Physical characteristics of fast neutron beams

Europ.07. CancerVol. 7, pp. 99-104. Pergamon Press 1971. Printed in Great Britain

Physical Characteristics of Fast Neutron Beams D. K. BEWLEY M.R.C. Cyclotron Unit, Hammersmith Hospital, Ducane Road, London W12, England Abstract - - The physical properties of fast neutrons which are important to understand the biological effects are discussed and a comparison is made with the analogousproperties of photons. Changes in radiation quality with neutron energy and with the establishment of secondary charged-particle equilibrium are more important than in the case of photons. Penetration of fast neutrons in tissue depends on the neutron energy. To match the penetration of e°Co T-rays it is necessary to use 14 MeV neutrons at 1 m or more, or to use neutrons generated by at least 30 MeV deuterons on beryllium. Scattering of neutrons by tissue is of smaUer importance than scattering of orthovoltage X-rays. Gamma-radiation is produced by interactions between neutrons and tissue and complicates the dosimetry. A biological dosimeter is a useful way of checking that depth-dose curves give a correct representation of biological effect. Fast neutrons give some degree of skin-sparing due to lack of secondary chargedparticle equilibrium on the surface. To match the degree of skin-sparing given by e°Co T-rays it is necessary to use at least 50 MeV deuterons on beryllium. Energy absorption depends on hydrogen concentration and is a minimum in bone and a maximum in fat. The variation of biologically effective dose between different tissues and across the build-up zone is not necessarily proportional to the observed ionization.

COMPARISON WITH X- AND GAMMARAYS WHEN CONSIDERING the physical properties of fast neutrons in relation to their biological effects, it is convenient to compare fast neutrons with X- or 7-rays whose properties are more familiar to radiotherapists and radiobiologists. In such a comparison the neutron has certain analogies with the photon. Both are uncharged and are therefore indirectly ionizing particles, unlike electrons, protons and ctparticles. O n the other hand the photon has a rest mass of zero while the mass of the neutron is almost the same as the mass of the proton. In the context of biological effect the vital difference lies in their modes of interaction with matter. Absorption of photons gives rise to recoil electrons which are responsible for the

biological effect. Fast neutrons, on the other hand, interact only with atomic nuclei, and energy is transferred to matter via heavy particles. In biological material these are mostly recoil protons, but nuclear reactions also give rise to alpha-particles and other products. Some energy is dissipated by recoil nuclei of C N and O. Gamma-radiation is also produced which complicates the dosimetry but usually has comparatively little biological effect.

RADIATION QUALITY The heavy secondary charged particles are more densely ionizing than electrons, and give a higher value of Linear Energy Transfer (LET) in tissue. The difference in biological effects of neutrons are usually put down to this increased ionization density relative to X-rays. LET spectra of four beams of fast neutrons

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have been calculated by Bewley [1]. They cover a wide range of LET, extending from 30 to 8000 MeV cm*g -1 (3-800keV/~t in unit density tissue). The LET spectrum of electrons, on the other hand, lies mostly below 10 MeV cm * g_l and does not exceed 300 MeV cm* g-1 [2]. Table 1 gives a summary of the L E T of the various secondary charged particles. Table 1. L E T of Various Charged Particles Range of LET (MeV cm2 g-l) 2- 300 50-1000

Particle Electrons (X, 7) Protons Products of nuclear reactions (mainly a-particles) Heavy recoils

400-2500 108-104

Up to 300 MeV cm2g -1 the response of mammalian cells shows little change with LET while changes became rapid at higher LET values [3, 4] (see Fig. 1). The response to Xrays and electrons therefore changes only slightly with the spectrum of the radiation. With neutrons on the other hand spectral changes can be important. Recoil protons deposit an important fraction of their energy below 300 MeV cm ~ g_1 and so behave to some extent like low LET radiation (i.e. X-rays). Recoil protons are also responsible for the major part of the energy absorption by tissue from neutrons, about 90% at low neutron energies falling to 70% at 14 MeV [5]. Attainment of secondary charged-particle equilibrium with fast neutrons is therefore accompanied by a change in RBE. A full explanation of the effects of fast neutrons on mammalian cells in terms of the effects of the associated charged particles has not so far been possible [6]. 4

P E N E T R A T I O N IN TISSUE From the point of view of radiation beam therapy the most important physical property of the beam is its penetration in tissue. Measurement of depth doses is complicated by a varying proportion of T-radiation and of thermal and epithermal neutrons. However, both theoretical calculations [7] and measurements [8] have shown that the dose from these low energy neutrons is always less than 1% of the incident neutron dose. Gamma-radiation is therefore the main problem. One way of measuring neutron depth doses is to measure the total dose with a tissue-equivalent chamber and to subtract the dose due to "t-rays measured by another method such as thermoluminescent dosimetry (TLD). Bewley and Parnell [8] preferred to use a special ionization chamber in which the effect of T-radiation is eliminated by internal compensation. Gamma-rays were measured separately with films or T L D (CaF2), and the depth doses were then checked using a biological dosimeter [9]. The ionization chamber is shown in Fig. 2. It consists of three coaxial chambers with a common central electrode. All chambers are air-filled. The central part has walls made of graphited polythene and has a high sensitivity to fast neutrons plus a slightly smaller sensitivity (in terms of charge per rad) to ~t-rays. The two outer parts have walls containing no hydrogen and have a sharply reduced sensitivity to neutrons while still responding to T-rays. The polarizing voltages on the central and outer parts are of opposite polarity. The chamber can be "balanced" to give zero response to X- and ~,-rays by adjusting the volume of the section at the end, and it then responds only to the fast neutron component of a mixed neutron and T field. Only a few measurements of neutron depth

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OER and RBE of human cells in vitro as a function of LET, from Barendsen et al. [4].

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Physical Characteristics of Fast Neutron Beams CABLE

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doses for limited field areas have been published at the present time. The most relevant ones are shown in Fig. 3 together with curves for 250 kV X-rays and e°Co ?-rays. Neutrons of mean energy 8 M e V and those of 14 M e V at 50 cm fsd give depth dose curves intermediate between 250 kV X-rays and e°Co ,/-rays. Lower energy neutrons are less penetrating and would not be suitable for deep therapy. To obtain penetration as good as that from 6°Co it is necessary to use 14 M e V neutrons at 100 cm fsd or more, or deuterons of at least 30 M e V on a beryllium target. It is also possible that lower energy deuterons on a deuterium target may give a high mean neutron energy and good penetration if used at 100 cm fsd or more [12]. Fast neutrons are scattered less in tissue than 250 kV X-rays. The effect of field size on percentage depth dose is therefore not very

great. Figure 4 shows central axis depth-dose curves for several field sizes for neutrons of mean energy 8 M e V from the M R C cyclotron. They were measured with the compensated chamber and refer to neutron dose only. Scattering outside the beam is also less important [8]. Back-scatter on the incident surface is also rather small, varying from 4 % to 10% over a range of field sizes from 20 to 400 cm ~, for neutrons of mean energy 8 M e V [8].

GAMMA RADIATION AND T O T A L BIOLOGICAL EFFECT The variation with depth of dose due to T-radiation has been measured for the neutrons of mean energy 8 M e Y [8, 9]. The gamma-dose as a percentage of the local neutron dose is shown in Fig. 5. The percentage rises steadily

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~fn= 8 M e V [8] E n = 14 M e V [I0] Neutrons from 50 M e V d on thin Be ['14] 6OCo "i-rays Neutrons from 30 M e V d on thin Be [11]

Physical Characteristics of Fast Neutron Beams with depth. The RBE of ?-radiation relative to neutrons is about one-third at the dose levels used in clinical practice; thus 15% of ?-dose at 10 cm depth represents about 5% of the biologically effective dose. Exit doses with neutrons of this energy are small and a correction for -/-radiation is not important, but ~t-radiation may be a more important factor in skin dosage with more penetrating beams especially for large field sizes. McNally and Bewley [9] used mammalian cells as a biological dosimeter to check whether depth doses measured with the compensated chamber, plus an allowance for ?-radiation, correctly represented the biologically effective dose. The results confirmed that this was so, showing that changes in neutron spectrum did not affect the relevance of the physical measurements. This result agrees with the work of Field and Parnell [13] who were unable to detect any significant change in neutron spectrum with depth. With mono-energetic neutron sources however, such as those producing 14 MeV neutrons, some reduction in mean energy must be expected at a depth, and it will be important to find out if this affects physical measurements or causes a change in RBE.

SKIN-SPARING A finite thickness of tissue is required to give secondary charged particle equilibrium. This effect is particularly marked at the skin where the neutron beam is incident on the patient, since recoil protons are emitted in the forward direction. Figure 6 shows the measured build-up of ionization below the surface for various beams of neutrons and for 6°Co 3,-rays.

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At the same kinetic energy the range of protons in tissue is at least 30 times less than the range of electrons and the thickness of the build-up zone is correspondingly reduced. A further difference between photons and neutrons is the fact, noted above, that in the build-up zone biological effect is not necessarily proportional to ionization. Charged particle equilibrium for heavy recoils and the products of nuclear reactions is attained in a very short distance, comparable to the dimensions of a single cell. Consequently the radiation near the incident surface has a higher effective L E T than the radiation under full charged-particle equilibrium. Broerse etal. [15], using 15 MeV neutrons, found that near the surface the radiation had a higher RBE and a lower O E R than under conditions of chargedparticle equilibrium.

ENERGY ABSORPTION Relative energy absorption, in units of rads/ kerma (analogous to rads/roentgen), is nearly proportional to the hydrogen content of tissue. Thus bone experiences the lowest absorption of energy and fat the highest, the reverse of the situation with orthovoltage X-rays. In Table 1 the calculated values of rads/kerma for 14 MeV neutrons and neutrons of mean energy 8 MeV is given [12]. At bone-tissue interfaces a finite distance is required for attainment of charged particle equilibrium [12]. One should note that in bone the situation is similar to that in the build-up zone, namely the mean L E T will be higher than in tissue. The variation in biologically effective dose between tissues will therefore be smaller than the variation in the calculated values of rads/kerma.

REFERENCES 1. D . K . BEWLEY, Calculated LET distributions of fast neutrons. Radiat. Res. 34, 437 (1968). 2. A. COLE, Absorption of 20-eV to 50,000-eV electron beams in air and plastic. Radiat. Res. 38, 7 (1969). 3. G. W. BARENDSEN,H. M. D. WALTER, J. F. FOWLER and D. K. BEWLEY, Effects of different ionizing radiations on human cells in tissue culture. Radiat. Res. 18, 106 (1963). 4. G.W. BARENDSEN,C. J. KooT, G. R. VAN KERSEN, D. K. BEWLEY,S. B. FIELD and C. J. PARNELL, The effect of oxygen on impairment of the proliferative capacity of human cells in culture by ionizing radiations of different LET. Int. 07. Radiat. Biol. 10, 317 (1966). 5. R.L. BACHand R. S. CASWEL~.,Energy transfer to matter by neutrons. Radiat. Res. 35, 1 (1968). 6. D. K. BEWLEY, A comparison of the response of mammalian cells to fast neutrons and charged particle beams. Radiat. Res. 34, 446 (1968). 7. J.A. AUXIER,W. S. SNYDERand T. D. JONES, Neutron interactions and penetration in tissue. In Radiation Dosimetry (Edited by F. H. ATTIXand W. C. ROESCH), Vol. 1, p. 275. (1968). Academic Press, New York.

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D . K . BEWLEY and C. J. PARNELL,The fast neutron beam from the M.R.C. cyclotron. Brit. 07. Radiol. 42, 281 (1969). N . J . MCNALLY and D. K. BSWLEY, A biological dosimeter using mammalian cells in tissue culture and its use in obtaining neutron depth dose curves. Brit. 07. Radiol. 42, 289 (1969). D. GmSENE and R. L. THOMAS, An experimental unit for fast neutron radiotherapy. Brit. 07. Radiol. 41, 455 (1968). L.J. GOODMAN,S. A. MARINOand J. T. BmSNNA~,Minimum cyclotron size for radiation therapy. Conference on cyclotrons in chemistry, metallurgy and biology, p. 209. Oxford (1969). D. K. BEWLEY, Fast neutron beams for radiotherapy. In Current Topics in Radiation Research (Edited by M. EBERTand A. HOWARD),Vol. 5, p. 251. NorthHolland, Amsterdam (1970). S.B. FmLD and C. J. PARNELL, The use of threshold detectors to determine changes in a fast neutron energy spectrum with depth in a phantom. Brit. 07. Radiol. 38, 618 (1969). A.C. LucAs and W. M. QUAM, Personal communication (1970). J . J . BRom~s~., G. W. BARENDSENand G. R. VAN K~RSEN, Survival of cultured human cells after irradiation with fast neutrons of different energies in hypoxic and oxygenated conditions. Int. 07. Radiat. Biol. 13, 559 (1968).