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Exit Displacement Exit Angle

Tracking Studies

Error on Energy Mean vs. Energy Resolution NEm /

Radiography Studies for Proton CTM. Petterson, N. Blumenkrantz, J. Feldt, J. Heimann, D. Lucia, H. F.-W. Sadrozinski, A. Seiden, D. C. Williams SCIPP, UC Santa Cruz, CA 95064 USA

V. Bashkirov, R. Schulte Loma Linda University Medical Center, CA 92354 USAM. Bruzzi, D. Menichelli, M. Scaringella, C. Talamonti INFN and Univ. of Florence, Italy

G.A. P. Cirrone, G. Cuttone, D. Lo Presti, N. Randazzo, V. Sipala INFN Sezione di Catania, , I-95123 Catania, Italy

Measure Proton Positions, Angles and Energy Loss

Apparatus

Common Readout of Silicon Strip + Calorimeter into FPGASi have binary readout with Time-over-Threshold (ToT)CsI read out with Photodiode into Charge-to-Time-Converter (CTC)

pCT: Principle and ApplicationR. W. Schulte, et al.,, “Conceptual design of a proton computed tomography system for applications in proton radiation therapy”, IEEE Trans. Nucl. Sci., vol 51, no.3, pp 866 – 875, June 2004.

One limitation to the precision: is the use of x-ray computed tomography (CT).

The resulting uncertainties can lead to range errors from several millimeters up to more than 1 cm depending on the anatomical region treated.

Additional uncertainties exist with respect to the target position relative to normal tissues in the treatment room.

Reasons for Proton Computed Tomography pCT

Challenges for pCT

Imaging with the Calorimeter Energy

(1.2 mm x 1.2 mm pixels) Fluence limit : n = 8-16 (blurred image and many white pixels = no valid fit)

Fluence Limit number of protons in pixel > 10!

Conclusions

The proposed pCT set-up permits tracking of protons within a phantom to better than 500mDensity variations can be resolved by measuring the energy in the exit calorimeterLower limits on the dose required for imaging are derived based on the energy reconstruction methodThe dose required for imaging scales with approximate d-3 where d is the voxel size.

Loma Linda University Medical Center

Loma Linda University Medical Center

The most likely path of an energetic charged particle

through a uniform medium

D C Williams Phys. Med. Biol. 49 (2004) 2899–2911

• Good agreement data – MLP within ~ 300 m

• Increasing difference with larger angles: need for Monte Carlo Study

• Resolution inside Absorber < 500 m vs. MLP width of 380 m

Phantom

Energy Calibration without PMMMA

Energy resolution E in CsI (measured and corrected for pedestal)

PMMA Data not described by NIST -> Nuclear Effects ?

The error on the energy mean can be improved with higher statistics = larger dose

Table 5-2: finding the minimum dose via calculating the difference on the mean of the voxels divided by the error on the mean. The Gaussian fits are shown in the appendix.

n # entries

voxel mean error on

mean # entries control

mean control

Significance

1 5598 655 0.5 6377 629.4 51.2

2 2758 654 0.8 3196 629.3 30.875

4 1374 652.2 1.4 1609 629.3 16.357143

8 671 652.40 1.00 797 628.5 23.9

16 337 652.60 1.20 394 628.7 19.916667

32 174 655.50 1.90 207 628.4 14.263158

64 92 655.60 4.30 87 632 5.4883721

128 52 654.40 11.70 38 634.2 1.7264957

200 19 630.70 12.30 24 635 -0.35

= 1.2 g/cm3. “counting limit”: requirement of at least 10 protons in the fit

200 and 2 path’200 and 2 path’

CsI Calorimeter Response

External Tracking of Proton predicts Path inside Absorber (MLP)

Phantom = 6th out of 12 PMMA plates (each 1.25 cm thick )

Determination of Fluence Limit by Data Reduction by factor n = 2,…, 64

Fluence Required for Imaging the large Voxel

5.384*5273.2*0002.0# 2 EECTC

Phantom HolesDiameter 1.0 cm, depth 1.25 cm (D)Diameter 0.6 cm, depth 0.6 cm (B, F)

Reconstructed energy (pixel size 2.4 mm x 2.4 mm)

Reconstructed energy (pixel size 1.2 mm x 1.2 mm).Black boxes = target and control voxels

Gaussian fit to the upper part of the CTC spectrum( ¼ of protons are useful in the determination of the mean energy)

200 MeV 100 MeV Pedestal

Energy stragglingIn PMMA dominant effect for energy resolution.

Dose D – Voxel Size d – Density Variation

( 8 mm x 8 mm pixels, n =1 to 200)Fluence limit is reached at about n=64

Required Significance >2mmES /

32

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Dose D for two voxel sizes d:d [cm] D [mGy]1.2 2.8*10-5 0.6 2.5*10-4

Ratio d3 0.13 Ratio D 0.11

Dose vs. Voxel Size

1.E-06

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Counting Limit

Single ProtonTracking in 10 Si planes:Single-sided, 192 strips, 236 m pitch, [GLAST 97 B.T.)Module = x-y pair with 90o rotated strips Entrance and exit telescopes + 1 “roving” inside absorberEnergy Measurement in CalorimeterOne CsI crystal 5 cm x 5 cm x 15 cm

Displacement in the “Roving planes” vs. exit displacement bins of 500 m (all angles).MLP (size of open symbol = MLP spread).

Proposed pCT System

Measure displacement with “Roving Plane”at 3 depth

Correlation between “Roving plane# 2” and exit parameters

Proton radiation therapy is one of the most precise forms

of non-invasive image-guided cancer therapy. Well defined range of protons in material, low entrance dose, a dose maximum (“Bragg peak”) and a rapid distal dose fall-off,

-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 2 4 6 8 10 12 14 16 18 20

Depth inside Absorber [cm]

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Figure 5-2: the energy for pixels sized 35 x 35 strips. The voxel is easily seen in the first few plots. After reducing events by a factor of 32 the energy of the neighboring pixels fluctuates, making distinction between the background and voxel more difficult. Note the center pixel on the top row has a higher energy – this corresponds to the small voxel.

CsI Calorimeter Resolution

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