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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) The pixel chamber   parallel plate ionization chamber   anode segmented in 1024 square pixels   pixel dimension = 7.5 x 7.5 mm 2   sensitive area = 24 x 24 cm 2   1 mm water equivalent thickness   front-end electronics, located around the chamber, to perform analog to digital conversion

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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) 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

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

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

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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) Beam test at GSI (D) 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

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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) Homogeneity of the response 18 × 18 cm 2 uniform field  = 1.1 %  = 2.0 %

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) 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 1. 1.Homogeneity, symmetry, penumbra, energy stability 2. 2.Calibration of the monitor system, stability. Conventional instrumentation:  Thimble ion chamber (Farmer) One point measurement  Diode array 1D measurement

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) 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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) Profiles Profile comparison with a Farmer in a water phantom Field 10 X 10 cm 2 Depth 10 cm Field 20 X 20 cm 2 Depth 10 cm

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) Profiles Field 5 X 5 cm 2 Depth 10 cm

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) Long-term stability Stability measured during a one month period Stability measured during a one month period Stability within 1.5% Stability within 1.5% Mechanical problem caused variations in the sensitive volume Mechanical problem caused variations in the sensitive volume ±1% ±5%

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) IMRT measurements 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. 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

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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco)

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

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

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

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) 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 <100 m

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) 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 Slow data acquisition

1 st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) 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

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