P. Pihet

Microdosimetric Specification of Radiation Quality in Neutron Radiation Therapy

The neutron beams used by various radiotherapy centres are of widely differing energies, and differences of up to 50 per cent in the relative biological effectiveness (RBE) between different beams have been found in radiobiological experiments. Moreover, at some facilities RBE variations have been observed with increasing depth in a phantom. In spite of this evidence, there is no quantitative and uniquely accepted specification of radiation quality used in practice. The urgency of an adequate solution of this problem is illustrated by the fact that in radiation therapy the usual accuracy requirement for the quantity of radiation, i.e. the absorbed dose to be delivered to the tumour, is 3.5 per cent (1 SD). In this paper a pragmatic solution for the specification of radiation quality for fast neutron therapy is proposed. It is based on empirical RBE versus lineal energy response or weighting functions. These were established by using existing radiobiological data and microdosimetric spectra measured under identical irradiation conditions at several European neutron irradiation units.

Measurement of kerma factors for carbon and A-150 plastic: neutron energies from 13.9 to 20.0 MeV

Neutron fluence and neutron kerma were measured under similar conditions for quasi-monoenergetic neutron fields with energies ranging from 13.9 to 20.0 MeV, in steps of about 0.5 MeV. The combination of two techniques, the proton recoil telescope to determine the neutron fluence and low-pressure proportional counters made of carbon and A-150 plastic to determine the kerma, enabled the kerma factors to be measured for each material with uncertainties lower than 8.0%. The kerma factor for carbon increases from 19.3 pGy cm2 to 32.4 pGy cm2 and for A-150 plastic from 61.4 pGy cm2 to 76.0 pGy cm2 for neutron energies ranging from 13.9 to 20 MeV. For neutron energies above 18 MeV in particular, present kerma factors for carbon are 10 to 15% lower than other experimental data and the values calculated from evaluated nuclear data by Caswell et al, but are in closer agreement with the values derived from nuclear models.

Risk assessment for cancer induction after low- and high-LET therapeutic irradiation

SummaryThe risk of induction of a second primary cancer after a therapeutic irradiation with conventional photon beams is well recognized and documented. However, in general, it is totally overwhelmed by the benefit of the treatment. The same is true to a large extent for the combinations of radiation and drug therapy.After fast neutron therapy, the risk of induction of a second cancer is greater than after photon therapy. Neutron RBE increases with decreasing dose and there is a wide evidence that neutron RBE is greater for cancer induction (and for other late effects relevant in radiation protection) than for cell killing. Animal data on RBE for tumor induction are reviewed, as well as other biological effects such as life shortening, malignant cell transformation in vitro, chromosome aberrations, genetic effects. These effects can be related, directly or indirectly, to cancer induction to the extent that they express a “genomic” lesion. almost no reliable human epidemiological data are available so far.For fission neutrons a RBE for cancer induction of about 20 relative to photons seems to be a reasonable assumption. For fast neutrons, due to the difference in energy spectrum, a RBE of 10 can be assumed.After proton beam therapy (low-LET radiation), the risk of secondary cancer induction, relative to photons, can be divided by a factor of 3, due to the reduction of integral dose (as an average).The RBE of heavy-ions for cancer induction can be assumed to be similar to fission neutrons, i. e. about 20 relative to photons. However, after heavy-ion beam therapy, the risk should be divided by 3, as after proton therapy, due to the excellent physical selectivity of the irradiation. Therefore a risk 5 to 10 times higher than photons could be assumed.This range is probably a pessimistic estimate for carbon ions since most of the normal tissues, at the level of the initial plateau, are irradiated with low-LET radiation.

Specification of radiation quality in fast neutron therapy: microdosimetric and radiobiological approach.

Specification of radiation quality is an important issue in fast neutron therapy since the biological effectiveness of the beams varies to a large extent with neutron energy. It must meet specific criteria, mainly derived from the accuracy requirement for absorbed dose delivery. A first approach to this problem consists in identifying physical parameters that can be related to Relative Biological Effectiveness (RBE) and which describe the beam production technique (e.g. neutron-producing reaction, p + Be or d + Be, energy of the incident particle). A second is based on microdosimetry, which provides a description of the secondary radiation components to which the biological consequences of irradiations are more directly correlated. A third approach consists in experimental RBE determinations in reference conditions: intestinal crypt regeneration in mice after irradiation to the whole body with single doses is proposed as a standard biological system for radiobiological calibrations of clinical fast neutron beams. Dosimetric, microdosimetric and radiobiological intercomparisons are encouraged since they provide a homogeneous set of data which facilitate the exchange of clinical information. They also constitute a basis for the clinical RBE approach and an overall check of the irradiation procedure. Therefore they should be recommended in every non-conventional radiation therapy facility.

Measurement of Neutron Kerma Factors for Carbon and A-150 Plastic At Neutron Energies of 26.3 Mev and 37.8 Mev

Kerma factors for carbon and A-150 tissue equivalent plastic were determined in two nearly monoenergetic neutron beams. Kerma was measured with low pressure proportional counters (PCs) with walls made of graphite and A-150 plastic. The neutron fluence was measured with a proton recoil telescope. The corrections for the kerma contributions from low energy neutrons in the beams were determined by time-of-flight measurements with an NE213 scintillation detector and with the Pcs. For neutron energies of (26.3+/-2.9) MeV and (37.8+/-2.5) MeV the kerma factors for carbon were (35.5+/-3.0) pGy cm2 and (42.6+/-3.9) pGy cm2, respectively, and for A-150 they were (75.6+/-5.4) pGy cm2 and (79.0+/-5.6) pGy cm2. At these energies the kerma factor ratio for ICRU muscle tissue to A-150 calculated on the basis of the new kerma factors, is 0.93.

Improvement of a P(65)+be Neutron Beam for Therapy At Cyclone, Louvain-la-neuve

The variable energy cyclotron of the Catholic University of Louvain is used to produce intense neutron beams for neutron therapy purposes. As a first step, neutrons were produced by bombarding a Be target with 50 MeV deuterons; at present they are produced by 65 MeV protons. This paper describes the improvements to the target system. A new (17 mm) Be target together with the old (10 mm) Be target are inserted in a movable support which allows the production of neutrons either by 65 MeV protons or by 50 MeV deuterons. Both targets can be removed for proton beam therapy. The dosimetric characteristics of the p(65)+Be and d(50)+Be neutron beams are compared: dose rate, gamma-contribution, depth dose and room activation.

Comparison of Neutron Therapy Beams Produced by 50 MEV Deuterons and 65 MEV Protons on Beryllium

Neutron beams produced by bombarding a 10 cm thick beryllium target with 50 MeV deuterons have been used at Louvain-la-Neuve since nearly 4 years for routine therapeutic applications. At the end of 1981 they were replaced by neutron beams produced by 65 MeV protons on beryllium, mainly in order to improve the beam penetration in tissues. However, produced of neutrons from the p leads to Be reaction implies some disadvantages, mainly a lower dose rate and a higher activation level. In order to solve the problem a new target configuration was designed, consisting of a remote handled system which permits the use of 2 different target assemblies. The irradiated target is automatically removed immediately after the irradiation which greatly protects against exposure to the staff. The dosimetric characteristics of the d(50)-Be and p(65)-Be neutrons beams are compared.

Results of 10 y of radiation protection dosimetry at the neutrontherapy facility in Louvain-la-Neuve.

After 10 y of routine operation, radiation hazards to the neutron therapy staff at Louvain-la-Neuve were evaluated. These hazards to the staff in neutron therapy are a matter of concern, not only because of the dose levels due to induced activation after treatment but also because of the difficulty of evaluating adequately the dose equivalent rates (including the quality factor, QF) during the irradiation. Build-up of radioactivity near the collimator and in the therapy room is reported. In the beam axis, dose rates due to activation can amount up to 5 mGy h-1. However, these rates are efficiently reduced to 0.3 mGy h-1 by automatically withdrawing the target after irradiation. Other problems of radioactive contamination (for instance, the choice of the Fe composition of the collimator) are discussed. Dose equivalent rates were determined during treatment at different positions inside and outside the treatment room. At these locations, neutrons of widely varying energies are present, accompanied by a significant proportion of gamma rays. The measurements of neutron-dose equivalent rates were performed with three commercial neutron detectors: a BF3 and a He3 remcounter (Nuclear Enterprises and Nardeux) and the Dineutron multisphere remcounter (Nardeux). The Dineutron also allowed the evaluation of QF. The results of these detectors were compared with the results of the integration of microdosimetry spectra obtained in free air with a TE proportional counter (Far West Technology). For the p(65) + Be neutrons, the dose equivalent rate was equal to 0.4 Sv h-1 in the treatment room (at 2.5 m from the collimator) and was reduced to 2-3 microSv h-1 at the room entrance. The gamma component increased from 55% to 95% at the same positions, respectively. Ratios of the responses of all detectors and QF values are compared and discussed at the different positions. The whole-body dose equivalents to the neutron therapy staff over the 10 y considered are presented; although they are higher than for conventional radiotherapy, they remain far below the ICRP recommended dose limits.

Comparison of the neutron fields produced by 50 MeV deuterons and 65 MeV protons on /sup 9/Be at the Louvain-la-Neuve cyclotron

Intense neutron beams are produced for therapy purposes at the cyclotron of the University of Louvain. The paper deals with the improvement of the target system which allows the use of a 65 MeV proton beam instead of the previously used 50 MeV deuteron beam on Be target. The optimum composition of the target assembly is described. The performance of the new neutron beam is compared with the previous one: dose rate, gamma contribution, depth doses and induced radioactivity.