Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?Gregory T. Betzel, Byong Yong Yi, Ying Niu, and Cedric X. Yu Citation: Medical Physics 39, 5882 (2012); doi: 10.1118/1.4749965 View online: http://dx.doi.org/10.1118/1.4749965 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/39/10?ver=pdfcov Published by the American Association of Physicists in Medicine

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?Gregory T. Betzel and Byong Yong YiDepartment of Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland 21201

Ying NiuDepartment of Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland 21201and Department of Physics, The George Washington University, Washington, DC 20052

Cedric X. Yua)

Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland 21201

(Received 13 February 2012; revised 19 August 2012; accepted for publication 20 August 2012;published 11 September 2012)

Purpose: Rotational IMRT has been adopted by many clinics for its promise to deliver treatmentsin a shorter amount of time than other conventional IMRT techniques. In this paper, the authorsinvestigate whether RapidArc is more susceptible to delivery uncertainties than dynamic IMRT usingfixed fields.Methods: Dosimetric effects of delivery uncertainties in dose rate, gantry angle, and MLC leafpositions were evaluated by incorporating these uncertainties into RapidArc and sliding windowIMRT (SW IMRT) treatment plans for five head-and-neck and five prostate cases. Dose distributionsand dose-volume histograms of original and modified plans were recalculated and compared usingGamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR).Results of Gamma analyses using passing criteria ranging from 1%–1 mm up to 5%–3 mm werereported.Results: Systematic shifts in MLC leaf bank positions of SW-IMRT cases resulted in 2–4 times higheraverage percent differences than RapidArc cases. Uniformly distributed random variations of 2 mmfor active MLC leaves had a negligible effect on all dose distributions. Sliding window cases weremuch more sensitive to systematic shifts in gantry angle. Dose rate variations during RapidArc mustbe much larger than typical machine tolerances to affect dose distributions significantly; dynamicIMRT is inherently not susceptible to such variations.Conclusions: RapidArc deliveries were found to be more tolerant to variations in gantry positionand MLC leaf position than SW IMRT. This may be attributed to the fact that the average segmentalfield size or MLC leaf opening is much larger for RapidArc. Clinically acceptable treatments maybe delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.© 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4749965]

Key words: IMAT, sliding window IMRT, RapidArc, delivery errors

I. INTRODUCTION

Despite recent debate on its theoretical justifications,1–4

the use of rotational intensity modulated radiation therapy(IMRT) has been readily adopted by many clinics for itspromise to deliver IMRT treatments in a shorter amountof time than other conventional IMRT techniques. As dif-ferent variations of intensity-modulated arc therapy (IMAT)(Refs. 5 and 6) have been implemented clinically, investiga-tors have steadily reported plan and dosimetric comparisonsfor several tumor sites as compared to other modalities.7–26

In addition to dynamic leaf motion as in dynamic-MLCIMRT, RapidArcTM (Varian Medical Systems, Palo Alto, CA)utilizes gantry rotation as well as variations in gantry speedand dose rate. Many studies have advocated that the inher-ent complexities of this technique require similar, but addi-tional, commissioning and quality assurance than that of con-ventional IMRT.27–29

An important task for dynamic MLC QA has been todetect systematic geometric errors, which can lead to large

dosimetric errors in the delivery of IMRT.30–32 Many sub-sequent studies have been performed focusing on primarilyfixed field dynamic IMRT to analyze the dosimetric impactof systematic as well as random leaf errors. For example, ina study of MLC and backup diaphragm errors for dynamicIMRT, Parsai et al.33 indicated that when MLCs or backup di-aphragms alone were perturbed, random errors of at least σ

= 1.5 mm were required to cause dose discrepancies greaterthan 5%, while systematic errors on the order of ±0.5 mmwere shown to result in significant dosimetric deviations. Ina study that proposed a Monte Carlo based IMRT dose veri-fication method, Luo et al.34 found that an average MLC leafpositional error of 0.2 mm can result in a target dose error ofabout 1.0%. Mu et al.35 studied the impact of random and sys-tematic MLC leaf position errors for head and neck IMRT pa-tients. They found that the dosimetric effect was insignificantfor random MLC leaf position errors up to 2 mm, but for a1 mm systematic error, the average changes in D95% were4% in simple plans versus 8% in complex plans with notabledifferences also seen in spinal cord, brainstem, and parotid

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glands. Rangel and Dunscombe36 also studied random andsystematic MLC positional errors in their study of dynamicIMRT. They found the impact of normally distributed ran-dom errors up to 2 mm to be negligible, but noted that if a2% change in equivalent uniform dose of the target and2 Gy for the organs at risk (OARs) were adopted as accept-able levels of deviation in dose due to MLC effects alone,then systematic errors in leaf position will need to be lim-ited to 0.3 mm. Ung et al.37 noted in their step-and-shootIMRT study that systematic MLC errors depended on the di-rection of movement of the MLC relative to the beam centralaxis.

The impact of MLC errors has also been investigatedbriefly with respect to Elekta’s implementation of IMAT,VMATTM. Ling et al.27 discussed MLC errors in their VMATcommissioning and QA study and observed a direct relation-ship between MLC leaf position errors and leaf speed. Ina VMAT study by Oliver et al.,38 a linear correlation wasobserved between MLC errors and generalized equivalentuniform dose (gEUD) for random and systematic errors.Specifically, the gEUD dose sensitivities for PTV70 foreight head-and-neck (HN) cases were −0.2, −0.9, −2.8, and1.9 Gy mm−1 for random, systematic shift, systematic close,and systematic open errors. They suggest that to create plansthat are robust against MLC errors, the total MU should beminimized and MLC gaps should be as large as possible with-out sacrificing plan quality. Tatsumi et al.39 used three differ-ent treatment planning systems to create VMAT plans for fiveprostate cases and tested pass rates when systematic MLC leafpositions errors were introduced directly into the linac con-troller. They found that the impact of leaf position errors ondose distributions depended upon the final optimization resultfrom each planning system due to a correlation between doseerror and average leaf gap width.

The dosimetric impact of random and systematic changesin gantry angle during IMRT delivery has been reportedby few authors. In their study of angular misalignments onfixed-portal IMRT, Low et al.40 presented a method that es-timates dose errors caused by unintended collimator, gantryand couch setting errors. Xing et al.41 noted that although an-gular setting misalignments play a smaller role than patientpositioning errors, they found that a 5◦ gantry error in onlyone of nine coplanar beams resulted in a 1.5% decrease in theminimum target dose or 5.1% in the maximum cord dose. Incontrast, it has been shown that the impact of slightly displac-ing the gantry angle of beam apertures is minimal to IMATdeliveries.42, 43

Oliver et al.42 reported on the dose sensitivity of deliv-ery errors on RapidArc prostate cases that included varyinggantry angle, MU, and MLC leaf positional errors. Systematicand random gantry position errors up to 1◦ were found to berelatively insignificant. Random MU errors up to 5% and ran-dom MLC position errors up to 2 mm were found to have aninsignificant effect on dose. Systematic MLC gap open/closeerrors had the greatest impact on gEUD. They described thedifference in gEUD(%) per mm of their RapidArc prostatecases versus other IMRT studies to be a function of the per-centage of mean gap widths < 2 cm.

Previous studies report on errors observed on specific de-livery systems primarily for fixed-field IMRT. As Oliver et al.indicated,42 a preferable comparison between two techniqueswould be to use the same patient cohort for the same deliveriesplanned with the same planning criteria. In this study, experi-ments were performed to assess the relative dosimetric impactof delivery errors using a dynamic IMRT technique versus aRapidArc technique. Unlike previous studies, the comparisonwas carried out with the same clinical cases using the sameoptimizer and dose calculation engine, which was modeledfor deliveries on the same linear accelerator.

II. METHODS

This study evaluated the susceptibility of RapidArc andsliding window (SW) IMRT to several types of delivery un-certainties in the delivery of five HN and five prostate cases.An EclipseTM treatment planning system (v8.6) with the an-alytical anisotropic algorithm was used to create all cases,which were planned for delivery on a Varian TrilogyTM witha 120 Millennium MLC (Varian Medical Systems, Palo Alto,CA). Using RapidArc, head-and-neck cases were planned us-ing a single arc rotation while all prostate cases were plannedusing two arcs. Seven-field IMRT plans using the sliding win-dow technique were then created on the same planning sys-tem. Beam angles of 0◦, 50◦, 100◦, 150◦, 210◦, 260◦, and310◦ were used for prostate plans. HN sites varied, and onlyone planned treatment volume (PTV) (of typically three) wasused to simplify the comparison of the delivery between thetwo techniques. HN beam angles were equidistant, but somevaried for sparing of critical structures by up to 10◦. Cases foreach site varied in complexity and target volume; planningTarget volumes (PTV) and total monitor units (MU) for casesusing either technique are listed in Table I. Critical structuresincluded, but were not limited to, brainstem, cochlea, opti-cal chiasm, optical nerves, parotid glands, and spinal cord forhead-and-neck patients, and bladder, lymph nodes, rectum,and small bowel for prostate. For HN cases, particular atten-tion was paid to the following dose constraints for violation:brainstem, 0.1 cc < 55 Gy; spinal cord, 10 cc < 45 Gy. Head-and-neck prescription doses were 50.4 Gy/28 fxs for PTV1,9 Gy/5 fxs for PTV2, and 10.8 Gy/6 fxs for PTV3. The sameoptimization objectives and penalties were used for either SWor RapidArc cases, which were checked for compliance. Aconformity index (CI) was used as a tightness-of-fit of thePTV to the prescribed isodose volume, which was defined asthe volume receiving 95% of the isodose divided by the PTV.A homogeneity index (HI) was also used to quantitativelycompare target dose homogeneity between original slidingwindow and RapidArc plans, which was calculated as(D2–D98)/ Dmean. Average CIs for HN plans were 0.97 ± 0.05and 0.98 ± 0.01 for SW and RapidArc, respectively; forprostate plans, 0.96 ± 0.05 and 0.94 ± 0.05 for SW andRapidArc, respectively. Average HIs for HN plans were0.12 ± 0.01 and 0.11 ± 0.02 for SW and RapidArc, respec-tively; for prostate plans, 0.12 ± 0.02 and 0.10 ± 0.02 for SWand RapidArc, respectively. Note that differences betweenmost individual sliding window and RapidArc plans were

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TABLE I. Planning target volumes and total monitor units for cases usingeither seven-field sliding window (SW) IMRT or RapidArc (RA). Cases 1–5are head-and-neck and cases 6–10 are prostate.

Total MU

Case PTV Vol [cm3] SW RA

1 58 346 2202 236 812 2333 437 661 2964 431 1360 3305 609 996 3546 1330 1329 3837 1166 2564 3978 840 1242 4329 1102 1439 510

10 884 2108 545

small; the difference between the CI and HI of 8 of 10 plans(4 HN, 4 prostate) were within 0.01 and 0.02, respectively.

Plans were exported as DICOM RT files from Eclipse toa separate computer workstation. To emulate delivery uncer-tainties, modifications were made to cloned DICOM files tocreate variations in MLC leaf positions, gantry angle, anddose rate using an in-house IDL program (v8.0, ITT VisualSolutions, Boulder, CO). The modified DICOM RT files werethen imported back into Eclipse for dose calculation. Sce-narios were created that exemplified two types of changes inMLC leaf positions. Systematic errors were created by shift-ing the entire MLC leaf bank (60 leaves) away from the cen-tral axis of the beam (X1 direction) for 0.5, 1.0, 1.5, and2.0 mm. A separate scenario involved adding uniformly dis-tributed random variations of ±2 and ±3 mm to individualleaves that were involved in shaping the beam aperture, i.e.,had any change in position during the delivery of the initialarc plan. Random MLC errors therefore included both initialcalibration errors as well as successive leaf positioning errorsduring delivery. Changes to the initial position or gantry an-gle were performed by rotating a SW or RapidArc plan by1◦, 2◦, 3◦, 4◦, or 5◦ in the positive (clockwise) direction. Tomatch the original cases and for calculations to proceed with-out error, any changes to the plan also required a start angleof 179◦ and a finishing angle of 181◦. Because of these twoangular constraints, any control point that fell between thesetwo angles was programmed to be placed in next closest openinterval and equidistant from adjacent control points. As a re-sult of the angular shift, plans were in practice rotated in thecounterclockwise direction.

Variations in nominal dose rate �MU/�t for RapidArccases resulted from a change in the gantry angle position for agiven control point. The MU weight �MU and control pointspacing �θ associated with a control point and its preced-ing control point were used to calculate �MU/�t for a givengantry angle θ , which can be defined as

(�MU

�t

)= �MU

(�θ

�t

)�θ−1,

where (�θ /�t) = 360◦/65 s = 5.54◦ s−1 is the referencegantry speed assumed to be constant for nominal dose rates≤600 MU min−1.27 With �MU and (�θ /�t) held constantusing this model, fluctuations in dose rate for modified planswere then reflected as a change in gantry angle, θ ′. In otherwords, the change in angular spacing forces a change in theactual MU per degree, which would emulate a change in theactual dose rate versus the planned dose rate for delivery.Dose rates for each segment were modified accordingly byadding a normally distributed random variation of σ = 10%,20%, 30%, 40%, or 50% of the calculated dose rate at eachcontrol point. As some changes in dose rate were large, theorder of control points was also allowed to change.

For the purposes of this study, the following physical con-straints were ignored while performing dose calculations inEclipse: leaf speed, gantry acceleration, and monitor units perdegree (MU/◦). This allowed the corresponding changes to bemade in this study including scenarios where apertures or con-trol points even changed order. Varying MU or gantry angleexplicitly is representative of potential errors in the planningstage and is not the focus of this study.

Dose statistics between original and modified plans werecompared using dose indices of PTVs and OARs fromdose-volume histograms (DVH), 2D Gamma analysis viaMapCHECK (Sun Nuclear Corp, Melbourne, FL) for pla-nar dose analyses and 3D Gamma analysis using inhouseMonte Carlo dose verification software. DVH statistics in-cluded minimum (Dmin), maximum (Dmax), and mean dose(Dmean). Gamma analysis criteria (% dose difference and mmdistance-to-agreement) ranged from 5%–3 mm down to 1%–1mm to evaluate both fine and coarse changes in the dose com-parison. Absolute dose and a threshold value of 10% (or per-cent contour above which plan points are included) was usedfor gamma comparisons. A mean leaf gap (MLG), which wascalculated using the distance between opposing MLC leavesthat participated in shaping the beam for delivery, was alsoused for plan and delivery comparisons in the study of sys-tematic MLC leaf position errors.

III. RESULTS

III.A. MLC leaf position errors

Figure 1 shows the results of DVH indices, e.g., averagepercent differences in Dmin, Dmax, and Dmean, for PTVs ofmodified head-and-neck and prostate plans following system-atic shifts in MLC leaf bank (X1) positions of 0.5, 1.0, 1.5, or2.0 mm. Average percent differences of PTV Dmean values permm bank shift using SW and RapidArc were 4.5% mm−1 vs.1.2% mm−1 for HN plans and 3.4% mm−1 vs. 1.0% mm−1

for prostate, respectively. The Dmean of most OARs were alsoabout three times as high for SW (∼6.5% mm−1) than forRapidArc (∼2.2% mm−1); mean dose changes to the brain-stem or spinal cord were <0.1% mm−1. Average maximumdose changes for brainstem and spinal cord in RA cases fora shift of 2.0 mm were 2.6% and 2.3%, respectively, and notviolate maximum dose constraints. For SW cases, however,one violated the maximum dose constraint with a bank shift

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FIG. 1. Comparison of average PTV minimum, maximum and mean dosevalues for original vs. modified (a) head-and-neck and (b) prostate cases us-ing either RapidArc (RA) or sliding window (SW) IMRT with systematicshifts in MLC (X1) leaf bank position.

of 1 mm. If the shift in the MLC bank was X2 and not X1, itis possible that other violations may have occurred for eitherSW or RA.

The average percent differences of PTV Dmean values permm bank shift for all plans were then plotted against MLG(Fig. 2). The figure illustrates that there is a clear differencebetween the two delivery techniques, and suggests that thereis a trend between dose sensitivity to systematic MLC errorsand MLG (power law curve fit, R2 = 0.94).

Figure 3 shows results of Gamma analyses performed tocompare differences in dose distributions using SW or Rapi-dArc while incorporating systematic shifts in an entire MLCleaf bank position. Distributions were evaluated using crite-ria of 1%–1 mm, 2%–2 mm, 3%–3 mm, and 5%–3 mm. Us-ing a criterion of 3%–3 mm, pass rates and therefore qual-ity of SW plans quickly fell below what may be requiredfor an acceptable delivery (>95%) for a bank shift <1 mm.Figure 4 illustrates the dosimetric effect of systematic shiftsin MLC leaf bank positions when using either SW or Rapi-dArc. In the coronal plane at isocenter, 2%–2 mm criterion

FIG. 2. Comparison of average PTV values per mm due to systematic shiftsin MLC (X1) leaf bank position vs. mean leaf gap using either RapidArc(RA) or sliding window (SW) IMRT for head-and-neck (HN) or prostate (P)cases. A power law curve is fitted for comparison.

FIG. 3. Side-by-side comparison of average Gamma pass rates for (a) head-and-neck and (b) prostate plans delivered using either RapidArc (RA) or slid-ing window (SW) IMRT cases with systematic shifts in MLC leaf bank posi-tion.

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FIG. 4. Dose distribution comparisons using 2%-2 mm Gamma analysis cri-terion illustrating pass rates for MLC leaf bank shifts of 0.5, 1.0, 1.5, and2.0 mm for (a) one HN case and (b) one prostate case. PTV contours areshown. Points that failed are indicated in red.

was used to show the relative changes in pass rates betweenthe use of SW and RapidArc. Points that failed the Gammatest are shown in red, i.e., a higher dose in comparison.

Uniformly distributed random shifts of up to 2 mm inactive MLC leaf positions showed no significant change inDmean of PTVs (<0.1%) for all plans using either RapidArcor SW. However, shifts of up to 3 mm resulted up to 0.7% and0.2% change in Dmean of PTV of head-and-neck and prostateplans, respectively. Average gamma pass rates of 99.8% werefound for shifts up to 2 mm using 1%–1 mm criterion; shiftsup to 3 mm ranged from 96.7% to 99.7% for head-and-neckplans and 98.8% to 100% for prostate plans, respectively.Data points that failed were found roughly equally inside andoutside PTVs. Differences in dose homogeneity were seen insome OARs when comparing the impact of random leaf er-rors. For example, differences in Dmin and Dmax values of thebladder and rectum when using SW were <1.5% as comparedto <0.7% when using RapidArc for prostate plans. However,this is most likely case-dependent given the beam angles usedfor a SW plan.

III.B. Gantry angle errors

When comparing averaged PTV minimum, maximum andmean dose values alone over the specified range of errors,no clear difference is seen between SW and RA as shown inFig. 5. Shifts in gantry angle slightly affected OARs (0.1%–0.2% deg−1) (not shown), and smaller changes were seen inPTVs (0%–0.05% deg−1). Some OARs were more susceptibleto changes in gantry angle due to close proximity to high dose

FIG. 5. Comparison of average PTV minimum, maximum and mean dosevalues for (a) head-and-neck and (b) prostate cases planned using either slid-ing window (SW) IMRT or RapidArc (RA) cases with systematic gantry an-gle variations.

gradients and distance to isocenter i.e. 0.4% deg−1 for cochleaand 0.1% deg−1 for bladder and small bowel. Steeper dosegradients were present in HN cases and thus roughly doubledpercent differences in Dmin and Dmax.

Figure 6 shows the results of gamma analyses performed tocompare the differences in dose distributions using SW IMRTor RapidArc with systematic shifts in initial starting beam an-gle of 1◦, 2◦, 3◦, 4◦, or 5◦. Unlike the DVH data, there is clearevidence that rotational delivery is less susceptible to the samesystematic changes in gantry angle. For example, using 3%–3mm for a 5◦ shift, prostate cases using SW passed at a rate ofonly 81.2% versus 96.4% for RapidArc. In addition, the sameSW prostate cases would fail a 95% pass rate with a shift ofonly ∼2◦.

The effect of systematic shifts in gantry angle is illustratedusing a HN case in Fig. 7. In the coronal plane at isocenter,a Gamma analysis criterion of 1%–1 mm was used to accen-tuate the relative changes in pass rates. Points that failed theGamma test are shown in either red or blue to indicate a higheror lower dose, respectively. The clear difference here is how

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FIG. 6. Average Gamma analysis pass rates for (a) head-and-neck and (b)prostate cases planned using either sliding window (SW) IMRT or RapidArc(RA) with systematic gantry angle variations.

FIG. 7. Dose comparisons illustrating Gamma pass rates (1%-1-mm) forsystematic gantry angle variations of 1◦, 3◦, and 5◦ for a HN case using SWIMRT (top) and RapidArc (bottom). PTV contours are shown. Red and blueindicate Fail Hot or Fail Cold, respectively.

FIG. 8. Comparison of average PTV minimum, maximum and mean dosevalues between original and modified head-and-neck (HN) and prostate (P)RapidArc cases with added random variations in dose rate.

quickly hot and cold pots appear in the overall SW dose dis-tribution with 1.7% of data points already failing after a 1◦

shift. The difference in pass rates of SW and RapidArc in-creases quickly and disproportionately as a function of shiftin gantry angle.

III.C. Dose rate

The resulting changes to DVH indices of RapidArc planPTVs with added random variations in dose rate are presentedin Fig. 8. An increase in dose rate variation for PTVs resultedin a statistical spreading of the delivered dose distributions,which explains the gradual decrease in Dmax and increase inDmin of PTVs; for most OARs, up to a 0.4% dose differencewas observed.

Gamma analysis of the effect of the added randomizeddose rate variations is shown in Fig. 9. For example, randomvariations in dose rate using σ = 10% and 20% had little ef-fect on RapidArc dose distributions (<0.2%) even when withgamma tests using 1%–1 mm criterion as illustrated in thecoronal plane at isocenter in Fig. 10. In the figure, a HN caseand a prostate case are presented with added random fluctua-tions of σ = 10%, 30%, and 50% of the original dose rate.

IV. DISCUSSION

Despite the added complexities of delivering RapidArc,this study provided evidence that a rotational IMRT techniquesuch as RapidArc is less susceptible to delivery errors than thesliding window IMRT technique.

Systematic MLC positional errors were shown to be ofgreater dosimetric impact on sliding window IMRT than Rap-idArc. The difference between the two techniques is in howintensity modulation is achieved: sliding window uses a largernumber of overlapping segments and MU, whereas RapidArcby means of rotation uses larger apertures and less MU toachieve the same target dose deposition. Geometric errors in

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FIG. 9. Average Gamma analysis pass rates for (a) head-and-neck and (b)prostate cases planned using RapidArc with added random variations in doserate.

smaller apertures, or those with smaller mean gap widths, willhave a larger impact on dosimetric errors. The susceptibilityto MLC leaf positional errors is inversely proportional to themean leaf gap width,38, 39, 42, 44 which account for the differ-ences seen in this study. The dosimetric error caused by thesystemic MLC leaf positioning errors should be roughly thequotient of the combined leaf position error of both opposingbanks and the mean segment width.

Random errors to active MLC leaf positions were found tohave little dosimetric effect on PTVs or OARs regardless ofwhich technique is employed, agreeing with previous studiesof IMRT and RapidArc.35, 36, 42

Previous studies have reported on several types of MLCerrors to observe their impact on patient dose distributions. Inthis study, a systematic shift in one leaf bank was sufficientto make a clear comparison between the two techniques,39

but is not the most significant type of MLC error as shownin previous studies, e.g., both leaf banks open.37, 38, 42 Largevariability in sensitivity to MLC errors (Dmean) between cases

FIG. 10. Dose comparisons illustrating Gamma pass rates (1%-1 mm) dur-ing a RapidArc delivery with dose rate variations of σ = 10%, 30%, and 50%for (a) a HN case and (b) prostate case. PTV contours are shown. Red andblue indicate “Fail-Hot” or “Fail-Cold”, respectively.

was not seen as compared to a previous study.42 However,variability was seen in Dmin and Dmax: σ RA = 0.3μ and σ SW

= 0.2μ for Dmax of prostate plans and σ RA = 0.4μ and σ SW

= 0.4μ for Dmax of head-and-neck plans. Differences betweenthe selected HN and prostate cases were due to differencesin dose conformity were achieved, as only one arc was re-quired for HN cases as compared to two arcs for prostatecases. Steep dose gradients accounted for the sharp drop ineither Dmin or Dmax following the addition of leaf positionalerrors.

By nature, RapidArc plans were optimized with fixed an-gles and at each angle with fixed field shapes but deliveredwith constantly changing field shapes and a constantly mov-ing gantry. Optimized MUs are never intended to be deliv-ered at the planned beam angle for the delivery of IMAT, andso dose deposition is fairly tolerant to small delivery errorsin gantry angle,6 which may include potential effects due togantry acceleration and deceleration.45 Using a small num-ber of fields in SW IMRT commits a plan to specific beampaths and corresponding intensities, which makes them moresusceptible to unwanted dose deposition to critical struc-tures. If acceptance criteria of patient-specific QA for IMRTare set at >95% for 3%–3 mm Gamma test criterion, sys-tematic errors up to ∼2◦ vs. 5◦ may be considered feasi-ble for the prostate plans in this study using SW vs. Rapi-dArc, respectively. Random changes in gantry angle were notexplicitly addressed here as it was changed implicitly dueto a change in dose rate. As noted previously, it has beenobserved that adding random gantry errors to beam aper-tures is minimal to IMAT deliveries,43, 46, 47 and, similarly, sowere the corresponding random variations in dose rate in thisstudy.

Adding random variations to dose rate had little effect tothe resulting dose distributions for RapidArc cases. As notedpreviously, dose rate variation is not an issue with dynamicIMRT as the only dependent variable during delivery is theshape of the aperture. Dose rate was defined as the time re-quired to deliver a fixed number of MU. A change in doserate thus required a change in gantry angle that was assignedto each segment, which in practice is compensation in gantryspeed and leaf speed velocity. This study modeled such com-pensations in the planning system only, and so this showedthat a capable linac should still deliver a clinically acceptabledose distribution.

As stated previously, errors in delivered MU were not ad-dressed explicitly in this study as it is treated as an indepen-dent variable; this applies to any dynamic delivery of IMRT.However, altering the shape of the aperture via shifts in MLCleaf positions clearly changed the total delivered MU per seg-ment, which, in our case, escalated the deposited dose. Studiesof “overshoot” or “undershoot” phenomena of MLC controlsystems that lead to MU delivery errors48, 49 have showed thatsuch effects are relatively small and do not compromise IMRTtreatments at higher dose rates (400 or 600 MU−1). Analy-sis of log files of an MLC control system also showed thatno clinically significant consequences were due to segmentdelivery errors, which were independent of planned segmentMUs.50 For RapidArc delivery, dose rate is variable and never

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turned off from segment to segment; as it is a different tech-nique, such control errors and thus MU error comparisons donot apply.

The results of the dosimetric comparisons presented sup-port the argument that RapidArc, and possibly other IMATand VMAT implementation, is not any more susceptible todelivery errors despite its added complexities. Therefore, QAprocedures for IMRT can similarly be used for IMAT de-liveries. A number of methods used for patient-specific QAof IMRT have been adapted for IMAT (see Yu and Tang6).This and other studies have demonstrated that detecting ge-ometric errors in MLC positioning is a primary concern formachine QA. Systematic errors during treatment delivery aremore significant rather than random errors in terms of clinicaloutcome;51 it is the same case at the planning stage, wherean accurate model of the MLC system in the treatment plan-ning system is also required. In either case, it is important toknow how sensitive a particular dosimetric technique, pairedto a particular linac, is to MLC leaf position errors as wellas other rotational or translational errors that could compro-mise quality. Masi et al.52 reported recently that DELTA4 andMAPCHECK dosimetric systems for VMAT QA varied insensitivity for some plans, where a 3◦ gantry angle offset in-troduced during delivery was rarely detected (with a pass-ratereduction below 4%), which we have shown to be expectedunless a more strict tolerance level is used. Rangel et al.53

pointed out that patient-specific IMRT QA cannot replaceroutine MLC QA as none of their IMRT QA criteria testedwere sufficiently sensitive to identify MLC offsets within atolerance of 0.3 mm on a single field basis. MLC log fileshave been used previously to quantify leaf positional errorsfor IMRT54, 55 as well as RapidArc.56, 57

We have offered evidence that attention must be paid to as-sure machine performance for its use in dynamic IMRT muchmore so than RapidArc. The relative differences in impact ofdelivery errors using these two techniques are large, but theyshould not be generalized despite slight differences in otherplanning and delivery systems; corresponding dose sensitivi-ties should ideally be quantified.38

V. CONCLUSIONS

This study compared the dosimetric impact of different de-livery errors in the radiation treatment of prostate and head-and neck cancers with two common IMRT delivery tech-niques: fixed-field IMRT and IMAT. RapidArc deliveries werefound to be more tolerant to variations in dose rate, gantryposition, and MLC leaf position than fixed-field IMRT withdynamic SW delivery. Dose rate variations during RapidArcmust be large to affect dose distributions significantly; dy-namic IMRT is inherently not susceptible to such variations.Clinically acceptable treatments may be delivered accuratelyusing RapidArc despite large fluctuations in dose rate andgantry position. Comprehensive QA procedures should be de-signed with the understanding of the different sources of er-rors and their relative dosimetric impacts to effectively ensurepatient safety and accuracy.

ACKNOWLEDGMENT

This work was supported, in part, by NIH Grant Nos.R01CA117997 and R01CA133539.

a)Electronic mail: cyu002@umaryland.edu; Telephone: +1 410 328 0324;Fax: +1 410 328 2618.

1T. Bortfeld, “The number of beams in IMRT—theoretical investigationsand implications for single-arc IMRT,” Phys. Med. Biol. 55, 83–97(2010).

2T. Bortfeld and S. Webb, “Single-arc IMRT?,” Phys. Med. Biol. 54, N9–N20 (2009).

3K. Otto, “Letter to the Editor on ‘Single-Arc IMRT?’,” Phys. Med. Biol.54, L37–L41 (2009).

4W. Verbakel, S. Senan, F. J. Lagerwaard, J. P. Cuijpers, and B. J.Slotman, “Comments on ‘Single-Arc IMRT?’,” Phys. Med. Biol. 54,L31–34 (2009).

5C. X. Yu, “Intensity-modulated arc therapy with dynamic multileaf colli-mation: an alternative to tomotherapy,” Phys. Med. Biol. 40, 1435–1449(1995).

6C. X. Yu and G. Tang, “Intensity-modulated arc therapy: principles, tech-nologies and clinical implementation,” Phys. Med. Biol. 56, R31–54(2011).

7M. C. Aznar, S. S. Korreman, P. M. Petersen, F. Kjaer-Kristoffersen, andS. Engelholm, “Doses to normal structures in the treatment of prostate can-cer using RapidArc versus 5 and 7 field IMRT,” Int. J. Radiat. Oncol., Biol.,Phys. 75, S726 (2009).

8J. Cai, R. McLawhorn, W. Yang, K. Wijesooriya, N. Dunlap, C. Geesey,K. Sheng, T. Rich, and S. Benedict, “Dosimetric comparison of 6 MV and15 MV RapidArc to helical tomotherapy for the treatment of pancreaticcancer,” Int. J. Radiat. Oncol., Biol., Phys. 75, S733–S734 (2009).

9D. Cao, T. W. Holmes, M. K. Afghan, and D. M. Shepard, “Comparisonof plan quality provided by intensity-modulated arc therapy and helical to-motherapy,” Int. J. Radiat. Oncol., Biol., Phys. 69, 240–250 (2007).

10I. Csiki, J. Crass, G. Ding, C. Coffey, D. H. Hallahan, and A. M.Malcolm, “RapidArc volumetric modulated therapy for localized prostatecancer: Comparison with intensity-modulated radiotherapy,” Int. J. Radiat.Oncol., Biol., Phys. 75, S731–S731 (2009).

11W. Eppinga, F. J. Lagerwaard, W. Verbakel, B. J. Slotman, and S. Senan,“Conventional IMRT versus volumetric modulated arc therapy (RapidArc)for pancreatic cancer,” Int. J. Radiat. Oncol., Biol., Phys. 75, S712–S712(2009).

12A. Fogliata, S. Yartsev, G. Nicolini, A. Clivio, E. Vanetti, R. Wyttenbach,G. Bauman, and L. Cozzi, “On the performances of intensity modulatedprotons, RapidArc and helical tomotherapy for selected paediatric cases,”Radiat. Oncol. 4, 2–20 (2009).

13J. D. Fontenot, M. L. King, S. A. Johnson, C. G. Wood, M. J. Price, andK. K. Lo, “Single-arc volumetric-modulated arc therapy can provide dosedistributions equivalent to fixed-beam intensity-modulated radiation ther-apy for prostatic irradiation with seminal vesicle and/or lymph node in-volvement,” Br. J. Radiol. 85(1011), 231–236 (2012).

14R. W. Kopp, M. Duff, F. Catalfamo, D. Shah, M. Rajecki, and K. Ahmad,“VMAT vs. 7-field-IMRT: Assessing the dosimetric parameters of prostatecancer treatment with a 292-patient sample,” Med. Dosim. 36, 365–372(2011).

15H. Mok, T. M. Briere, M. K. Martel, S. Beddar, M. E. Delclos, S. Krishnan,C. H. Crane, and P. Das, “Comparative analysis of volumetric modulatedarc therapy versus intensity modulated radiation therapy for radiotherapyof anal carcinoma,” Pract. Rad. Oncol. 1, 163–172 (2011).

16M. A. Morales-Paliza, C. W. Coffey, and G. X. Ding, “Evaluation of thedynamic conformal arc therapy in comparison to intensity-modulated radi-ation therapy in prostate, brain, head-and-neck and spine tumors,” J. Appl.Clin. Med. Phys. 12, 5–19 (2010).

17M. Oliver, W. Ansbacher, and W. A. Beckham, “Comparing planning time,delivery time, and plan quality for IMRT, RapidArc and tomotherapy,” J.Appl. Clin. Med. Phys. 10, 117–131 (2009).

18D. Palma, E. Vollans, K. James, S. Nakano, V. Moiseenko, R. Shaffer,M. McKenzie, J. Morris, and K. Otto, “Volumetric modulated arc therapyfor delivery of prostate radiotherapy: Comparison with intensity-modulatedradiotherapy and three-dimensional conformal radiotherapy,” Int. J. Radiat.Oncol., Biol., Phys. 72, 996–1001 (2008).

Medical Physics, Vol. 39, No. 10, October 2012

5890 Betzel et al.: Delivery error impact on dynamic and rotational IMRT 5890

19Y. Rong, G. Tang, J. S. Welsh, M. M. Mohiuddin, B. Paliwal, and C. X. Yu,“Helical Tomotherapy Versus Single-Arc Intensity-Modulated Arc Ther-apy: A Collaborative Dosimetric Comparison Between Two Institutions,”Int. J. Radiat. Oncol., Biol., Phys. 81, 284–296 (2011).

20M. Scorsetti, M. Bignardi, A. Clivio, L. Cozzi, A. Fogliata, P. Lattuada,P. Mancosu, P. Navarria, G. Nicolini, G. Urso, E. Vanetti, S. Vigorito, andA. Santoro, “Volumetric modulation arc radiotherapy compared with staticgantry intensity-Modulated radiotherapy for malignant pleural mesothe-lioma tumor: A feasibility study,” Int. J. Radiat. Oncol., Biol., Phys. 77,942–949 (2010).

21D. E. Spratt, J. G. Phillips, R. Diaz, J. Crass, W. Kirby, S. Stinson, andA. J. Cmelak, “Comparison of IMRT with planar RapidArc in diffuse cere-bral meningiomatosis,” Int. J. Radiat. Oncol., Biol., Phys. 75, S666 (2009).

22G. Tang, M. A. Earl, S. Luan, C. Wang, M. Mohiuddin, and C. X. Yu,“Comparing radiation treatments using intensity-modulated beams, multi-ple arcs and single arc,” Int. J. Radiat. Oncol., Biol., Phys. 76, 1554–1562(2009).

23S. Ulrich, “Comparison of arc-modulated cone beam therapy and helicaltomotherapy for three different types of cancer,” Med. Phys. 36, 4702–4710 (2009).

24W. Verbakel, S. Senan, F. J. Laerwaard, D. Hoffmans, B. J. Slotman, andJ. P. Cuijpers, “RapidArc vs. IMRT planning: A comparative study withdosimetric validation for head and neck, glioma and pancreas cancer,” Int.J. Radiat. Oncol., Biol., Phys. 72, S596–597 (2008).

25D. Wagner, H. Christiansen, H. Wolff, and H. Vorwerk, “Radiotherapy ofmalignant gliomas: Comparison of volumetric single arc technique (Rap-idArc), dynamic intensity-modulated technique and 3D conformal tech-nique,” Radiother. Oncol. 93, 593–596 (2009).

26D. C. Weber, H. Wang, L. Cozzi, G. Dipasquale, H. G. Khan, O. Ratib,M. Rouzaud, H. Vees, H. Zaidi, and R. Miralbell, “RapidArc, intensitymodulated photon and proton techniques for recurrent prostate cancer inpreviously irradiated patients: a treatment planning comparison study,” Ra-diat. Oncol. 4, 34–44 (2009).

27C. C. Ling, P. Zhang, Y. Archambault, J. Bocanek, G. Tang, and T. Losasso,“Commissioning and quality assurance of RapidArc radiotherapy deliverysystem,” Int. J. Radiat. Oncol., Biol., Phys. 72, 575–581 (2008).

28A. Van Esch, D. P. Huyskens, C. F. Behrens, E. Samsøe, M. Sjölin,U. Bjelkengren, D. Sjöström, C. Clermont, L. Hambach, and F. Sergent,“Implementing RapidArc into clinical routine: A comprehensive programfrom machine QA to TPS validation and patient QA,” Med. Phys. 38,5146–5166 (2011).

29J. O’Daniel, S. Das, Q. J. Wu, and F.-F. Yin, “Volumetric-modulated arctherapy: Effective and efficient end-to-end patient-specific quality assur-ance,” Int. J. Radiat. Oncol., Biol., Phys. 82(5), 1567–1574 (2012).

30G. J. Budgell, J. H. Mott, P. C. Williams, and K. J. Brown, “Requirementsfor leaf position accuracy for dynamic multileaf collimation,” Phys. Med.Biol. 45, 1211–1227 (2000).

31C. Burman, C.-S. Chui, G. Kutcher, S. Leibel, M. Zelefsky, T. LoSasso,S. Spirou, Q. Wu, J. Yang, J. Stein, R. Mohan, Z. Fuks, and C. C. Ling,“Planning, delivery, and quality assurance of intensity-modulated radio-therapy using dynamic multileaf collimator: A strategy for large-scale im-plementation for the treatment of carcinoma of the prostate,” Int. J. Radiat.Oncol., Biol., Phys. 39, 863–873 (1997).

32T. LoSasso, C. S. Chui, and C. C. Ling, “Comprehensive quality assurancefor the delivery of intensity modulated radiotherapy with a multileaf colli-mator used in the dynamic mode,” Med. Phys. 28, 2209–2219 (2001).

33H. Parsai, P. S. Cho, M. H. Phillips, R. S. Giansiracusa, and D. Axen, “Ran-dom and systematic beam modulator errors in dynamic intensity modulatedradiotherapy,” Phys. Med. Biol. 48, 1109–1121 (2003).

34W. Luo, J. Li, R. Price, L. Chen, J. Yang, J. Fan, Z. Chen, S. McNeeley,X. Xu, and C. Ma, “Monte Carlo based IMRT dose verification using MLClog files and R/V outputs,” Med. Phys. 33, 2557–2564 (2006).

35G. Mu, E. Ludlum, and P. Xia, “Impact of MLC leaf position errors onsimple and complex IMRT plans for head and neck cancer,” Phys. Med.Biol. 53, 77–88 (2008).

36A. Rangel and P. Dunscombe, “Tolerances on MLC leaf position accu-racy for IMRT delivery with a dynamic MLC,” Med. Phys. 36, 3304–3309(2009).

37N. Ung, C. Harper, and L. Wee, “Dosimetric impact of systematic MLCpositional errors on step and shoot IMRT for prostate cancer: a planningstudy,” Australas. Phys. Eng. Sci. Med. 34, 291–298 (2011).

38M. Oliver, I. Gagne, K. Bush, S. Zavgorodni, W. Ansbacher, and W.Beckham, “Clinical significance of multi-leaf collimator positional errorsfor volumetric modulated arc therapy,” Radiother. Oncol. 97, 554–560(2010).

39D. Tatsumi, M. N. Hosono, R. Nakada, K. Ishii, S. Tsutsumi, M. Inoue,T. Ichida, and Y. Miki, “Direct impact analysis of multi-leaf collimator leafposition errors on dose distributions in volumetric modulated arc therapy: apass rate calculation between measured planar doses with and without theposition errors,” Phys. Med. Biol. 56, N237–N246 (2011).

40D. A. Low, X. R. Zhu, J. A. Purdy, and S. Söderström, “The influence of an-gular misalignment on fixed-portal intensity modulated radiation therapy,”Med. Phys. 24, 1123–1139 (1997).

41L. Xing, Z.-X. Lin, S. S. Donaldson, Q. T. Le, D. Tate, D. R. Goffinet,S. Wolden, L. Ma, and A. L. Boyer, “Dosimetric effects of patient displace-ment and collimator and gantry angle misalignment on intensity modulatedradiation therapy,” Radiother. Oncol. 56, 97–108 (2000).

42M. Oliver, K. Bush, S. Zavgorodni, W. Ansbacher, and W. A. Beckham,“Understanding the impact of RapidArc therapy delivery errors for prostatecancer,” J. Appl. Clin. Med. Phys. 12, 32–43 (2011).

43G. Tang, M. A. Earl, and C. X. Yu, “Variable dose rate single-arc IMATdelivered with constant dose rate and variable angular spacing,” Phys. Med.Biol. 54, 6439–6456 (2009).

44T. LoSasso, C. S. Chui, and C. C. Ling, “Physical and dosimetric aspects ofa multileaf collimation system used in the dynamic mode for implementingintensity modulated radiotherapy,” Med. Phys. 25, 1919–1927 (1998).

45Y. Song, P. Zhang, C. Obcemea, B. Mueller, C. Chandra, and B. Mychal-czak, “Dosimetric Effects of Gantry Angular Acceleration and Decelera-tion in Volumetric Modulated Radiation Therapy,” in Proceedings of WorldCongress on Medical Physics and Biomedical Engineering, Vol. 25(1),edited by Dössel, Olaf, and Wolfgang C. Schlegel (Munich, Springer,2009), pp. 1046–1050.

46S. M. Crooks, X. Wu, C. Takita, M. Watzich, and L. Xing, “Aperture mod-ulated arc therapy,” Phys. Med. Biol. 48, 1333–1344 (2003).

47C. Wang, S. Luan, G. Tang, D. Z. Chen, M. A. Earl, and C. X. Yu,“Arc-modulated radiation therapy (AMRT): A single-arc form of intensity-modulated arc therapy,” Phys. Med. Biol. 53, 6291–6303 (2008).

48G. A. Ezzell and S. Chungbin, “The overshoot phenomenon in step-and-shoot IMRT delivery,” J. Appl. Clin. Med. Phys. 2, 138–148 (2001).

49V. Y. Kuperman and W. C. Lam, “Improving delivery of segments withsmall MU in step-and-shoot IMRT,” Med. Phys. 33, 1067–1073 (2006).

50A. M. Stell, J. G. Li, O. A. Zeidan, and J. F. Dempsey, “An extensive log-file analysis of step-and-shoot intensity modulated radiation therapy seg-ment delivery errors,” Med. Phys. 31, 1593–1602 (2004).

51J. R. Palta, S. Kim, J. G. Li, and C. Liu, “Tolerance limits and action lev-els for planning and delivery of IMRT,” in Intensity-Modulated RadiationTherapy: The State of the Art (Medical Physics, Wisconsin, USA, 2003),pp. 593–612.

52L. Masi, F. Casamassima, R. Doro, and P. Francescon, “Quality assuranceof volumetric modulated arc therapy: Evaluation and comparison of differ-ent dosimetric systems,” Med. Phys. 38, 612–621 (2011).

53A. Rangel, G. Palte, and P. Dunscombe, “The sensitivity of patient spe-cific IMRT QC to systematic MLC leaf bank offset errors,” Med. Phys. 37,3862–3867 (2010).

54P. Zygmanski, J. H. Kung, S. B. Jiang, and L. Chin, “Dependence of fluenceerrors in dynamic IMRT on leaf-positional errors varying with time and leafnumber,” Med. Phys. 30, 2736–2749 (2003).

55D. W. Litzenberg, J. M. Moran, and B. A. Fraass, “Verification of dynamicand segmental IMRT delivery by dynamic log file analysis,” J. Appl. Clin.Med. Phys. 3, 63–72 (2002).

56R. A. Popple, J. B. Fiveash, I. A. Brezovich, and J. A. Bonner, “RapidArcradiation therapy: First year experience at the University of Alabama atBirmingham,” Int. J. Radiat. Oncol., Biol., Phys. 77, 932–941 (2010).

57T. Teke, A. M. Bergman, W. Kwa, B. Gill, C. Duzenli, and I. A. Popescu,“Monte Carlo based, patient-specific RapidArc QA using Linac log files,”Med. Phys. 37, 116–123 (2010).

Medical Physics, Vol. 39, No. 10, October 2012