In some cases Half- life will be very short. â¡ ... Half-Life of C-14 â 5730 years. Brunckhorst .... Li TLD pairs at different locations of bunker (13/01/2012). 6. 9. 22.
Neutrons in Radiotherapy Bunkers
Classification
Baryon
Composition
1 up quark (+2/3 e), 2 down (-1/3 e) quarks
Mass
1.674927351(74)×10−27 kg
Rest Energy = 939.565378(21) MeV
Derrick M Wanigaratne, PhD Prof. Tomas Kron, Petermac Cancer Centre, Melbourne Asso. Prof. Rick Franich, RMIT Melbourne
Radiotherapy in Cancer treatment Soon after the accidental discovery, X-rays played a major role in diagnostic as well as in therapy.
1895: Roentgen Stumbles Upon X-Rays
2
Radiotherapy in Cancer treatment Statistics show that 1 in 3 people will get cancer in their lifetime, 50% of cancer patients need radiotherapy Become a major treatment modality Aim is to get as much dose to tumour and as little as possible to normal structures
Radiation therapy using high-energy photon beams proved to have a significant effect on the quality of cancer treatments. To deliver the prescribed dose to the target volume and avoid normal tissue irradiation, the beams are usually delivered from several angles while the gantry rotates around the treatment couch.
High energy radiation (especially photons) have a dosimetric advantage … greater penetration depth …. enhance the skin sparing (eg prostate cancer)
Isodose Curves 6MV & 18MV 6 MV
18 MV
Dmax Dmax
http://ozradonc.wikidot.com
Note: deeper isodose lines in 18 MV
patients get bigger, …
6
Better rectal constrains with 18X IMRT
7
Three patients – similar results
8
•But.. one common concern (≥7 MV), … inadvertent production of neutrons •In treatments with high-energy X-ray beams photo-neutrons are produced through (γ, n) interactions mainly with linac head components
•Potential contamination of the treatment rooms and entrance maze ….. • …..resulting patient/staff being exposure to unaccounted scatter doses - likely increase the risk for secondary cancers. •Even small scatter doses to OAR can elevate the probability to induce secondary cancers in patients.
Photon Interaction with Matter
Compton effect is predominance at Intermediate energies
Photoelectric effect Ekmax = hν-ϕ
Pair Production Photon disappears and e – and e+ pair with a combined kinetic energy of hν-2mec2 is produced
Photonuclear reaction or Photodisintegration reactions At very high energies, the photon can deposit energy into the nucleus causing partial or complete disintegration. Nuclear binding energies are typically around 6-7 MeV per nucleon or higher, hence probability of disintegration increases at energies>7 MeV resulting in emission of one or more nuclear particles, neutron (x,n), or proton (x,p) Photodisintegration is a source of low-level neutron production that must be considered when designing radiation shielding around high-energy linear accelerators.
Nuclear interaction cross section
cross section - characterise the probability that a nuclear reaction will occur.
can be quantified physically in terms of "characteristic area" - larger area means a larger probability of interaction.
The standard unit (denoted as σ) is the barn, which is equal to 10−28 m² or 10−24 cm².
Cross sections can be measured for all possible interaction processes together, in which case they are called total cross sections, or for specific processes, distinguishing elastic scattering and inelastic scattering; of the latter, amongst neutron cross sections the absorption cross sections are of particular interest.
Sources of Neutron – Interaction of photons and electrons with heavy metals (Linac head components - target, MLCs, jaws, flattening & wedge filters.. – Khan 2003)
Continue…
Metals embedded in the walls and in some instances radiosensitive structures in patients
The production of neutrons is more probable from γ-n interaction than e-n reactions as the cross section for (γ,n), reaction is about 10 times larger than (e,n) reactions
When a photon (with energy larger than Neutron separation energy) interact with the nucleus, it emits a neutron. In most cases a transformation of the nucleus into a active product.
X- Ray
A Z
X → X
A−1 Z
1 0
+ n
Possibly a radioactive element 15
Photonuclear cross sections for selected material
(Maximum photon energy studied in Siemens PRIMUS is marked by the vertical line – EXFOR database – Nuclear Energy Agency)
Why neutrons are creepy? •Neutrons are heavy particles without charge. These characteristics allow the neutrons to travel relatively free in matter even at very low energies. •Like x-rays, neutrons are indirectly ionizing.
•However, their mode of interaction with matter is different and complex •Concern is not only the doses from neutron, but also the radioactivity that is induced through (x, n) reactions •Neutrons are given a higher radiation weighting factor in estimation of equivalent dose, H T = ∑ w DT , R T
Activation of materials
Neutrons can activate materials in their beam After high energy photon irradiation, beam modifiers such as wedges or compensators may become activated
Greatest concentration of activity will be found in the target which consists of tungsten (W) bonded to a thick Cu backing.
Photo-neutron emitted from the target will produce radioactive Cu-62 with a t1/2 of 9.8 min and Cu-64 with a t1/2 of 12.8 Hrs. Threshold energy for neutron production in Copper is just about 10 MeV.
Radioactivity will also be induced in the W collimator and the jaw system, flattening filter and bending magnets (these are 20 times less radioactive than Cu). In some cases Half- life will be very short.
After prolonged use of high energy photons (e.g. for commissioning) it is advisable to let activation products decay prior to entering the room (>10min) (IAEA Training Material on Radiation Protection in Radiotherapy – Part 7)
Interaction of neutron with matter
Neutrons can have several types of interactions with nucleus Scattering or Capture
Cross section of the interaction vary with neutron energy as well as target nucleus ..
Classification of Neutrons as per the energy
• Elastic (total Ek of neutron or nucleus is unchanged)
In elastic neutronscattering, the neutron bounces off the bombarded nucleus without exciting or destabilising it.
•Scattering events
• Inelastic (nucleus undergoes an internal re-arrangement onto an excited state from which it eventually releases radiation when returning to ground state)
Neutron Capture Neutrons may be captured and variety of emission may follow. The nucleus may rearrange its internal structure and release one or more gamma. Charged particles may also be emitted (eg: Protons, deuterons and alpha)
Two Neutron Capture reactions are of Special Interest in terms or Radiobiology (a) Radioactive capture by Hydrogen – - Incoming neutron is bound to the proton, forming deuterium. Binding energy is released as a single γ of 2.23 MeV (interaction cross section 0.33 barns/ atom)
1 1
1 0
2 1
H +n → H + γ (b) Captured by Nitrogen (cross section 1.84 barns/atom) -Nitrogen captures the thermal neutrons and releases a proton of 0.580 MeV and radioactive C-14 (beta emitter) – Can contribute to significant doses to human tissues)
14 7
1 0
14 6
1 1
Brunckhorst, 2009
N +n → C + p Half-Life of C-14 – 5730 years
Recoil proton
Most efficient absorbers of a neutron beam are the hydrogenous materials like paraffin wax, polyethylene etc. Dose deposited in tissue from a neutron beam is predominantly contributed by recoil protons. Due to higher Hydrogen content, dose absorbed in fat is ~20%greater than in muscle. So nuclear disintegration induced neutrons results in emission of heavy charged particles, γ rays can contributed to about 30% of the tissue dose
Higher energy IMRT and arc therapy Beneficial or detrimental ?
Current feasibility on high energy IMRT (which uses higher number of MU and relatively longer treatment time) and arc therapy using 15MV and 18MV
Neutron Dosimetry •It is clear that accurate measurement of neutron doses are important
•However, due to diverse secondary radiation produced by neutron interactions, the neutron dosimetry is relatively complicated. •Until now only a few direct measurements of the neutron dose inside a photon field and inside a phantom were reported in the literature
•Scope of the research project is to evaluate a robust measurement method and apply this to obtain a thorough understanding of neutron contamination in photon/neutron mixed fields in radiotherapy bunkers.
Use of TLDs (thermoluminescence detectors)
Non-conductive crystals, and at ambient temperatures all electrons are confined to valance band.
Due to ionising radiation some electrons gain sufficient energy to get into conduction band
Imperfections in TL material can trap electrons from conduction band at an energy state between conduction and valance band
No of trap electrons is a function of the incident radiation intensity
Depend on the depth of these traps and temperature electrons can gain enough energy to escape back to conduction band.
With rising temperature, electrons gain enough energy to raise to the conduction band and release energy (light) when recombine with positive holes in the valance band.
6LiF
and 7LiF chips doped with Mg, Cu and P (MCP, Krakow)
http://cdf-radmon.fnal.gov/dosimetry/MCP_Physical_Constants.pdf
•The advantage of the combination is that 6LiF has a large neutron capture cross section for thermal neutrons compared to 7LiF
Brunckhorst, 2009
Glow Curves
7LiF
6LiF
d=80cm, 18MV, FS=10x10 cm2
Uncertainties in TLDs
It’s a relative dosimetric technique and too many factors relate to the dose Accuracy depends on the reproducibility Generally, with care, reproducibility of ± 2% or better can be achieved Accuracy can be achieved by using a set (say 50) of TLD and which are calibrated individually (known sensitivity) and several of dosimeters (say10) of a given set can be exposed to a known dose at approximately same time as the measurements. This will overcome problems associated with energy response, supralinearity, fading
30
Batch Sensitivity – 6LiF Li -6 Batch Sensitivity Correction Factor 1.20
BatchSensitivity (Normalised by
1.15 1.10 1.05 1.00 0.95 0.90 0.85
06 Jan2012 7 Dec2011
0.80
15 March 2012 Ave BCF
0.75
Series7 0.70 0
10
20
30
40
50
TLD #
Correction was also done to adjust the differences in Photon sensitivity
Batch Sensitivity 7LiF TL 7 - Batch Sensitivity Correction Factor 1.30
Batch Sensitivity (normalised by the average)
1.20
1.10
1.00
0.90 07Dec2011 19 Dec 2011 0.80
06 Jan 2012 15 March 2012 Average BCF
0.70
0
10
20
30
40
50
TLD no #
32
Preliminary Neutron Measurements
6
Li & 7Li TLD pairs at different locations of bunker (13/01/2012)
Reading (Corrected for Sensitivity)
3000.00
2500.00 TL6 TL7
2000.00
1500.00
4
1000.00
33
24
6
23 22
5
31
32 34
7
9 8
500.00
26
16 11 13
15
21
25
17
36 35
27 29
10
38
28
14
18 19
404142
44
37
30 12
39
43
20
50 46 45 4748 49
0.00 0
4
8
12
16
20
24
28 TLD No#
32
36
40
44
48
52
56
6LiF
– Greater cross section for thermal neutrons
Requires a good moderating medium (to slow neutron but minimum neutron absorption)
35
Suitable moderating medium
18x Exposure with Various Buildup
700
600
TL6
Perspex
TL7
Corrected reading
500
Wax
400 Domes 300 Pb
Open
200 Domes
Domes
100
Wax Perspex
Pb
Open
0 0
10
20
30
40
50
60
TLD #
36
Comparison of various materials Difference in TL6- TL7>=0
Net Difference (TL76 - TL7) Sensitivity corrected
600
500 Perspex Wax 400
300
NO BU 200
Pb
OPEN
41-45
46-50
No BU
100
Domes 0 21-25
26-28
29-30
31-35 TLD#
36-40
Measurements with various thickness of Perspex
700
600
Raw Reading
500
400 Li-6 Li-7 300
200
100
0 0
10
20
30 TLD #
40
50
60
Measurements with various thickness of Perspex Difference in TL6 & TL 7 Reading (Corrected) 600.0
∆ (TL6-TL7) (Csensitive correced)
500.0
400.0
2
y = -12.595x + 126.28x + 174.75 2 R = 0.9973
300.0
200.0
100.0
0.0 0.0
1.0
2.0
3.0 Build Up - Perspex (cm)
4.0
5.0
6.0
Calibration of TLDs using a known neutron source preliminary calibration was done at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). ARPNASA has a neutron sources (Am-Be) that produce neutron fields which are well characterised
d
Neutron and Photon readings as a function of distance from Central axis (CAX) 2
Neutron reading as a function of distance from CAX (18MV , 10x10 cm FS) 1000000
100000
Neutron reading (TL6TL7)
10000
1000
100
10
Neutron reading (Difference between TL6 & TL7) Corrected TL7 (for Photon) 1 0
20
40
60
80
100
120
140
Distance from Central Axis (cm)
160
180
200
220
Readings converted to Gy 2
Neutron reading as a function of distance from CAX (18MV , 10x10 cm FS) 1 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
0.1
Dose in Gy
Neutron Dose (Gy)
0.01
0.001
0.0001
Distance from Central Axis (cm)
Photon Dose in Gy
210
With a weighting factor =10 Neutron reading as a function of distance from CAX (18MV , 10x10 cm2 FS)
10
1
Dose Equivalent -QF=10 (Sv) Dose Equivalent Sv
Dose in Sv
0.1
0.01
0.001
0.0001 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Distance from Central Axis (cm)
160
170
180
190
200
210
Neutron Shielding Different concept from X Ray shielding Neutrons scatter more Attenuation (and scatter) depend VERY strongly on the neutron energy Best shielding materials contain hydrogen or boron (with high cross section for thermal neutrons) 45
TENTH VALUE DISTANCE MAZE LENGTH
LINAC & PATIENT SHIELDING & PROTECTION STAFF & PUBLIC
• Long maze - many ‘bounces’ 46
BUNKER WALL
Does the Shielding for photon will give the adequate protection from Neutron?
1. PHOTON S I H (TREATMENT) T F BEAM I OK
IS
RADIATION
2. NEUTRON CONTAMINATION
3.NEUTRONINDUCED However there are conflicting PROMPT claims … RADIATION
LL I W E S E K H O T BE
4.NEUTRONINDUCED DELAYED RADIATION
47
Aims of the project Validate a robust and accurate measurement technique (use of pairs of 6LiF and 7LiF) for neutron measurements and use it for neutron survey measurements in bunkers Evaluate neutron contamination at 18X conformal therapy, high energy IMRT, arc therapy etc.. Comprehensive understanding of neutron contamination will enable to evaluate the advantages / disadvantages of proposed high energy techniques Review current radiation shielding practices with presence of neutron contamination 48
