Dosimetry of Small IMRT Fields

Transcription

1 Dosimetry of Small IMRT Fields Parham Alaei, Ph.D. Department of Radiation Oncology University of Minnesota NCCAAPM Symposium-October 10,

2 IMRT and Small Fields IMRT beams typically contain small fields or segments How small depends on the planner/planning system/institutional policy Minimum segment size used in planning should be equal or larger than that for which beam data was collected 2

3 Beam data for planning systems Without IMRT: 4x4 cm 2 40x40 cm 2 With IMRT: 2x2 cm 2 Or smaller 40x40 cm 2 3

4 Small field data challenges Detector size (affects profile and output factor measurements) Alignment of water tank, detector, etc. Jaw/MLC positioning errors Beam modeling in TPS IAEA and AAPM guidelines for absolute dosimetry of small fields TG-155: Small fields and non-equilibrium condition photon beam dosimetry 4

5 Focus on: Output Factors Output factors needed for treatment planning system commissioning Measurement field sizes/depths depend on the planning system used: Pinnacle: Range of field sizes (small->large), 10 cm depth, 100 cm SSD Eclipse: Range of field sizes (small->large), 10 cm depth, 100 cm SSD Xio: Range of field sizes (4x4-40x40), 10 cm depth Monaco: Range of field sizes (1x1-40x40), 10 cm depth, 100 cm SSD MasterPlan: Range of field sizes (5x5-20x20, smaller fields if needed), 10 cm depth, 100 cm SAD 5

6 TG 120 Recommendations Med. Phys. 38(3): (2011) 6

7 ASTRO Recommendations Practical Radiation Oncology (2011) 1,

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10 Total scatter factor S cp versus field size measured with various available radiation detectors for (a) 6 MV and (b) 15 MV beams. 10 Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry Med. Phys. 35: (2008)

11 S. Dieterich and G. W. Sherouse: Comparison of seven commercial dosimetry diodes for SRS Med. Phys. 38: (2011) 11

12 Focus on: Output Factors and IMRT If the output factors measured for input into the treatment planning system are wrong What is the consequence for small field plans (IMRT, SBRT, SRS, )? 12

13 Small fields and IMRT A typical IMRT plan has 5-10 segments per beam There are at least ~2-3 large segments per beam So there may be ~ 0-7 small segments A fraction of these are < 4 cm 2 Assuming 4 cm 2 is the cut-off for potential erroneous measurements So the error introduced by inaccurate small field beam data should be minimal This also depends on the planning system and/or delivery system (linear accelerator) 13

14 Medical Physics, Vol. 39, No. 8, August 2012 pp Pinnacle treatment planning system Elekta and Varian accelerators Results may be different for other planning systems! 14

15 Method Two five-field IMRT plans created on TomoTherapy cheese phantom (one for each linac) Plans optimized to deliver 200 cgy to a cylindrical target of 2 cm diameter and 2 cm length Four structures used as organs-at-risk DMPO, minimum segment area: 2 cm 2, minimum segment MU: 1 Ion chamber location contoured 15

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17 Output factor vs. field size 10 cm depth) Elekta Varian Relative Output Factors x2 3x3 4x4 5x5 6x6 8x8 10x10 Field Size (cm) 17

18 Method (Cont d) Small field output factors (2x2 and 3x3 cm 2 ) altered by ±5, 10, and 20% and machine re-commissioned 18

19 Altering small field output factors Relative Output Factors Measured -20% -10% -5% 5% 10% 20% x2 2x2 3x3 3x3 4x4 4x4 5x5 5x5 6x6 6x6 8x8 8x8 10x10 10x10 Field Size (cm) 19

20 Method (Cont d) Plan re-optimized every time Small change in segments between plans (0-2) All plans were delivered to the phantom with the ionization chamber positioned at isocenter In order to confirm the variations in output factors in the planning system, single static 2 2 cm 2 fields were created for each re-commissioned machine and delivered to the phantom 20

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23 Summary of plans Elekta Varian Plan Name No. of segments Monitor Units 20% Decrease % Decrease 530 5% Decrease 515 Base % Increase % Increase % Increase 468 X: cm Y: cm 40% of segments 3x3 cm 2 or smaller Plan Name No. of segments Monitor Units 20% Decrease % Decrease 51 5% Decrease 50 Base 53 5% Increase 53 10% Increase 50 20% Increase 55 X: cm Y: 4.0 cm 55% of segments 3x3 cm 2 or smaller 23

24 Measured vs. Calculated dose-elekta Percentage Dose Difference (cgy) % decrease 10% decrease 5% decrease Base 5% increase 10% increase 20% increase Percent Output Factor Variation 24

25 Measured vs. Calculated dose-varian 5.00 Percentage Dose Difference (cgy) % decrease 10% decrease 5% decrease Base 5% increase 10% increase 20% increase Percent Output Factor Variation 25

26 Measured vs. Calculated dose-summary Elekta % Difference (Meas. vs. Calc.) Varian % Difference (Meas. vs. Calc.) Plan Name Plan Name 20% Decrease % Decrease 3.6 5% Decrease 0.8 Base 0 5% Increase % Increase % Increase % Decrease % Decrease % Decrease -0.1 Base 0 5% Increase % Increase % Increase % of segments 3x3 cm 2 or smaller 26

27 Why the difference? Elekta X-diaphragm traces MLCs, Y-jaws conform to each segment Varian X & Y jaws fixed for the delivery of all segments, encompassing the largest segment 27

28 Why the difference? Elekta Handled as a conformal jaw machine in Pinnacle MUs calculated by employing small field output factors as a consequence of conforming the collimator jaws to each of the segments Varian Handled as a non-conformal jaw machine in Pinnacle MUs calculated by employing output factors of the larger-encompassing jaw size, and modeling blockedfield effects Varian segment MUs not sensitive to small field OF changes 28

29 Verifying the results using static fields Single, 6 MV, 2 x 2 cm 2 field incident on phantom Same machines with modified small field OF used to compute the dose with the TPS Dose calculated and measured at 3 cm depth for all plans 29

30 Verifying the results using static fields Single jaw/mlc-defined 2x2 cm 2 field Elekta % Difference Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.) 20% Decrease % Decrease % Decrease Base % Increase % Increase % Increase Single jaw-defined 2x2 cm 2 field Varian % Difference Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.) 20% Decrease % Decrease % Decrease Base % Increase % Increase % Increase Correlation between OF change and dose/mu delivered

31 Verifying the results using static fields Single MLC-defined 2x2 cm 2 field (jaws at 10x10) Varian % Difference Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.) 20% Decrease % Decrease 0.4 5% Decrease 0.0 Base 0.0 5% Increase % Increase % Increase 0.2 No correlation between OF change and dose/mu delivered 31

32 Results Incorrect output factors doaffect the dose delivered for a conformal jaw machine (Elekta, Siemens, Varian TrueBeam with jaw tracking) But they do not affect the dose delivered for a non-conformal jaw machine (Varian) As applicable to Pinnacle TPS! 32

33 Conclusion It is very important to have accurate output factor data in the TPS, as it is to have accurate beam profiles Inaccurate data could lead to miscalculation of dose If you don t have reliable data for small fields/segments, don t use them! 33

34 Questions? 34