Physical and Dosimetric Aspects of MLC LoSasso

Physical and dosimetric aspects of a multileaf collimation system usedin the dynamic mode for implementing intensity modulated radiotherapy

Thomas LoSasso,a) Chen-Shou Chui, and C. Clifton LingDepartment of Medical Physics, Memorial Sloan–Kettering Cancer Center, 1275 York Avenue, New York,New York 10021

~Received 19 May 1997; accepted for publication 3 August 1998!

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapybecame a reality at our institution in 1995. Unlike treatment with static fields using a multileafcollimator, there are significant dosimetric issues which must be assessed before dynamic therapycan be implemented. We have performed a series of calculations and measurements to quantifyhead scatter for small fields, collimator transmission, and the transmission through rounded leafends. If not accounted for, these factors affect the delivered dose to the prostate by 5%–20% for atypical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileafcollimator, achieved during leaf position calibration and maintained during dose delivery, is alsodiscussed. ©1998 American Association of Physicists in Medicine.@S0094-2405~98!02510-3#

Key words: multileaf collimator, intensity modulation, dosimetry, quality assurance

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I. INTRODUCTION

Recent advances in computer application and linac techogy have provided new tools for creating and delivering otimized radiation treatments. Several groups have desigcomputer algorithms, based on the ‘‘inverse planninmethod, which automatically generate optimized dose disbutions by modulating the photon fluence in the radiatfields.1,2 In parallel, software for operating multileaf collimators ~MLCs! in the dynamic mode~i.e., the leaves are inmotion during radiation delivery! has been developed~Varian Associates, Palo Alto, CA!. Converting the radiationfluence profiles to the leaf motion patterns is a third softwmodule.3 These capabilities, in combination, have beimplemented in our center for the delivery of intensity modlated radiotherapy~IMRT!.

MLCs are available from several manufacturers at ttime. Their designs differ in the way they couple with coventional collimators and in their physical and dosimetcharacteristics. MLC hardware is already familiar in maclinics, as MLCs have been used for static field therapyseveral years. Several papers have discussed the dosimethese devices as a replacement for metal alloy blocks ufor static fields.4–7 Varian MLCs ~Varian Associates, PaloAlto, CA! have been used clinically at our institution to dliver static field treatments since 1992 and to deliver intsity modulated field treatments with dynamic multileaf colimation ~DMLC! since 1995.8 In general, the dosimetry oradiation fields is similar whether shaped by the Varian Mor metal alloy blocks. Specifically, factors such as geomecal accuracy, depth dose, transmission, and outputcomparable.4–7 In addition, the dose distributions are similfor a multifield treatment at the boundary of the plannitarget volume, when electron transport and patient setupcertainty are properly considered.7 However, based upon ouexperience, these studies are inadequate to predict the do

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etry of DMLC used for intensity modulated treatments. Tapplication of MLCs to intensity modulation engendersunique set of problems, relating to accurate and reproducdose delivery, which need to be resolved before DMLtherapy can be successfully implemented in the clinic. Pviously, we have reported the results of tests designedexamine certain mechanical aspects of DMLC and theirplications on dose delivery.9 Tests examined the stability oleaf speed, the effect of acceleration and deceleration ofmotion, the positional accuracy of the leaves, and the efof lateral disequilibrium on dose profiles due to the varyiintensity levels between adjacent leaves. Based upon thesults, the mechanical aspects of the Varian MLC were judto be accurate and reliable. Dosimetric verification of IMRprostate fields in phantom have shown the dose accuachievable with this technique.10 It was also clear that furthestudies were needed in order to characterize fully the dosetric aspects of DMLC.

In this study we carefully examined the accuracy andproducibility of the leaf gaps, keeping in mind that the accracy of the delivered dose with DMLC is directly dependeon the precision of the gap between opposed leaves. FoMLC used in our study, this depends upon the gap calibtion methods of the manufacturer. Another factor whichfects the gap precision is the ability of the leaves of the Mto move to their prescribed coordinates at the prescritimes. For this reason, a proprietary software system intetively monitors the leaf motions and interrupts radiation dlivery if the leaves are not at the specified positions to witha certain tolerance. Thus, we evaluate the impact of thfactors on the dose delivery.

We also assessed how the MLC design influencesphoton fluence produced by DMLC with the ‘‘sliding window’’ technique.3 In this method, because of the relativesmall gaps between opposed leaves and because mos

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gions are shielded by leaves much of the time, the delivephoton fluence is quite sensitive to the transmissions throthe leaves and the rounded leaf ends, the leakage betweeleaves, and the head scatter, factors which are of lessernificance in the use of static fields. If not properly accounfor, these influences could result in substantial errors in ddelivered. Whereas this study involves only MLC of a spcific design, the concepts and findings have broad impltions for intensity modulated therapy with DMLC and mabe applicable to MLC of other designs as well.

II. MATERIALS AND METHODS

A. MLC description

The Varian MLC~Mark II! is mounted below the conventional block collimators on a Varian C-Series acceleratorconsists of 26 pairs of tungsten alloy leaves, approximatecm thick, each projecting a 1.0 cm leaf width at isocentwith the midleaf position at about 51 cm from the sourcThe leaves, mounted within carriages, move in a rectilinfashion, in the same direction as the lower block collimatoThe range of travel of individual leaves is approximatelycm at isocenter relative to the carriage, and120 to 216 cmabout isocenter when combined with carriage motion. Mamum leaf speed is 3 cm per second at isocenter. For DMoperation, only the leaf motion is permitted~with the currentversion of the software!; the carriages remain fixed durindose delivery. The upper collimators define the superiorinferior borders, and the lower collimators are positionedcm distal to the most retracted leaf position at each sidethe field, similar to the procedure for static fields. In orderensure a relatively constant penumbra at different positiin the beam, the ends of the leaves are rounded as showFig. 1~a!. The central 3 cm portion of each leaf end is circlar with a radius of curvature of 8.0 cm. Beyond this, the leend is straight, and at an angle of 11.3° relative to the vecal axis. Leakage between adjacent leaves is minimizedan interlocking tongue and groove arrangement@Fig. 1~b!#.

The rectilinear leaf motion and the rounded leaf edgecombination, introduce a nonlinear dependence of field son leaf position. As a leaf is retracted away from the cenaxis, the field-defining point on the rounded end is no lonthe center of the leaf, but moves away from the source. Aleaf is extended across the central axis, the field-definpoint moves toward the source. In either case, the resufield size is slightly smaller than that predicted by the linechange in leaf position. Varian provides a generic correctfile to compensate for the nonlinear dependence of fieldwith leaf position. In this way, the digital readout is linkedthe light and radiation fields, and not the physical displaments of the leaves. Since the MLC leaves are precismachined, the parameters in this file are common toVarian MLCs and should not require adjustment. A secofile, the Positioning Calibration File, allows the user to copensate for misalignment between the MLC and the cenaxis. This is performed by adjusting four parameters,skewness of the left and right leaf banks individually, the gbetween opposed leaf banks, and the centering of the

Medical Physics, Vol. 25, No. 10, October 1998

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banks on the central axis. It is notable that the gap adjment, which is most critical for dynamic dose delivery, cbe accurately performed using a feeler gauge for a small fisize ~e.g., 0.1 cm!. Other fields sizes are set relative to threference field relying upon the precision of the individuleaf motor encoders. After these correction files are avated, the digital readout should agree with the projeclight field.

The planning and execution of intensity modulation trement at Memorial Sloan–Kettering Cancer Center~MSKCC!is based upon an inverse plan optimization which strivesachieve a uniform dose within the target while keepingnormal tissue doses below their specified tolerances.10 Dosecalculations are performed using the convolution of an omized intensity distribution with a pencil beam kernel.11 The‘‘sliding window’’ technique, where all leaves start at onend of the field and move unidirectionally, with differenspeeds, to the other end, is used to deliver the intenmodulated profiles.3

B. Mechanical accuracy

Given its importance in IMRT with the sliding windowtechnique, the accuracy in leaf position and the relatedwidth were studied and monitored by several techniquFirst, the reproducibility of the leaf position readout is rotinely checked by static light field projection at isocentewith a spatial uncertainty of about 0.25 mm. Second, ahas been devised to check the accuracy and reproducibof leaf positions and gap widths in DMLC mode.9 Briefly, anintensity-modulated radiation field consisting of narrobands~1 mm wide! of exposure spaced at 2 cm intervals

FIG. 1. Views of the leaves~a! from the side and~b! from the front. The leafface is rounded near the center with a radius of curvature of 8 cm. The oportions of the face are straight, 11.3° off the vertical.

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produced on film. By this method, it is possible to visuadetect errors greater than 0.2 mm in an individual leaf potion or gap width.

A third test involves periodically measuring with an iochamber the dose for a uniform field delivered dynamicawith a small, 0.5-cm-wide, sweeping gap, and normalizinwith the dose from a static 10310 cm2 field defined by thejaws. Since the dose can be measured precisely and sincvery sensitive to any width change of this small gap, this tis capable of detecting less than 0.1 mm deviation. Tabove test, performed with a cylindrical ion chamber in airthe isocenter, was also repeated for different gantry an~0°, 90°, 180°, and 270°! to assess the effect of gravity on thperformance of the MLC in the dynamic mode. Other factwhich could affect the delivered dose, such as chambersition and beam output variation with gantry angle, weaccounted for by repeating the measurements with the fireference field defined by the jaws.

The accuracy of dose delivered during DMLC operatiis dependent, in part, on a proprietary software modwhich controls the movement of the individual leaves durradiation delivery, according to the data in a DMLC fiwhich specifies the leaf positions as a function of moniunits ~MU!. As described in our previous publication, thDMLC file is generated, based on the intensity modulabeam profiles produced by an inverse planning algorithma software module designed by Spirou and Chui at MSKC3

Every 55 ms during DMLC delivery, the control softwamonitors the leaf positions and compares them to their pscribed positions. The beam is interrupted momentarilyany leaf position is outside a user-defined tolerance, seable from 0.1 to 5.0 mm. In theory, this should never happbecause in designing the MSKCC software which createsDMLC file, the capability of the MLC in the dynamic mod~i.e., the maximum leaf speed!, was explicitly considered. Inactuality, there are occasional ‘‘out-of-tolerance interruptfor reasons which are not fully understood, but which cobe due to MLC motor fatigue and effects of leaf acceleratand deceleration. To assess the influence of leaf positionerance on DMLC operation, relative dose measuremewere made for a range of tolerance settings at the centraland at off-axis points using an ion chamber and film inpolystyrene phantom for several typical DMLC fields.

C. MLC transmission

Average transmission through the MLC was determinwith ion chamber measurements in a polystyrene phantomparallel plate chamber~2.5-cm-diam collection volume! wasplaced at 100 cm source to chamber distance~46 cm fromthe bottom of the leaves! on the central axis and at 7 cm oaxis. Since the chamber’s diameter spanned several leathe measurement averages over midleaf and interleaf trmissions. Measurements were made for fields definedthe upper and lower jaws, with the MLC in the open aclosed positions. The ratio of these measurements istransmission. For the measurements on the axis,‘‘closed’’ MLC, in order to avoid leakage between th


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rounded ends, the junction between the opposed leavesplaced off axis and under the jaw. For measurements at 7off axis, the lower jaws were positioned asymmetricalMeasurements on the axis were made for jaw settings636, 10310, and 14314 cm2, and at depths ofdmax through30 cm, for 6 and 15 MV x rays; off-axis measurements wemade for 10310 cm2 fields only.

Additional measurements were made with radiograpfilm to separately quantify midleaf and interleaf transmsions for both 6 and 15 MV x rays. For jaw settings10310 cm2, Kodak XV2 film was placed at 15 cm depth ia polystyrene phantom perpendicular to the central axisirradiated to the same average optical density,;1.0, for boththe open and closed MLC fields by adjusting the MU. Tfilm was scanned with a scanning densitometer with a 1 mmaperture. Doses were derived from densities using film cbrated at 15 cm depth for a 10310 cm2 field. The transmis-sion profile for the leaves was obtained by dividing the tramitted dose profile by the open field dose profile afcorrecting for the relative MUs delivered.

D. Head scatter

An intensity modulated~IM ! field generated by thesliding-window technique is essentially a composite of marelatively small fields. Accurate dose calculations for anfield, given the rapid change in output factor with field sifor small fields, requires detailed knowledge of the ‘‘etended source’’ distribution. Thus, we measured in-air pton output and extracted the source distribution from therivative of the measured output as a function of field siThe derivative is approximated with primary and scatcomponents. In our current model, the primary componenassumed to be a delta function, and the scatter is descrby two conic distributions. The heights and base radii oftwo cones were chosen so that the calculated and measoutput factors agree to within 1% for field sizes up to315 cm2. The primary fluence at any point can then be coputed by integrating the source function over the part ofsource seen by the point through the aperture.

For these measurements the upper and lower jaws wset to 15 cm at the isocenter. The MLC was used to defields from 10310 cm2 to less than 131 cm2 with squareand rectangular shapes. Two methods have been usemeasure output in air. In one method, a small cylindricalchamber was used with a 3-mm-thick brass buildup cap wa combined outer diameter and axial length of 1.2 andcm, respectively. For field sizes of 3 cm and less in eitdimension, a film technique was used to supplement thechamber measurements. Kodak XV2 film in ready pack wsuspended in air perpendicular to the beam central axistween a pair of 0.5-mm-thick polystyrene plates. To achiesufficient buildup for 15 MV x rays, lead disks, 6 mm idiameter and 3 mm in thickness, were placed at the ficenter in contact with the film jacket, upstream and dowstream from the film, with slight pressure asserted byplates. In this configuration, the density at the center offield was converted to dose, extending the output in-air m

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surements to field sizes less than 1 cm2. Relative outputswere measured for 6 and 15 MV x rays.

E. Rounded leaf end transmission

MLCs with rounded leaf ends are offered by sevemanufacturers. Two approaches were used to assess thetribution of the radiation transmitted through the roundleaf ends. First, photon attenuation by the leaf end wasculated based on an accurate description of the MLCgeometry@Fig. 1~a!# and the measured linear attenuation cefficient. The attenuation coefficient was derived from tmeasured average MLC transmission and the knownthickness for the manufacturer’s design. Whereas an ividual leaf is nominally 6 cm thick, we calculated the aveage thickness to be only 5.65 cm, due to guides at the topthe bottom, and the tongue and groove at the [email protected]~b!#. Profiles were calculated for leaf ends positioned oncentral axis and at 10 cm off axis.

A second approach involves measuring the in-phandose profiles of static MLC fields with gap widths of 0–1cm, and then integrating over the measured profile. Thetegration should yield the sum of~a! the fluence transmittedthrough the MLC leaves,~b! the fluence through the gaopening, and~c! the fluence transmitted through the roundleaf ends. The sum of~b! and ~c! should be proportional tothe effective gap opening, i.e., the nominal gap plus anset. The assumption that the offset is a constant, largelydependent of leaf position, is a good approximation with65 cm about the central axis, as our calculated resultsshow.

For both 6 and 15 MV x rays, Kodak XV2 film waplaced at 100 cm from the source at a depth of 5 cm ipolystyrene phantom. The upper and lower jaws were se5.0 and 14.0 cm, respectively, in symmetric mode. The Mgaps, symmetric about the central axis, varied from 0~with opposing leaf ends in contact! to 10 cm ~to give a 5310 cm2 field!. A WP102 scanning film densitometer~Well-hofer, West Germany! was used to scan each film parallelthe leaf motion. Each film was scanned ten times, with escan separated by 1 mm~in the direction perpendicular to thleaf motion! covering the 1 cm region around the centaxis, to include midleaf and interleaf contributions. Aftconversion to dose, all ten scans for each field were ingrated over the 12 cm region, symmetric about the cenaxis.

Another measurement was made to obtain the MLC tramission for this experimental geometry. The MLC was sezero field size and its center positioned 7.5 cm off the cenaxis ~and 0.5 cm into the shadow of the lower jaw!. Theexposed film was scanned and integrated over the 12extent about the central axis. This transmission, calculaper unit length of leaf, is then used to subtract the contrition of MLC transmission~a! from the integrated dose foeach of the open fields described above. A plot of theintegral dose~b! plus ~c! versus gap width, should yield thoffset to the nominal gap width, when extrapolated to zdose.


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III. RESULTS

A. Mechanical accuracy

A number of characteristics are important in the use oMLC in the dynamic mode: mechanical precision, stabiliand electromechanical reliability. Routine quality assuranhas been performed with the Varian MLCs at our institutifor several years before the commencement of DMLC opetion. These tests have indicated that the calibration ofMLC is very stable. Based upon visual examination of tlight fields, the reproducibility of leaf positioning is, geneally, 60.25 mm. Furthermore, the operation of the MLChave been very reliable, i.e., essentially trouble free opetion with an average use factor for the MLCs on three mchines of approximately 70%.

During the development of the DMLC programMSKCC, a number of acceptance and QA tests were devusing the dynamic functions.9 One QA test which is rou-tinely used consists of narrow bands of radiation produon film by a start and stop leaf pattern in dynamic mode. Ttest results confirmed that the precision of leaf position

FIG. 2. Effect of the leaf tolerance setting on~a! delivered dose and~b!treatment time for two of the five fields, the left anterior oblique~LAO! andthe posterior~POST!, used to deliver 180 cGy in our standard IMRT protate plan.

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0.25 mm or better for both Varian MLCs used for DMLCFor one of these MLCs, more sensitive measurements ogap width, based upon the output measured with anchamber for a small scanned field, were made at differgantry angles. The results showed that the precision wwhich the gap of the Varian MLC can be set and maintainhas been found to be better than 0.1 mm, independengantry angle. Measurements using the same field at off-points are consistent with the reduced off-axis leaf end tramission presented in Sec. III E.

In the test of the effect of the tolerance setting, astolerance was decreased from 5.0 down to 0.1 mm, the bhold off invoked by the software becomes more frequeindicating that the leaves are at times unable to reach tprescribed positions, and that such incidences are more liat tighter tolerance settings. The effect of tolerance on deered dose is shown in Fig. 2~a! for two of five fields used todeliver 1.8 Gy in our standard five field intensity modulatprostate plan.12 The maximum dose variation is less than 1for each field, occurring for tolerances between 0.1 and

FIG. 3. The average of midleaf and interleaf transmissions measureisocenter in phantom with a parallel plate ion chamber, 2.5 cm in diamfor ~a! 6 MV x rays and~b! 15 MV x rays. The transmission is the ratio othe central axis dose for the MLC blocked field to the open field forcollimator settings and depths shown.


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mm. Further testing revealed that this variation is not dueerror in gap width, but to beam instability caused by tincrease in beam hold off incidence at the tighter tolerasetting.

B. Treatment time

As shown in Fig. 2~b!, the treatment time is essentiallconstant for tolerances greater than 2 mm. As the toleranare reduced, the beam was increasingly held off, therlengthening the treatment time. For the two examples shothe treatment time more than doubles, as the tolerancreduced from 2.0 to 0.1 mm.

C. MLC transmission

The average of midleaf transmission and interleaf leakis shown in Figs. 3~a! and 3~b! for 6 and 15 MV x rays,respectively, normalized to the output of the open field. Ttransmission, over a range of clinically useful field sizes~636 to 15315 cm2! and depths~dmax to 20 cm!, is about2.0%. The transmission increases with jaw opening for benergies; the exact relationship is not known, althoughcreased scatter from the MLC is likely. The transmission6 MV x rays also increases with depth of measuremewhile for 15 MV x rays it is almost constant. This may bdue to beam hardening for 6 MV x rays within the MLresulting in a more penetrating transmitted beam thanopen beam. The increase in pair production within the Mfor the higher energies in the 15 MV beam may have offthe beam hardening effect, as there is little change wdepth. The transmission at 7 cm off axis is essentiallysame as it is on the central axis for each energy.

Transmission profiles obtained for a 10310 cm2 field at15 cm depth in polystyrene using film are displayed in Fig.The average transmissions were 2.0% and 2.1% for 6 anMV x rays in good agreement with the values found with tion chamber. The range of transmission over the central p

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FIG. 4. The midleaf and interleaf transmissions measured at isocentephantom with radiographic film for 6 and 15 MV x rays. The transmissionthe ratio of the MLC blocked field profile to the open field profile for10310 cm2 field defined by the jaws and a depth of 15 cm.

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tion of the MLC was from 1.7% at midleaf to 2.7% betweleaves. The variation in transmission through different leais less than 0.1%, while interleaf values vary by as much0.5%. For purposes of treatment planning calculations, 2%used for static transmission to points outside the field. Hoever, for DMLC fields using the ‘‘sliding window’’ tech-nique, each point in the field is shielded for a longer timethe leaves relative to the time it spends under the windresulting in an effective transmission of 4%–6% of the tofluence.

D. Head scatter

Relative output measurements were made in air usingion chamber, and radiographic film to extend the measuments to fields less than 1 cm2. The results are shown inFigs. 5~a! and 5~b! for fields of various sizes and shapeRelative to that of a 10310 cm2 field, the output for a 333 cm2 field is reduced by 2%, and for a 130.6 cm2 field by5%–6%. This reduction is small, about half that reportedothers when fields were defined by the jaws for a simmachine.13 This difference is probably due to the fact that t

FIG. 5. Output measurements in air at the isocenter with an ion chambewith radiographic film for~a! 6 MV x rays and~b! 15 MV x rays. The width~W! and length~L! are dimensions defined by the MLC. For all measuments, the jaws are set to 15315 cm2.


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Varian MLC is located below the lower jaws, and therefothe field size dependence is less sensitive to the distribsource and head scatter.

E. Leaf end transmission

Based upon the measured average transmission throthe MLC of 2.0% and the calculated effective leaf thickneof 5.65 cm, the linear attenuation coefficient is taken to0.692 cm21. Using this value and assuming a point sourthe photon transmission profiles are calculated for theedge positioned at the central axis and at 10 cm off axis~Fig.6!. The primary transmission under the leaf falls off to 50at 0.3 mm~projected distance at isocenter!, to 20% at 1.5mm, and to 4% at 6 mm. Beyond 8 mm the transmission fto 2%. The falloff is slightly faster when the leaf edge is oaxis.

The effect of the transmission through the rounded leafthe dose delivered by DMLC can be approximated by amm offset applied to the leaf position. The offset is repsented in Fig. 6 by the rectangle, which is equal to the aunder the transmission curve for the leaf end, except for2% MLC transmission. Thus, the added transmission duthe rounded leaf ends is equivalent to enlarging the field sof a focused collimator by 2.0 mm~1.0 mm for each opposedleaf!. As an example of the significance of this, the transmsion through the leaf ends will contribute 10% of the todose delivered by a nominal 2.0 cm gap moving at a consspeed across the field, since the effective gap is 2.2Although the ‘‘offset’’ varies slightly with leaf position~at10 cm off axis, it is 0.8 mm!, a single value is a good approximation.

The results of film dosimetry, i.e., integrating cross-fiedose profiles of static MLC fields of various widths, a

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FIG. 6. Calculated primary beam transmission through the rounded leafThe central axis and off-axis transmissions are based upon the geomethe leaf and the measured effective attenuation coefficient for the MLC.area under the central axis transmission curve~less the 2% corresponding tothe transmission for full leaf thickness! is equivalent to the rectangular arefrom 0 to 1 mm, indicating that the transmission due to the roundededges may be approximated in a dynamic treatment as an offset of the leby 1 mm.

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given in Fig. 7, with the relative integral dose plotted agaithe nominal field width. Regression analysis of the dyielded offsets of 2.0 and 1.7 mm for 15 and 6 MV, respetively, in excellent agreement with that derived from calclated transmission curves of the MLC leaf end.

IV. DISCUSSION

In the sliding window technique of DMLC applicationthe delivered dose is directly related to the gap betweenposed leaves as they sweep across the field. To appreciaimportance of gap width accuracy in DMLC, we related erin delivered dose to error in gap width for different nominwindow widths. As shown in Fig. 8, the dose error is larfor small gap width and large gap error. However, as psented earlier, the MLC position precision is better thanmm. In addition, for clinical intensity modulated treatmenat this time, relatively little dose is delivered with gap

FIG. 7. Integrated dose vs MLC gap width measured in phantom withdiographic film for 6 and 15 MV x rays. The lines are linear fits to the dusing least-squares regression. Extrapolation to zero integral dose dmines the effective gap offset. The uncertainties are the standard errothe data.

FIG. 8. Calculated results relating the error in the dose delivered to the ein the gap for a range of gap widths.


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,1 cm. Therefore, the dose error due to gap error is noissue. It is noteworthy that subtle changes in individual lepositions may be caused by gravity, drag due to friction,simply misalignment with the central axis; however, eachthese factors tends to shift both leaves in the same direcwith the result that the gap width is unchanged.

Figure 2~b! indicated that a leaf position tolerance settiof 0.1 mm could increase the treatment time of a typiprostate field from about 30 to 80 s. On the other haaccuracy in delivered dose either on the central [email protected]~a!# or at off-axis points~data not shown! is hardly affectedby the choice of tolerance setting. Therefore, for reasonstreatment time efficiency, dose accuracy, and potential hware problems with the leaves during DMLC operation,mm was chosen as the leaf position tolerance. Thus, unnormal operating conditions the leaf position tolerancenearly passive; however, it acts as a safety interlock shouleaf behave abnormally, e.g., becoming stuck during irradtion.

The implications of MLC transmission and transmissithrough the rounded leaf end are more serious for DMthan for static MLC treatments. The transmissions throuthe leaves and the leaf ends contribute to the dose througthe target, not just near or outside the field boundary asstatic MLC fields. Furthermore, the relatively small fieldassociated with the ‘‘sliding window’’ technique of DMLCtreatments tend to amplify the importance of the transmitphotons. Typically 1- to 4-cm-wide gaps are used for ttreatment of the prostate, which are uncommon for stafield treatments. The idealized curves in Fig. 9 indicatemagnitudes of these effects for a typical prostate treatmwith 15 MV. The curves are based upon the assumptiona gap of fixed width moves with constant speed acrosstarget. The fractional contribution of the rounded leaf endapproximately the ratio of the effective gap offset, 2 mm,the gap width. The dose transmitted through the MLC, wtypically vary from 3%–6% of the total target dose andrelated to the target width~7 cm for this example! divided bythe gap width. For DMLC in the treatment of prostate ca

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FIG. 9. Comparative influences of MLC transmission, rounded edge tramission, and head scatter on the target dose.

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cer, these combined transmissions will add 5%–20% oftarget dose for 1- to 4-cm-wide gaps. For static fields, onother hand, the transmissions produce only slight broadeof the penumbra and 2% primary fluence outside the fieboth of which can be managed as for cerrobend blocking

For a dynamic field, the influence of the MLC on outputmuch less than the curves in Fig. 5 suggest. As thewindow moves across the target, each point in the ta‘‘sees’’ overlapping portions of the extended source at dferent times. As a result, the source distribution associawith a dynamic field as a whole is nearly the same as thata static field with the same outer shape. It follows then, asstatic fields for this MLC design, that overall head scattera dynamic field is essentially independent of the MLCshown in Fig. 9; head scatter is defined mainly by the jsetting. Actually, due to the nonuniform intensity patterwithin a dynamic field, the contribution of head scatter wvary slightly within the field.

In our scheme, the head scatter and leaf transmissionaccounted for during the conversion from optimized intensprofiles to leaf motion patterns. The inclusion of MLC tranmission is explained in detail in a previous publication.3 Theeffects of head scatter and leaf end transmission are hanin the following manner. First, without considering thesefects, the leaf motions are generated from the ‘‘intendeintensity distribution. Then the head scatter contributioneach point, calculated by backprojecting the field openidefined by MLC and jaws, onto the source plane, and theend transmission profile are convolved with the leaf motpatterns to produce a ‘‘delivered’’ intensity distribution. Lemotions are then generated from a third ‘‘working’’ intensidistribution, which is modified until the ‘‘delivered’’ and‘‘intended’’ distributions agree to within 1%~<3 iterations!.That the corrections are properly applied is evidencedroutine comparisons of measured and calculated dose. Inverification study, shown in Fig. 10, the five intensity modlated fields for a particular prostate patient were used topose radiographic film parallel to the central axis in a cyldrical polystyrene phantom, 25 cm in diameter. The fidensities were converted to dose by calibrating the film12.5 cm depth. Dose calculations were then performedthe five fields in a similar phantom geometry using the thrdimensional treatment planning system. The agreemwithin and around the target region is very good.

Finally, something should be said about tongue agroove effects. These effects have been evaluated forclinical fields used for the intensity modulated prostate trements of our patients. They result in a reduction in the dbetween leaves, which tends to counteract the interleaf leage. For all the DMLC patients treated to date, film dosietry has been performed in a flat homogeneous phantomeach field and the measured dose distributions are compwith similarly calculated dose distributions. For the majorof the fields, the measured dose variation at the interregions is not detectable. The maximum variation obserwas a 5% dose reduction along a line 3 cm in length witfull width at half-maximum of 2 mm for one of five prostatfields for an intensity modulated plan. Obviously, the sev


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ity is lessened when the composite dose for all five fieldsconsidered. In addition, a new optimization strategy has bintroduced which should alleviate tongue and grooeffects.14 This will be tested in conjunction with our inversplanning optimization algorithm to determine its benefits.

V. SUMMARY

Commissioning a MLC for DMLC application requireconsiderable effort. This paper addresses the important isconfronting the Varian MLC user. The results presenabove can be divided into two categories. Those whichrelated to the alignments and calibration of the MLC needbe monitored carefully by the user to establish their accurand precision, since the DMLC application is much less terant of misalignment and poor calibration than conventiostatic field treatments. The second category comprisesavoidable factors which affect the output and which mustaccurately quantified and accounted for during the plantimization or dose delivery process. These include head ster and transmissions associated with midleaf, interleaf,leaf end components of the MLC.

ACKNOWLEDGMENTS

The authors wish to thank Dr. Wendell Lutz, Dr. RadMohan, Dr. Spiridon Spirou, and Dr. Jorg Stein for helpfdiscussions, and Dr. Chandra Burman for generating theculated dose distribution in Fig. 10. This work was supporin part by Grant No. CA 59017 from the National CancInstitute, Department of Health and Human Resources,thesda, MD.

FIG. 10. Comparison of calculated and measured dose distributionscorrections for MLC transmissions and head scatter. The distributionsgenerated using the five intensity modulated fields for a particular prospatient, superimposed upon a homogeneous cylindrical phantom.

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Physical and Dosimetric Aspects of MLC LoSasso

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