Dynamic collimator and dose rate control: Enabling technology for Enhanced Dynamic Wedge

Copyright

ELSEVIER

Medical Dosimetry, Vol. 22. No. 3, pp. 167-170, 1997 0 1997 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0955.3947197 $17.00 + .oo

PII s0958-3947(97)00013-7

DYNAMIC

COLLIMATOR

TECHNOLOGY

AND DOSE RATE FOR ENHANCED

CONTROL: ENABLING DYNAMIC WEDGE

DENNIS D. LEAVITT, PH.D.,’ CALVIN HUNTZINGER, M.S.,’ THANOS ETMEKTZOGLOU, M.S.2 ’ Departmentof RadiationOncology. University of Utah Schoolof Medicine,Salt Lake City, UT: ’ Varian Oncology Systems. PaloAlto, CA Abstract-The application are dynamic control implementation Key Words:

key features that make dynamic dose delivery possible in the Enhanced Dynamic Wedge computerized position and control of the independent collimating jaws and computerized of the linear accelerator dose rate. These features will be described and related to the current of Enhanced Dynamic Wedge. 0 1997 American Association of Medical Dosimetrists.

Enhanced

dynamic

wedge,

Conformal

therapy.

within individual segments.This is illustrated in Fig. 1. The cumulative dose is displayed as a line graph as a percentage of the total dose delivered; the differential dose is displayed as a bar graph showing the percentage of total dose delivered per cm segmentof travel of the moving jaw. (The moving jaw is stopped 0.5 cm before reaching the stationary jaw; therefore, the fluence profile will be normalized to the value 0.5 cm less than the position of the stationary jaw.) The STT for a specific field is derived from this fluence profile, or “Golden STT.” The generation of a derived STT for a given field width and wedge angle consistsof the following steps:

INTRODUCTION The Varian Enhanced Dynamic Wedge (EDW) combined variable dose rate capabilities with a new generation of computerized dynamic collimator control to deliver wedge fields. This implementation supports standardwedge anglesof 10, 15,20,25. 30,45, and 60”, and allows any intermediate wedge angle by superposition of open field and 60” wedge. The full range of field widths from 4-30 cm wide is achieved by using a single table of dose delivery instructions. This table of instmctions, the SegmentedTreatment Table (STT), definesthe fraction of total dose to be delivered vs. position as the moving collimator jaw smoothly traversesits path across the field while the beam is on. Truncation of this single table and combination with open field doseprovides dose delivery instructions for any field width lessthan 30 cm for any wedge angle from zero degreesthrough 60 degrees. The techniques that are used to provide these capabilities to the operator will be discussedin this paper.

SEGMENTED

TREATMENT

1. Read the fluence (“Golden STT”) for the selected energy from disk. 2. Derive the fluence for selected effective wedge angle. 3. Truncate fluence to selectedfield size. 4. Normalize fluence to total dose. 5. Compute dose rate and collimator speedsfor all segments.

TABLE These steps are illustrated in the flow charts of Fig. 2. The following differences are noted between the golden STT and the derived STT: The golden STT is stored on computer disk in a tabular representation of dose vs. collimator position. The golden STT is expressedas a fraction of the dose to be delivered, while the derived STT will be expressedin actual dose for a specific EDW setup.The conversion from fractional dose to actual dose can only be done after the operator specifies the actual Monitor Units (MU) for the specific EDW treatment. Additionally, the derived STT covers the specific field width for the treatment, while the golden STT covers the entire 30 cm field. Finally, the derived STT corresponds to the effective wedge angle, while the golden STT is specifically for the 60” wedge angle. The 60” golden STT is combinedwith the open field

A separatesingle predefined fluence profile set exists for each available photon energy. These fluence profiles are referred to as “Golden STTs.” However, for a given photon energy, the same Auence profile set is usedfor all linacs producing that photon energy. Thus, a Varian Clinac 2 1OOCwith 6 MV and 18 MV photons would usedifferent fluence profile setsfor 6 MV and 18 MV. But a Varian 600C with 6 MV photon mode would use the same Auence profile set applied to the 6 MV modeof the 2 1OOC,analogousto using the samephysical wedge for both classesof machine. The profile can be evaluated as a table of cumulative dose vs. moving jaw position, or as a table of differential dose delivered Reprintrequests to: DennisLeavitt.Ph.D., ation Oncology, City. UT 84132.

University

of Utah

School

Department of Radiof Medicine, Salt Lake

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Volume;22,Number 3, 1997

ocm Jaw Position

-1 Ocm

k

(cm)

Fig. 3. Weightedaveragingof the 60” goldenSTT and the open-fieldfluenceto generatethe derivedSTT for intermediate wedgeangles.The weightingof the openand60” goldenSTT aredeterminedbasedon theratio of tangentsmethoddescribed in the text.

The derived STT is then computed as the weighted averageof the dosein the open field and 60” golden STT, using the formula: Dose,= (Dose,,pen)Wopm + (Dose60.W6,,

I

I

a

I

I

I.

I

I

I

I

Jaw

Position

(cm)

Fig. 1. Percentof MU deliveredvs. jaw positionfor fluence profile correspondingto C121OOC 6X, 60” wedge.Profile is displayedas a solid line for the cumulativedose vs. jaw position,and as a bar graphfor the differential dosevs. jaw position. to generatethe derived STT correspondingto the desired effective wedge angle. The relative weighting of the 60” golden STT and the open field is determined using the Ratio of Tangentsmethod,’ where Wopen =

tan60”- tan0 tan60”

Figure 3 illustrates an intermediate angle derived STT formed asthe weighted sumof the open field fluence and the 60” golden STT. Once this intermediate angle derived STT has been formed, it is truncated to the field width desired. This is illustrated in Fig. 4. Pl and P2 represent the jaw positions corresponding to the actual field sizes desired. Thesejaw positionscan define any field within the limits of 20 cm to - 10 cm. Thus, asymmetric fields can be defined and the fluence will be truncated to fit that field. The derived fluence acrossthe actual field size is identically the sameas the fluence acrossthat fraction of the

t

,.; ::

tan0 W60~ = tan60”

x’.p”

0’

Jaw Position

Fig. 2. Flow chartdescribinggenerationof the STT from the fluenceprofile.

(cm)

$

Fig. 4. Truncationprocessto generatethe reducedwidth derived STT. The cumulativedosethat would have beendelivered in the fraction of the full field fluencefrom 20 cm to positionPl is now deliveredasthe openfield segmentat Pl. The fractionof cumulativedosethat wouldhavebeendelivered from P2 + 0.5 to - 10 cm is deleted.

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Dynamic collimator and dose rate controls 0 D. D. LEAVIR et al.

entire 30 cm field. In terms of actual dose delivery, this is equivalent to adding up all of the differential dose contributions that would have beendelivered from 20 cm to the Pl position of the reduced field size, and delivering this fraction as the initial open field component. As the moving jaw reaches a position 0.5 cm from P2, dose delivery stops. That fraction of the full field fluence remaining is not delivered. This is the truncation. So the cumulative dosethat would have been delivered from the maximum position of the moving jaw (20 cm) to the actual start position of the reduced field width (Pl) is delivered as the open field segmentusing the field width defined by the distance between Pl and P2; the additional dose that would have been delivered between the stop position of the moving jaw (P2-0.5 cm) and the final closure (full field effective angle tluence) is truncated. One of the benefits of this truncation process is that the effective wedge factors vary smoothly with wedge angle and field width. The truncated dose fluence is normalized by proportionally scaling the doseso that the final dose(doseat P2-0.5 in Fig. 4) is the total dose (Monitor Unit value) programmedby the Clinac operator. The derived STT is completely defined at the end of this step. Once the STT has been normalized, the dose rate and collimator speed is to be used for each segmentof the EDW treatment are calculated.

OPTIMIZATION OF DOSE RATE COLLIMATOR SPEED

AND

The control system automatically calculates the doserate and collimator speedfor each segmentindividually so that the segment is delivered in the shortest possibletime. Treatment time is minimized by choosing the maximum collimator velocity for each field segment that allows delivery of the required monitor units within that segment.Thus, in segmentsrequiring a small number of monitor units to be delivered, the maximum collimator velocity is set, while the doserate is lessthan the ceiling dose rate set by the operator. For segmentsrequiring a large number of MU, a slower collimator velocity is set, while the dose rate is changed to the maximum selectedby the operator. In order to minimize the time required to deliver an EDW dose, the doserate should be as high as possible and the collimator jaw speedshould be as fast as possible.The maximum jaw speedusedin EDW fields is I .Ocmlsec. Figures 5(a) and 5(b) illustrate the optimization of doserate and collimator speedduring a representative EDW field. The EDW treatmentscan be viewed asconsisting of two phases:an open field phaseand a collimator sweepphase.A11EDW treatments start with a fraction of the dose being delivered to the full field, that is, before the collimator starts moving to sweep the field, During this open field segment, the doserate is set to maximum and the collimator is not moving. At the completion of the open field

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Fig. 5.(a) Dose rate progression vs. time. The treatment starts

with the open-fieldsegment deliveredat maximumdose.At the completionof this segment,the doserate is reducedto deliver the requireddifferentialdoseacrossthe nextsegment.Thedose rate thenincreases asneededacrossthe remainingsegments to deliverthe requireddifferentialdoseacrosseachsegment, until the maximumdoserate is reached. Fig. 5. (b) Collimatorspeedprogression vs. time.The coliimator is stationary during the delivery of the open-fielddose segment.It then accelerates to its maximumspeedof 1 cm/set for the sweepacrossthe field. For the segments in which the doseto be deliveredis largerthan achievableusingmaximum doserate,the collimatorspeedisreducedto allow a largerdose delivery within that segment.Thisis illustratedby thelastthree segments, wherethe doserate is maximumandthe collimator speedis reduced. segment,the collimator acceleratesform zero to its maximum speedof 1.0 cmkec, and the doserate drops to a low-enough value that the required differential doseto be delivered in that segment will be uniformly delivered across the segment.At the completion of this segment, the dose rate is automatically adjusted upward so that a higher differential dosewill be delivered acrossthe next segment. This is illustrated in Fig. 5a as an increasing stair-step pattern in doserate in each segmentuntil the dose rate again reachesits maximum. (This corresponds to approaching the maximum in the MU vs. Jaw Position curve of Fig. 1.) At this point, a greater dose is required to be delivered within the segmentthan can be done with the jaw moving at maximum speed,so a lower jaw speed is used in order to deliver the required dosewithin each segment. This is displayed in Fig. 5b as a reduction in collimator speedvs. time. While the dose rate and jaw

170

Medical

Dosimetry

&oci~ may follow a stairstep pattern, dose delivery and &l.imator motion are, however, inontinuous.

3b?J’T DELIVERY

AND VIERIFICATION

Once the SIT for an EDW &eatment is computed, it w the dose vs. position path that the control ?ystern is mmmitted to follow during the treatment. The &se &&ered vs. position uniqudly determinas’the isoSince an EDW treatment first d$l&ersithe portion of the dose, the treatment !must start with the -al collimator positions match&g @3I.XJ aud COLKZ (points Rl and P2 in Fig. 4). /an &ii&l Position Mock (.IJ??i%@ ensures that treatmentdwnot start until the collirm&Brs are placed at their pmlpm%a%ting positioas- The @‘&N iinterlock lis not cleati unik?ss the cunmt colIima%m etions are within 0.1 ccun&mm the specitied COLL Xi a-&l COLLY2 values. Qsrneithe beam-oa is pressed & i&e Jreatment starts, d6B.e Lanll Imeans of the Wmae position coatrol are guarantees &at hip impli&l I@ the STT is accurately fohw&l. llf &her the collim&s~ position or the dose delivered is nnti:ti&hin specificaltismss, then the DPSN interlock is a.~&. The DPSN interI& guarantees that throughout the E39W Imeatment, no dose is delivered unless the collimator p&&on is within 0.2 cm of its required position. Sin&Q, the control system guarantees that at any point during an BDW treatment the total dose is always within 0.2 MU #of its required value. Thus, in terms of the dose vs. @tion path followed, the control system guarantees that at any point during an EDW treatment, the collimator position and dose does not deviate from the derived STT specified dose vs. position path by more than 0.2 MW and 0.2 cm, respectively.

TRACING ACCURACY STATISTICS FOR DOSE AND COLLIMATOR POSITION In addition to the DPSN interlock mechanism, the control system tracks dose and position accuracy statistics for each EDW treatment. Dose and position deviation statistics are related to the DPSN interlock. The 0.2 MU and 0.2 cm values associated with the DPSN interlock are interlock levels. If these levels are exceeded at any point during the treatment, the treatment is stopped by the DPSN interlock. However, actual errors during typical treatments are much smaller than 0.2 MU or 0.2 cm. The purpose of the statistics is to provide a measure of the actual deviation. The control system samples the actual dose and position throughout the EDW treatment in real-time. Any deviations are continuously logged in the control system memory at very fine, regular time intervals (samples). Typically, hundreds of samples are taken throughout an

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EDW treatment. The actual number of samples is proportional to treatment duration. At the.end of treatment, the dose and dose weighted position standard deviations are computed and displayed to the operator in the Treatment Complete message. Dose weighting of the position deviation reflects the fact that posittin deviations are irrelevant unlessdose is being delivenzd. The two standard deviations.are also logged onto the control system hard disk. These two statistical errors are the most informative representation of how closely the SlT specified dose/position path was followed during a specific EDW treatment.

EDW 3XEATMENT

RECORDS

.)Whenever amEDW treatment is performed, inforlmation about the WC treatment is stored in a spci81 %le, t.called a “D&B IfiIe.” ‘These Ales contain ?the ifallowing informati: ate and time the treatment i-s @&mimed; treatmentt B&Q [parameters (treatment ape, ene~g&, MU); the ti m. 1pMsition tracking accurqy +&ttisti~s; the derived m,, &at iis, the SIT that was emted and used fax t&e mlar treatment; &I nxxktime dose and posit&m szm~~~&m
CONCLUSION Enhanced dynamic wedge achieves the same clinical endpoint as conventional physical wedges. The importance of EDW is that it provides the first step toward more advanced conformal techniques. The promise exhibited with prototype research systems2-4 is now widely available for clinical use in the form of EDW and will serve as the basis for further advances in conformal therapy.

REFERENCES 1. Petti, P.L.; Siddon, R.L. Effective wedge angles with a universal wedge. Phys. Med. Sol. 30:985-991; 1985. 2. Kijewski, P.K.; Chin, L.M.; Bjamgard, B.E. Wedge shaped dose distributions by computer-controlled collimator motion. Med. Phys. 5~426-429; 1978. 3. Leavitt, D.D.; Martin, M.; Moeller, J.H.; Lee, W.L. Dynamic wedge field techniques through computer controlled collimator motion and dose delivery. Med. Phys. 17:87-91; 1990. 4. Leavitt, D.D. Dynamic beam shaping. Med. Dosim. 15:47-50; 1990.