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

Transcription

1 Characterization of interplay errors in step-and-shoot IMRT of the lung Travis J. McCaw Under the supervision of Dr. Larry DeWerd Medical Radiation Research Center Department of Medical Physics University of Wisconsin, Madison, WI NCCAAPM Fall Meeting October 24, 2014 Medical Radiation Research Center

2 Target motion during IMRT: Blurring 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 P. J. Keall, G. S. Mageras, J. M. Balter, et al., Med Phys 33, (2006). 2

3 Target motion during IMRT: Interplay Dose perturbations observed within an oscillating target volume in a simulated tomotherapy treatment 1-3 Attributed to interplay of beam and target motion TARGET Figure courtesy Kissick et al J. Yang, T. Mackie, P. Reckwerdt, et al., Med Phys (1997). 2. C. Yu, D. Jaffray, and J. Wong, Phys Med Biol 43, (1998). 3. M. Kissick, S. Boswell, R. Jeraj, et al., Med Phys (2005). 3

4 Interplay Interference Calculated tomotherapy delivery errors up to 9% for measured target motion with 3 mm peak-to-peak amplitude 1 Attributed to interference between target and couch motion frequencies Reduced to 1.3% when target motion within couch frequency range was filtered out Figure courtesy Kissick et al G. S. Tudor, S. V. Harden, and S. J. Thomas, Med Phys 41, (2014). 2. M. W. Kissick, R. T. Flynn, D. C. Westerly, et al., Phys Med Biol 53, (2008). 4

5 Motivations and goals TG-76 recommendations based exclusively on motion amplitude 1 Frequency-dependent errors due to interference can be substantial Unique to each treatment modality 2 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 1. P. J. Keall, G. S. Mageras, J. M. Balter, et al., Med Phys 33, (2006). 2. M. W. Kissick and T. R. Mackie, Med Phys 36, (2009). 5

6 Linear accelerator model Modeled 6 MV beam from Varian Clinac 21EX accelerator using BEAMnrc 1 Included Millennium MLC 120 leaf model Measured and simulated profiles agree within 2%/1mm MLC not to scale 1. D. W. Rogers, B. A. Faddegon, G. X. Ding, et al., Med Phys 22, (1995). 6

7 Film stack dosimeter 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 T. J. McCaw, J. A. Micka, and L. A. DeWerd, Med Phys 41, (2014). 7

8 Treatment plan preparation 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, D 98% = 100% Coplanar fields ITV margins 1 : LR-1 mm, AP-1 mm, SI-7 mm PTV margins: uniform 5 mm RUL target 48 Gy/4 fractions (RTOG 0915), D 95% = 100% Non-coplanar fields ITV margins 1 : LR-1 mm, AP-2 mm, SI-3 mm PTV margins: uniform 5 mm 1. Y. Seppenwoolde, H. Shirato, K. Kitamura, et al., Int J Radiat Oncol Biol Phys 53, (2002) 8

9 Interplay measurements Measured IMRT delivery with film stack dosimeter undergoing respiratory motion 1 Simulated measured dose and compared with measurement Motion phantom Film stack dosimeter Thorax phantom 1. M. W. Kissick, R. T. Flynn, D. C. Westerly, et al., Phys Med Biol 53, (2008). 9

10 Interplay simulations Model target motion Model beam delivery Simulate treatment delivery Reconstruct delivered dose Simulations of treatment delivery used DOSXYZnrc t 1 t 2 t 3 t 4 Target position binned into 1.25x1.25x1.25 mm 3 voxels 10

11 Comparison of simulations and measurements Plan Motion 3%/3mm gamma (%) IMRT SBRT 98.7 Static D a D b Static D c D b 97.5 a 30 s period, 20 mm amplitude b Motion parameters from Y. Seppenwoolde, et al., Int J Radiat Oncol Biol Phys 53, (2002) c 40 s period, 10 mm amplitude 11

12 Interplay frequency dependence (1) 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 position Weighted sum of resulting distributions based on time at each position Isolated interplay errors from blurring errors 12

13 Interplay frequency dependence (2) Dose differences normalized to prescription dose D 98 recommended by ICRU 83 as near-minimum dose surrogate Intrafield Segment Intrafield Interfield Segment Interfield 1. V. Grégoire, T. R. Mackie, W. D. Neve, et al., ICRU Report 83 (2010). 13

14 Interplay frequency dependence (3) Dose differences normalized to prescription dose Segment Intrafield Interfield Segment Intrafield Interfield 14

15 Conclusions and future work Conclusions Negligible interference over typical respiratory periods (3 to 5 s) Greatest interference over timescales of intra- and interfield modulation Reduction in D 98 relative to blurring dose in excess of 4% Up to 2% reduction in D 98 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 15

16 Acknowledgements 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 16

17 Summary of characterization 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) T. J. McCaw, J. A. Micka, and L. A. DeWerd, Med Phys 41, (2014). 17

18 Sensitivity of verification Investigate sensitivity of verification to phase shifts in the motion Successive phase shifts of π/20 applied to simulated waveform Exposure Plan Phase offset Gamma (%) IMRT π/ IMRT π/ IMRT 3π/ IMRT π/ IMRT π/ IMRT 3π/ SBRT π/ SBRT π/ SBRT 3π/ SBRT π/ SBRT π/ SBRT 3π/