Build-up and depth-dose characteristics of different fast neutron beams relevant for radiotherapy

Fast neutron and proton therapy sources

Neutrons from the reactions of protons or deuterons of energies between 35 and 70 MeV on beryllium are most suitable for therapy because of the high yield and good penetration. Lower energy sources are used for limited applications. Proton therapy is conducted mainly in two energy ranges, viz. 60–75 MeV (exclusively for the treatment of eye lesions) and ≥200 MeV for general applications. Characterisation of both neutron and proton therapy beams in terms of spectral fluence is important for accurate determination of dosimetry parameters, for interpreting biological effects, for intercomparison purposes and to perform accurate radiation transport calculations.

Biological intercomparisons of neutron beams used for radiotherapy generated by p(+)-->Be in hospital-based cyclotrons

The new generation of hospital-based neutron therapy facilities involve cyclotrons using protons on beryllium. The spectrum of neutrons produced includes a large and variable proportion of low-energy neutrons that are poorly penetrating but biologically effective. Cells cultured in vitro were used to compare the three US facilities at Seattle, M.D. Anderson and UCLA, together with the UK facility at Clatterbridge. Cyclotrons were compared within a given experiment on the same day using cells from a common suspension. Among the three US facilities, the relative potency factor at a depth of 25 mm differs by about 11%, with Seattle the least and UCLA the most biologically effective. Clatterbridge was compared directly with M.D. Anderson and found to be less effective by about 5%; it has a slightly lower biological effectiveness than any of the US facilities. There is evidence for an increased biological effectiveness in the build-up region, which reduces the effective skin sparing potential. There is not much difference in build-up between the three US facilities. Using the proton-on-beryllium neutron production process results in a wide spectrum of neutrons with a large but variable low-energy component. The biological effectiveness of the beam depends on target design and thickness as well as the design of the collimating system. Consequently the biological effectiveness of neutron beams generated by this process must be assessed on an individual basis. It cannot be assumed that because cyclotrons have similar accelerating energies that the relative biological effectiveness will be the same.

Physical characteristics of the M.D. Anderson Hospital clinical neutron beam

The physical characteristics of the M.D. Anderson Hospital (MDAH) clinical neutron beam are presented. The central-axis percent depth-dose values are intermediate between a 4 and 6 MV X-ray beam. The build-up curves reach a depth of maximum dose at 1.2 cm and have surface dose values of approximately 30%. Teflon flattening filters are employed to flatten the beam at the depth of the 75% dose level. Two wedges are available for shaping the beam; they are made of Teflon and produce wedge angles of 31 degrees and 45 degrees as defined by the ICRU. Output factors ranged from 0.88 for a 4 x 4 cm field to 1.12 for a 20 x 20 cm field. Tungsten blocks reduced the dose received at Dmax to 25% of the unblocked value but only 52% of the unblocked value at a depth of 22.8 cm.

Dosimetry with tissue-equivalent ionisation chambers in fast neutron fields for biomedical applications

The use of calibrated tissue-equivalent (TE) ionisation chambers is commonly considered to be the most practical method for total absorbed dose determinations in mixed neutron-photon fields for biomedical applications. The total absorbed dose can be derived from the charge produced within the cavity of an ionisation chamber employing a number of physical parameters. To arrive at the charge produced in the cavity several correction factors have to be introduced which are related to the operational characteristics of the chambers. Information on the operational characteristics of four TE ionisation chambers is presented in relation to ion collection, density and composition of gas in the cavity, wall thickness and effective point of measurement. In addition, some recent results from an ionisation chamber operated at high gas pressures (up to 8 MPa(80 bar)) are presented. The total absorbed doses derived from TE ionisation chambers show agreement within the uncertainty limits with results from other independent dosimetry methods, i.e., differential fluence measurements and a TE calorimeter. Conscientious experimentation and a common data base can provide dosimetry results with TE ionisation chambers with variations of less than +/- 2%.

Results of d + T fast neutron irradiation on advanced tumors of bladder and rectum

From November, 1975 to November, 1981, around 400 patients were irradiated with 14 MeV d + T fast neutrons at the Antoni van Leeuwenhoek Hospital in Amsterdam. Special interest was focused on inoperable tumors of bladder and rectum. During the pilot phase of the study 47 patients were treated, mostly via two parallel opposed ports with dosages that ranged from 18 to more than 22 Gy. Although persistent local control was achieved in 23 patients (48%), 14 patients (29%) died of severe complications. By the introduction of a six field technique, the fatal complication rate could be reduced significantly. Since May 1978 patients were randomized in a three arm trial, using two dose levels on the neutron site. The preliminary results of a group of 91 patients show a similar survival in the three treatment arms with a somewhat better local control rate for high dose neutrons. An attempt was made to estimate RBE values for tumor control and normal tissue reactions by comparing the data for neutron irradiation with the data obtained with photons on a similar group of patients. From the values derived it must be concluded that the gain for neutron irradiation on these tumors in the pelvis will be negligible.

Dose-effect relations for tumour control and complication rate after fast neutron therapy for pelvic-tumours

The effects of fast neutrons and high-energy photons were analysed for tumour control and skin and bowel damage after treatment of bladder and rectal cancers. A combination of statistical models was used for this analysis. A steep dose-effect curve was found for local tumour control as well as for the risk of complications. Some estimates were made of RBE values for control of bladder tumour and for normal-tissue damage. The implication of these data for neutron therapy is discussed.

A liquid ionisation chamber for neutron dosimetry

Theories of ionisation in liquid and the use of liquid ionisation chambers in mixed neutron field dosimetry have been studied. Theoretical models developed by Jaffe and by Onsager were used for comparison with the experimental measurements. The Jaffe method predicts a higher collecting efficiency than does the Onsager model. Gamma and neutron sensitivities of a liquid chamber can be calculated by the Bragg-Gray principle and Onsager's theory. The calculation is subject to relatively large uncertainties, mainly due to insufficient knowledge of the ion distribution in ionisation tracks and of W/e values for iso-octane. The chamber constructed for clinical use should work best for mixed neutron fields with gamma-neutron ratios of 10-20%; however, it would be difficult to use a liquid ionisation chamber to detect small neutron variations in mixed fields with large gamma components.