Measurement of neutron spectra and neutron doses at the Munich FRM II therapy beam with Bonner spheres
Introduction
In Munich, neutrons have been used for radiation therapy since 1985, and 715 patients with different types of tumours were treated at the first research reactor FRM I in Garching until its shut-down in 2000 (Wagner et al., 2008). Since June 2007, neutron irradiation treatments are performed at the new Medical Application facility (MEDAPP) at the Research Neutron Source Heinz Maier-Leibnitz (FRM II), at the beamline SR10. This new reactor was designed to have a greater neutron fluence rate, primarily at low neutron energies. These low-energy neutrons are converted by two uranium plates to high-energy neutrons for medical application at the MEDAPP beamline. Filters are used to change the neutron-to-photon ratio in order to match them to those of the earlier FRM I beam. Hence, treatment protocols acquired at FRM I can also be applied at FRM II. As main improvements to the earlier facility, the new FRM II facility produces an about 3 times higher total neutron fluence rate and has an about 6 times larger field size (Wagner et al., 2008) as well as a multi leave collimator that can be used to adapt the field shape to the shape of the treatment volume (conformal therapy). Therefore, treatment time decreases, improving treatment conditions and quality.
The next desirable improvement would be to introduce a computer-based treatment planning system rather than the present practice of using water phantom depth dose curves. Before such a treatment planning system is developed, however, the neutron spectrum at the patient’s location must be known. The neutron spectrum represents a major input information for any treatment planning system, and is a prerequisite for calculating the dose inside a patient. In order to be able to compare any biological results obtained at FRM II with those obtained at FRM I, it is also of great interest to know the neutron spectrum in detail. Breitkreutz et al. (2008) already simulated the beam with the MCNP Monte Carlo code, adjusted the neutron spectrum obtained with results from threshold probe measurements and showed the convergence with results of transmission calculations. Additionally, they measured the total fluence rate by means of the water bath-gold probe method. The present paper describes the neutron spectrum and the total neutron fluence rate as measured in the treatment room and outside the shielding with a Bonner sphere spectrometer (BSS). The results of this experiment are presented here and discussed in terms of neutron dose measurements.
Section snippets
Measurement principle and locations
The used BSS system consisted of a set of polyethylene (PE) spheres of different sizes (varying between 2 and 15 inches in diameter). In the center of each sphere, a detector is placed which is sensitive to thermal neutrons. Depending on the thickness of the PE (that is the size of the sphere), the incident neutrons are partly moderated to thermal neutrons. Additionally, a bare detector (without any PE) is used to detect thermal neutrons. Typically, this thermal neutron detector includes a
Count rates measured with the BSS system
Fig. 5 shows the gold foil activities obtained in the patient treatment and the radiography room. For a better comparison, both datasets were normalised to the value of the 7 inch sphere (i.e., 209 Bq/g/s for the patient treatment room and 127 Bq/g/s for the radiography room). Because the different-sized spheres do not provide totally independent results, the points of the two datasets are expected to lie on a smooth curve, which is actually the case within the measurement uncertainties. As
Conclusion
The neutron spectrum inside and outside the patient treatment room, and that inside the radiography room at the beamline SR10 of the FRM II was measured using a Bonner sphere spectrometer including gold foils and 3He proportional counters as thermal neutron detectors, respectively.
The resulting spectra in the patient treatment and the radiography room were compared to a neutron spectrum determined by calculations with MCNP and its adaptation to measurements of threshold probes (Breitkreutz
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