Francesco d’Errico

In vivo neutron dosimetry during high-energy Bremsstrahlung radiotherapy

PURPOSE A new technique is presented for in vivo measurements of the dose equivalent from photoneutrons produced by high-energy radiotherapy accelerators. METHODS AND MATERIALS The dosimeters used for this purpose are vials of superheated halocarbon droplets suspended in a tissue-equivalent gel. Neutron interactions nucleate the formation of bubbles, which can be recorded through the volume of gel they displace from the detector vials into graduated pipettes. These detectors offer inherent photon discrimination, dose-equivalent response to neutrons, passive operation, and small sensitive size. An in vivo vaginal probe was fabricated containing one of these neutron detector vials and a photon-sensitive diode. Measurements were carried out in patients undergoing high-energy x-ray radiotherapy and were also repeated in-phantom, under similar irradiation geometries. RESULTS AND CONCLUSION Neutron doses of 0.02 Sv were measured in correspondence to the cervix, 50 cm from the photon beam axis, following a complete treatment course of 46.5 Gy with an upper mantle field of 18-MV x-rays. This fraction of dose from neutrons is measured reliably within an intense photon background, making the technique a valid solution to challenging dosimetry problems such as the determination of fetal exposure in radiotherapy. These measurements can be easily carried out with tissue-equivalent phantoms, as our results indicate an excellent correlation between in vivo and in-phantom dosimetry.

The importance of the direction distribution of neutron fluence and methods of determination

For the estimation of non-isotropic quantities such as personal dose equivalent and effective dose, and for the interpretation of the readings of personal dosemeters, it is necessary to determine both the energy and direction distributions of the neutron fluence. In fact, for workplace fields, the fluence and dose-equivalent responses of dosemeters and the relationships of operational and protection quantities, are frequently more dependent on the direction than on the energy distribution. In general, the direction distribution will not be independent of the energy distribution, and simultaneous determination of both may be required, which becomes a complex problem. The extent to which detailed information can be obtained depends on the spectrometric properties and on the angle dependence of the response of the detectors used. Methods for the determination of direction distributions of workplace fields are described.

High-energy neutron detection and spectrometry with superheated emulsions

The response of some superheated emulsions was investigated using quasi-monoenergetic neutron beams in the 46– 134 MeV energy range at the Universit! Catholique de Louvain, Louvain la Neuve, Belgium and at The Svedberg Laboratory, Uppsala, Sweden. In order to determine the detector response to the high-energy beams, the spectra of incident neutrons were folded over functions modeled after the cross-sections for the neutron-induced production of heavy ions from the detector elements. The cross-sections for fluorine and chlorine were produced in this work by means of the Monte Carlo high-energy transport code HADRON based on the cascade-exciton model of nuclear interactions. r 2003 Published by Elsevier Science B.V.

Energy- and angle-differential neutron fluence measurements with superheated drop (bubble) detectors

Abstract One of the latest additions to the field of neutron spectrometry is based on the active control of the response functions of superheated emulsions. By varying the superheat of the detectors, either changing their operating temperature or applied pressure, it is possible to generate a matrix consisting of nested responses suitable for few-channel energy spectrometry. In the device presented here, a detector is embedded in a recess milled on the surface of a moderating sphere. This sphere has the purpose of introducing an angular dependence in the otherwise nearly isotropic response of the detector. The device relies on the acoustical detection of bubbles and on temperature stepping to vary the superheated emulsion thresholds. In correspondence to each temperature/threshold, measurements are sequentially performed at different angular orientations of the sphere. The response matrix of the system to monoenergetic neutrons was determined as a function of angular position by means of Monte Carlo neutron transport simulations. The directional spectrometer was tested by means of irradiations with a californium neutron source. Energy- and angle-differential unfolding of the detector readings was performed by means of a maximum-entropy technique which does not require a-priori information. The spectrometer operates well with large energy-angle groups, and produces accurate integral values of total fluence, which can be used to derive quantities such as ambient dose equivalent H *(10) or directional dose equivalent H ′(10). However, the device presents limitations in unfolding spectra over a finer group structure, and will require the future developments outlined in the conclusions.

A wide-range direction neutron spectrometer

A new device is presented which has been developed for measuring the energy and direction of distribution of neutron fluence in fields of broad energy spectra (thermal to 100 MeV) and with a high background of photon, electron and muon radiation. The device was tested in reference fields with different energy and direction distributions of neutron fluence. The direction-integrated fluence spectra agree fairly well with reference spectra. In all cases, the ambient and personal dose equivalent values calculated from measured direction-differential spectra are within 35% of the reference values. Independent measurements of the directional dose equivalent were performed with a directional dose equivalent monitor based on superheated drop detectors.

System for automated magnetic resonance imaging of a superheated emulsion chamber for brachytherapy dosimetry

This manuscript describes a pressure cycling and brachytherapy source insertion/removal system developed to automate magnetic resonance imaging (MRI) of a superheated emulsion chamber (SEC), a position-sensitive radiation dosimeter. Previous MRI studies of a SEC demonstrated its efficacy for obtaining two-dimensional relative radiation dose profiles from brachytherapy sources. The SEC detector is a 0.5 l chamber containing an emulsion of highly superheated halocarbon droplets suspended in a tissue-equivalent glycerin-based gel. When irradiated by a source, the halocarbon droplets vaporize into microbubbles. Relative dose profiles are determined by mapping, via MRI, the resulting microbubble distributions. During use the SEC is repeatedly cycled through a series of steps including source insertion, exposure, and source removal, MRI data collection, and chamber pressurization and depressurization. To acquire microbubble counts sufficient to derive dose profiles having good statistics, it is necessary to col...

Evaluation of air photoactivation at linear accelerators for radiotherapy

High-energy x-rays produced by radiotherapy accelerators operating at potentials above 10 MV may activate the air via (γ, n) reactions with both oxygen and nitrogen. While the activation products are relatively short-lived, personnel entering the accelerator room may inhale some radioactive air, which warrants internal dosimetry assessments. This work illustrates a method based on the use of ammonium nitrate solutions for the evaluation of photon-induced air activation and for the estimate of internal doses to radiotherapy personnel. Air activation and internal dosimetry assessments based on our method are presented for some widespread radiotherapy linear accelerator models. Our results indicate that the equivalent dose to the lungs of radiotherapy personnel is negligible for beam energies below 18 MeV.

Application of a Bonner sphere spectrometer for the determination of the angular neutron energy spectrum of an accelerator-based BNCT facility

Experimental activities are underway at INFN Legnaro National Laboratories (LNL) (Padua, Italy) and Pisa University aimed at angular-dependent neutron energy spectra measurements produced by the (9)Be(p,xn) reaction, under a 5MeV proton beam. This work has been performed in the framework of INFN TRASCO-BNCT project. Bonner Sphere Spectrometer (BSS), based on (6)LiI (Eu) scintillator, was used with the shadow-cone technique. Proper unfolding codes, coupled to BSS response function calculated by Monte Carlo code, were finally used. The main results are reported here.

Calculated neutron air kerma strength conversion factors for a generically encapsulated Cf-252 brachytherapy source

The 252Cf neutron air kerma strength conversion factor (SKN/mCf) is a parameter needed to convert the radionuclide mass (μg) provided by Oak Ridge National Laboratory into neutron air kerma strength required by modern clinical brachytherapy dosimetry formalisms indicated by Task Group No. 43 of the American Association of Physicists in Medicine (AAPM). The impact of currently used or proposed encapsulating materials for 252Cf brachytherapy sources (Pt/Ir-10%, 316L stainless steel, nitinol, and Zircaloy-2) on SKN/mCf was calculated and results were fit to linear equations. Only for substantial encapsulation thicknesses, did SKN/mCf decrease, while the impact of source encapsulation composition is increasingly negligible as Z increases. These findings are explained on the basis of the non-relativistic kinematics governing the majority of 252Cf neutron interactions. Neutron kerma and energy spectra results calculated herein using MCNP were compared with results of Colvett et al. and Rivard et al.

A comparison of different neutron spectroscopy systems at the reactor facility VENUS

Abstract The VENUS facility is a zero-power research reactor mainly devoted to studies on LWR fuels. Localised high-neutron rates were found around the reactor, with a neutron/gamma dose equivalent rate ratio as high as three. Therefore, a study of the neutron dosimetry around the reactor was started some years ago. During this study, several methods of neutron spectroscopy were employed and a study of individual and ambient dosemeters was performed. A first spectrometric measurement was done with the IPSN multisphere spectrometer in three positions around the reactor. Secondly, the ROSPEC spectrometer from the Fraunhofer Institut was used. The spectra were also measured with the bubble interactive neutron spectrometer. These measurements were compared with a numerical simulation of the neutron field made with the code TRIPOLI-3. Dosimetric measurements were made with three types of personal neutron dosemeters: an albedo type, a track etch detector and a bubble detector.