Neutron spectra around a tandem linear accelerator in the generation of (18)F with a bonner sphere spectrometer.

A Bonner sphere spectrometer was used to measure the neutron spectra produced at the collision of protons with an H2(18)O target at different angles. A unique H2(18)O target to produce (18)F was designed and placed in a Tandem linear particle accelerator which produces 8.5MeV protons. The neutron count rates measured with the Bonner spheres were unfolded with the MAXED code. With the GEANT4 Monte Carlo code the neutron spectrum induced in the (p, n) reaction was estimated, this spectrum was used as initial guess during unfolding. Although the cross section of the reaction (18)O(p,n)(18)F is well known, the neutron energy spectra is not correctly defined and it is necessary to verify the simulation with measurements. For this reason, the sensitivity of the unfolding method to the initial spectrum was analyzed applying small variation to the fast neutron peak.

A new online detector for estimation of peripheral neutron equivalent dose in organ.

PURPOSE: Peripheral dose in radiotherapy treatments represents a potential source of secondary neoplasic processes. As in the last few years, there has been a fast-growing concern on neutron collateral effects, this work focuses on this component. A previous established methodology to estimate peripheral neutron equivalent doses relied on passive (TLD, CR39) neutron detectors exposed in-phantom, in parallel to an active [static random access memory (SRAMnd)] thermal neutron detector exposed ex-phantom. A newly miniaturized, quick, and reliable active thermal neutron detector (TNRD, Thermal Neutron Rate Detector) was validated for both procedures. This first miniaturized active system eliminates the long postprocessing, required for passive detectors, giving thermal neutron fluences in real time. METHODS: To validate TNRD for the established methodology, intrinsic characteristics, characterization of 4 facilities [to correlate monitor value (MU) with risk], and a cohort of 200 real patients (for second cancer risk estimates) were evaluated and compared with the well-established SRAMnd device. Finally, TNRD was compared to TLD pairs for 3 generic radiotherapy treatments through 16 strategic points inside an anthropomorphic phantom. RESULTS: The performed tests indicate similar linear dependence with dose for both detectors, TNRD and SRAMnd, while a slightly better reproducibility has been obtained for TNRD (1.7% vs 2.2%). Risk estimates when delivering 1000 MU are in good agreement between both detectors (mean deviation of TNRD measurements with respect to the ones of SRAMnd is 0.07 cases per 1000, with differences always smaller than 0.08 cases per 1000). As far as the in-phantom measurements are concerned, a mean deviation smaller than 1.7% was obtained. CONCLUSIONS: The results obtained indicate that direct evaluation of equivalent dose estimation in organs, both in phantom and patients, is perfectly feasible with this new detector. This will open the door to an easy implementation of specific peripheral neutron dose models for any type of treatment and facility.

Estimation of neutron-equivalent dose in organs of patients undergoing radiotherapy by the use of a novel online digital detector.

Neutron peripheral contamination in patients undergoing high-energy photon radiotherapy is considered as a risk factor for secondary cancer induction. Organ-specific neutron-equivalent dose estimation is therefore essential for a reasonable assessment of these associated risks. This work aimed to develop a method to estimate neutron-equivalent doses in multiple organs of radiotherapy patients. The method involved the convolution, at 16 reference points in an anthropomorphic phantom, of the normalized Monte Carlo neutron fluence energy spectra with the kerma and energy-dependent radiation weighting factor. This was then scaled with the total neutron fluence measured with passive detectors, at the same reference points, in order to obtain the equivalent doses in organs. The latter were correlated with the readings of a neutron digital detector located inside the treatment room during phantom irradiation. This digital detector, designed and developed by our group, integrates the thermal neutron fluence. The correlation model, applied to the digital detector readings during patient irradiation, enables the online estimation of neutron-equivalent doses in organs. The model takes into account the specific irradiation site, the field parameters (energy, field size, angle incidence, etc) and the installation (linac and bunker geometry). This method, which is suitable for routine clinical use, will help to systematically generate the dosimetric data essential for the improvement of current risk-estimation models.

SU-E-T-391: Modelling Peripheral Photon Dose in TomoTherapy Treatments.

PURPOSE: The delivery of the therapeutic radiation dose to the tumour in photon radiotherapy, also implies dose deposition in distant organs (peripheral dose) related to secondary cancers induction (Hall and Wuu, Int J Radiat Oncol Biol Phys 56:83-88, 2003). Therefore, peripheral dose estimation in MU-demanding techniques, such as Helical TomoTherapy (HT), becomes relevant. TLD measurements and Monte Carlo modelling were compared by D'Agostino (Strahlenther Onkol 187:693, 2011). The purpose of this work was to find out experimental models predicting the equivalent photon dose as a function of the distance to the isocenter for different treatment types. The prostate case is presented here. METHODS: A HT prostate plan was delivered to an anthropomorphic phantom mimicking a male adult. The phantom was made of polyethylene blocks whereas light wood was used for lungs. 16 points distributed along the phantom, covering different depths, were selected (Sánchez-Doblado IFMBE, World Congress Med Phys & Biomed Eng, 259-261, 2009). Additionally, a polyethylene sheet was inserted in the phantom to measure the off-axis dose profile at midplane depth. Measurements were carried out with standard TLD-100 pairs of dosimeters (calibrated in a 137Cs source). RESULTS: Two-exponential-terms curve fitting was carried out to model separately the scatter and leakage contribution (f=a*exp(-b*x)+c*exp(-d*x)). The former resulted predominant in the proximal region (10=x=14cm) and the latter in the distal re gion (x=14cm). Both components equate at 18cm. Scatter contribution becomes negligible for x=23cm. Points at 5cm were not used for the model as they are too close to the isocenter to be considered as peripheral dose. Model fits well experimental data (13% mean deviation). Only depths behind the build-up region could be properly modelled. CONCLUSIONS: Peripheral photon dose profiles in HT treatments have been modelled by a two-exponential-terms curve modelling separately scatter and leakage.