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Neutron Interactions Part II

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1 Neutron Interactions Part II
Rebecca M. Howell, Ph.D. Radiation Physics B1.4580 Today, I will present the second of a 2-part lecture on neutron interactions.

2 Why do we as Medical Physicists care about neutrons?
Neutrons in Radiation Therapy Neutron Therapy Fast Neutron Therapy Boron Neutron Capture Therapy Contamination neutrons in high energy x-ray therapy and proton therapy. Patient and personnel dose Important component in shielding design The majority of radiation therapy is delivered using high energy photons or electrons and at a few centers protons are being used. So, why do we as medical physicists care about neutron interactions? There are 2 important scenarios where we encounter neutrons in radiation therapy. First, they can be used for radiation therapy and there are in fact 3 dedicated neutron radiotherapy centers in the US. Second, they can be a source of contamination dose in both high energy photon therapy and in proton therapy. This contamination dose is relevant for patients, personnel and this is an important component in shielding design.

3 Neutron Radiotherapy Fast Neutron Therapy Beams
Boron Neutron Capture Therapy History and Current Facilities Treatment sites The first half of today’s lecture will focus on neutron radiotherapy. I will discuss two types of therapy including fast neutron therapy and boron neutron capture therapy. Then I will briefly provide some details about the history of neutron therapy and current facilities and the types of tumors that have been successfully treated with neutrons.

4 Fast Neutrons Methods of Production
Neutrons can be produced in a cyclotron by accelerating deuterons or protons and impinging them on a beryllium target. Protons or deuterons must be accelerated to ≥50 MeV to produce neutron beams with penetration comparable to megavoltage x-rays. In photon therapy, we use linear accelerators with either magnetrons or klystrons to accelerate electrons and then impinge these fast moving electrons on a high Z-Bremsstrahlung target to generate high energy photons. Neutrons like photons are neutral and we accelerate charged particles and then impinge these charge particles on a target to generate fast neutrons. However to generate fast neutrons, we accelerate either protons or deuterons rather than electrons. And we impinge them on low Z targets rather than high Z targets. Specifically, neutrons can be produced using a cyclotron by accelerating deuterons or protons and impinging them on a beryllium target. The protons or deuterons must be accelerated to ≥50 MeV to produce neutron beams with penetration comparable to megavoltage x-rays.

5 Fast Neutrons Methods of Production
Accelerating deuterons to ≥50MeV Requires very large cyclotron, too large for hospital. Accelerating protons to ≥50MeV Much smaller cyclotron b/c proton has ½ the mass of deuteron. When designing a neutron therapy facility, the decision whether to use protons or dueterons to generate a fast neutron beam, is greatly effected by the amount of space available for the facility because accelerating deuterons requires a cyclotron that is approximately twice the size of a cyclotron to accelerate protons because the mass of a deuteron is twice that of a proton. Such a large cycletron is impractical for a hospital and would more likely be found at a research facility.

6 Fast Neutrons from Deuteron Bombardment of Be
Stripping Process – Proton is stripped from the deuteron. Recoil neutron retains some of the incident kinetic energy of the accelerated deuteron. For each neutron produced, one atom of Be is converted to B. The first method of generating a fast neutron beam is a stripping process. In this method, deuterons are accelerated to a high energy > 50 Mev and then impinged on a Berilium-9 target. If we consider an example with a single deuteron impinging a single Be-9 atom, the following steps lead to the production of a fast neutron. The deuteron collides with the Be-9 target and a proton is stripped from the deuteron and “added” to the Be-9 converting it to Boron-10. The recoil neutron from the proton stripped deuteron retains some of the incident kinetic energy of the accelerated dueteron. P+ n n + g

7 Fast Neutron Spectra from Deuteron Bombardment of Be
Neutron spectra consists of a single peak, with a modal value of about 40% of the energy of the incident deuterons. Fig 24.2a Hall fig 24.2a This figure shows two neutron spectra resulting from Deuteron Bombardment of Be. The spectrum on the left is from deuterons accelerated to 16 MeV and the spectrum on the right is from deuterons accelerated to 50 MeV. In both spectra there is a single peak, with a modal value of approximately 40% of the energy of the incident deuterons.

8 Fast Neutrons from Proton Bombardment of Be
Knock-out Process Protons impinge target of beryllium, where they knock-out neutrons. For each neutron “knocked-out”, one atom of Be is converted to B. The second method of generating a fast neutron beam is a knock-out process. In this method, protons are accelerated to a high energy > 50 Mev and then impinged on a Be-9 target. If we consider an example with a single proton impinging a single Be-9 atom. The following steps lead to the production of a fast neutron. The proton collides with the Be-9 target. The proton is “added” to the Be-9 and a neutron is “knocked-out” of to the Be-9 converting it to B-9. The neutron retains some of the kinetic energy of the incident proton. P+ n + g

9 Fast Neutron Spectra from Proton Bombardment of Be
The neutron spectra spans a wide range of energies. Necessary to filter out the low energy neutrons to achieve acceptable depth dose distribution. According to Hall text book a polyethylene filter was used to harden the beam. Based on our knowledge of neutron interactions why would polyethylene be a good choice for removing the low energy component from the neutron beam? Would polyethylene alone solve the problem or make it better? Hall fig 24.2b Fig 24.2b This figure shows two neutron spectra resulting from proton Bombardment of Be. The dashed line is an unfiltered spectrum that was produced from protons accelerated to 43 MeV. This spectrum spans a wide range of energies from 0 up to the energy of the incident protons. The low energy neutrons in this spectrum are not penetrating enough for therapy and must be filtered-out. This figure is from the Hall text book which says that a low Z material such as polyethylene was used to filter the beam. Polyethylene is a long chain of hydrocarbons. I think this may be only part of the story and suspect that boronated polyethylene was used to filter the beam. Recall from the previous lecture that elastic scatter dominates for lower energy fast neutrons and that nuclei with lower mass are more effective on a “per collision” basis for slowing down neutrons. So, a low Z material like polyethylene would reduce the energy of these neutrons to thermal energies. But, just using polyethylene would in my opinion just make the problem worse because the beam would have even more low energy neutrons. However, if , the polyethylene was doped with a material like boron which has a high thermal neutron cross section, it would effectively remove the thermalized neutrons from the beam achieving an overall harder spectrum as is shown by the solid line. Give this some thought, I would be interested to hear what you think…. You can read more about fast neutron beams in chapter 24 of the Eric Hall textbook, Radiobiology for the radiologist.

10 Isodose Distribution Bewley, Fig 4.3 Neutron beam (produced from 50-MeV protons or deuterons) has comparable depth dose distribution/isodose to 6MV photon beam. 6MV is the most commonly used beam energy in photon radiation therapy and thus provides a good frame of reference. The dmax for a 6 MV is 1.5 cm and percent depth dose at 10 cm is approximately 65%. This figure compares isodoses for a fast neutron beam and 6MV beam. Here, we see that isodose values ≥ 70% are very similar for the two beams. However, Isodose lines less than 70 % are at more shallow depths for neutrons than for photons, resulting in a somewhat steeper fall-off in dose. Note: differences incearse for low isodose lines

11 Long Treatment Distances
Neutron beam treatment distances are 100 to 140 cm due to large collimator Collimator materials: Hydrogenous material to slow the neutrons Absorber material to remove thermal neutrons Pb or other high Z material to absorb g-ray component (remember that activation follows absorption, g-photon is often the result) Again, if we compare a neutron beam to a 6MV photon beam, there are differences in the machine design. For example, a photon beam uses high Z materials such as lead or tungsten to collimate the beam. These materials are easily accommodated within the treatment gantry by a target to isocenter distance of 100 cm. However, neutron beams require a greater distance between the target and isocenter to accommodate thicker collimators. Neutron beams are typically collimated with several different materials designed to remove neutrons. The neutrons are first slowed down with hydrogen rich materials, then an absorbing material is used to remove the thermal neutrons. This is usually followed by a high Z-material to absorb the g-photons that are a byproduct of activation which follows absorption.

12 Clinical Experience with Fast Neutrons
First experience at Lawrence Berkeley Laboratory Hammersmith Hospital in London 3 Neutron Therapy Facilities in the US Northern Illinois University Institute for Neutron Therapy at Fermilab University of Washington Medical Center Gershenson Radiation Oncology Center at Harper University Hospital, Detroit The first clinical experiences with fast neutron beams were at Lawrence Berkeley Laboratory and Hammersmith Hospital in London. I do not have time to discuss these experiences in detail. However, if you are interested. More details can be found in in chapter 24 of the Eric Hall textbook, Radiobiology for the radiologist. Currently, there are 3 Neutron Therapy Facilities in the United States. Northern Illinois University Institute for Neutron Therapy at Fermilab University of Washington Medical Center Gershenson Radiation Oncology Center at Harper University Hospital, Detroit

13 Modern Neutron Therapy Facilities
University of Washington Medical Center Cyclotron accelerates protons (50.5MeV) Rotating gantry MLC equipped Gershenson Radiation Oncology Center Karmanos Cancer Center/Wayne State University (KCC/WSU) Gantry mounted superconducting cyclotron accelerates deuterons (48.5 MeV) Rotating Gantry MLC equipped

14 University of Washington Neutron Clinical Neutron Therapy System (CNTS)
University of Washington CNTS Lower Floor Schematic Rotating gantry The University of Washington operates a proton cyclotron that accelerates protons  to 50.5MeV. Then, a beamline transports the accelerated proton beam from the cyclotron to a gantry system. The gantry system contains magnets for deflecting and focusing the proton beam onto a beryllium target. The advantage of having a beam transport and gantry are that the cyclotron can remain stationary, and the radiation source can be rotated around the patient. radiation can be directed from virtually any angle by varying the orientation of the treatment couch and gantry position. This slide shows the schematic of the CNTS facility. Notice that the cyclotron is separately housed from the room with the rotating gantry. While I have never seen this facility, from the schematic, it appears that there is also a fixed beam line in a separate treatment room. Fixed beam line?

15 University of Washington CNTS MLC
University of Washington Neutron Clinical Neutron Therapy System (CNTS) University of Washington CNTS MLC The CNTS gantry head contains dosimetry systems to measure the dose along with the MLC and other beam shaping devices. This slide shows a photograph of the MLC.

16 Neutron therapy facility at the Gershenson Radiation Oncology Center KCC/WSU
Schematic gantry mounted superconducting cyclotron GMSCC The neutron therapy facility at the Gershenson Radiation Oncology Center bears some similarities to the CNTS at the University of Washington, but also has many unique characteristics. The facility produces its neutron beam by accelerating 48.5 MeV deuterons onto a beryllium target. This method produces a neutron beam with depth dose characteristics roughly similar to those of a 4MV photon beam. The deuterons are accelerated using a gantry mounted superconducting cyclotron (GMSCC), eliminating the need for extra beam steering magnets and allowing the neutron source to rotate a full 360° around the patient couch. A schematic of the gantry mounted superconducting cyclotron is shown in this figure.

17 Neutron therapy facility at the Gershenson Radiation Oncology Center KCC/WSU
MLC Schematic MLC Photo Neutron therapy facility at the Gershenson Radiation Oncology Center is equipped with an MLC beam shaping device. This slide shows both a schematic a photograph of the MLC. This facility is capable of delivering Intensity Modulated Neutron Radiotherapy (IMNRT).

18 Fast Neutron Therapy Considerations:
Who should be treated with neutrons? Subgroups of patients that may benefit from neutrons. Slower growing tumors. Cancers w/ good response to neutron Therapy: adenoidcystic carcinoma (cancer of parotid glands) locally advanced prostate cancer locally advanced head and neck tumors inoperable sarcomas cancer of the salivary glands There is evidence that certain subgroups of patients may benefit from neutron therapy compared to high energy photon therapy. Specifically, neutrons have been shown to be effective for patients with slower growing tumors such as adenoidcystic carcinoma (cancer of parotid glands), locally advanced prostate cancer, locally advanced head and neck tumors, inoperable sarcomas, and cancer of the salivary glands. Slower growing tumors are generally considered to be “later” responding and have lower a/B ratios than early responding tumors. The effectiveness of neutrons for these types of cancer is likely related to the higher LET of neutrons. Remember the dose from neutrons is actually delivered by high energy charged particles set in motion following interactions between neutrons and nuclei in the tissue. Recall from radiation biology that high LET radiation is more likely to result in double strand breaks where as low LET photons are more likely to result in single strand breaks. Further discussion of this topic is beyond the scope of this course and is covered in Radiation Biology.

19 Neutrons for Radiation Therapy
A few references…………… Fast neutron radiotherapy for locally advanced prostate cancer. Final report of Radiation Therapy Oncology Group randomized clinical trial. (American Journal of Clinical Oncology Apr; 16(2):164-7) Fast neutron irradiation of metastatic cervical adenopathy: The results of a randomized RTOG study. (International Journal of Radiation Oncology Biology Physics, Vol. 9, pp ) Neutron versus photon irradiation for unresectable salivary gland tumors: Final report of an RTOG-MRC randomized clinical trial. (International Journal of Radiation Oncology Biology Physics, Vol. 27, pp ) Fast neutron radiotherapy for soft tissue and cartilaginous sarcomas at high risk for local recurrence. (International Journal of Radiation Oncology Biology Physics, Vol. 50, No. 2, pp. 449–456) Photon versus fast neutron external beam radiotherapy in the treatment of locally advanced prostate cancer: results of a randomized prospective trial. (International Journal of Radiation Oncology Biology Physics, Vol. 28, pp ) In addition to the information in the information in chapter 24 of the Eric Hall textbook, Radiobiology for the radiologist, you may also find the following references of interest. I will do not expect you to read these, it is just a list for those who are particularly interested in the topic.

20 Boron-Neutron Capture Therapy
The idea: Preferentially deliver Boron containing drug to the tumor. Then deliver thermal (0.025eV) neutrons, which interact with the boron to produce alpha particles. Recall 10B has large thermal cross section: s = 3837 barns The 10B absorbs the thermal energy neutron and ejects an energetic short-range alpha particle (1.47MeV) and lithium ion (0.84MeV) which deposit most of their energy within the cell containing the original 10B atom. Let’s switch topics and discuss Boron neutron capture therapy (BNCT). BNCT uses a neutron beam that interacts with boron injected into a patient. BNCT depends on the interaction of slow neutrons with boron-10 to produce alpha particles and lithium nuclei. Patients are given an intravenous injection of a boron-10 tagged chemical that preferentially binds to tumor cells. Then thermal neutrons are delivered and interact with the Boron-10 which has a high thermal neutron absorption cross section of 3837 barns. The Boron-10 absorbs the thermal energy neutron and ejects a 1.47MeV short-range alpha particle and 0.84MeV lithium ion which deposit most of their energy within the cell containing the original Boron-10 atom.

21 Why Boron??? Several nuclides have high thermal neutron s, but 10B is the best choice for several reasons: it is non-radioactive and readily available, comprising approximately 20% of naturally occurring boron; Emitted particles (a and 7Li) have high LET Combined path lengths are approximately one cell diameter; i.e., about 12 microns, theoretically limiting the radiation effect to those tumor cells that have taken up a sufficient amount of 10B, and simultaneously sparing normal cells Chemistry of boron is well understood and allows it to be readily incorporated into a multitude of different chemical structures. In the previous lecture, we saw that there were several nuclei with high thermal absorption cross sections. So, why do we use boron over some of these other Nuclei such as Cadmium-113 which as a cross section that is 5 times greater than Boron 10? First, it is non-radioactive and readily available, comprising approximately 20% of naturally occurring boron; Second, the Emitted particles (a and 7Li) have high LET and their combined path lengths are approximately one cell diameter; i.e., about 12 microns, theoretically limiting the radiation effect to those tumor cells that have taken up a sufficient amount of 10B, and simultaneously sparing normal cells, and Finally, the chemistry of boron is well understood and allows it to be readily incorporated into a multitude of different chemical structures.

22 Boron-Neutron Capture Therapy
Beam Energy Selection Limited penetration of thermal neutrons. Thermal neutrons rapidly attenuated by tissue. HVL only about 1.5cm. Not possible to treat depths greater than a few cm. Can use epithermal neutrons (1eV-10keV), which are theramlized by tissue (via collisions w/ H). Peak in dose occurs at 2 to 3cm; Avoid high surface doses, but still poorly penetrating! An important consideration for BNCT is that thermal neutrons are not very penetrating and external irradiation with thermal neutrons would really only be effective for very shallow tumors. Therefore in reality, the patient is irradiated with epithermal neutrons, which are thermalized within the superficial tissues via elastic scatter collisions with hydrogen. While this improves the penetration compared to thermal neutrons and reduces the skin dose, the peak doses are still somewhat shallow occurring at 2 to 3 cm.

23 Boron-10 Neutron Interaction
An epithermal beam rapidly loses energy by elastic scattering in tissue. The thermal neutrons are captured by the 10B atoms which become 11B atoms in the excited state for a very short time (~ s). The 11B atoms then splits into alpha particles, 7Li recoil nuclei and in 94% of the reactions, gamma rays. This slide is a schematic of Boron-10 interaction capitalized upon in BNCT.

24 BNCT Neutron Source at MIT/Harvard
The MIT/Harvard group makes use of a fission converter based epithermal neutron beam at the MITR-II Research Reactor. filtered by aluminum, Teflon, cadmium,  and Lead. provides a broad spectrum epithermal beam with low incident gamma and fast neutron contamination while maintaining an incident neutron flux of ~5 x 109  neutroncm-2sec-1.  permits irradiations for clinical trials to be conducted in fractions in 10 minutes or less This slide provides some details about the BNCT source that is currently in use at the MIT/Harvard facility.

25 Treatment Sites for BNCT
Clinical Trials for: Glioblastoma Multiform (GBM) Sweet and colleagues first demonstrated that certain boron compounds would concentrate in human brain tumor relative to normal brain tissue.1 Melanoma Sweet and colleagues first demonstrated in 1952 that certain boron compounds would concentrate in human brain tumor relative to normal brain tissue. Shortly thereafter, clinical trials were initiated at Brookhaven National Laboratory and at MIT. Unfortunately, these trials failed to show any evidence of therapeutic efficacy. It became clear later, that there were two major reasons for their lack of success. First the study used thermal neutrons which were insufficiently penetrating and second the boron compounds that were used in the trials were freely diffusible and did not achieve selective localization in the tumor. Later, more encouraging results were observed in clinical studies done in Japan for the treatment of malignant gliomas and melanoma.  At present, there are several groups in the U.S. and abroad working on BNCT for melanoma and gliomas.  1(Sweet, W.H., Javid, M., "The possible Use of Neutron-capturing Isotopes such as boron-10 in the treatment of neoplasms," I. Intracranial Tumors, J. Neurosurg., 9: , (1952) )

26 Production of Secondary Neutrons
The second half of today’s lecture will focus on the production of secondary neutrons i.e. contamination neutrons in radiotherapy.

27 Secondary Neutrons Radiation Therapy
X-Ray Therapy Neutrons can be produced via (g-n) reactions primarily with high atomic number materials within the treatment head. Proton Therapy Neutrons can be produced via (p,n) reactions, not limited to high Z materials. At the high energies other reactions are also possible…………….. Contamination neutrons in x-ray therapy result from g,n reactions with high atomic number materials in the linac head. Contamination neutrons in proton therapy result from p,n reactions with high Z materials in the nozzle, but can also be the result of p,n reactions within the patient. Both (g,n) and (p,n) reactions are threshold reactions. So why do we observe p,n and not observe g,n reactions in the patient? It has to do with the energies of the photon and proton beams and the answer to this question should be clear by the end of today’s lecture.

28 Production of Neutrons
(g,n) Reactions

29 Bremsstrahlung Spectrum
This plot is an example of a spectrum for an 18 MV photon beam. The maximum energy of the photon spectrum is determined by the energy of the electrons incident on the Bremsstrahlung target. In this example, the maximum energy of the electrons was 18MeV. So, the photon beam has a range of energies from 0 to 18MeV, with an average spectrum energy of approximately 6 MeV. Now, lets consider this spectrum in the context of (g,n) reaction cross sections and threshold energies…..

30 Particle Production Cross Sections ENDF/B-VII Incident-Gamma Data
(g,n) cross section High energy 18MV x-ray treatment beam has max energy of 18MeV and an average energy of 6MeV. Secondary neutron Production is possible in high Z components of linac head. No neutrons for 6MV x-ray beam all photons below threshold This plot, shows the g,n reaction cross section for Pb-207 in black. Note that the threshold energy is slightly greater than 6 MeV and increases rapidly between 6 and about 18 MeV, and then begins to drop. So, in the context of the bremstrahlung spectrum on the previous slide, secondary neutrons can be generated from interactions between high energy photons in that spectrum and Pb-207 in the linac head. The g,n reaction cross sections for other high Z materials in the linac head such as tungsten are similar to that of Pb-207. Threshold energy for (g,n) Note: Magnitude of the cross sections  barns

31 18 MV beam has more photons above the (g,n) threshold and most are in the region where cross section dramatically increases. Now let’s compare the spectra for 18 MV and 15 MV photon beams. Notice that 18 MV beam has more photons above the (g,n) threshold and most are in the region where the g,n cross section dramatically increases. So, how would we expect this to effect neutron contamination. We would expect to see substantially more contamination neutrons for a 15 MV beam compared to an 18 MV beam. And there is in fact 2 fold increase is observed in neutron dose for 18 MV beams compared to 15 MV beams for certain linacs.

32 (g-n) Reaction Cross-Sections Elements in Tissue (C,O,N)
Note: Threshold energies are much higher compared to high Z The magnitude of the reaction x-sections are an order of magnitude lower than for high Z (0.005 – 0.02). These data are from the T-2 Nuclear Information Service. Now, let’s consider the reaction cross sections for nuclei in the human body. This is a plot of g,n reaction cross sections for C, O, and N. Nitrogen has the lowest threshold energy at about 8 Mev and Carbon has the highest at about 19 MeV. These thresholds are much higher than those observed for high Z materials and there are very few photons in our 15 MV and 18 MV spectra above these thresholds. Also, the magnitude of the reaction x-sections are an order of magnitude lower than for high Z materials. Therefore, we can conclude that secondary neutron production is tissue is extremely unlikely and essentially negligible.

33 Secondary Neutron Spectra from Clinical Photon beams
The initial distribution of secondary neutrons generated in the linac head from (g,n) reactions is approximately isotropic and resembles a fission spectrum. Then, The neutron energy decreases as a consequence of their transport through the components of the treatment head (primary collimators, flattening filter, secondary jaws, MLC, etc). The primary mechanisms of energy loss in high Z materials in the linac head are inelastic scattering and (n,2n) reactions. The initial distribution of secondary neutrons generated in the linac head from (g,n) reactions is approximately isotropic and resembles a fission spectrum. Then, The neutron energy decreases as a consequence of their transport through the components of the treatment head (primary collimators, flattening filter, secondary jaws, MLC, etc). The primary mechanisms of energy loss in high Z materials in the linac head are inelastic scattering and (n,2n) reactions.

34 Photoneutron Spectra Effect of Collimators and room shielding
Photoneutron spectrum for 15MeV electrons striking W target (designated 15MeV W PN bare) Spectrum with 10 cm of W shielding surrounding W target. Spectrum with 10 cm of W shielding surrounding W target inside a concrete room A 252Cf fission spectrum shown for comparison NCRP-79 Fig 25 This figure from NCRP-79 compares a 252Cf fission spectrum to a photoneutron spectrum for 15MeV electrons striking W target (designated 15MeV W PN bare). You can see that these two spectra are very similar. Also shown are the photoneutron spectrum with 10 cm of tungsten to simulate materials in the gantry head. Notice that the energy is decreased as a consequence of inelastic scatter and n,2n reactions Also shown is a spectrum with 10 cm of W shielding inside a concrete room to simulate materials in the gantry head in an actual treatment vault. The energy is further reduced from elastic scatter with low Z materials in the concrete.

35 Secondary Neutron Spectra Measured for Varian 18MV Linac
This figure shows photoneutron spectra measured for a Varian linac operated at 3 different beam energies 15, 18, and 20 MV. Notice that qualitatively these spectra are similar, with fast neutron peaks centered at approximately 0.23 MeV and have similar average energies. Each spectrum also has a low-energy tail that arose from neutrons scattered throughout the treatment vault. The obvious distinction between the three spectra is the total neutron fluence per MU. A 1.7-fold increase in fluence was observed as the energy increased from 15 to 18 MV and a 1.4-fold increase was observed as energy was increased from 18 to 20 MV. Howell et al. Medical Physics, Vol. 36, No. 9, (2009)

36 Production of Neutrons
(p,n) Reactions

37 Proton Spectra Clinical proton beams have a much smaller energy spread compared to photon beams (Gaussian distribution). Also, the maximum energies are considerably higher and clinical beam energies may include, 100 MeV, 160 MeV, 200 MeV, and 250 MeV beams. Clinical proton beams have a much smaller energy spread compared to photon beams (Gaussian distribution). Also, the maximum energies are considerably higher and clinical beam energies may include, 100 MeV, 160 MeV, 200 MeV, and 250 MeV beams.

38 Particle Production Cross Sections ENDF/B-VII Incident-Proton Data
Note: High Z material (e.g. Pb-207): Magnitude of the (p,n) cross sections are higher than (g,n) and continue to increase with increasing energy. The proton beam energies can be as high as 250MeV, well above the threshold. This plot, shows the p,n reaction cross section for Pb-207 in black. Note that the threshold energy is approximately 10MeV and increases rapidly with energy. So, in the context of the energies used for clinical proton beam therapy, neutrons can be generated from interactions between high energy protons in that spectrum and Pb-207 in the nozzle. Also, the magnitude of the (p,n) cross sections are higher than (g,n). The p,n reaction cross sections for other high Z materials in the linac head such as tungsten are similar to that of Pb-207.

39 Particle Production Cross Sections ENDF/B-VII Incident-Proton Data
Note low Z material (e.g. C-12): Magnitude of the (p,n) cross sections are much lower than in high Z materials energy thresholds for (p,n) in low Z are higher compared to high Z Energy threshold similar to (g,n) Now, let’s consider the reaction cross sections for nuclei in the human body. This is a plot of p,n reaction cross sections for C-12. The thresholds is somewhat higher than for Pb-207, but still well below the energy of a clinical proton beam. Therefore, we can conclude that in contrast to our observation that g,n reactions in tissue are extremely unlikely for a clinical photon beam, p,n reactions in tissue can occur in a clinical proton beam and this neutron contamination dose is not negligible. November 2007  Rebecca M. Howell, Ph.D.

40 Particle Production Cross Sections ENDF/B-VII Incident-Proton Data
Note low Z material (e.g. C-12): Magnitude of the (p,n) cross sections are much lower than in high Z materials energy thresholds for (p,n) in low Z are higher compared to high Z Energy threshold similar to (g,n) November 2007  Rebecca M. Howell, Ph.D.

41 Secondary Neutron Spectra for Clinical Proton Beams
These data are a plot of Monte Carlo simulated neutron spectra for clinical proton beams. These neutron spectra were generated via p,n reactions. There are two pronounced peaks in all neutron spectra, joining each other at approximately 10 MeV. The low-energy peak is similar in shape for all proton energies, with neutron energies ranging from 0 to about 10 MeV and having a modal energy of about 1 MeV. These low energy neutrons are mainly produced from evaporation processes and are isotropically distributed. The high-energy neutron peak starts at about 10 MeV and extends up to the maximum proton energy, with the modal energy varying with the proton energy. The high-energy peak contains forward-peaked neutrons from direct (nucleon–nucleon) reactions produced in intranuclear cascades and neutrons ejected from the compound-nucleus and pre-equilibrium processes. Major difference between secondary neutrons from proton and photon beams is the energy; neutrons from proton beams have much higher energy. This results from the much higher max energy of proton beams (~range: 100 MeV to 250 MeV) verses the max energy of photons (~ range: 10 MV to 25 MV) energy of the incident photon Zhang et al. Phys. Med. Biol. 53 (2008) 187–201

42 References Eric J. Hall. Radiobiology for the Radiologist 5th Ed. (2000) Frank H. Attix. Introduction to Radiological Physics and Radiation Dosimetry. (1986) Patton H. McGinley. Shielding Techniques for Radiation Oncology Facilities 2nd ed. D.K. Bewley. The Physics and Radiobiology of Fast Neutron Beams (1989) AAPM Report 7 Protocol for Neutron Beam Dosimetry ICRU 45 Clinical Neutron Dosimetry NCRP 79 Neutron Contamination from Medical Electron Accelerators NCRP 151 Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities (2005) ICRP74/ICRU57 (Jointly published by both ICRU and ICRP) Conversion Coefficients for use in Radiological Protection against External Radiation ICRP 60 Recommendations of the International Commission on Radiological Protection ICRU 66 Determination of Operational Dose Equivalent Quantities for Neutrons T2.lanl.gov

43 End This concludes the material from the neutron interactions lectures that will be covered on the exam in this course.

44 Extra Information The material in the remaining slides is covered in Radiation Protection and is beyond the scope of today’s lecture. In previous years, I have covered this material in this course. This year, I have decided to cut this material to minimize overlap with other courses. However, I decided to make the slides available to you should you be interested, but will not be testing you on this material.

45 Shielding Considerations for Secondary Neutrons
High Energy X-Ray Beams

46 Shielding for Photoneutrons
High beams (>10MV) are contaminated with neutrons. Produced by high energy x-rays and e-s incident on various materials (target, flattening filter, collimators, etc.). Many more neutrons in x-ray beam than in e- beam. X-section for (e,n) reactions smaller than x-section for (g,n) by factor of 10. In electron mode beam current is about 1000X less than in x-ray mode due to inefficiency of Brems. Production.

47 Shielding for Photoneutrons
Neutron contamination increases rapidly with energy from 10 to 20 MV, then remains approx. constant above 20MV (recall that there are very few linacs with max energies greater than 25MeV). Neutron contamination at cax for 16-25MV x-ray beam is approx. 0.5% of x-ray dose, and falls off to about 0.1% outside the field. Higher for IMRT more beam on time to achieve same photon dose.

48 Particle Production Cross Sections ENDF/B-VII Incident-Gamma Data
(g,n) cross section Neutron contamination increases rapidly with energy from 10 to 20 MV, then remains approx. constant above 20MV (recall that there are very few linacs with max energies greater than 25MeV). Threshold energy for (g,n) Note: Magnitude of the cross sections  barns November 2007  Rebecca M. Howell, Ph.D.

49 Shielding for Photoneutrons
Concrete barriers designed for x-ray shielding are sufficient for photoneutrons. Door must be protected against neutrons that diffuse into the maze and reach the door. Required door shielding can be minimized with a good maze. Maze > 5m desirable. This length is chosen because the TVL for the photoneutrons entering the maze is approximately 5m (MKcGinley, pg 71)

50 Example of Door in High Energy Vault shielded for Neutrons
Note: Figure (5.5) is taken from Shielding Techniques 2nd ed. by Patton McGinley Note: the average energy of the neutrons at the Maze entrance is approximately 100keV (NCRP, 1984) Note: there are many ways to design a door, in some cases, the lead is placed before the polyethylene, and in some cases it is sandwiched between two layers of lead, this is just one example of a door design.

51 Door Design Photoneutron Shielding
After multiple scattering interactions in the polyethylene (high H content) the neutrons are thermalized. Thermal neutrons can undergo neutron capture releasing high energy g-rays (n,g)  g-rays energies can exceed 8MeV, and have an average energy of 3.6MeV. How do we eliminate these high energy g-rays? Add 5% Boron to the polyethylene. Boron absorbs (high thermal absorption cross section) the low energy neutrons before they have a chance to undergo (n,g) reactions. But the reaction also results in a 0.48MeV g. Lead is placed after the boronated polyethylene, where it attenuates the photons produced in the boron (0.48MeV) and any capture gamma rays generated in the maze by neutron capture in the concrete walls, ceiling, and McGinley, 2002

52 Boron Interaction with Thermal Neutrons
Materials with B are effective absorbers of thermal neutrons because of the high thermal neutron cross section The thermal neutrons are captured by the 10B atoms which become 11B atoms in the excited state for a very short time (~ s). The 11B atoms then fissions producing a-particles, 7Li recoil nuclei, and in 94% of the reactions, gamma rays (0.48MeV).

53 Door Design for Neutron Shielding Details
Boronated polyethylene: The polyethylene (high H content) slows (moderates) the fast and intermediate energy neutrons to thermal energies. The 5% Boron absorbs the low energy neutrons (high cross section for thermal neutron absorption). Lead absorbs the 0.48 MeV photon that results from the (n,a) and capture gammas ( from maze ceiling, and floor). Lead Steel Casing Maze Polyethylene 5% Boron

54 Activation of Materials in Linac components
Reaction Mode of Decay T1/2 Photon E (MeV) 27Al(n,g)28Al b- 2.3 min 1.780 63Cu(n,g)62Cu b+ 9.7 min 0.511 55Mn(n,g)56Mn 2.6 hr 0.847 63Cu(n,g)64Cu b+/b- 12.7 hr 1.346 65Cu(n,g)64Cu 186W(n,g)187W 23.9 hr 0.479/0.686 58Ni(n,g)57Ni 36.0 hr 1.378/1.920 Reproduced from Table 4.4, McGinley

55 Activation Material in Air
Air is made radioactive by medical accelerators operated above 10 MeV primarily by: Each reaction produces a positron emitter with a relatively short half-life. Patients/personnel can be exposed to MeV annihilation photons. Reaction Half-life (sec) Threshold Energy (MeV) 14N(g,n)13N 600 10.5 16O(g,n)15N 122 15.7

56 Activation Material in Air
Maximum permissible concentration in air (MPCa) based on a 40-hr work week and typical treatment vault (air volume). Isotope Organ MPCa (Bqm-3) DI (nSvs-1) 13N Skin 1.5x106 41.7 Whole Body 8.5x106 6.95 15O 7.4x105 Reproduced from Table 7-5, McGinley

57 Activation Material in Air
McGinley at al (1984) calculated annual total dose equivalent to radiation therapists skin Conservative Calculation Assumptions: 40 patients per day 5 days per week 4Gy/fx Daily treatment time = 120-s Therapist stay time of 600s per patient Far below the Maximum Permissible Skin dose (0.5Sv). Air activation presents minimal hazard.

58 Two main Categories of “Neutron Dosimetry”
Neutron Dose Two main Categories of “Neutron Dosimetry” Clinical Neutron Beam Dosimetry Dose from neutron beams used for patient treatment. Protection Dosimetry for unwanted neutron dose (low neutron doses)

59 Quantities used in Radiation Protection
Quantities used in Radiological Protection There are two types of quantities used in radiological protection: Protection Quantities - defined by the International Commission on Radiation Protection (ICRP). Operational Quantities - defined International Commission on Radiation Units and Measurements (ICRU).

60 ICRP Protection Quantities
The (ICRP) defines limiting or protection quantities as dosimetric quantities specified in the human body. Recommended dose limits are expressed in terms of protection quantities. These quantities are not directly measurable but may be related by calculation to the radiation field in which the exposure occurs.

61 ICRU Operational Quantities
Operational quantities were designed by the ICRU in response to ICRP recommendations on radiological protection. Used to demonstrate compliance with dose limits. Operational quantities provide a reasonable estimate of the protection quantities and serve as calibration quantities for dosimeters used in monitoring.

62

63 Protection Quantities

64 ICRP Protection Quantities
Absorbed Dose, DT, is the mean absorbed dose in a specified tissue or organ of the human body, T, Equivalent Dose, HT, is the absorbed dose averaged over the tissue or organ, T, irradiated in a radiation field consisting of several different radiations with different values of WR, Effective Dose, E, is the sum of the weighted equivalent doses in all the tissues and organs of the body, where, mT is the tissue or organ mass, DT is the absorbed dose in the mass element dm. where, DT,R is the average absorbed dose from radiation, R, in tissue T. where, HT is the equivalent dose in the tissue or organ T and WT is the weighting factor for the tissue.

65 ICRP Protection Quantities
ICRP-60 Tissue Weighting Factors Tissue or Organ Tissue Weighting Factor, WT Gonads 0.2 Bone marrow (red) 0.12 Colon Lung Stomach Bladder 0.05 Breast Liver Esophagus Thyroid Skin 0.01 Bone Surface Remainder Note: In the case in which a single one of the remainder organs receives an equivalent dose in excess of the highest dose in any of the 12 organs for which a weighting factor is specified: a weighting factor of should be applied to that tissue and a weighting factor of applied to the average dose in the rest of the remainder organs.

66 Radiation Weighting Factors

67 Radiation Weighting Factors

68 Operational Quantities

69 ICRU Operational Quantities Dose Equivalent
The Dose Equivalent, H as defined by the ICRU is the product of Q and D at a point in tissue: where D is the absorbed dose Q is the quality factor at this point The SI unit for H is the Sievert (Sv).

70 ICRU Operational Quantities Dose Equivalent
The quality factor depends on the unrestricted linear energy transfer, L, for charged particles in water, specified in ICRP publication 60. For photons and electrons the quality factor is unity. For neutrons, the quality factor is strongly energy dependent (ICRU Report 66).

71

72 Determining Dose Equivalent
Dose equivalent is essentially unmeasurable. But, as shown in the previous diagram, it can be determined by calculation or by a combination of calculations and measurements. Quantities needed to determine dose equivalent are: absorbed dose and the quality factor or fluence and fluence-to-dose equivalent conversion coefficients.

73 Additional Information on Neutron Interaction Cross-sections
This information will not be on your exam.

74 Transport of Secondary Neutrons
Strongly dependent on absorbing material and incident neutron energy. Example: 207Pb The next 15 slides of the presentation are provided as an example of the data that are available for a given isotope, given time constraints, these can not be covered in detail.

75 Great Reference for Nuclear Data T2.lanl.gov

76 View PDF files for various elements/ isotopes
T2.lanl.gov What Data are provided in this file? Let’s consider an example: For 207Pb, the PDF file has 179 pages of data. View PDF files for various elements/ isotopes

77 Principal Cross Sections Low Energy (Log-Log Plot)

78 Principal Cross Sections High Energy (Linear Plot)

79 Inelastic Cross Sections are provided on Separate Plots

80 Inelastic Cross Sections
What is (n,n*1), (n,n*2), etc. There is a discrete energy band gap between energy levels in the nucleus. *1 refers to promoting a neutron into the first excited state. *2 refers to promoting a neutron into the second excited state. (n,x) – Anything above 20th excited level (not n,g) How much energy are we talking?

81 Select Levels and Gamma Search
NuDat 2.4 Select Levels and Gamma Search

82 Enter Isotope of Interest
Search

83 Nuclear Level and Gamma Search
1st excited State 2nd excited State 3rd excited State

84 Threshold Reactions

85 Angular Distributions (provided for all elastic and inelastic Scattering)
Angular Distribution of Elastic Scatter (0-30MeV) Angular Distribution of inelastic (n,n*1)Scatter (0-30MeV)

86 Definitions from ICRP Report 63
Elastic denotes a reaction in which incident projectile scatters off target nucleus with total KE being conserved (final nucleus is the same as bombarded nucleus). Non-elastic is a general term referring to nuclear reaction that are not elastic (i.e. KE is not conserved). Inelastic refers to specific type of non-elastic reaction in which KE not conserved, but final nucleus is the same as bombarded nucleus.


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