Volumetric Modulated Arc Therapy Volumetric Modulated Arc Therapy (VMAT): The future of IMRT?(VMAT): The future of IMRT?

Daliang CaoDaliang Cao

Swedish Cancer Institute, Seattle, Washington, USASwedish Cancer Institute, Seattle, Washington, USA

AcknowledgementsAcknowledgements

�� David M. ShepardDavid M. Shepard�� Matthew EarlMatthew Earl�� Timothy HolmesTimothy Holmes�� Muhammad AfghanMuhammad Afghan�� Cedric YuCedric Yu�� ShuangShuang LuanLuan

History of VMATHistory of VMAT

�� VMAT, formally known as Intensity Modulated Arc Therapy VMAT, formally known as Intensity Modulated Arc Therapy (IMAT), was first brought up by Dr. Cedric Yu in 1995.(IMAT), was first brought up by Dr. Cedric Yu in 1995.

IMAT (1)IMAT (1)

�� 19951995-- Initial paper described the delivery Initial paper described the delivery technique and demonstrated feasibility.technique and demonstrated feasibility.

Basics of IMATBasics of IMAT

�� IMAT is a rotational IMRT that can delivered IMAT is a rotational IMRT that can delivered using conventional linear accelerators with using conventional linear accelerators with conventional MLC.conventional MLC.

�� Radiation is on while gantry is rotating with Radiation is on while gantry is rotating with MLC leaves moving continuously.MLC leaves moving continuously.

�� Intensity modulation is created by overlapping Intensity modulation is created by overlapping arcs.arcs.

ARC 1

ARC 2

ARC 3

From Cedric Yu

IMAT DeliveryIMAT Delivery

Benefit of Rotational IMRTBenefit of Rotational IMRT

254425440.1550.1550.9330.9330.0380.0380.1510.1513333

254525450.1710.1710.9120.9120.0490.0490.1760.1762121

254225420.1800.1800.9080.9080.0530.0530.1870.1871515

257025700.1860.1860.8790.8790.0580.0580.2020.2021111

259925990.1920.1920.8550.8550.0640.0640.2220.22299

259725970.2060.2060.8670.8670.0640.0640.2420.24277

256425640.2150.2150.8140.8140.0900.0900.3180.31855

273327330.4880.4880.7470.7470.1240.1240.6650.66533

Total Integral Total Integral DoseDose

Mean Dose To Mean Dose To the Region At the Region At

RiskRisk

Minimum Dose Minimum Dose Cover ing 90% Cover ing 90% of the Target of the Target

(1.0 = Max)(1.0 = Max)

Standard Standard Deviation In Deviation In The Target The Target

DoseDose

Objective Objective Function ValueFunction Value

Number of Number of Beam Beam

DirectionsDirections

Improvement found by increasing the number of beam angles

Independent of the number of beam angles

D.M . Shepard, et al, Medical Physics, 26 pp. 1212 (1999); T.R. M ackie, et al, Intensity Modulated Radiation Therapy – The State of the Art, pp. 247 (2003)

IMAT IMAT –– Dynamic Conformal ArcsDynamic Conformal ArcsClinical Trial

�� In 2000, a Phase I clinical trial was conducted at In 2000, a Phase I clinical trial was conducted at the University of Maryland.the University of Maryland.

�� Plans were created using forward planning.Plans were created using forward planning.

�� Arcs were approximated as multiple shaped fields Arcs were approximated as multiple shaped fields spaced every 5 to 10spaced every 5 to 10ºº. .

�� Both coplanar and nonBoth coplanar and non--coplanar arcs were coplanar arcs were considered and wedge filters were incorporated in considered and wedge filters were incorporated in selected plans.selected plans.

�� Two sets of bilateral arcs.Two sets of bilateral arcs.

�� 1 set of arcs matches BEV of PTV.1 set of arcs matches BEV of PTV.

�� 1 matches BEV of PTV minus rectum.1 matches BEV of PTV minus rectum.

�� The weights of the arcs are optimized.The weights of the arcs are optimized.

IMAT IMAT –– Dynamic Conformal ArcsDynamic Conformal ArcsExample 1: Prostate

55--arc treatmentarc treatment

IMAT IMAT –– Dynamic Conformal ArcsDynamic Conformal ArcsExample 2: Spinal Ependymoma

IMAT IMAT –– Dynamic Conformal ArcsDynamic Conformal ArcsInitial ExperienceInitial Experience

�� 50 patients were treated in this trial: central 50 patients were treated in this trial: central nervous system (17 patients), head and neck nervous system (17 patients), head and neck (25 patients) and prostate (8 patients).(25 patients) and prostate (8 patients).

�� Results demonstrate that IMAT can be Results demonstrate that IMAT can be delivered safely and efficiently on a generaldelivered safely and efficiently on a general--purpose linear accelerator. purpose linear accelerator.

�� Average treatment time was 7.5 minutes.Average treatment time was 7.5 minutes.

IMAT-Dynamic Conformal ArcsCommercial Solutions

�� Multiple vendors offer solutions for planning and Multiple vendors offer solutions for planning and delivering dynamic conformal arcs.delivering dynamic conformal arcs.

�� Elekta offers this capability with the 3DLine Elekta offers this capability with the 3DLine mMLCmMLCand Ergo++ treatment planning.and Ergo++ treatment planning.

IMAT IMAT –– Current StatusCurrent Status

�� IMAT has largely withered on the vine due to a IMAT has largely withered on the vine due to a lack of appropriate delivery control systems and a lack of appropriate delivery control systems and a lack of robust inverse planning tools.lack of robust inverse planning tools.

�� Good NewsGood News:: Linear accelerator and treatment Linear accelerator and treatment planning vendors are currently addressing these planning vendors are currently addressing these needs and we will soon be able to realize the full needs and we will soon be able to realize the full potential of IMAT.potential of IMAT.

New Developments in IMATNew Developments in IMAT

IMAT to VMAT IMAT to VMAT –– New DevelopmentsNew DevelopmentsDelivery Control Systems

�� Elekta and Varian have introduced new linac control Elekta and Varian have introduced new linac control systems that will systems that will be able to change the MLC leaf be able to change the MLC leaf positions and dose rate while the gantry is rotating.positions and dose rate while the gantry is rotating.

�� Elekta Elekta →→ PreciseBeam InfinityPreciseBeam Infinity®®

�� Varian Varian →→ RapidArcRapidArc®®

�� Both are using the term Both are using the term Volumetric Modulated Arc Volumetric Modulated Arc Therapy (VMAT)Therapy (VMAT)..

RapidArc always uses single arc to deliver treatment

RapidArcRapidArc™™ vs. vs. ““ ConventionalConventional”” IMRTIMRT

Courtesy of Dave Mellenberg

RapidArcSingle-Arc plan

496 MU Conventional7-field IMRT

1685 MU

Elekta VMAT can be delivered using single or multiple arcs

Elekta: VMATElekta: VMAT

�The complex nature of VMAT treatment planning has been a primary barrier to routine clinical implementation.

�From one angle to the next in each VMAT arc, one must account for the interconnectedness of the beam shapes.

VMAT VMAT –– Inverse PlanningInverse Planning

�� Leaf motion between adjacent angles is limited by leaf Leaf motion between adjacent angles is limited by leaf travel speed and gantry rotation speed.travel speed and gantry rotation speed.

�� For example, if the gantry speed is 10 degree/sec and For example, if the gantry speed is 10 degree/sec and the leaf travel speed is 3 cm/sec, then the maximum leaf the leaf travel speed is 3 cm/sec, then the maximum leaf travel distance between two adjacent angles is 3 cm.travel distance between two adjacent angles is 3 cm.

d = 5 cmd = 0 cm

Gantry angle = 30

Not Not allowedallowed

Gantry angle = 40

Interconnectedness of Adjacent Beam ShapesInterconnectedness of Adjacent Beam Shapes

� We have developed an “arc-sequencing”algorithm that translates optimized intensity maps into deliverable VMAT arcs.

� The key feature is that the VMAT delivery constraints are included in the arc-sequencing.

� Highly conformal dose distributions can be achieved with limited number of arcs.

VMAT VMAT –– Arc SequencingArc Sequencing

How do we create VMAT plans?How do we create VMAT plans?

�� A stepA step--andand--shoot treatment plan is created is shoot treatment plan is created is created in the Pinnaclecreated in the Pinnacle33 treatment planning treatment planning system with beams separated by 10 degrees.system with beams separated by 10 degrees.

�� The optimized intensity maps are extracted and The optimized intensity maps are extracted and sent to our arcsent to our arc--sequencing algorithm.sequencing algorithm.

�� The sequencer produces a VMAT plan that is The sequencer produces a VMAT plan that is read back into Pinnacleread back into Pinnacle33 for a final for a final convolution/superposition dose calculation.convolution/superposition dose calculation.

Arc Sequencer Arc Sequencer –– DetailsDetails

�� The algorithm uses a simulated annealing based The algorithm uses a simulated annealing based optimization and minimizes the discrepancy between optimization and minimizes the discrepancy between the optimized and sequenced intensity maps.the optimized and sequenced intensity maps.

�� The algorithm iteratively changes the leaf positions The algorithm iteratively changes the leaf positions and aperture weights and rejects any change that and aperture weights and rejects any change that violates an VMAT delivery constraint. violates an VMAT delivery constraint.

A prostate caseA prostate case

A 4-arc VMAT plan

95%

50%

GTV

PTV

Bladder

Rectum

A lung caseA lung case

4-arc IMAT plan Helical tomotherapy plan

The isodose plot on a same axial slice suggests very similar dose distr ibutions for both VMAT and tomotherapy plans.

DVH comparisons for the lung caseDVH comparisons for the lung case

Another prostate caseAnother prostate case

5 Arc VMAT plan

100%

PTV1PTV1

PTV2PTV2Bladder

Another Prostate Case (II)Another Prostate Case (II)

A headA head--&&--neck case with three prescription levelsneck case with three prescription levels

axial coronal

� Three targets with prescr iption levels of 60, 54, and 51 Gy

� A VMAT plan with 9 350º axial coplanar arcs was generated

� The five isodose levels in the above figures are 60, 54, 51, 35, and 24 Gy.

CTV1

CTV2CTV2

CTV2

CTV1

CTV3

LT Parotid

RT Parotid

DVH comparisons for this H&N caseDVH comparisons for this H&N case

An orbit case using sagittal arcAn orbit case using sagittal arc

�� The prescriptions to The prescriptions to GTV and PTVGTV and PTV--GTV GTV are are 7070 and and 63 Gy63 Gy, , respectively. respectively.

�� A VMAT plan with A VMAT plan with four four 210210ºº sagittal arcs sagittal arcs were created for this were created for this case.case.

A Brain GBM caseA Brain GBM case

axial sagittal� Both coplanar and non-coplanar VMAT plans were

generated for this case.

� The above isodose plots are from the non-coplanar VMAT plan. Five isodose levels are plotted: 100%, 80%, 70%, 50%, and 30%.

DVH comparisons for this brain caseDVH comparisons for this brain case

New development of the Arc SequencerNew development of the Arc Sequencer

�� Capable of generating both SingleCapable of generating both Single--Arc Arc & Multiple& Multiple--Arc VMAT plans Arc VMAT plans

95%

GTVPTV

Bladder

RectumHips

Single Arc: A Prostate CaseSingle Arc: A Prostate Case

�� MU for this case is 702 per fraction with 200cGy per fraction.MU for this case is 702 per fraction with 200cGy per fraction.�� Delivery time is about 2 minutes.Delivery time is about 2 minutes.

95%

Single Arc: Another Prostate CaseSingle Arc: Another Prostate Case

98%

Single Arc: Another Prostate Case (II)

�� 554 MUs to deliver 180 554 MUs to deliver 180 cGycGy; Delivery time ; Delivery time ≈≈ 2 minutes2 minutes�� 7 fields IMRT 7 fields IMRT -- 626 MUs, 6 to 8 minute delivery626 MUs, 6 to 8 minute delivery

Single Arc: A Brain CaseSingle Arc: A Brain Case

�� MU: 267 per fraction with 200 MU: 267 per fraction with 200 cGycGy per fractionper fraction

�� Delivery time about 2 minutesDelivery time about 2 minutes

Single Arc: A Brain Case (II)Single Arc: A Brain Case (II)

Solid lines: VMAT single-arcDashed lines: 9-field IMRT

Cord

PTV

Single Arc: HeadSingle Arc: Head--&&--Neck Case (I)Neck Case (I)

VMAT: single-arc, 532 MU; IMRT: 89 segments, 728 MU

Single Arc: HeadSingle Arc: Head--&&--Neck Case (II)Neck Case (II)

�� 566 MU to deliver 180 566 MU to deliver 180 cGycGy per fraction; per fraction; ~2 minutes to deliver~2 minutes to deliver

Single Arc: HeadSingle Arc: Head--&&--Neck Case (II)Neck Case (II)

172° 170° 168° 166° 164°

162° 160° 158° 156° 154°Max leaf motion is 2cm per 2 degree

VMAT vs. StepVMAT vs. Step--&&--Shoot IMRTShoot IMRT

7-9266842280

(7flds)1Lung

8-10272853289

(9flds)1H& N (3)

15-20

10-15

15-20

6-8

6-8

1045

1010

1120

418

626

169(9flds)

127(9flds)

172(9flds)

57(7flds)

69(7flds)

IMRTIMRTIMRT VMATVMATVMAT

24241H& N (4)

24522H& N (2)

25661H& N (1)

22671Brain

25541Prostate

Estimated Delivery time (minutes)

number of MUnumber of

Arcs/Segments

SummarySummary

�� VMAT is a rotational approach to IMRT that can VMAT is a rotational approach to IMRT that can be delivered using conventional linear be delivered using conventional linear accelerators with conventional MLC.accelerators with conventional MLC.

�� VMAT can provide highly conformal dose VMAT can provide highly conformal dose distribution and improve the IMRT delivery distribution and improve the IMRT delivery efficiency significantly.efficiency significantly.

�� Both Elekta and Varian will be offering VMAT Both Elekta and Varian will be offering VMAT delivery capability.delivery capability.

Thank you!Thank you!