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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