Commissioning and quality assurance for Mlc-Based IMRT

PII: S0958-3947(01)00066-8

COMMISSIONING AND QUALITY ASSURANCE FOR MLC-BASED IMRT

CHENG B. SAW, PH.D., KOMANDURI M. AYYANGAR, PH.D., WEINING ZHEN, M.D.,ROBERT B. THOMPSON, M.D., and CHARLES A. ENKE, M.D.

Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE

( Accepted8 March 2001)

Abstract—The commissioning and quality assurance (QA) associated with the implementation of linear accel-erator multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) at the University ofNebraska Medical Center are described. Our MLC-based IMRT is implemented using the PRIMUS linearaccelerator interface through the IMPAC record and verification system to the CORVUS treatment planningsystem. The “step-and-shoot” technique is used for this MLC-based IMRT.

Commissioning process requires the verification of predefined parameters available on the CORVUS andthe collection of some machine data. The machine data required are output factor in air and output factor inphantom, and percent depth dose for a number of field sizes. In addition, inplane and crossplane dose profilesof 4 3 4 cm and 203 20 cm field sizes and diagonal dose profiles of a large field size have to be measured.Validation of connectivity and dose model includes the use of uniform intensity bar strips, triangular-shapednonuniform intensity bar strip, and N-shaped target.

QA procedure follows the recommendation of the AAPM Task Group No. 40 report. In addition, the leafposition accuracy and reproducibility of the MLC should be checked at regular intervals. The dose validation isimplemented through the hybrid plan where the patient beam parameters are applied to a flat phantom.Independent dose calculation method is used to confirm the dose delivery plan and data input to the CORVUS.© 2001 American Association of Medical Dosimetrists.

Key Words: Intensity, modulation, IMRT, Commissioning, Quality assurance, MLC, Photon beam

INTRODUCTION

There has been significant investigational effort directedat altering the course of radiotherapy to intensity-modu-lated radiation therapy (IMRT).1–8 The interest to pursueIMRT has been our inability to deliver higher radiationdose to the target volume due to the unavoidable irradi-ation of organs at risk. IMRT uses nonuniform intensitywithin a field, which is different from conventional treat-ment technique. As such, this dose delivery techniqueallows the use of lower beam intensity and hence irradi-ation of organs at risk at a lower dose in order to deliverthe desired doses to the target volume. Consequently,IMRT leads to delivering reduced dose to organs at risk;hence, improving morbidity and/or providing the oppor-tunity to give additional doses to the target volume. Earlyclinical outcomes of IMRT have been promising andsupport the continued development of IMRT.9,10

Currently, there are 3 delivery methods for perform-ing IMRT. The simplest method is the use of intensitymodulator.11 Intensity modulator is similar to compen-sator except the former is used to modulate the radiationbeam instead of compensating for missing tissue. It re-quires fabrication and manual insertion of the modulatorinto the tray mount. This technique is considered labo-rious and complex, especially in those cases where mul-

tiple gantry angles or portals are used in a treatment. Thenext method uses a binary modulator, which is an add-oncomponent to a linear accelerator, to modulate the radi-ation beam.12–14 Although the binary modulator is com-puterized and automated compared to the intensity mod-ulator, the add-on makes the transfer of patients from onelinear accelerator to another inconvenient. The third meth-od uses conventional multileaf collimator (MLC), whichis a part of a linear accelerator, to modulate the radiationbeam.15–17This method allows automated beam shapingand also gantry movement, and therefore should reducetreatment time. Furthermore, the transfer of patients fromone linear accelerator to another linear accelerator is alsoconvenient because there is no add-on components.

The aim of this paper is to present our experience inthe implementation of MLC-based IMRT, emphasizingcommissioning and quality assurance (QA). At this time,there is no protocol established by AAPM on the com-missioning and QA of IMRT.18 As such, what should beconsidered as appropriate commissioning and QA testsand how these tests should be carried out are left to thediscretion of the responsible radiation oncology physi-cist. Published QA data on MLC-based IMRT has alsobeen limited.19

EQUIPMENT

MLC-based IMRT relies on an automated dose de-livery system where the complex movement of the MLC

Reprint requests to: Cheng B. Saw, Ph.D., Department of Radi-ation Oncology/UNMC, 987521 Nebraska Medical Center, Omaha, NE68198-7521. E-mail: [email protected]

Medical Dosimetry, Vol. 26, No. 2, pp. 125–133, 2001Copyright © 2001 American Association of Medical Dosimetrists

Printed in the USA. All rights reserved0958-3947/01/$–see front matter

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leaves is controlled by the computer of the linear accel-erator. The leaf movements are complex because theyhave to overcome the limit imposed by the design of theMLC to create the desired nonuniform field.20 The staticand dynamic leaf sequencing techniques are currentlyavailable. For the static leaf sequencing also referred toas “step-and-shoot” technique, a nonuniform field is de-composed into a number of uniform intensity subfields orsegments of different beam shapes. When the dose isdelivered through these segments at a gantry angle, theresulting dose distribution is equivalent to the plannednonuniform field. On the other hand, the dynamic leafsequencing also referred to as “sliding window” tech-nique uses varying speed and dose rate to generate thenonuniform field. The leaf configurations used to createeach segment have to be downloaded from the treatmentplanning system. Because current linear accelerators donot allow a direct downloading, the transfer of the seg-mentation data has to pass through an interface.

At University of Nebraska Medical Center (UNMC),our MLC-based IMRT is implemented on our PRIMUSlinear accelerator (Siemens, Concord, CA) interfacethrough the IMPAC record and verification system(IMPAC, Mountain View, CA) to the CORVUS treat-ment planning system (NOMOS, Sewickley, PA) vianetwork. Patient data derived from CT, MRI, and PETlocated at various sites around the hospital can be down-loaded to the CORVUS via network for planning pur-pose. When the various equipment is connected, theMLC-based IMRT functions as a single turnkey systemfrom the operator standpoint. As such, the CORVUSgenerates the intensity map and thereafter it is down-loaded to the PRIMUS ready for patient treatment.

Linear acceleratorThe primus linear accelerator at UNMC shown in

Fig. 1 is a dual-energy machine capable of producing 6-

and 23-MV photon beams, as well as 5 electron beamenergies. The MLC replaces the lower moveable jaws inthis machine. The MLC consists of 27 pairs of leaveswith 1.0-cm width at isocenter and 2 pairs of outer leaveswith 6.5-cm width, as depicted in Fig. 2. The end con-figurations are double focus and straight end. Each leafcan travel 10 cm across the beam central axis.The MLC-based IMRT program implemented at ourinstitution uses the “step-and-shoot” technique. The Pri-mus is capable of delivering each segment automaticallythrough an auto sequencer available on the IMPACrecord verification system.

IMPAC record and verification systemThe IMPAC record and verification system is a

database system that has access to the Primus through adirect keyboard interface. In other words, the Primusviews every instruction from IMPAC as if it is typed inby an operator from the keyboard. The system is auto-mated when IMPAC responds to requests and submitsinstructions to the linear accelerator without assistancefrom the operator. As such, the delivery of a number ofuniform intensity static fields at a particular gantry angle,followed by movements of the gantry to a differentangle, can be done automatically. The automation pro-cess can only be carried out using the auto sequencer,provided the segmentation data are located in theIMPAC database.

Unfortunately, the IMPAC system at our site is notcapable of receiving the segmentation data directly fromthe CORVUS. As such, this link requires the assistanceof an operator. After the segmentation is created inCORVUS, the segmentation must be written into a file inASCII format unique to the IMPAC system, referred toas the RTP file. This file is pulled into the IMPAC via thenetwork using the TCP/IP protocol (FTP format). Afterthe file is available on the IMPAC server, the segmen-

Fig. 1. Primus linear accelerator setup for IMRT includingmodifying the treatment couch to accommodate the head &neck baseplate, attachment of the CRANE for indexing treat-ment couch movement, and portal imager for image capture.

Fig. 2. The MLC leaf configuration of the Primus linearaccelerator.

Medical Dosimetry Volume 26, Number 2, 2001126

tation must be reviewed and manually pushed in theIMPAC database. Because of this procedure, some ex-perience of networking is required for the radiation on-cology physicist to install connectivity between thePRIMUS and CORVUS.

CORVUS planning systemCORVUS is a treatment planning system designed

to perform IMRT. It is designed as an integrated systemwith either MIMiC or MLC as its beam-modulatingdevice. Using patients’ data based on imaging modalitiessuch as CT or MRI scans, the planning system creates aplan for a treatment delivery system and generates inten-sity maps to a compatible modulating system that wouldfacilitate the delivery of IMRT.

The planning system uses an inverse planning algo-rithm instead of a forward planning algorithm. In inverseplanning algorithm, the desired goals to the target andorgan at risks are specified. Based on the desired dosespecifications, the algorithm performs trial and errorprocedure using an objective function to optimize the

plan. As such, the CORVUS treatment plan is thereforea computer-intensive process compared to the forwardplanning algorithm. The optimization procedure is basedon simulated annealing criteria.

The patient treatment planning sequence is sepa-rated into modular form in the CORVUS system. Theseare patient information, image registration, image fusion,anatomy, prescription, and display results. After CT im-ages are read into the planning system, patient informa-tion can be changed in the patient information module.After the information has been verified, the images areprepared for planning in the image registration module.Here, the immobilization system, anatomical set, win-dow level, fiducial markers, and patient orientation aredefined. In the case where coregistration is required, it is

Fig. 3. The CORVUS prescription module for MLC-basedIMRT.

Fig. 9. Hybrid plan where the beam parameters are applied toa flat phantom for ion chamber measurements. Isodose lines are10%, 30%, 50%, 70%, and 90% of the maximum dose value.

The red area represents the position of the ion chamber.

Fig. 12. A plan with 3 gantry angles vs. another plan with 5 gantry angles showing different dose distribution for thesame target and specified goals. Isodose lines are 10%, 30%, 50%, 70%, and 90% of the maximum dose value.

Commissioning and QA for MLC-based IMRT● C.B. SAW et al. 127

done in the image fusion module. After defining theimages, the targets and organs at risk are outlined in theanatomy module. Once the targets and organs at risk aredefined, the desired goals of doses to the target andorgans at risk are specified in the prescription module. Inaddition, uncertainties associated with localization andimmobilization are defined. The treatment machine mustbe selected and the beam orientation must be defined,as illustrated in Fig. 3. In addition, the collimator anglecan be automatically selected based on minimizing thebeam’s-eye view area. After defining these parameters,the data are submitted to the dose engine for calculations.After completing the calculations, the dose distribution isdisplayed in the display results module. Here, the dosedistribution is reviewed and approved, and thereafter, theintensity map is stored on disk and a report is printed ashard copy.

COMMISSIONING

Description on the commissioning procedure of theCORVUS for MLC-based-IMRT in the section of thevendor’s Beam Utility Manual is limited.21 This isbecause the CORVUS planning system evolved fromthe Peacock plan, which was designed to support theMIMiC-based IMRT. Support from the vendor is there-fore crucial for those radiation oncology physicists whoare not familiar with the system.

Commissioning of the CORVUS consists of creat-ing a treatment unit data file, determining the calibrationfactor, and verifying the dose distribution using the plan-ning system. The treatment unit data are created using aset of measured data from the linear and entered usingthe CORVUS beam utilities software. As described in aprevious publication by Sawet al.,14 the machine dataare entered into CORVUS in modular forms. There are 7modules or sections, as outlined in Table 1. The detailsof each module have been presented in the previousspecial issue dedicated to MIMiC-based IMRT.14 To pro-ceed with MLC-based IMRT commissioning, the MLCoption in the “Treatment Unit Information” module mustbe checked instead of the MIMiC option. Althoughthe data entry modules are the same, there are variousareas where the data entered are different compared toMIMiC-based commissioning, as identified below.

Configuration moduleThere is a list of predefined machine data in the

configuration module. One of the available machine datasets is the Siemens with MLC option. Once this option isselected, the preset values are shown for assessment, asillustrated in Fig. 4. The task of the radiation oncologyphysicist here is to verify that the preset values areconsistent with specified and measured data. Measure-ments that must be made are the leaf transmission, jawtransmission, and the radiation field offset from leafposition. The latter is intended to account for the round-ed-edge configuration of the MLC leaf construction.

Dosimetry moduleData measurements for the MLC-based IMRT are

essentially the same as for MIMiC-based IMRT. Themeasured data include the output factors in air and inphantom for a number of field sizes. In addition, thepercent depth doses for these field sizes are required. TheCORVUS accepts data measured from a number of scan-ning systems and, in our case, the Wellhofer water scan-ning system (IBA, Bartlett, TN). Diagonal profiles attwice the depth of maximum dose for a large field sizeare also required. The difference between the data for theMIMiC-based and MLC-based data is the inplane andcrossplane profiles. The field sizes of 43 4 cm and 20320 cm for the inplane and crossplane profiles at twice thedepth of maximum dose are used for MLC-based IMRTdata commissioning.

Table 1. Beam Utility Modules in CORVUS

Modules Parameters

1. Treatment unitinformation

a) Treatment unit nameb) Institution namec) Linac serial no.d) Local linac name

2. Angle system a) Gantry angleb) Couch anglec) Collimator angle

3. Configuration a) Predefined Treatment unitb) Beam energyc) Nominal Dmax

d) Calibration field sizee) Max MU settingf) SADg) Dose rate used in IMFASTh) Min source to tissue distancei) # crossplane pencil beamsj) Leaf transmissionk) Jaw transmissionl) IMFAST maximum dose tolerancem) IMFAST RMS dose toleranceo) Leaf synchronization methodp) Static leaf segmentation algorithmq) Radiation field offset from leaf posr) Impact tolerance tables) Default leaf transmission sett) Default patient positioneru) Commissioning setup

4. Gantry angle range a) Couch start and end anglesb) Gantry start and end angles

5. Dosimetry Output factors and %DDa) Relative output factor in airb) Relative output factor in phantomc) Percent depth dosed) SSDe) Output calibration parametersPenumbra and diagonal profilesa) Inplane & crossplane (43 20; 4 3 20)b) Inplane & crossplane (203 20; 203 20)c) Diagonal 303 30

6. Results a) %DDb) Plamda fitc) Profilesd) Penumbra fit

7. Verification plans a) FSPB intensity setb) Verification templatec) Calculation queue


IMRT equipment connectivity verificationAn essential aspect of MLC-based IMRT is the

automated process from planning to dose delivery. Theprocess requires the transfer of CT images to the COR-VUS planning and thereafter the intensity maps to thelinear accelerator. In general, the connection from the CTto CORVUS is done through the network. Likewise, theintensity map is also transferred to the Primus via net-work through IMPAC. Lastly, each segment must bepushed into the database of the IMPAC before it can beused by the Primus to deliver the dosage.

The connectivity of the MLC-based IMRT is vali-dated using bar strip of uniform intensity, as shown inFig. 5. The bar strip of uniform intensity is created usingthe CORVUS beam utility software. The bar strips aredelivered to a film positioned in a flat phantom at depthof maximum dose to complete testing the connectivityaspects of MLC-based IMRT. In addition to connectiv-ity, Fig. 5 also reveals the movement and positional errorof the leaves. If the intensity bar strips do not have astraight edge, the leaf positions have to be recalibrated asillustrated in the figure.

Intensity level verificationThe essential characteristic of MLC-based IMRT is

the ability to deliver nonuniform intensity within a field.The delivery of different intensity levels has to be vali-dated. To validate this feature, triangular bar strips ofdifferent intensities, as depicted in Fig. 6, were used.Again, the bar strip pattern in triangular form was createdusing the CORVUS beam utility software. The beamutility software created the intensity map, for delivery toa film positioned in a flat phantom at the depth ofmaximum dose. The different gray scales represent thedifferent intensity levels. The triangular design is used toverify the collimator rotation. At UNMC, the collimatorangle definition in CORVUS has been changed to cor-respond to the angle defined on our Primus linear accel-erator.22

Dose verificationThe principal concern of any new modality has been

the accurate delivery of the prescribed dose to the pa-tient. Two forms of dose validation are required. The firstdose verification is the relative dose distribution, while

Fig. 4. Treatment unit configuration module on CORVUS for commissioning.


the second is the absolute dose measurement. Becausethe principal advantage of IMRT is the delivery of highconformal isodose dose around a concave target, we havechosen an N-shape target to validate the dose distribu-tion. The selection of the appropriate shape target isnecessary to highlight the features of IMRT, namely thecreation of segmentation. A “C” shape target has beencommonly used.

The relative dose distribution is planned using 3gantry angles of 90°, 180°, and 326° delivering 1.8 Gy tothe N-shaped target. The plan results in 14 segments at90° gantry angle, 12 segments at 180°, and 11 segmentsat 326°. The plan was implemented to deliver the dosageto the NOMOS verification cassette with EC film sand-wiched between slabs of high-impact polystyrene. Afterirradiation, the optical density was converted to dose andits isodose distribution was compared to the planneddoses, as depicted in Fig. 7.

For point dose measurement, flat phantom of thesize 303 30 cm3 22 cm that can accommodate an ionchamber was used. The planning procedure involvesdefining 2 targets, one surrounding the other. The innertarget was defined around the shape of the ion chamber;the outer target was a concentric ring, as illustrated inFigs. 8 and 9. The purpose of such target definitions wasto allow CORVUS to determine the mean dose to thechamber while the outside target was used to aid inimproving the uniformity of dose to the inner target. Inthis particular validation, a treatment technique using 5fields separated by 72° was used. The difference betweenthe measured dose and planned dose is within 1%.

Use of hybrid planThe unique feature of CORVUS has been its ability

to provide a hybrid plan.22 A hybrid plan represents aplan in which the beam parameters, such as the beamorientation and beam intensity that have been generatedfor a patient, can be applied to a phantom. As such, theeffect of patient contour has to be recalculated and thedose distribution generated. Although the resulting dosedistribution is not exactly like a patient dose distribution,it gives confidence that a known dose is delivered to thepatient if the measured dose in a phantom agrees with theplanned dose. An example of the hybrid plan is shown inFig. 9. A hybrid plan allows an indirect measurement ofdoses to patients. As such, a hybrid plan confirms thedose calculation as well as dose delivery.

Immobilization/fixation supportBecause of the implementation of nonuniform dose

field in IMRT, accurate and highly reproducible patientsetup that is more stringent than conventional techniqueis required. During commissioning, the available patientfixation devices must be reviewed and improved if nec-essary. Fixation devices for IMRT have been reviewedby Sawet al.23 in the first special IMRTMedical Dosi-metryissue. Modification of the treatment couch may benecessary to improve reproducibility accuracy. At UNMC,our treatment couch has been modified, as shown in Fig.10, to accommodate the Head & Neck base-plate. In ad-dition, the width of the treatment couch is reduced to pro-

Fig. 5. Uniform intensity bar strips created using CORVUSBeam Utility software and delivered using MLC.

Fig. 6. Triangular bar strips of different intensities from 100%to 10% in 10% decrement for testing intensity level and colli-

mator angle definition.


vide a larger range of gantry angle for our MIMiC-basedIMRT.

Lasers are also used to align patients undergoingIMRT. Because of the precision required, remounting ofthe laser may be necessary. The criteria of 2-mm toler-ance, as specified in AAPM Task Group No. 40, may notbe sufficient for IMRT.18

QUALITY ASSURANCE

The QA procedures for MLC-based IMRT followthe recommendations of AAPM Task Group No. 40because there are no add-on components. However, thetolerance for the leaf positioning and leaf speed and doserate should be reviewed. The leaf positioning is criticalfor the “step-and-shoot” technique, while the leaf speedis critical for the “sliding window” technique. If eachsubfield is not properly shaped, it can change the dosedelivered in the step-and-shoot technique. Checkingthese parameters at regular intervals is necessary becausethese issues can be easily corrected.

A major effort in IMRT has been directed at patient-related QA. This includes a dry run of a plan to avoidequipment collision and the avoidance of beam directedthrough a treatment bar or side rails and phantom mea-surements for dose verification. Patient localization is oflesser importance because the localization technique is

well established. At UNMC, dose validation for eachpatient has been performed using the hybrid plan createdthrough the application of patient beam parameters onto

Fig. 7. Plan vs. measurement using film dosimetry of relative dose distribution. Isodose lines are 10%, 30%, 50%, 70%,and 90% of the maximum dose value.

Fig. 8. Ion chamber measurement using flat phantom. Iso-dose lines are 10%, 30%, 50%, 70%, and 90% of the maximum

dose value.


a flat phantom. This procedure is applied to all patients,although it seems to be a laborious process. No adjust-ment of MUs is made if the dose measurement is within5% of the dose predicted by the hybrid plan. Thus far, wehave not made any adjustment or repeated the plan basedon this criterion. We are in the middle of changing thedose validation based on measurement to the use ofan independent dose calculation method described byAyyangar et al.24 The independent dose calculationmethod merely confirms the planning methodology butnot the delivery aspect of IMRT. As such, we will con-tinue to perform the hybrid plan until we have confidencethat the dose is delivered correctly.

In addition to MLC and dose verification, alignmentlasers should be checked daily. Lasers are used to alignfiducial markers placed on fabricated masks acting as analignment point. The CRANE used for patient setup hasto be checked. An improperly bolted CRANE to the sideof the treatment couch can cause shearing of threads onthe device that holds the CRANE to the treatment couchrails. The accuracy and functionality of the LCD readouton the CRANE should be checked, and should corre-spond to the distance traveled by the treatment couch.

QA for the CORVUS planning system is limited.The maintenance of the system is limited to purgingtemporary files, the removal of raw patient data fromother modalities pushed to the CORVUS workstationand archiving of patients who had completed treatmentsat regular intervals. Rebooting the workstation computerwill purge the temporary files. Documentation of ap-proval number for each patient is important. A restoredpatient file would not have the same study number orapproval status.

DISCUSSION

The implementation of MLC-based IMRT is theresult of a highly complex automated process of deliv-

Fig. 10. Treatment couch modification to accommodate thehead and neck baseplate.

Fig. 11. Use of the electronic portal imager to measure inten-sity map and compared to planned intensity map.


ering radiotherapy. Because automation involves com-puters that can perform self-checks with a stored toler-ance table, the treatment process would not proceed if thelinear accelerator component’s readout is outside thetolerance value. Regular maintenance is therefore moreimportant, while the role of QA becomes less important.When the tolerance is tight, the movement of the linearaccelerator is more precise. For example, the treatmentwould not initiate unless the gantry angle was within 0.3°of the present value. However, a QA procedure may beused as a routine check to identify failing componentsbefore they actually fail during treatment. This is partic-ularly important in evaluating the integrity of the equip-ment, such as tightening loose screws or correctingdented parts.

While linear accelerator technology and computertechnology will inevitably proceed in the direction ofself-checking and IMRT automation, QA is still a vitalcomponent to ensure that the patient receives qualitycare. At this juncture, the electronic portal imager seemsto be a good tool in assisting the QA procedure, as de-scribed by Partridgeet al.25 The imager can be used tocheck the mechanical performance of the leaf or measureleaf penumbra, speed, and position. For patient-related QA,the imager can be used to take a gantry port for local-ization or intensity maps, and serves as a verification asdepicted in Figure 11. The imager is essentially a re-placement of film and hencereduces the overall treatmenttime. Lastly, the imager may become a tool in performingdosimetric measurements.

An inverse planning system such as the CORVUS isa semi-automatic system requiring an experienced user.Although the system uses an optimization algorithmbased on the specified desired goals to the targets andorgans at risks, 2 degrees of freedom are not included inthe optimization process. These are the gantry angle andtreatment couch angle. The effect of only the choice ofgantry angle is illustrated in Fig. 12. Figure 12a uses 3gantry angles of 90°, 180°, and 326°, while Figure 12buses 5 gantry angles, equally spaced, planned with thesame target and organs at risk goal. It should be obviousthat the dose distributions from the 2 plans are different.The use of 5 fields for this N-shaped target is obviouslyinappropriate, leading to high doses outside of the target.The dosimetric experience of the user is therefore im-portant in creating a plan that is appropriate for theindividual patient.

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Commissioning and quality assurance for Mlc-Based IMRT