A solution for dosimetry and quality assurance in

A solution for dosimetry and quality assurance in IMRT and hadrontherapy: the pixel ionization chamber. S. Amerioa, S. Bellettib, A. Borianoc, R. Ciriod, S. Codaa, M. Donettie, d, B. Ghedib, A. Lupariad, E. Madonf, F. Marchettod, U. Nastasia, C. Peronid, C. J. Sanz Freired, g, E. Trevisiolf, A. Urgesif Servizio di Fisica Sanitaria, Ospedale S. Giovanni A. S. , V. Cavour 31, I-10123 Torino b Servizio di Fisica Sanitaria, Spedali Civili di Brescia, P. le Ospedale 1, I-25123 Brescia c ASP, V. le Settimio Severo 65, I-10133 Torino d University and INFN of Torino, V. Giuria 1, I-10125 Torino e TERA Foundation, V. Puccini 11, I-28100 Novara f OIRM S. Anna, V. Baiardi 43, I-10126 Torino Villa Olmo, October 15 Advanced Technology and Particle f partially supported by IBA, Ion Beam Application, Louven la-Neuve, Belgium -19, 2001 Physics a

Introduction In Intensity Modulated Radiation Therapy (IMRT) techniques like step and shoot, sliding window, dynamic wedge and in hadrontherapy with scanning beam the delivered dose in the treated volume is not just function of the space but also of the time. An instrument useful in dose measurements has to be fast, reproducible and able to measure in several points. We have developed, built and tested a fast ionization chamber segmented in pixels that measures the dose in a plane which can help the physicist in the dose measurements and in quality assurance. The outline of the talk is the following detector design read out electronics data acquisition test performed results 2

Detector design 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 cathode made of aluminized mylar foil total dimension = 64 X 3 cm 3 front end electronics, placed around the chamber, perform the analog to digital conversion 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 3

Detector design Thin version When used like monitor chamber the detector has to be thin to minimize the effects 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. 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 4

Detector design Thick version When used in plastic phantom the detector can be thick. With photon or electron beam air gap has to be minimized. 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. 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 5

Read out electronics The read out electronics is housed next to the electrodes. We developed a Very Large Scale Integration (VLSI) chip. Every chip has 64 channels that convert the collected charge in digital output. The name of the last version is TERA 05 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 6

Read out electronics TERA 05 characteristics • Very Large Scale Integration • i f conversion • output Qint • 100 f. C

Read out electronics ecycling Integrator architecture. 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 8

Read out electronics The excellent linearity over a large range is due to recycling integrator architecture. QC = 600 f. C 10 p. A < I < 2 A QC = 100 f. C Linearity better than 0. 3 % 20 p. A < I < 0. 6 A 7 th International Conference on Advanced Technology and Particle Linearity better than 0. 7 % Villa Olmo, Como 15 -19 October 2001 9

Read out electronics I = 49. 96 n. A 26 chips Charge quantum have been measured over different chips in order to evaluate the spread of the value. Results show an RMS 1% 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 10

Read out electronics Charge quantum reproducibility has been measured. I = 49. 96 n. A Qc = 600 f. C 24 ºC < T < 27 ºC 1 month 6 measures 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 11

Data acquisition Slow data acquisition • connection by twisted pair flat cables (100 m max) • max rate transfer = 1 MHz = 2 Mb/s • read out transfer time = 1 ms • PCI DAQ card • Lab. VIEW software • cheap solution • easy to handle 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 12

Data acquisition Fast data acquisition • connection by twisted pair flat cables (100 m max) • max rate transfer = 10 MHz = 40 Mb/s • read out transfer time = 50 s • read out cycle total time = 500 s (FIFO operation + data transfer) • real time operation system • expensive solution 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 13

Hadron beam test Aims GSI - Darmstadt Germany • spatial resolution voxels • homogeneity of response Beam characteristics • raster-scan delivery system • 270 Me. V/u, C+6, 8. 8 mm (FWHM) • fast data acquisition system • data acquisition synchronized with raster-scan 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 14

Hadron beam test Spatial resolution pixel dimensions = 7. 5 × 7. 5 mm 2 beam dimension = 8. 8 mm (FWHM) counts Spatial per voxel 103 resolution < 0. 2 mm 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 15

Hadron beam test The calibration accounts for the different gain of each channel (gas gap variations and electronics) Response homogeneity 18 × 18 cm 2 uniform field before calibration = 2. 0 % 7 th International Conference on Advanced Technology and Particle after calibration = 1. 1 Villa Olmo, Como 15 -19 October 2001 % 16

Photon beam S. Giovanni A. S. and S. Anna hospitals – Torino Quality Assurance Aims • Measurements reproducibility • Depth dose profile measurements and comparison to reference ion chamber (Farmer) results • Profile measurements In such measurements there aren’t DAQ time constraints. Anyway the system can measure the delivered dose 7 International Conference on in real time. Villa Olmo, Como 15 -19 October 2001 Advanced Technology and Particle th 17

Photon beam Several measures 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 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 18

Photon beam Depth dose profiles have been measured for different photon energy. We use a Farmer ion chamber in a plastic phantom to compare the results. An excellent agreement has been found for every photon energy. 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 19

Photon beam Profile comparison with a water phantom have been performed. Field 10 X 10 cm 2 Depth 10 cm 7 th International Conference on Advanced Technology and Particle Field 20 X 20 cm 2 Depth 10 cm Villa Olmo, Como 15 -19 October 2001 20

Photon beam IMRT measurements Dynamic wedge case: • 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 2 D dose in 1024 points. 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 21

Photon beam 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 2 D delivered dose The pixel ion chamber has a fast read out. Every 1 ms (slow DAQ system) a complete read out can be pe It is possible to monitor the 2 D dose in real time. Delivered dose in function of time 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 Total dose delivered 22

Conclusions We developed and tested a pixel ionization chamber that can be used to measure and monitor the treatment in IMRT and hadrontherapy. Its time resolution allows to perform 2 D dose measurements in real time. Two prototypes have been built in order to use the same technique both as monitor chamber and to perform relative dose measurements. Future development: By stacking several chambers 3 D measurements can be obtained. 7 th International Conference on Advanced Technology and Particle Villa Olmo, Como 15 -19 October 2001 23