RRFM 2009 Transactions - European Nuclear Society

Fig. 3. Scan of an etch track film which was irradiated together with a whole-body section of
a mouse. The darker the area the higher the concentration of boron. The tumour contains
about 5 ppm 10 B (courtesy of D. Gabel, University of Bremen).
3.2 Medical and biological use (standard fast neutron beam)
Biological and clinical applications are principally motivated by the high linear energy transfer
(LET) up to 100 keV/µm of fission neutrons. The high LET leads to biological radiation effects
different from photons of the same energy, e.g., a much weaker dependence of the cell
deactivation on the oxygenation. The quick energy loss, however, goes hand in hand with a
low penetration depth, therefore fission neutron therapy is primarily suited for tumours
situated near to the body surface.
There is already an expertise from 715 patients with tumours who were treated at the fission
beam of the former research reactor FRM [5-7] until July 2000. The best suited tumours for
neutron treatment are radiation resistant adenoid cystic carcinoma of the salivary glands,
head and neck tumours, malign melanoma, lymph node metastases or skin metastases from
various cancer diseases, and chest wall metastases of breast cancer. In the latter case, the
steep depth dose curve is an advantage because the radiation dose to the lungs and the
heart is generally not prohibitive even with vertical incidence. At FRM II since June 2007, 36
patients have undergone 209 irradiations. 40 % of the patients have been treated with
(locally) curative intent, the others for palliation.
In parallel to the clinical applications, the dose-response relationship of dicentric
chromosomes in human lymphocytes for fission neutrons has been investigated at different
depths in a polyethylene phantom [8]. The dose-response curves for dicentrics suggest a
significantly lower biological relative effectiveness RBE (compared to 60 Co γ-rays) with
increasing depth (Fig. 4).
Fig 4: RBE of the MEDAPP beam vs. dose at depths of 2 and 6 cm, resp., in a PE-phantom.
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