Travis J. McCawUnder the supervision of Dr. Larry DeWerd

Medical Radiation Research CenterDepartment of Medical Physics

University of Wisconsin, Madison, WI

NCCAAPM Fall MeetingOctober 24, 2014

Characterization of interplay errors in step-and-shoot IMRT of the lung

MedicalRadiationResearchCenter

Treatment plans are prepared using a static image of patient anatomy

Patient motion during treatment alters delivered dose relative to planned dose

Dose blurring Averaging of delivered dose distribution over the path of motion Dose gradients are reduced or “blurred” Managed with expanded treatment margins and motion restriction AAPM TG-761

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Target motion during IMRT: Blurring

1. P. J. Keall, G. S. Mageras, J. M. Balter, et al., Med Phys 33, 3874-3900 (2006).

TARGET

Dose perturbations observed within an oscillating target volume in a simulated tomotherapy treatment1-3

Attributed to interplay of beam and target motion

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Target motion during IMRT: Interplay

1. J. Yang, T. Mackie, P. Reckwerdt, et al., Med Phys 24 425-436 (1997).2. C. Yu, D. Jaffray, and J. Wong, Phys Med Biol 43, 91-104 (1998).3. M. Kissick, S. Boswell, R. Jeraj, et al., Med Phys 32 2346-2350 (2005).

Figure courtesy Kissick et al.3

Calculated tomotherapy delivery errors up to 9% for measured target motion with 3 mm peak-to-peak amplitude1

Attributed to interference between target and couch motion frequencies Reduced to 1.3% when target motion within couch frequency range was

filtered out

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Interplay Interference

1. G. S. Tudor, S. V. Harden, and S. J. Thomas, Med Phys 41, 031704 (2014).2. M. W. Kissick, R. T. Flynn, D. C. Westerly, et al., Phys Med Biol 53, 4855-4873 (2008).

Figure courtesy Kissick et al.2

TG-76 recommendations based exclusively on motion amplitude1

Frequency-dependent errors due to interference can be substantial Unique to each treatment modality2

Interference with characteristic modulation frequencies of conventional linac treatments has not been considered

Goals Identify characteristic modulation frequencies in SS-IMRT lung

treatments Quantify potential interplay errors Progress toward frequency-dependent motion-management criteria

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Motivations and goals

1. P. J. Keall, G. S. Mageras, J. M. Balter, et al., Med Phys 33, 3874-3900 (2006).2. M. W. Kissick and T. R. Mackie, Med Phys 36, 5721-5722 (2009).

Modeled 6 MV beam from Varian Clinac 21EX accelerator using BEAMnrc1

Included Millennium MLC 120 leaf model Measured and simulated profiles agree within 2%/1mm

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Linear accelerator model

1. D. W. Rogers, B. A. Faddegon, G. X. Ding, et al., Med Phys 22, 503-524 (1995).

MLC not to scale

Gafchromic® EBT2 films Films were laser cut with a

tolerance of ±0.08 mm Positioning uncertainty within

phantom of ±0.1 mm

1 mm-thick Virtual Water™ spacers interleaved between films Improved spacing uniformity

Film stack dosimeter specifications: 3.8 cm diameter 2.7 cm height 22 films and 21 spacers

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Film stack dosimeter

T. J. McCaw, J. A. Micka, and L. A. DeWerd, Med Phys 41, 052104 (2014).

Prepared two lung treatment plans using Eclipse v10 TPS 7 field, step-and-shoot delivery Modified patient CTV to dimensions of film stack dosimeter housing

RLL target 70 Gy/35 fractions, D98% = 100% Coplanar fields ITV margins1: LR-1 mm, AP-1 mm, SI-7 mm PTV margins: uniform 5 mm

RUL target 48 Gy/4 fractions (RTOG 0915), D95% = 100% Non-coplanar fields ITV margins1: LR-1 mm, AP-2 mm, SI-3 mm PTV margins: uniform 5 mm

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Treatment plan preparation

1. Y. Seppenwoolde, H. Shirato, K. Kitamura, et al., Int J Radiat Oncol Biol Phys 53, 822-834 (2002)

Measured IMRT delivery with film stack dosimeter undergoing respiratory motion1

Simulated measured dose and compared with measurement

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Interplay measurements

1. M. W. Kissick, R. T. Flynn, D. C. Westerly, et al., Phys Med Biol 53, 4855-4873 (2008).

Motion phantom

Thorax phantom

Film stack dosimeter

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Interplay simulationsModel target motion

Model beam

delivery

Simulate treatment delivery

Reconstruct delivered

dose

t1 t2 t3 t4 Target position binned into 1.25x1.25x1.25 mm3 voxels

Simulations of treatment delivery used DOSXYZnrc

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Comparison of simulations and measurements

Plan Motion 3%/3mm gamma (%)

IMRT

Static98.7

99.7

1Da99.2

98.6

3Db98.4

99.8

SBRT

Static99.8

99.1

1Dc98.4

97.7

3Db98.6

97.5a30 s period, 20 mm amplitudebMotion parameters from Y. Seppenwoolde, et al., Int J Radiat Oncol Biol Phys 53, 822-834 (2002)c40 s period, 10 mm amplitude

Investigated interplay errors for 1D respiratory motion 1 s to 180 s period 5 mm to 15 mm amplitude 20 initial phases 600 MU/min

Blurring dose reconstruction Reconstruct delivery of entire treatment to each target positionWeighted sum of resulting distributions based on time at each position

Isolated interplay errors from blurring errors

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Interplay frequency dependence(1)

Dose differences normalized to prescription dose D98 recommended by ICRU 83 as near-minimum dose surrogate

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Interplay frequency dependence(2)

1. V. Grégoire, T. R. Mackie, W. D. Neve, et al., ICRU Report 83 (2010).

Segment IntrafieldInterfield Segment

Intrafield

Interfield

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Interplay frequency dependence(3)

Dose differences normalized to prescription dose

Segment Segment

Intrafield IntrafieldInterfield

Interfield

Conclusions Negligible interference over typical respiratory periods (3 to 5 s)

Greatest interference over timescales of intra- and interfield modulation Reduction in D98 relative to blurring dose in excess of 4% Up to 2% reduction in D98 for 5 mm motion amplitude

Amplitude-based motion-management criteria may provide sufficient mitigation of interplay errors in SS-IMRT

Future work Incorporate motion irregularities Investigate impact of fractionation Repeat SBRT study for FFF dose rate

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Conclusions and future work

Dr. Larry DeWerd Dr. Wes Culberson John Micka Dr. Michael Kissick

Ben Palmer Cliff Hammer Scott Johnson (Med-Cal, Inc.)

Dr. Jennifer Smilowitz

Dan Anderson Jeff Radtke

UWMRRC students and staff UWADCL customers for their continued support of our research program

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Acknowledgements

Energy independent within 1.5% at 6 MV

Orientation independent within ±1.5%

Water equivalent within ±1.5%

Consistent with TLD measurements within overall measurement uncertainty of 5.8% (k = 2)

Measurements of IMRT distribution had 98% agreement with TPS (3%/2mm)

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Summary of characterization

T. J. McCaw, J. A. Micka, and L. A. DeWerd, Med Phys 41, 052104 (2014).

Investigate sensitivity of verification to phase shifts in the motion Successive phase shifts of π/20 applied to simulated waveform

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Sensitivity of verification

Exposure Plan Phase offset Gamma (%)

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IMRT π/20 96.0

IMRT π/10 95.1

IMRT 3π/20 94.4

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IMRT π/20 94.3

IMRT π/10 95.8

IMRT 3π/20 93.0

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SBRT π/20 97.7

SBRT π/10 93.2

SBRT 3π/20 90.0

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SBRT π/20 93.8

SBRT π/10 81.7

SBRT 3π/20 76.3