not possible to neglectone of the dimensions individually. The system will alwaystry to optimize all dimensions even when they are verysmall. Therefore it makes no sense to define an action levelfor the 6D setup correction. In the verification of the setupon the other hand an action level of 2 mm is defined fortranslational deviations and 1 for rotational deviations.

An important advantage of the robotics system in combi-nation with the Exact couch is that there is no limitationfor translational corrections and the vertical rotation canbe corrected up to 10.0. The HexaPODTM Robotics TreatmentCouch (Medical Intelligence, Schwabmunchen, Germany),what was the only commercially available alternative for6D patient setup correction at the time of the study, limitsthe translational corrections to 4.0 cm in the verticaldirection and 3.0 cm in the lateral andlongitudinal direction.All rotations are limited to 3.0for the HexaPODTM.

The application of the automatic setup of ET/NB in com-bination with the robotics system allow the radiation tech-

nologists to put less effort in a correct pre-positioning ofthe patient what leads to an intrinsic time gain during thepatient setup procedure.

The patient setup was always performed based on bonystructures and none of the patients had internal markers im-planted. Inter- and intra-fraction motion of the target vol-ume is therefore not taken into consideration in thisstudy. Intra-fraction motion of the prostate is expected tobe small when the patent is treated always with an emptyrectum. Litzenberg et al. described a typical intra-fractionmotion less than 3.0 mm AP and SI and less than 1.0 mmLR, with exceptions up to 10.0 mm, even without rectal orbladder filling instructions given to the patient [9]. In our

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department no instructions are given to the patient withrespect to rectum filling, only instructions are given withrespect to bladder filling. Taking this into considerationand based on our own experience with implanted markers[14], the CTVPTV margins used in the TPS (6.0 mm LR,10.0 mm AP and SI) are enough to take the inter- andintra-fraction motion of the target volume into account.

ConclusionThe majority of the measured secondary shifts and rota-

tions are within the tolerance range and it can concludedthat the tilt and roll of the 6D patient setup correctionhardly introduces secondary patient motion. If any second-ary motion is detected it is strongly patient dependent.

The strategy is therefore proposed to verify the setupduring the first 5 fractions of the treatment and to use theseresults as a predictor for the remainder of the treatmentfractions. The acquisition of verification images will bestopped when during the first fractions the tolerance limitsof 3.0 mm and 2.0are not exceeded. In the other situation,verifications will be taken during the remainder of the treat-ment fractions prior to treatment and secondary shifts androtations will be corrected accordingly.

Considering the fastness, accuracy and low dose impactof this approach to correct the patient setup, the applica-tion of stereoscopic X-ray imaging in combination with the6D setup correction is extended to all cranial and extra-cra-nial fractionated treatments at the Novalis with the strat-egy defined by this study.

Acknowledgement

This work was supported in part by BrainLAB AG and the Fondsvoor Wetenschappelijk Onderzoek Vlaanderen Project FWOAL 390.

* Corresponding author. Nadine Linthout, Medical Physics,Department of Radiotherapy, Universitair Ziekenhuis Brussel,Oncologisch Centrum, Laarbeeklaan 101, B-1090 Brussels, Bel-gium. E-mail address:nadine.linthout@uzbrussel.be

Received 9 March 2007; accepted 22 April 2007; Available online 17May 2007

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174 Patient motion during couch top rotation

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