Development of in-vivo

diamond dosimetry for

Brachytherapy

J. Burger(1), V. Cindro(2), A. Gorišek(2), G. Kramberger(2) , I. Mandić(2), M. Zavrtanik(2), M. Mikuž (2,3)

(1) Institute of Oncology, Ljubljana

(2) Jožef Stefan Institute, Ljubljana

(3) Faculty for Mathematics and Physics, Department of Physics, University of Ljubljana

Motivation and goals Patient dose verification at the point of delivery is an important part

of quality assurance in radiotherapy treatment. It is recommended or obligatory within the whole EU (European Directive 97/43/EURATOM)

Radiotherapy treatments in Ljubljana currently without online verification. Specific interest in Brachytherapy – previous collaborations with the Institute of oncology

A lot of experience (and partners) in technologies required for in-vivo dosimetric arrays:

Development of flexible fine pitch printed circuits

Sensor development

Read out electronics

Our goal : for development of technologies which lead to construction of one or two dimensional dosimetric sensors fields for in-vivo dosimetry in medical applications, mainly radiotherapies. Arrays consist of few (up to 10 sensor elements)

Sensor arrays (I) Two detector technologies selected:

RadFET

Diamond sensors (reported in this talk)

Sensor array consists of up to 8 individual sensors with readout

Why diamond detectors?

single crystalline

Diamond

Silicon consequence

Z Z=6 Z=14 Tissue equivalence Zeff=6.3 in case of fields with

mixed/unknown g energies more precise dose

determination

Bang gap 5.5 eV 1.12 eV Large temperature dependence of dark current for Si –

must be stable or precise correction is needed. No

problem with diamond

Ionization

energy

13.6 eV/e-h 3.6 eV/e-

h

Larger signal for Si

Density 3.52 g/cm3 2.33 g/cm3 Improves the signal a bit for the diamond

Technology Cheap expensive Silicon is far more mature in all respects

Bias Requires outside

bias

No-bias Complicated operation

Silicon operates as p-i-n diode while diamond has both contacts ohmic.

Radiation hardness affects less diamond – no increase of leakage current with irradiation.

In-vivo dosimetry for Brachytherapy (I)

HDR Brachy-therapy uses a single radioactive seed (192Ir, 1-10 Cu) which is

moved in the catheters. The treatment plan is fulfilled by:

• dwell time

• seed location – different catheters

In-vivo dosimetry for Brachytherapy (II) Ideally a dosimeter array can be installed in irradiation catheter – probably

too thin, but another catheter with array inserted close or into tumor (for

prostate cancer for example in rectum) is possible

Measurements in many points (sensor plans) can be compared (verified)

with irradiation plan

There are many fold :

On-line verification of treatment plan

On-line verification of source (seed) location within catheters (prevention of

different accidents – swap of the catheter, lost seed, larger movement of

the catheters). “GPS” algorithm based on measured dose-rates.

dwell points

sensor location

GEANT4 simulation of the operation (I) All relevant physics process included (Compton, Photoelectric effects,

bremsstrahlung, e+ - e- creation, multiple scattering, delta electrons, Bethe-Bloch).

Thresholds set to low energy operation.

192Ir, 1 Cu source used in simulation the same as used by therapy machine

4 sensors/array 1 cm apart on simplified flexible circuit

Phantom made of plexi-glass as used/will be used for demonstrator studies

GEANT4 simulation of the operation (I) All relevant physics process included (Compton, Photoelectric effects,

bremsstrahlung, e+ - e- creation, multiple scattering, delta electrons, Bethe-Bloch).

Thresholds set to low energy operation.

192Ir, 1 Cu source used in simulation the same as used by therapy machine

4 sensors/array 1 cm apart on simplified flexible circuit

Phantom made of plexi-glass as used/will be used for demonstrator studies

GEANT4 simulation of the operation (II) Example of 100 irradiated photons (green-photons, red-electrons)

Dimensions 1.5 x 1.5 x 0.5 mm3, sensors 1 cm apart, arrays ~3 cm apart

Source position in the center between two arrays

Green lines photons, red electrons, yellow dots (interaction points)

GEANT4 simulation of the operation (III)

Simulated currents and dose rates (I)

Sensitivity

𝐼𝑖𝑜𝑛 =𝑒0∙

𝑑𝑁𝑑𝐸

𝐸𝑑𝐸∞

0

13.6 eV ∙ 𝑡𝑎𝑐𝑞 1 2 3 4 𝐷 =

𝑑𝑁𝑑𝐸

𝐸𝑑𝐸∞

0

𝜌 ∙ 𝑉 ∙ 𝑡𝑎𝑐𝑞

𝐷

𝐼𝑖𝑜𝑛= 3.43

μGy

s pA

Bottom line Top line

tacq=270 ms

Simulated currents and dose rates (II)

192Ir source close to the sensor – maximum

dose rate for 1 Cu

The size of 1.5x1.5 mm2 for the diamond

sensor is enough to cover the whole dynamic

range (for HDR brachytherapy using 10 Cu

source the sensors can be smaller)

1 2 3 4

Bottom line Top line

Determination of the position Position in the target volume determined by finding the point

where dose rate and distance squared to that point is minimum:

Wi,j=1 in the simplest form (weights err. on dose measurements)

The position found is precise within a 1 mm in the whole target

volume for integration time of > few ms. No significant dependence on minimization algorithm

Converges to one of the solution in case of degeneracy

Convergence improves with smaller uncertainty on dose rate measurement

χ = 1

𝑊𝑖,𝑗𝐷 𝑖 𝑟𝑖 (𝑟𝑖 − 𝑅)2−𝐷 𝑗(𝑟𝑗)(𝑟𝑗 − 𝑅)2

𝑖

𝑗=1

𝑁

𝑖=1

𝑅 = min (χ)

1 2 3 4

5 6 7 8

Measurements First prove-of-principle detectors: single crystalline CVD diamonds

Diamonds supplied by Element VI (~4.3x4.3x0.5 mm3)

Identical metallization done by Ohio State University (partner in the project). Note that the

diamond can, unlike silicon, be reused/remetallized.

Tests with 90Sr (2.3 MBq) – lab environment

Ke 6517 electrometer used for current measurements

~2 mm

XY - table

Experimental setup

Ionization current

Algoritem za dolocitev polozaja izvira

Pokazati v več točkah ali da algoritem

primeren rezultat

Stable operation after applying bias after few minutes:

Leakage current on the level of ~1 pA equal for both polarities

Stable current after exposure to 90Sr source (some drift for the positive polarity)

Agreement with the simulation is very good – cross check of both measurements

and simulations (simulation is a key tool for system design)

Removal of the source clearly seen in the signal

0.5 cm distance to the sensor from the source nozzle

simulation prediction

Ionization current (rise and decay)

The rise and fall of the signal are almost prompt; take into account 1Hz

readout and finite movement speed of the source

It seems that removal maybe has small “leg”.

Removal of the source for 30 s

removal of the source

introduction of the source

leakage current

Simulation of the measurements

Good agreement with measurements:

90Sr setup with also 10x more active

source will be used for characterization

brachytherapy sensors before the use

with therapy source in the phantom

Verification of the simulation –

simulation used for optimization of

devices and setting the system

requirements

Spectrum of deposited energy

[1s acq. time]

Iion ~15 pA

dN

/dE

Prototype diamond sensors

scCVD diamond detectors have just been cut and metallized

Small 1.1x1.1 mm2

Large 2x2 mm2 examples

Readout system

Readout electronics requirements:

Huge dynamic range (1 pA – 65 nA)

8 channel version parallel fast readout with few Hz (required by the dwell times)

simple connectivity to PC

PC 16 bit ADC

mC (ARM M3)

pA meter (analog) #8

pA meter (analog) #1

To flexible circuit

Example of such a flexible circuit (not appropriate though)

1.2 mm

Flexible circuit: double sided (vias)

70 mm line pitch

Sandwich

configuration for

pickup shielding

Should fit into ~2 mm

diameter catheter

Summary and conclusions

Simulations of dosimeter arrays for Brachytherapy (192Ir) were done

using GEANT4 framework

required range of ionization currents is between 1 pA – 100 nA

Localization of the source on the s scale is with 1mm3 precision is

possible

Simulation and measurements cross-checked with 90Sr electrons on

the test bench. Good agreement is found

First diamond detectors of appropriate size at hand and will be

tested soon