[en] Neutron flux monitoring in nuclear reactors is frequently performed by foil activation. Most of the theoretical models for calculating corrections for self-shielding only hold for isotropic neutron fields, although small anisotropies can occur in dependence of the irradiation position. The influence of such anisotropies on the resonance self-shielding factors in thin foils is investigated. We make use of a formalism developed for the multiple scattering resonance self-shielding factor, whose structure enables the consideration of different external fields without duplication of computing time. The main isolated resonances Mn-55, Au-197 and Co-59 are considered. The upper and lower limits for the deviations from the isotropic case are given for small anisotropies. They may account for the differences between experimental and calculated resonance self-shielding factors. (orig.)

[en] Thermal neutron self-shielding factors are calculated for different materials used in reactor dosimetry. The effect of multiple scattering inside the probe is taken into account within a new theoretical formalism recently proposed |1| with application to foils. Here, the method is extended to probes of cylindrical geometry. A fast algorithm is developed for the calculation of the self-shielding based on an approximation equivalent to nearly six collisions inside the probe, which gives quite satisfactory results in the thermal region. (author)

[en] Published in summary form only

[en] A flexible multiple source model capable of fast reconstruction of clinical electron beams is presented in this paper. A source model considers multiple virtual sources emulating the effect of the accelerator head components. A reference configuration (10 MeV and 10x10 cm² field size) for a Siemens KD2 linear accelerator was simulated in detail using the GEANT3 Monte Carlo code. Our model allows the reconstruction of other electron energies and field sizes, as well as other beam configurations of similar accelerators, using only the reference beam data. Electron dose calculations with the reconstructed beams were performed in a water phantom and compared with experimental data. An agreement of 1-2%/1-2 mm was obtained, comparable to the accuracy of full Monte Carlo accelerator simulation. The source model reduces accelerator simulation CPU time by a factor of 7500 relative to full Monte Carlo approaches. The developed model was then interfaced to DPM, a fast radiation transport Monte Carlo code for dose calculation. Dose distributions and dosimetric parameters computed in the presence of air gaps using this procedure agree within 1%/1 mm with experimental data. The obtained accuracy corresponds to a significant improvement when compared to the treatment planning system PLATO, currently used in clinical practice. Globally, a two order of magnitude efficiency gain was achieved with respect to the full Monte Carlo approach

[en] In this paper, we present a novel implementation of a dose calculation application, based on the GEANT4 Monte Carlo toolkit. Validation studies were performed with an homogeneous water phantom and an Alderson-Rando anthropomorphic phantom both irradiated with high-energy photon beams produced by a clinical linear accelerator. As input, this tool requires computer tomography images for automatic codification of voxel-based geometries and phase-space distributions to characterize the incident radiation field. Simulation results were compared with ionization chamber, thermoluminescent dosimetry data and commercial treatment planning system calculations. In homogeneous water phantom, overall agreement with measurements were within 1-2%. For anthropomorphic simulated setups (thorax and head irradiation) mean differences between GEANT4 and TLD measurements were less than 2%. Significant differences between GEANT4 and a semi-analytical algorithm implemented in the treatment planning system, were found in low-density regions, such as air cavities with strong electronic disequilibrium

[en] The use of plane parallel ionization chambers for the dosimetry of electron beams has been extensively recognized in national and international recommendations and codes of practice. The construction details of different chambers and their influence on the measure, which should be converted in suitable perturbation factors, have also been published. Updated information, new data, refined investigations have recently been gathered in IAEA TRS 398 where the major efforts of Primary Standard Dosimetry Laboratories (PSDLs) in providing calibration factors in terms of absorbed dose to water at a reference quality is reflected. From the users point of view, and specially concerning electron beams, one can probably get confused with different methodologies and procedures in order to accurately determine absorbed dose to water. The aim of this work is to explore the different methodologies and procedures concerning absolute dose determinations, in a hospital and using different chambers, in order to appreciate the relative deviations in absolute dose values. Despite the fact that Markus chamber does not meet all the minimum requirements namely concerning scattering perturbation effects due to the geometry and dimensions of the guard electrode, it is still a quite used chamber type in current clinical practice. So we have included dose determinations with this plane parallel chamber (PTW 23343) and also with a Roos chamber (PTW 34001). Markus chamber is provided with a standard calibration certificate in terms of absorbed dose to water (N_W) and also in terms of absorbed dose to air (N_A), referred to a high energy electron beam whereas Roos chamber has a calibration factor in terms of absorbed dose to water referred to ⁶⁰Co. Starting from these materials different methodologies have been applied: i) Using NA of Markus chamber according to the users instruction manual which refers DGMP Report No.6 Praktische Dosimetrie von Elektronenstrahlung und ultraharter Rontgenstrahlung, 1989. ii) With that N_W value and the high energy electron quality taken as Q₀, the formalism of TRS 381 has been applied, using the beam quality correction factors k_Q,Q0 for our electron beam qualities (nominal energies: 6,8,10,12,15 and 18 MeV). iii) The cross calibration methodology using the electron energy beam quality of the certificate taken as Q_cross at the highest electron energy available (18 MeV), has been used according to TRS 398. iv) To become independent of the calibration certificate, the same cross calibration procedure has been used but this time against a calibrated reference cylindrical chamber calibrated in ⁶⁰Co by the Portuguese SSDL. v) For the Roos chamber, and in order to appreciate the influence of chamber type, we have used the general N_D,W based formalism with k_Q quality correction factors. We have also compared the results of TRS 381 and TRS 398 as these codes of practice differ in electron beam quality specification - TRS 381 uses E₀, the mean energy at the phantom surface, whereas TRS 398 uses R₅₀, the half-value depth of absorbed dose in water as the beam quality index. vi) Also with Roos chamber, we have used the cross calibration methodology against two different reference cylindrical chambers (PTW 31003 and PTW 30006 Farmer) calibrated both in ⁶⁰Co, but in different SSDLs. Whenever a ratio of measures is involved an external monitor chamber has been used in order to minimize the effect of any variation in the accelerator output. The analysis has been done by chamber type (in Gy/C to became independent of the measure itself) and the deviations encountered can reach up to 2.5%. An example is presented in the figure. Also dose determinations have been compared for each nominal electron energy and independently of chamber type. An uncertainty analysis has also been done including an estimate of the combined uncertainty associated to each methodology. For a common user in a typical hospital where different chambers may exist, many are indeed the possibilities in terms of dose determination methodologies. The ultimate choice should rely on an external international audit like ESTRO Quality Assurance Network (EQUAL)

[en] In vivo dosimetry, by entrance and exit dose measurements, is a vital part of a radiotherapy quality assurance program. The uncertainty associated with dose delivery is internationally accepted to be within 5% or inferior depending on the tumor pathology. Thermoluminescent dosimetry is one of the dosimetric techniques used to verify the agreement between delivered and prescribed doses. Nevertheless, it requires a very accurate calibration methodology. We have used LiF chips (4.5 mm diameter and 0.8 mm thick) calibrated towards a PTW ionization chamber of 0.3 cc, in three photon energies: Co-60, 4 and 6 MeV. The TLD reader used was a Rialto 688 from NE Technology and the annealing oven the Eurotherm type 815. The calibration methodology relies on the experimental determination of individual correction factors and on a correction factor derived from a control group of dosimeters. The exit and entrance dose measurements are performed in quite different situations. To be able to achieve those two quantities with TLD, these should be independently calibrated according to the measurement conditions. Alternatively, we can use a single calibration, in entrance dose, and convert the result to the exit dose value by introducing some correction factors. These corrections are related to the different measurement depths and to the different backscattering contributions. We have proved that within an acceptable error we can perform a single calibration and adopt the correction factors which are energy and field size dependent. (author)

[en] In the scope of medical radiological exposures and according to European recommendations and national legal requirements [14], the maximum responsible for an installation must assure the establishment of local dose levels for each type of radiological examination and also assure that they are available for the doctor who prescribes the examination. In the absence of national reference dose levels, the so called Local Diagnostic Reference Levels (L.D.R.L.) should be in agreement with the European Diagnostic Reference Levels published for the different types of medical exposures. The aim of this work was to establish a protocol of measurement for each type of more frequent examination, namely in conventional radiology, in CT and in mammography performed in our hospital. For each kind of examination the recommended dose descriptor was adopted and directly measured or derived from basic charge measurements. The patient sample corresponded in each case to at least a minimum of 10 standard-sized patients, as recommended, in order to obtain averages that constitute for each type of radiological examination the L.D.R.L. that could be compared with the European D.R.L.. The results obtained are in the large majority of the situations below the corresponding D.R.L.. Nevertheless we have identified some situations that deserve more attention.In conventional radiology all L.D.R.L. are below the reference levels. However we have detected some skull post anterior (skull P.A.) exposures where entrance skin doses exceeded the standard value. This may be due to equipment age problems but, as always, improvement of staff education will contribute to better practices and we hope that this work can contribute to this objective. In CT the L.D.R.L. that corresponds to single slice scan meet the standards whereas complete examinations described by dose length product (D.L.P.) values show a larger variation and also some situations where the reference level is exceeded. It is possible that doctors are requiring too large ranges for complete examinations. Mammography was perhaps the area that deserved more evident alert. The results clearly showed that the majority of entrance surface dose (E.S.D.) values determined in mammography exceeded the recommended reference level. In fact the mammography unit used to establish the dose measurement protocol was dismounted soon after the end of this work. A new digital mammography unit is now installed. We hope that new L.D.R.L. that will soon be obtained can accomplish the dose requirements. The results obtained so far may not be the best ones but it was the beginning of a very important way towards better practices, more information to staff and patients and improved quality assurance of clinical radiological examinations. As a last conclusion we would like to state that this was a pioneer work in Portugal. We hope that this may lead the way to further developments. (authors)

[en] Dose measurements of narrow photon beams used in radiosurgery are complicated by the lack of lateral electron equilibrium which is a requirement namely for ionometric methods. The details of basic dosimetry for these narrow beams are still quite unknown. To overcome this difficulty Monte Carlo simulation is a privileged tool to assess the processes of the energy deposition phenomena in such narrow photon beams. Some simulations had already been performed to calculate percent depth doses in a water phantom of the narrows beams used in our hospital (Centre Regional de Oncologia de Coimbra-Portugal) and the agreement with experimental data was good. A more specific analysis of the calculated and experimental dose measurements in the build-up region revealed that the depth of the dose maximum d_max increases with the size of the additional collimators which is the opposed behavior presented by radiotherapy conventional radiation fields. To fully understand this phenomenon, Monte Carlo simulations are performed in order to verify if it is due to processes occurring in the generation of the narrow photon beams or in processes occurring in the water phantom. The Monte Carlo code used in these studies is MCNP-4c, the accelerator is a Siemens Mevatron KD2 in 6MV photon mode and the size of the additional collimators goes from 5mm to 23mm (geometrical dimension). For the analysis in air, various scoring planes were placed just after the collimators (one for each collimator). Phase space data from the scoring planes is characterized in terms of: type of particles, energy and spatial distributions. Contributions of the various elements of the accelerator head are scored. The photons that reach the scoring plane have two different origins: photons coming from the elements of the accelerator head and photons that had suffered interactions inside the additional collimators. Regarding the photons coming directly from the head structures, most of them were generated by Bremsstrahlung process in the target; the contributions of the other head elements become more important as the size of the collimator increases. The ratio between photons coming from the accelerator head and photons originated in the additional collimator also increases as the size of the collimator increases. The analysis of the various scoring planes shows also that the electron contamination of the additional collimators is quite negligible. Finally, the average energy of the incident photons upon the water phantom decreases as the size of the collimator increases. These results seem to contradict the behavior of d_max. The in-water analysis includes a scoring cell array along the central axis in the buildup region. As the radius of the additional collimators increases, the number of electrons that contributes to the energy deposition increases for each scoring cell as it was expected. The spatial distribution analysis of these electrons shows that some electrons are generated in the scoring cells or very close to them due to primary photons that reach them and other electrons, equally generated by primary photons, are created relatively far away from those cells. The energy analysis shows that as the collimator radius increases for each electron spatial distribution, the electron average energy increases in the same scoring cells and also increases as the depth of the scoring cells increases for a given collimator. In fact, as the size of the collimators increases, the electrons of these two spatial distributions will contribute to the energy deposition deeper in the buildup region increasing the depth of the dose maximum d_max. Through this detailed analysis, we have concluded that characterizing the photon and electron spectra in air is not sufficient to explain the increase of d_max with the increase of the size of the additional collimator. Only the processes occurring in water explain this behavior

[en] In the context of optimization of radiological exposures, the European Directive 97/43/Euratom refers to the concept of diagnostic reference levels (DRL). These levels are expected not to be exceeded for standard procedures when good and normal practice regarding diagnostic and technical performance is applied. According to national legal requirements, the maximum responsible for a radiological installation must assure that the radiological exposures are in agreement with accepted dose reference levels. The dose information of each type of examination should be available to the doctor who prescribes it and also to the patient. A measuring protocol has been established in terms of the most convenient dose indicators for each type of the more common examinations carried out in the Imaging Department of the Instituto Portugues de Oncologia de Coimbra (IPOC-FG, E.P.E.). The aim of this work is to compare the dose reference levels locally determined with the European DRLs in conventional radiology, computed tomography and mammography. Also, the relevant information to be given to the patient is discussed and a general model for that information is proposed. DRLs are expressed either in terms of entrance surface dose (ESD), for conventional radiology and mammography, or in terms of computed tomography dose index (CTDI_w) or dose length product (DLP) for CT examinations. This dose information and the exposure parameters for each type of examination are used as input values to the code PCXMC, version 1.5.2., in order to assess patient organ doses and effective dose values in conventional radiology. In the same way, the program CTDosimetry is used in computed tomography. The Average Glandular Dose (AGD) is determined in mammography. In order to give to the patient and also to the doctor who prescribes the examination readily accessible dose information, the concept of BERT (Background Equivalent Radiation Time) has been used. The average value of effective dose for each type of radiological exposure was converted in time of natural background radiation having 1 mSv/year as a reference value. Also the estimated risk associated to each type of examination was included in the patient dose information form. The estimated risk was obtained using the nominal probability coefficient for stochastic effects of 7.3% per Sv. An example of the patient dose information form is presented