1. Lotte Verbunt Investigation of leaf positioning accuracy of two types of Siemens MLCs making use of an EPID

2. Overview background goals calibration image acquisition method I method II measurements comparison results conclusions recommendations

3. Dr. Bernard Verbeeten Institute Tilburg independent institute for radiotherapy and nuclear medicine: - 11 radiotherapy oncologists - 3 nuclear physicians - 6 medical physicists - 1 general doctor 2360 new radiation treatment patients a year (2004) background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

4. Linear accelerator and EPID background goals calibration acquisition method I method II measurements comparison results conclusions recommendations acceleration bending through patient conversion into light reflection by mirrordetection by camera

5. Multileaf collimator two types of Siemens MLCs: MLC 1: 29 leaf pairs; 2 mm accuracy MLC 2: 41 leaf pairs; 1 mm accuracy background goals calibration acquisition method I method II measurements comparison results conclusions recommendations Y jaw leaf bank central axis

6. Project goals development of a leaf verification method applicable to a Siemens linac using a CCD-camera based EPID verification of the leaf positions of two different types of Siemens MLCs and to check whether the MLCs and the current leaf calibration method are accurate enough for IMRT background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

7. Why is this of interest? Intensity Modulated Radiation Therapy (IMRT): - several beams from different directions - each beam consist of several (abutting) segments background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

8. Leaf calibration method: light field performed with EPACtool using light field and grid lines from graph paper 4-points calibration: 20, 10, 0, and -10 cm background goals calibration acquisition method I method II measurements comparison results conclusions recommendations left leaf bankright leaf bank -20-1001020

9. Leaf calibration method: light field performed with EPACtool using light field and grid lines from graph paper 4-points calibration: 20, 10, 0, and -10 cm uncertainties because: –a difference between light and radiation field –a blurred light field edge –a possible rotation of graph paper background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

10. Leaf verification methods: radiation field method I: 50 % dose value method II: from dose change to leaf positions both methods use same input from the EPID background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

11. Image acquisition 6 segments, 5 cm wide, 2 mm overlap of the leaves black = blocked by the leaves white = unblocked  irradiated part background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

12. Image acquisition 6 segments, 5 cm wide, 2 mm overlap of the leaves barrel distortion correction background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

13. Image acquisition 6 segments, 5 cm wide, 2 mm overlap of the leaves barrel distortion correction radiation center and pixel dimensions background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

14. Method I take a mean profile for each segment and for each leaf pair fit a polynomial function between 20 and 80% of the dose value determine leaf position (50% dose value) background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

15. Accuracy method I images: –nine 3 cm wide segments –flat side of the leaves –90  rotation accuracy leaf position determination: 0.08 ± 0.18 mm (1 SD) shift of ± 0.45 mm at abutment leaf ends for MLC 2 background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

16. Method II sum all segments take a mean profile for each leaf pair calculate the dose change at each abutment convert the dose change into gap width extreme dose average dose background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

17. Calibration function method II calibration function f : six tests with different gap widths (-1 to 4 mm) plot: intended gap width versus dose change dose change is 15-20 % per mm gap width background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

18. Accuracy calibration function method II difference between measured (line) and expected gap (dots) width. accuracy calibration functions: –0.09 mm (1 SD) for linac 1 –0.12 mm (1 SD) for linac 2 background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

19. Measurements gap width and gap position: long-term reproducibility: segments from left to right (20 times in 10 weeks) short-term reproducibility: segments from left to right (5 times in succession) hysteresis: –segments from right to left (5 times in 10 weeks) –segments in random order (5 times in 10 weeks) background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

20. Comparison t-test for comparison of both methods: –difference is 0.004 ± 0.14 mm (1SD) statistical: different because of many measurements clinical: difference not relevant only results obtained with method I are shown background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

21. Results results of MLC 2 are split because of calibrations * Vieira et al. Radiother. Oncol. (2005) mean gap width error: 1.26 mm due to: –a difference between light and radiation field –a blurred light field edge background goals calibration acquisition method I method II measurements comparison results conclusions recommendations gap width error [mm]gap position error [mm] MLC 11.12 ± 0.43-0.10 ± 0.32 MLC 2 (1-7)1.26 ± 0.49 0.43 ± 0.30 MLC 2 (8-13)1.30 ± 0.35-0.07 ± 0.60 MLC 2 (14-20)1.61 ± 0.51-0.32 ± 0.58 Vieira et al. *1.21 ± 0.22

22. Results criterion dose error: 3% criterion position error: 3 mm mean gap width error: 1.26 mm  under dosage of 19-25% (15-20% per mm)  4-5% per plan (if plan consists of 5 beams) calibration method is not accurate enough for IMRT background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

23. Results: dependency gap width and gap position not dependent on: –abutment position –leaf pair number background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

24. Results: gap width error in time MLC 1 MLC 2 drift (0.35 mm) background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

25. Results: gap position error in time MLC 1 MLC 2 background goals calibration acquisition method I method II measurements comparison results conclusions recommendations rotation: 0.6  * * Bayouth et al. Med. Phys. 30 (2003)

26. Results: reproducibility gap width short-term reproducibility (1 SD): 0.17 mm and 0.12 mm for MLC 1 and MLC 2 long-term reproducibility (1 SD): 0.22 mm and 0.18 mm for MLC 1 and MLC 2 3% dose error criterion  15% dose error in beam (using 5 beams)  maximum allowed gap width error: 0.75 mm (3 SD) (using 20% per mm) maximum gap width deviation for each leaf pair from its average: 0.75 mm and 0.70 mm for MLC 1 and MLC 2  MLCs are accurate enough for IMRT background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

27. Results: hysteresis gap width: –no hysteresis gap position: –small hysteresis for MLC 1 (  0.40 mm) –no hysteresis for MLC 2 background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

28. Conclusions an accurate leaf verification method has been developed the current leaf calibration method using the light field is not accurate enough for IMRT the day-to-day variation in gap width is accurate enough for typical IMRT plans background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

29. Recommendations not possible to calibrate the leaves using this leaf verifation method method can be used for: –checking the accuracy of the leaf calibration –verifying the leaf positions in order to change the encoder value using MLCCHK  reduce calibration frequency investigate hysteresis for different gantry angles background goals calibration acquisition method I method II measurements comparison results conclusions recommendations

30. Questions?