intensity modulation delivery techniques: ‘‘step & shoot ... · objectives of other...

Intensity modulation delivery techniques: ‘‘Step & shoot’’ MLCauto-sequence versus the use of a modulator

Sha X. Chang,a) Timothy J. Cullip, and Katharin M. DeschesneDepartment of Radiation Oncology, University of North Carolina at Chapel Hill, Chapel Hill,North Carolina 27514

~Received 1 June 1999; accepted for publication 1 March 2000!

Two intensity modulation radiotherapy~IMRT! delivery systems, the ‘‘step & shoot’’ multileafcollimator ~MLC! auto-sequence and the use of an intensity modulator, are compared with empha-sis on the dose optimization quality and the treatment irradiation time. The intensity modulation~IM ! was created by a dose gradient optimization algorithm which maximizes the target doseuniformity while maintaining dose to critical structures below a set tolerance defined by the user interms of either a single dose value or a dose volume histogram curve for each critical structure. Twoclinical cases were studied with and without dose optimization: a three-field sinus treatment and asix-field nasopharyngeal treatment. The optimization goal of the latter case included the sparing ofseveral nearby normal structures in addition to the target dose uniformity. In both cases, the targetdose uniformity initially improved quickly as the IM level increased to 5, then started to approachsaturation when the MLC technique was used. In the absence of the both space and intensitydiscreteness intrinsic to the MLC technique, the modulator technique produced greater tumor doseuniformity and normal structure sparing. The latter showed no systematic improvement with in-creasing IM level using the MLC technique. For the sinus tumor treatment of 2 Gy the treatmentirradiation time of the modulator technique is no more than that of the conventional treatment. Forthe MLC technique the irradiation time increased rapidly from 4.4 min to 12.4 min as the IM levelincreased from 2 to 10. Both clinical cases suggested that an IM level of 5 offered a good com-promise between the dose optimization quality and treatment irradiation time. We showed that arealistic photon source model is necessary for dose computation accuracy in the MLC-IM treat-ments. © 2000 American Association of Physicists in Medicine.@S0094-2405~00!03305-8#

Key words: intensity modulation, multileaf collimator, compensator, dose optimization

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I. INTRODUCTION

There are several different techniques available for rouclinical treatment delivery of intensity modulation radiatiotherapy ~IMRT! designed by dose optimizatioalgorithms.1–3 Multileaf collimator ~MLC! techniques utilizea built-in or added-on functionality of modern medicaccelerators.4,5 The MLC techniques deliver an intensitmodulated photon field by either moving the collimatleaves during irradiation or by irradiating a sequence of stMLC ports. The former is often referred to as the ‘‘dnamic’’ MLC technique2,3,6 and the latter as the ‘‘step &shoot’’ MLC auto-sequence technique.7–10 A less commonmethod of IMRT treatment delivery is the use of a compesatorlike intensity modulator. At the University of NortCarolina at Chapel Hill we developed a modulator techniqin early 1993 to deliver IMRT, which has been successfuimplemented in routine clinical application since 1996.

Compensators of different types have been used overyears to improve dose distributions.11,12 Initially, they wereused to merely compensate for the ‘‘missing tissue’’ in tactual patient geometry compared to a rectangular phanAlthough these compensators were developed at a time w3D image-based treatment planning and dose calculawere not yet available, they are still used today with theof skin contour definition tools such as video systems,13 CT

948 Med. Phys. 27 „5…, May 2000 0094-2405Õ2000Õ27„5

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scanners,14 and Moire topography.15 In recent years moresophisticated techniques including the one used in this stwere developed whose objectives were no longer to compsate for ‘‘missing’’ tissue but for the missing dose.16–21 Weuse this technique as an alternative to the MLC techniquedeliver intensity modulated treatments designed by atreatment planning system via a dose optimizatalgorithm.22 To mark the distinct difference in objectives btween the ‘‘missing tissue’’ type compensators and thistensity modulation technique we designate this techniquea modulator IMRT technique. We strongly believe that tobjectives of other compensator techniques should no lonbe ‘‘skin deep,’’ rather, they should adapt to a new role asalternative IMRT delivery tool.

The major advantage of the ‘‘step & shoot’’ MLC delivery technique over the modulator technique is treatmentlivery automation. This advantage can become very imptant when the number of IM treatment fields is large. Tadvantages of the modulator technique over the ‘‘stepshoot’’ MLC technique are much less apparent. A basic dference between the two IM delivery techniques is the relution of the intensity modulation delivered. The modulattechnique generates a continuously varying intensity molation, as it is created by the dose optimization of cliniccases. Characterized by the finite width of the MLC leaf a

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949 Chang, Cullip, and Deschesne: Intensity modulation delivery techniques 949

the fact that the treatment is delivered one segment at a tthe ‘‘step & shoot’’ MLC-IM technique delivers‘‘skyscraper’’-like intensity modulation maps that are dicrete in both intensity level and spatial variation within tIM plane. The actual dose optimization quality deliveredrelated to how close the delivered IM map is to the intendcontinuous IM map. This match can be improved by increing the number of MLC segments used, which is proptional to the intensity resolution as shown in the Resusection of this paper.

In the following sections of the paper we evaluate bothe ‘‘step & shoot’’ MLC technique and the modulator tecnique for IMRT treatments of two clinical cases. The evaation is performed in terms of two clinically relevant factordose optimization quality specified by the dose optimizatgoal used and treatment irradiation time. In addition,present the results of the dependence of the dose optimtion quality and the treatment irradiation time on the numof MLC segments. Some other issues important to the stpresented will also be discussed.

II. METHODS AND MATERIALS

A. Treatment planning and dose optimization

An in-house 3D treatment planning system, PLanUN~PLUNC!, with a dose gradient optimization algorithm22 wasused for the study. The variables in the dose optimizationrestricted to the intensity modulation of the fields and thbeam weightings. The number of fields and the geometreach field including its port must be defined by the user pto dose optimization. There are different treatment optimition goals used in dose optimization algorithms ranging frdose optimization goals such as dose uniformity of a groof points, within a defined 2D plane, or within a volume;dose biological effect goals characterized by tumor conand accepted normal tissue complication probabilities,23,24aswell as hybrids of both dosimetric and dose effeconcerns.25,26 In this study the dose optimization goal wasachieve as uniform a target dose distribution as posswhile maintaining the critical structure doses below atolerance defined by the user in terms of either dose voluhistogram curves or specified maximum dose values for estructure of interest.

The original dose gradient optimization algorithm, usfor the first clinical case, is briefly described below. Penbeams of different intensities are used to assemble the insity modulation of a field. When the optimization goaltarget dose uniformity, the dose gradient~or variation! in thetarget volume~due to the otherwise open field irradiation! isminimized by adjusting the pencil beam intensities to crethe opposite dose gradient thus the net dose gradient is mmized. Only the component of the dose gradient parallethe IM plane of the field can be minimized by the intensmodulation of the field. The dose gradient component ppendicular to the IM plane can be minimized by varyingbeam weighting and/or by modulating the intensity map~s! ofother beam~s!. Along each pencil beam that passes throuthe target volume, the point doses were sampled within

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target volume. The pencil beam-averaged target dose,fined as the sum of the target point doses along the pebeam divided by the total number of the points sampledcalculated. Note that the dose at each sampling point istotal dose contributed from all fields, with and without IMThe same calculation is performed for every pencil beamfor all IM fields, then the relative intensity of each pencbeam is adjusted to attempt to give equal pencil beaaveraged target dose. Because the dose to a point is conuted by almost all pencil beams under the influence of tisdensity heterogeneity~a modified BATHO27 tissue inhomo-geneity correction method was used in the dose computat!a full-scale 3D dose computation is performed after eaiteration. Several iterations are needed for the optimizatprocess to converge. When the optimization goal inclunormal structure dose tolerances the intensity of pebeams passing through the structures is limited by the toance dose set by the user.

Pencil beams used in the calculation had infinitesimwidth. Sixty-four pencil beams were sampled uniformlyeach IM field. The intensities of the pencil beams betwethe sampled pencil beams were derived via linear interption. The iterative process continues until every pencil bein the IM fields has the same pencil beam-averaged tadose unless it is limited by the normal structure dose toance. When this is achieved, the projection~or the compo-nent! of the average dose gradient~over the treatment target!in the IM plane of each IM field is minimized, i.e., the treament target dose gradient itself is minimized, and the tardose uniformity is thus optimized. The optimization procenormally converges within 10 iterations in 30 s of time in oclinical application. The convergence can be convenienmonitored by user on computer display of target volume dferential DVH, whose peak height initially grows then ceato grow with further iterations. PLUNC is running on a nework of Compaq Professional Workstation XP1000~True64 Unix! ~Compaq, Inc., Houston, Texas! with an X11window system using Open GL extensions~Silicon GraphicsComputer Systems, Inc., Mountain View, California!. TheAlpha 21264 processor runs at 500 MHz with 512 MBytesmain memory.

Prior to dose optimization the user is required to setinitial condition by assigning conventional beam weightifor each field and by defining which field~s! will be intensitymodulated. Treatment fields whose intensities are designby the user not to be modulated~they still participate in thedose optimization process! can be either open or wedgefields. The beam weightings are the only optimization vaables for these fields. The practical benefits of this optimition flexibility will be discussed later in the paper.

Recently we have modified the original dose optimizatialgorithm and added important versatility in the definitionthe optimization goal. The goal can now be defined in terof either a single dose value or a user-defined dose voluhistogram curve for each anatomical structure of interest.index-dose concept is used to accomplish the new dosetimization utilizing the original dose gradient approach.brief an index-dose at a point is a product of the physi

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dose and a number specific to the location and/or strucassociation of the point. The index distribution can be mnipulated in such a way so the optimization of index-dogradient throughout the entire treatment volume will genate a dose distribution approaching to the quality specifiethe dose optimization goal. This index-dose optimizationused in the second clinical case presented below. Detaithis index-dose approach are beyond the scope of this sand will be published separately.

B. Clinical case 1: Sinus tumor

A three-field sinus tumor treatment is shown in Fig.The extensive tumor was primarily located in the ethmsinus region between the globes, making tumor dose unmity and dose sparing to the left globe a treatment plannchallenge. The critical structures~globes! were blocked in allthree fields. Using the same beam setup, treatment planwas performed using conventional open/wedged fields, doptimization via modulators, and dose optimization v‘‘step & shoot’’ MLC sequences of different IM resolutionsThe original dose gradient optimization algorithm was usfor the optimization. The treatment planning goal wassame independent of the treatment technique used: to mmize dose uniformity in the treatment target volume. Athree beam ports were defined by MLC regardless oftechnique used. This is important for dose volume histogranalysis, where whether the volume is defined by a contous block or a jagged MLC edge can make a sizable difence. The part of the tumor between the globes was blocin the two lateral fields, therefore it was considerably unddosed using the conventional wedge technique. All th

FIG. 1. A sinus tumor treated with a standard three-field setup. The glowere blocked via MLC in all three beams. The same beam setup andwere used in all IMRT and conventional treatment techniques studied.


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fields were intensity modulated in the dose optimization. Fure 2 shows the resulting intensity modulation maps forthree fields.

C. Clinical case 2: Six-field nasopharynx tumortreatment

Figure 3 shows the six-field nasopharynx tumor treatmeThere are a number of normal structures in the vicinity oftumor in this case and they require important considerain addition to the treatment target dose uniformity. The nmal structures are cord, globes, chiasm, and the left and rparotid glands. The index-dose gradient optimization alrithm was used for optimization. The dose optimization go

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FIG. 2. The intensity modulation~IM ! maps of the three-field sinus tumotreatment~Fig. 1! generated by the dose gradient optimization algorithmPLUNC. The height of the map represents the relative photon fluence insity within the field.

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for this case includes the dose uniformity~70 Gy! of thetumor volume and a user-defined dose criterion for eachthe normal structures. The cord and the chiasm were limto a maximum dose of 40 Gy and 30 Gy, respectively. Tgoal for the globes, left parotid gland, and the right parogland were defined in terms of maximum dose volume htograms~DVHs!. There is no penalty if the resulting normstructure DVH is better than initially specified. Figure 4 dplays an example of the user-defined DVH curve~for globes!as the dose optimization goal. Also displayed in Fig. 4 isDVH of the initial condition prior to dose optimization. Alsix fields were intensity-modulated during optimizationthe nasopharynx treatment. Two examples of the resulIM maps are shown in Fig. 5. Compared to the sinus tumcase, where there was less concern of nearby normal tisparing, the IM maps in this case had a larger intensity vation with higher spatial frequency. When the maximurange of intensity modulation set in PLUNC is reached o

FIG. 3. A nasopharynx tumor treated with a six-field~co-planar! setup.Equal angler separation between adjacent fields was used. The treatarget volume, cord, and chiasm are illustrated. The normal structureconcern not shown in the figure are globes and the left and right paglands.

FIG. 4. Dose optimization goal specification in terms of the desired Dcurve ~chiasm! for the nasopharynx case. The dots are created by moclicking on the DVH curve and used to manipulate the desired DVH shaThe DVH calculated from the initial condition prior to dose optimizationalso shown.


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ewould see the high intensity in the IM map truncated. Tmaximum IM magnitude setting in PLUNC is 1.8% of thopen field intensity. The IM range of the modulator tecnique is discussed in the section below.

D. Modulator-IM technique

The modulator-IM technique can deliver the IM treatmethe way it was originally designed by the dose optimizatialgorithm. After the dose optimization is completed a modlator file for each IM map is created. The photon fluenattenuation through the modulator is calculated in PLUNusing the exponential equation,e2mt, where t is the pathlength of the pencil beam in the modulator. The lineartenuation coefficientm was determined from the measuredata in the following manner. Wedgelike modulators~corre-sponding to 30°–60° wedges! were made and the beam profiles in the water phantom were measured. We used alength-dependentm value to reflect the beam hardening efect, m(t)5m01ct. The constants,m0 ~0.217 for 6 MV;

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FIG. 5. Two example IM maps of the six-field nasopharynx tumor treatm~Fig. 3!, field RPO~upper! and field LAO ~lower!.

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0.175 for 15 MV! andc ~20.005 for 6 MV; 20.003 for 15MV !, are selected so that the calculated modulator profimatch the corresponding measurement for a variety offield sizes, depths, and the wedge angles. The increasedter dose from this medium-density modulator was notserved in percentage depth dose measurements. The stuChanget al.28 on contralateral breast dose from different tagential breast irradiation techniques including tmodulator-IM and the MLC-IM techniques showed that tmodulator techniques generated slightly less scatter dosthe contralateral breast than the conventional wedge tniques. The same study also showed that the MLC-IM tenique together with the virtual wedge technique producedleast scatter dose to the contralateral breast.

The modulator file @specifically designed for thecomputer-controlled milling machine~Par Scientific, ModelACD-3, Odense, Denmark!# and the accelerator type~Si-emens Medical Systems, Inc., Concord, California! wasdownloaded from PLUNC to the PC computer that operathe milling machine. The milled styrofoam mold was visally verified for its pattern, dimension, and orientation wrespect to the printout of the IM map. A thin plastic shewas used to cover the opening of the mold with a helpdouble-sided tape. Through a small filling hole the mold wthen tightly packed with small tin granules to a consistdensity with the help of an electric massager. The finishedgranule modulator was labeled and placed into a stuacrylic box, which can withstand daily clinical handling, ancan be easily and correctly inserted into the wedge slotthe accelerator. The styrofoam mold is precisely sizedsnugly fit the modulator box, which in turn fits into thwedge slot in the accelerator head as the wedge does. Tmisalignment of a modulator during treatment is unlikely

The QA procedure for the modulator-IM treatmentscludes beam profile and patientin vivo dose measuremenand comparison. Two IM beam profiles, inplane and croplane with user-defined offsets, were calculated at the msurement condition and compared to the measuremeneach modulator prior to the first treatment. The Profilertector system~SunNuclear Corporation, Melbourne, Florida!,a PC computer-controlled beam profile measurement sysequipped with a linear array of diode detectors separated5 mm, was used for the measurement and its real time cparison to the calculation. The calculated beam profiles wdownloaded to the Profiler computer prior to the measument. Using the ‘‘overlay’’ display feature, the comparisbetween the calculated and measured beam profile is acplished in real time during the irradiation. We find this retime comparison to be a very valuable feature for routclinical modulator application where timely and informativQA procedures are much needed.

A patient skin dose measurement using MOSFET detors ~Thomson & Nielsen Electronics Ltd., Ontario, Canad!is also performed and compared to the dose calculatedPLUNC within the first 10 Gy of dose delivery. The mesured dose is compared with the calculated dose at the msurement location. The IMRT modulator QA procedure29

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dosimetric verification processes for all clinical modulatoA summary of the QA results of the clinical IMRT modulators for the past several years, together with the dose optzation results, is currently being prepared for publicatioThe lowest photon intensity achieved by the current tin grule type of modulator is 38.3% and 44.9% of the open fifor 6 MV and 15 MV beam, respectively. However, thrange can be easily increased by replacing the tin granwith a material of higher effective density and/or by increaing the maximum thickness of the modulator~currently 5cm! without further modification of the technique.

E. ‘‘Step & shoot’’ MLC technique

To deliver the IM fields via the MLC technique, eachthe original IM maps~Figs. 2 and 5! generated by the dosoptimization was converted into the correspondi‘‘skyscraper’’-like discrete map. The discrete IM map is crated from the original map using a user defined leveldiscreteness, i.e., IM level. If an IM level of 5 is used, toriginal map is divided into 5 levels of intensity after thcommon background subtraction. Figure 6 shows a sediscrete IM maps converted from the original IM map of tleft lateral field in the sinus tumor case. Each of the discrIM maps has a different intensity resolution representedthe number of IM levels used in constructing the IM maOnce the dose optimization was performed and the ‘‘stepshoot’’ MLC technique was chosen, PLUNC converted toriginal IM maps to their corresponding discrete maps wituser-defined number of IM levels.

The discrete IM maps were then input to a stand-aloMLC sequence optimization software system IMFAST~Si-

FIG. 6. Discrete IM maps@~a!–~e!# converted from the smooth IM map~f!originally generated by the dose optimization algorithm for the left latefield of the three-field sinus treatment. The intensity modulation resoluof each map is represented by an IM level, which is the number of intenlevels~after common background subtraction! used to construct the IM mapThe IM levels used in the maps were~a! 2, ~b! 3, ~c! 5, ~d! 7, and~e! 10 asshown.

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emens Medical Systems, Inc., Concord, California!, whichgenerated an optimal sequence of MLC segments andcorresponding monitor units~MU! for a given fractionaltreatment dose. IMFAST takes into account the mechanconstraints of the MLC leaf positioning, the ‘‘tonguegroove’’ effect,30 a simple photon source model fitted to thaccelerator beam data, and other relevant parameters iMLC sequence optimization. There are several differentquence optimization methods available in IMFAST allwhich minimize the number of MLC segments, the treatmdelivery time, and to deliver the IM field as close to the inpdiscrete IM map as possible. Siochi31 has reported the detailof the MLC sequencing optimization method used for tstudy. Figure 7 shows the MLC sequence generated byFAST for the left lateral field of the sinus tumor case usin5 IM level discrete IM map. The MLC-IM treatment is delivered on a Siemens digital Mevatron accelerator retrwith MLC using Primeview~Siemens Medical Systems, IncConcord, California!, a treatment delivery automationrecord, and verify system when used together with LanRecord & Verify system~Siemens Medical Systems, IncConcord, California!. All segments of the same IM field cabe grouped together and delivered automatically with a kboard operation similar to the delivery of a single field.

Using the same MLC segment optimization methoPLUNC independently generated a set of MLC segmewhich were identical to the one by IMFAST but which hadslight difference in monitor units for segments which wevery narrow in one dimension. This is primarily due to tdifference in photon source distribution modeling betwePLUNC and IMFAST. PLUNC uses a two-source model32 inwhich each source has a Gaussian intensity distributwhereas IMFAST assumes the source intensity distributioexponential. Realistic photon source modeling plays anportant role in the accuracy of dose/MU computation wheportion of the field is very narrow, a situation uncommonseen in conventional treatment fields but frequently encotered in the ‘‘step & shoot’’ MLC technique. We adjustethe parameters in the two-source model~relative strength,relative location, and the size of each source! until the cal-culated dose distribution was in good agreement with msurement under several irregular MLC segment configutions. Figure 8 shows a beam profile comparison betwmeasurement and calculation for a test MLC field compo

FIG. 7. The MLC segments~8! of the left lateral field of the sinus treatmenfor the ‘‘step & shoot’’ MLC-IM treatment where 5 IM levels were useThe segment with the largest area, or the base segment, shows the treaportal.


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III. RESULTS

Treatment plans were made for each clinical case wand without dose optimization via intensity modulatioIMRT delivery techniques used here are ‘‘step & shooMLC-IM and modulator-IM.

A. IM resolution versus MLC segments

We found that for the MLC-IM treatment technique theis a linear proportionality between the number of IM leveused and the number of MLC segments required as calated by IMFAST as shown in Fig. 9. The proportionalitybetween 1 to 2 segments per IM level per field for boththree-field sinus tumor treatment~left Y-axis! with relativelysimple IM patterns and the six-field nasopharynx tumtreatment~right Y-axis! with more complex IM patterns. Therelationship of IM resolution, dose optimization quality, antreatment irradiation time are presented in the following stions.

B. Dose optimization quality

The dose optimization quality of each treatment techniqwas judged by how well the defined optimization goal wreached for each case.

1. Sinus tumor case

The dose optimization goal for the sinus tumor treatmwas dose uniformity in the target volume. The quality

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FIG. 8. Comparison of calculated beam profiles by two different phosource models~used in the treatment planning system! and the measuremenon a test MLC pattern. The realistic photon source model, composeddual Gaussian-distribution source, produced a calculated dose profile~solidline! very similar to the measurement~circles!. The calculated dose from asource model having a single small Gaussian distribution only~dashed line!generated a considerable discrepancy with the measurement under thMLC configuration.

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critical structure~globe! sparing was not considered in thcomparison because~1! globes were outside the treatmeports and~2! each treatment port was defined by the saMLC leaf configurations for all techniques. For this casedefined the dose optimization quality as Uniform Dose Vume ~UDV! alone. Derived from the differential dose voume histogram of the target volume, shown on Fig. 10~a! forthe nasopharynx case, UDV is the percentage of the trment target volume that receives within65% of the nominaldose. The dependence of UDV on the intensity resolutionthe IM map is displayed in Fig. 10~b! ~left Y-axis!. The solidhorizontal line in Fig. 10~b! represents the UDV value of thmodulator technique. The data show that for the three-fisinus treatment at least 3 IM levels were required forMLC-IM treatment to achieve better dose uniformity compared to the conventional treatment technique (UD575%). The UDV quickly improved with increasing IMlevel initially then approached saturation at an IM level ofThe UDV only improved from 82% to 83% as the IM levchanged from 5 to 10. The gap between the UDV valuesthe modulator-IM ~87%! and the 10 IM level MLC-IM~83%! is one-third of the total difference between the coventional treatment~75.5%! and the optimized treatmenwith no limit in IM resolution ~87%!.

2. Nasopharynx tumor case

The dose optimization quality of this nasopharynx tumtreatment is judged by how well all the prerequisites spefied in the dose optimization goal have been reached. Tinclude target volume dose uniformity and a set of specifiDVH curves or single dose values for the nearby normstructures of interest in addition to tumor dose uniformi

FIG. 9. The total number of MLC segments required vs the IM level usedthe ‘‘step & shoot’’ MLC-IM treatment technique for the three-field sin~left Y-axis! and the six-field nasopharynx~right Y-axis! treatment. The datashow that 1.2 and 1.45 MLC segments per field are required for eachlevel used for the treatments, respectively.


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Figure 10~b! ~right Y-axis! shows one aspect of this multfactored dose optimization quality, the UDV, as a functionIM level for the MLC technique together with the data frothe modulator-IM technique. One sees a similar trend independence of UDV on the IM levels for both clinical caspresented: improvement in UDV slows down after the Ilevel reaches 5. The gap in UDV value between tmodulator-IM technique ~93.1%! and the 10 IMlevelMLC-IM technique~90.4%! is smaller than that of thesinus tumor case. We want to clarify that the IM magnitudof some of the fields in this case exceed the limit of tcurrent tin granule modulator. The label ‘‘modulator-IM’’ inFigs. 10 and 11 is replaced by ‘‘smooth IM’’ to reflect th

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FIG. 10. ~a! Differential DVH of nasopharynx target volume; the peaheight is related to UDV value.~b! Uniform Dose Volume~UDV! as afunction of IM level for the three-field sinus tumor~left Y-axis! and thesix-field nasopharynx tumor~right Y-axis! treatment. UDV represents tumodose uniformity and is defined as the percentage of the tumor volumeceiving 65% of the nominal dose. The same solid horizontal line in F10~b! represents the UDV values of the modulator/smooth IM techniqueboth clinical cases. The smooth IM can be achieved using a modusimilar to the one discussed in this paper. The treatment ports defineMLC are identical for all IM delivery techniques.

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Manual treatment planning using the conventional trement technique~wedges and open fields! was extremely dif-ficult and planner-dependent in this six-field case withcomplex dose optimization goal. It was relatively easymanually achieve tumor dose uniformity but not whilemultaneously sparing the normal structures as specifiethe dose optimization goal. For this reason this plannerpendence of the conventional treatment plan should be nif the plan is used for a reference of comparison.

In terms of the other aspect of the dose optimization quity, the normal structure sparing, the evaluation method uis much less systematic and quantitative but informatiFigure 11 shows the degradation in the dose sparing ofMLC-IM technique with different IM resolutions for some othe normal structures~cord, chiasm, and left parotid! in termsof DVH. Similar results are also seen in DVHs of othnormal structures not shown here. The data from the cosponding conventional and the modulator-IM techniquesalso displayed in Fig. 11. It is interesting to see thatdifferences in DVHs between the modulator-IM techniqand the MLC-IM techniques are not closely related to thelevel used. This phenomenon strongly suggests that thetial resolution of the MLC-IM technique (1 cm31 cm),which is unchanged as the IM level increases, is responsfor the difference in the nearby normal structure sparingtween the modulator-IM and the MLC-IM techniques.

C. Treatment irradiation time

The treatment irradiation time is defined as the timelapsed between the initiation of the treatment on the acerator console by pushing the ‘‘rad on’’ button to thcompletion of the irradiation. Besides the treatment irradtion time defined above, the conventional and tmodulator-IM techniques require additional time for thepists to enter the treatment room to exchange the moduor wedge between treatment fields. This beam modifierchange time is generally no more than 45 s in our clinic. Wdefined equivalent treatment irradiation timeas the treat-ment irradiation time plus the beam modifier exchange tiwhen applicable for a meaningful inter-comparison of trement irradiation time among the MLC-IM technique, thmodulator-IM technique, and the conventional techniqThe treatment irradiation times were measured using a swatch for a treatment dose of 2 Gy. Figure 12 displaysdependence of theequivalent treatment irradiation timeonthe IM resolution or the IM level for the three-field sinutumor treatment. The modulator-IM and the conventiowedge technique were included on the graph for comparisFigure 12 reveals that theequivalent treatment irradiationtime increased quickly from 5.6 to 13.8 min as IM leveincreased from 2 to 10. The modulator-IM and the convtional treatment techniques required substantially less trment irradiation time~3.2 min!.


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D. Comparison of the IM delivery techniques

The inter-dependence between the dose optimizaquality and the treatment irradiation time for the MLC-IM

FIG. 11. DVHs of the normal structures from conventional and differentdelivery techniques for the nasopharynx case:~a! chiasm,~b! cord, and~c!left parotid gland.

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technique in the sinus tumor treatment is shown jointlyFigs. 10~b! and 12. The former required more IM levels fobetter dose optimization quality which in turn demandmore treatment irradiation time. The dose optimization quity represented by UDV approached saturation at an IM leof 5 whereas the treatment irradiation time increased briswith increasing IM levels. An IM level of 5 seemed to begood compromise between the dose optimization qualitythe treatment irradiation time for the MLC-IM techniquEven though the optimization goal included both normstructure sparing and treatment volume dose uniformitythe nasopharynx tumor case, the IM level effect of tMLC-IM technique manifested itself primarily in the targdose uniformity, as evidenced by Figs. 10 and 11.

We have demonstrated that the modulator techniquetablished the upper limit for the quality of dose optimizatiowhereas the conventional technique via open and wedbeams established the baseline for the treatment irradiatime. It is apparent from Fig. 12~three-field sinus treatment!that the modulator technique required no additional timetreatment delivery compared to the conventional techniwhile the MLC-IM techniques required considerably longtime ~100%–400%! to deliver the treatment. The total patietreatment time, which includes but is not limited to the irrdiation time described thus far, will be discussed later.

E. IMFAST verification

An ‘‘error map’’ is defined in IMFAST as the differencbetween the input~skyscraper! intensity map and theIMFAST-produced ‘‘step & shoot’’ MLC-IM intensity map.This ‘‘error map’’ is a quick yet informative presentation othe quality of the MLC segment sequence optimizationusers. We independently verified an ‘‘error map’’ along tinplane direction on the central axis for the left lateral sin

FIG. 12. The Equivalent Treatment Irradiation Time as a function of thelevel for the three-field sinus tumor treatment. In addition to the irradiattime itself, the equivalent treatment irradiation time includes the extra trequired~45 s per exchange! for a therapist to enter the treatment room ato manually exchange the wedge/modulator between treatments of difffields when applicable. The data of the conventional wedge techniquethe modulator-IM technique were also included in the graph for the sakcomparison.


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field. The measured ‘‘error map’’ is the difference betwethe discrete intensity map calculated by PLUNC andmeasurement from the MLC-IM field delivery. The averadeviation between the predicted~by IMFAST! and measured‘‘error map’’ in the measurement region was less than 2results in other regions of the IM map were similar.

F. IM quality evaluation

Figure 13 shows the profile comparison between the ornal IM map generated by the dose optimization algorithand the actual IM maps delivered by the modulator andMLC techniques for the sinus case~a! and the nasopharynxcase~b!. The Profiler array detector system was used

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FIG. 13. Profile comparison of the original IM map generated by the doptimization algorithm and the actual measured IM maps delivered byferent IM treatment delivery techniques~modulator technique and ‘‘step &shoot’’ MLC-IM techniques of different IM level!. ~a! Right lateral field ofthe sinus treatment case, and~b! RAO field of the nasopharynx treatmencase.

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measurement and comparison. The measurement setup80 cm SSD, depths of 2 cm and 4 cm for 6 MV and 15 Mphoton beams, respectively. The quality of IM of each tenique reflected in the beam profile comparison appears tconsistent with the dose optimization quality shown in Fi10–11 previously.

IV. DISCUSSION

A. Fractional monitor unit „MU…

If the total number of segments per MLC-IM treatmentlarge due to either a large number of IM fields and/or a lanumber of IM levels, many segments might deliver only vesmall MUs. When the accelerator cannot deliver fractioMUs the dose round-off error in this case could be sizabFor the sinus tumor case, the minimum segment MU wafor the 10 IM level MLC-IM treatment of 2 Gy per fractionIn this case the round-off error for these segments washigh as 25%. The cumulative round-off error over the entreatment, however, was significantly smaller, and thissupported by the study of IM profile of different IM leveshown in Fig. 12. However, eliminating those MLC sements which have only a few MUs has become an acceptand common practice in ‘‘step & shoot’’ MLC-IMRT. Oneshould realize that doing so further deteriorates the restion of the IM delivered and therefore widens the differenbetween the original IM and the actual IM delivered. Thfractional MU issue is irrelevant to the calculated results psented in this paper because PLUNC considers fractioMU in its dose computation. The MU truncation is peformed when the treatment setup data are downloaded fPLUNC to Lantis treatment record and verify system.

B. The UDV saturation behavior

The UDV saturation behavior seen in Fig. 10~b! can beinterpreted as the upper limit of the IM level effect on ttarget dose uniformity for the MLC-IM technique. An IMtreatment delivered by the modulator technique canviewed as a high resolution MLC-IM treatment with hudreds of IM levels and hundreds of MLC leaves. The gbetween the saturation UDV value of the MLC-IM techniqand that of the modulator-IM technique is related to the finspatial resolution~1 cm! of the MLC technique, which iskept the same for all IM levels used. The UDV saturatibehavior of the MLC technique is suspected to be relatethe closeness of the collimator angle used to the opticollimator angle. The collimator angle, or the orientationthe MLC leaves, can have significant influence on the dcrepancy between the discrete ‘‘sky-scraper’’ IM map cated for~and delivered by! the MLC technique and its corresponding original smooth map. The effect of the collimaangle is similar to that in conforming a MLC opening togiven treatment portal defined by a block. An optimal comator angle can minimize the field edge jaggedness; antimal collimator angle can also reduce the difference betwthe discrete IM map and its original smooth map. The oritation of MLC leaves~the collimator angle! should be con-


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sidered as a variable in the MLC-IM treatment delivery otimization process. We are currently in the processincorporating such a concept into PLUNC for the MLC-IMtreatment delivery technique.

C. Treatment automation and treatment time

Gantry and couch movements of newer digital acceletors, which are often equipped with MLC functionality, caalso be controlled remotely from the accelerator consoThese functions, coupled with Primeview or other advanctreatment automation and record and verify systems, cansult in substantial timesaving in total patient treatment deery. However, this was not included in our treatment irradtion time analysis because these timesavings are not unto the ‘‘step & shoot’’ MLC-IM technique—they apply to altreatment techniques studied which were delivered fromsame accelerator. The combined potential timesaving duthe newer generation of digital accelerators and recordverify systems plus the ‘‘step & shoot’’ MLC-IM techniquitself is highly clinically relevant and deserves separstudy.

D. Time required other than treatment delivery

Although we intended to focus on the dose optimizatiquality and the treatment irradiation time in this study, tcomparison between the two IMRT delivery techniquwould be incomplete without mentioning the efforts requiroutside treatment irradiation. This includes the work atime involved in IM treatment preparation and QA procdures. In our clinic it normally takes a trained medical phyics technician, who is also responsible for block fabricatioan average of 20 min to complete one modulator. This aincludes the time to perform the appropriate QA checks ding the fabrication process. Obviously, the MLC-IM tecniques eliminate this time consuming IM preparation pcess. However, once the modulator is made, there is virtuno difference between IMRT via modulator and a convetional treatment in terms of treatment delivery, record keing, QA via port film and chart check procedures. As dscribed earlier, the dosimetric QA procedure for tmodulator-IM technique includes~1! the IM profile measure-ment and its comparison to the calculation and~2! in vivopatient dose measurement and its comparison to calculaThe amount of time for clinical physicists to conduct theQA procedures is on average 30 min per case.

E. Optimization with open and wedged beams

Independent of the treatment delivery technique chosthe workload required to accomplish an IMRT treatment cbe significant. The workload is directly proportional to thnumber of intensity modulated fields used. For this reawe have purposely been minimizing the number of intensmodulated fields in the dose optimization. Because the doptimization algorithm is based on dose gradient rather tdose itself there are a number of approaches possible to mmize the existing dose gradient. It gives the user the flexi

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ity to designate each beam in the plan to be either opwedged, or intensity-modulated. All beams participate indose optimization process, i.e., the dose optimizationbased on the dose distribution from all beams. The dosedient optimization algorithm finds the most appropriate beweightings and/or the intensity modulation map for eabeam. In many clinical situations, we found that reducingnumber of intensity-modulated fields considerably reduthe workload involved for both types of IMRT delivery techniques. The reduction was done only when there wouldminimal compromise on the quality of dose optimizatioWhen the IM magnitude required is very large and outsthe limit of the modulator technique we often use a comnation of wedged and IM fields in the dose optimizationlong as the desired IM has a uniform slope component whcan be created by the wedged field.

V. CONCLUSION

We have presented comparisons between the ‘‘stepshoot’’ MLC auto-sequence and the modulator techniqfor intensity modulated treatments created by a dose gradoptimization algorithm for two clinical cases. We havshown that for the MLC-IM technique the treatment irradtion time increased swiftly as the IM resolution improveHowever, target volume dose uniformity initially improvewith the increasing IM resolution then approaches saturabelow the value achieved by the modulator technique. Tquality of normal structure sparing in the nasopharynx cis less than that of the modulator technique; there wasclose correlation to the IM resolution of the MLC-IM technique. We found an IM level of five to be a good comprmise between irradiation time and dosimetric quality for bocases. The more favorable results of the modulator-IM tenique can be attributed to its high IM resolution. The usea modulator can deliver IMRT as it was originally designwithout degradation.

Finally, we would like to point out that the main objectivof this paper was to evaluate the two IMRT treatment deery techniques. Although only two related dose optimizatalgorithms and one MLC segmentation method were usethis study we believe the conclusions are likely to hold,least qualitatively, for other MLC segmentation methods adose optimization algorithms which generate continuousmaps.

ACKNOWLEDGMENTS

The authors are grateful to Drs. Edward L. Chaney aJoel E. Tepper for the helpful discussion and the construccomments during the preparation of the original manuscrand Larry Potter for his valuable comments on the reviversion. The authors would also like to acknowledgeAlfredo Siochi of Siemens Medical Systems for providinassistance with the IMFAST application, and ElizabethMiller for her contribution to the early development of thmodulator IMRT technique, and, finally Dr. Julian Roseman for his suggestion of the modulator material andpursuing modulator-IM clinical application.


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