A pixel chamber as beam monitor for IMRT

1. A pixel chamber as beam monitor for IMRT S. Belletti 4, A.Boriano 6, F.Bourhaleb 5,7, R. Cirio 6, M. Donetti 5,6, B. Ghedi 4, E. Madon 2, F. Marchetto 6, U. Nastasi 3, C. Peroni 6, C.J. Sanz Freire 1,6 , E. Trevisiol 2 1 IBA, Ion Beam Applications, Louvain la-Neuve, Belgium 2 OIRM S. Anna, Torino, Italy 3 Servizio di Fisica Sanitaria, S. Giovanni A.S. Hospital, Torino, Italy 4 Servizio di Fisica Sanitaria, Spedali Civili, Brescia, Italy 5 TERA Foundation, Novara, Italy 6 University and INFN, Torino, Italy 7 University Mohamed First, Oujda, Morocco

2. Outline Mechanical structure of the pixel ionisation chamber • IMRT measurements Read-out: electronics and performances Characterization of the detector with hadron beams Characterization of the detector with photon beams 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

3. The pixel chamber • parallel plate ionization chamber • anode segmented in 1024 square pixels • pixel dimension = 7.5 x 7.5 mm2 • sensitive area = 24 x 24 cm2 • 1 mm water equivalent thickness • front-end electronics, located around the chamber, to perform analog to digital conversion 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

4. Detector design When used in plastic phantom the detector can be thick. Anode and cathode are glued to a plastic slab, which is a grid of 1024 holes. Each hole can be considered as an independent sensitive volume and the material around provides lateral electronic equilibrium. 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

5. Detector design 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

6. Front-end electronics • located around the chamber • we have developed a full custom chip (TERA05) • every chip has 64 channels. Each one converts the charge collected by a pixel into counts 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

7. Linearity at 100 fC charge quantum • 20 pA < I < 0.6 A • Linearity better than 0.7 % 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

8. Linearity at 600 fC charge quantum • 10 pA < I < 2 A • Linearity better than 0.3 % 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

9. Reproducibility of charge quantum • I = 49.96 nA • Qc = 600 fC • 24 ºC < T < 27 ºC • 1 month • 6 measures 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

10. Profiles measurement ok Uniformity of the response ~0.9% Beam test at PSI (CH) • PSI test with a 138 MeV proton beam • 7 mm FWHM beam • Spot-scanning 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

11. Beam characteristics • raster-scan delivery system • C+6, beam dimension = 8.8 mm (FWHM) • data acquisition synchronized with raster-scan Aims • spatial resolution • homogeneity of response Beam test at GSI (D) 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

12. Spatial resolution: 16 spot 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

13. Spatial resolution  < 0.2 mm Spatial resolution 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

14. 18 × 18 cm2 uniform field  = 2.0 %  = 1.1 % Homogeneity of the response 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

15. Homogeneity, symmetry, penumbra, energy stability • Calibration of the monitor system, stability. Q.A. with photon beams A Quality Assurance protocol establish: • Physic and dosimetric parameters to be measured • Procedures followed for the measurements • Frequency of the controls • Reference values Conventional instrumentation: • Thimble ion chamber (Farmer) One point measurement • Diode array 1D measurement 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

16. Q.A. with photon beams S. Giovanni A.S. and S. Anna hospitals – Torino Aims • Depth dose profile measurements and comparison to a reference ion chamber (Farmer) • Profile measurements and comparison Measurements to a reference ion chamber • Stability of the response To perform these kind of measurements, there are no time constrains. The only information needed is the final measurement 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

17. Depth dose curve Depth dose profiles have been measured for different photon energy. We have used a Farmer ion chamber in a plastic phantom as a reference An excellent agreement has been found for each photon energy. 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

18. Profiles Profile comparison with a Farmer in a water phantom Field 20 X 20 cm2 Depth 10 cm Field 10 X 10 cm2 Depth 10 cm 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

19. Profiles Field 5 X 5 cm2 Depth 10 cm 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

20. ±1% ±5% Long-term stability • Stability measured during a one month period • Stability within1.5% • Mechanical problem caused variations in the sensitive volume 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

21. IMRT measurements Dynamic wedge case • By moving each side of the collimator with different speed one obtains a wedge shape of the dose • At the end of the treatment one has to check the delivered dose in different points The pixel ion chamber can measure precisely the 2D dose in 1024 points. 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

22. IMRT measurements Dynamic multi leaves collimator (DMLC) case: • independent leaves can be moved to shape the beam field • one needs to check as a function of time the 2D delivered dose The pixel ion chamber has a fast read out. Every 1ms a complete read out can be performed. It is possible to monitor the 2D dose in real time. 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

23. Total dose delivered IMRT measurements Example: small square dose distribution going to large square dose distribution 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

24. Time: 5 sec Time: 13 sec Time: 0 sec Time: 1 sec Time: 2 sec Time: 3 sec Time: 4 sec Time: 14 sec Time: 7 sec Time: 8 sec Time: 9 sec Time: 10 sec Time: 11 sec Time: 12 sec Time: 6 sec IMRT measurements Delivered dose in function of time 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

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26. Detector design Thin version When used as monitor the detector has to be thin to minimize the changes on the beam energy and shape. For this configuration, anode and cathode are glued to frames in order to assure the mechanical rigidity and the gap thickness . Water equivalent thickness < 1 mm. 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

27. Read out electronics Recycling Integrator architecture. 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

28. channel # 1 channel # 64 TERA05 • Very Large Scale Integration (VLSI) • if conversion • output  Qint • 100 fC<charge quantum<800 fC • Imax= 4 A • 64 channels • 16 bit wide counters • multiplexed digital output • dead-time free readout • max read out rate = 10 MHz 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

29. <100 m Fast Data acquisition • Signals (RS422 standard) are delivered via twisted pair flat cables (100 m maximum length) • max transfer rate = 10 MHz = 40 Mbyte/s • read out transfer time = 50 s • read out cycle total time = 100 s • real time operating system 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

30. Slow Data acquisition Slow data acquisition • connection by twisted pair flat cables (100 m max) • max rate transfer = 1 MHz = 2 Mbyte/s • read out transfer time = 1 ms • PCI DAQ card • LabVIEW software • cheap solution • easy to handle 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

31. Reproducibility Several measurements have been performed to evaluate the reproducibility. After a pre-irradiation reproducibility is better than 1% 100 MU per irradiation field bigger than sensitive volume 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

32. OUTPUT FACTOR Disagreement between the pixel chamber and the Farmer 3.4% No solution so far: Higher sensitivity of the pixel chamber to low energy photons due to a different structure 1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)