A pixel chamber as beam monitor for IMRT S. Belletti 4, A.Boriano 6, F.Bourhaleb 5,7, R. Cirio 6, M....

A pixel chamber as A pixel chamber as beam monitor for beam monitor for

IMRT IMRT S. Belletti S. Belletti 44, A.Boriano , A.Boriano 66, F.Bourhaleb , F.Bourhaleb 5,75,7, R. Cirio, R. Cirio 6 6, M. Donetti , M. Donetti

5,65,6, B. Ghedi , B. Ghedi 44, E. Madon , E. Madon 22, F. Marchetto , F. Marchetto 66, U. Nastasi , U. Nastasi 33, C. , C. Peroni Peroni 66, C.J. Sanz Freire , C.J. Sanz Freire 1,61,6 , E. Trevisiol , E. Trevisiol 22

11 IBA, Ion Beam Applications, Louvain la-Neuve, Belgium IBA, Ion Beam Applications, Louvain la-Neuve, Belgium22 OIRM S. Anna, Torino, Italy OIRM S. Anna, Torino, Italy

33 Servizio di Fisica Sanitaria, S. Giovanni A.S. Hospital, Torino, Italy Servizio di Fisica Sanitaria, S. Giovanni A.S. Hospital, Torino, Italy44 Servizio di Fisica Sanitaria, Spedali Civili, Brescia, Italy Servizio di Fisica Sanitaria, Spedali Civili, Brescia, Italy

55 TERA Foundation, Novara, Italy TERA Foundation, Novara, Italy66 University and INFN, Torino, Italy University and INFN, Torino, Italy

77 University Mohamed First, Oujda, Morocco University Mohamed First, Oujda, Morocco

1st Euro-mediterranean meeting on radiotherapy –Oujda (Morocco) [email protected]

OutlineOutline

IMRT measurementsIMRT measurements

Mechanical structure of the pixel ionisation chamberMechanical structure of the pixel ionisation chamber

Read-out: electronics and performancesRead-out: electronics and performances

Characterization of the detector with hadron beamsCharacterization of the detector with hadron beams

Characterization of the detector with photon beamsCharacterization of the detector with photon beams


The pixel The pixel chamberchamber

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


Detector designDetector 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.


Detector designDetector design


Front-end electronicsFront-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


Linearity at 100 fC charge Linearity at 100 fC charge quantumquantum

20 pA < I < 0.6 A

Linearity better than 0.7 %


Linearity at 600 fC charge Linearity at 600 fC charge quantumquantum

10 pA < I < 2 A

Linearity better than 0.3 %


Reproducibility of charge quantumReproducibility of charge quantum

I = 49.96 nA Qc = 600 fC 24 ºC < T < 27 ºC 1 month 6 measures


Beam test at PSI (CH)Beam test at PSI (CH)

PSI test with a 138 MeV proton PSI test with a 138 MeV proton beambeam

7 mm FWHM beam7 mm FWHM beam Spot-scanningSpot-scanning

Uniformity of the Uniformity of the response response ~0.9%~0.9%

Profiles measurement okProfiles measurement ok


Beam test at GSI (D)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


Spatial resolution: 16 spotSpatial resolution: 16 spot


Spatial Spatial resolutionresolution

Spatial resolution < 0.2 mm


Homogeneity of the Homogeneity of the responseresponse

18 × 18 cm2 uniform field18 × 18 cm2 uniform field

= 1.1 % = 1.1 %

= 2.0 % = 2.0 %


Q.A. with photon beamsQ.A. with photon beams

A Quality Assurance protocol A Quality Assurance protocol establish:establish:

Physic and dosimetric parameters Physic and dosimetric parameters to be measuredto be measured

Procedures followed for the Procedures followed for the measurementsmeasurements

Frequency of the controlsFrequency of the controls

Reference valuesReference values

1. Homogeneity, symmetry, penumbra, energy stability

2. Calibration of the monitor system, stability.

Conventional instrumentation:Conventional instrumentation:

Thimble ion chamber (Farmer)Thimble ion chamber (Farmer) One point One point measurementmeasurement

Diode array 1D measurementDiode array 1D measurement


Q.A. with photon beamsQ.A. with photon beamsS. 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


Depth dose curveDepth 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 referenceAn excellent agreement has been found for each photon energy.


ProfilesProfilesProfile comparison with a Farmer in a water phantom

Field 10 X 10 cm2

Depth 10 cm

Field 20 X 20 cm2

Depth 10 cm


ProfilesProfiles

Field 5 X 5 cm2

Depth 10 cm


Long-term stabilityLong-term stability

Stability measured during a one month periodStability measured during a one month period Stability withinStability within 1.5%1.5% Mechanical problem caused variations in the sensitive Mechanical problem caused variations in the sensitive

volumevolume1.050

0.950

1.000

1.050

0.950

1.000

±1% ±5%



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.



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



Total dose delivered

Example: small square dose distribution going to large square dose distribution



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


Detector Detector designdesign

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.


Read out electronicsRead out electronics

Recycling Integrator architecture.


TERA05TERA05Very Large Scale Integration (VLSI)if conversionoutput Qint

100 fC<charge quantum<800 fCImax= 4 A64 channels16 bit wide countersmultiplexed digital outputdead-time free readoutmax read out rate = 10 MHz

channel # 1

channel # 64


Fast Data Fast Data acquisitionacquisition

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


Slow Data acquisitionSlow 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


ReproducibilityReproducibility

Several measurements have been performed to evaluate the reproducibility.After a pre-irradiation reproducibility is better than 1%

100 MU per irradiationfield bigger than sensitive volume


OUTPUT FACTOROUTPUT FACTOR

10

..D

DFO r

3.4% No solution so far:No solution so far:

Higher sensitivity of Higher sensitivity of the pixel chamber to the pixel chamber to low energy photons low energy photons due to a different due to a different structurestructure

Disagreement Disagreement between the pixel between the pixel chamber and the chamber and the FarmerFarmer

A pixel chamber as beam monitor for IMRT S. Belletti 4, A.Boriano 6, F.Bourhaleb 5,7, R. Cirio 6, M....

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