J. Zoetelief

A comparison of MCNP4C electron transport with ITS 3.0 and experiment at incident energies between 100 keV and 20 MeV: influence of voxel size, substeps and energy indexing algorithm

The condensed-history electron transport algorithms in the Monte Carlo code MCNP4C are derived from ITS 3.0, which is a well-validated code for coupled electron-photon simulations. This, combined with its user-friendliness and versatility, makes MCNP4C a promising code for medical physics applications. Such applications, however, require a high degree of accuracy. In this work, MCNP4C electron depth-dose distributions in water are compared with published ITS 3.0 results. The influences of voxel size, substeps and choice of electron energy indexing algorithm are investigated at incident energies between 100 keV and 20 MeV. Furthermore, previously published dose measurements for seven beta emitters are simulated. Since MCNP4C does not allow tally segmentation with the *F8 energy deposition tally, even a homogeneous phantom must be subdivided in cells to calculate the distribution of dose. The repeated interruption of the electron tracks at the cell boundaries significantly affects the electron transport. An electron track length estimator of absorbed dose is described which allows tally segmentation. In combination with the ITS electron energy indexing algorithm, this estimator appears to reproduce ITS 3.0 and experimental results well. If, however, cell boundaries are used instead of segments, or if the MCNP indexing algorithm is applied, the agreement is considerably worse.

Optimisation of mammographic breast cancer screening using a computer simulation model

To optimise breast cancer screening protocols, risk (induction of fatal tumors) versus benefit (reduction in the number of fatal tumors) analyses are performed for a simulated stable Swedish female population, using the Model for evaluation of Breast cancer Screening (MBS). The present study comprises, the influences of various screening parameters, i.e. ages at which screening is started and stopped, interval period between successive sessions, tumor detection limits for screening and average glandular dose per screening session. When the results of the present study are expressed in terms of numbers of fatal breast tumors, it appears that starting and stopping ages for screening of 40 and 80 years, respectively, seem realistic. An increased screening frequency results in a larger reduction of breast cancer mortality. This reduction is significant for ages between 40 and 51 years but only marginal for ages above 70 years. High resolution screening, i.e., the detection of tumors at smaller size, results in a larger benefit but does not indicate a younger age for starting of screening. The average glandular dose per screening session does only influence the risks of screening. As separate risk and benefit results are presented, a change in average glandular dose on the total effect of screening can easily be calculated.

Simulation of image detectors in radiology for determination of scatter-to-primary ratios using Monte Carlo radiation transport code MCNP/MCNPX

PURPOSE The purpose of this study was to compare and validate three methods to simulate radiographic image detectors with the Monte Carlo software MCNP/MCNPX in a time efficient way. METHODS The first detector model was the standard semideterministic radiography tally, which has been used in previous image simulation studies. Next to the radiography tally two alternative stochastic detector models were developed: A perfect energy integrating detector and a detector based on the energy absorbed in the detector material. Validation of three image detector models was performed by comparing calculated scatter-to-primary ratios (SPRs) with the published and experimentally acquired SPR values. RESULTS For mammographic applications, SPRs computed with the radiography tally were up to 44% larger than the published results, while the SPRs computed with the perfect energy integrating detectors and the blur-free absorbed energy detector model were, on the average, 0.3% (ranging from -3% to 3%) and 0.4% (ranging from -5% to 5%) lower, respectively. For general radiography applications, the radiography tally overestimated the measured SPR by as much as 46%. The SPRs calculated with the perfect energy integrating detectors were, on the average, 4.7% (ranging from -5.3% to -4%) lower than the measured SPRs, whereas for the blur-free absorbed energy detector model, the calculated SPRs were, on the average, 1.3% (ranging from -0.1% to 2.4%) larger than the measured SPRs. CONCLUSIONS For mammographic applications, both the perfect energy integrating detector model and the blur-free energy absorbing detector model can be used to simulate image detectors, whereas for conventional x-ray imaging using higher energies, the blur-free energy absorbing detector model is the most appropriate image detector model. The radiography tally overestimates the scattered part and should therefore not be used to simulate radiographic image detectors.

Glandularity and mean glandular dose determined for individual women at four regional breast cancer screening units in The Netherlands

The nationwide breast cancer screening programme using mammography has been in full operation in The Netherlands since 1997. There is concern that the mean glandular doses due to mammography might be differing between different regions of the country due to differences in glandularity and compressed breast thickness. To investigate regional differences, glandularity, compressed breast thickness and mean glandular dose were determined for individual breasts during screening at mammography units at four locations in The Netherlands. Differences in glandularity were observed, which could be related qualitatively to differences in age of the participants at the different locations. Mean glandular dose depends on compressed breast thickness, glandularity and technical conditions of screening. The lowest average value of the mean glandular dose was found for the unit in Amsterdam. This is most likely due to the use of the Mo/Rh anode/filter combination at this unit, in addition to the Mo/Mo combination. At the other three units, almost exclusively the Mo/Mo anode/filter combination was used. Differences in mean glandular dose averaged per unit could be related mainly to differences in tube-current exposure-time product values. Consequently, it is concluded that differences in mean glandular dose at different units are marginal.

Results of a European dose survey for mammography

For the dose study, a semi-automated method of data collection is used in this study. The participating centres were asked to fill out a spreadsheet with all necessary data and return it. For direct digital (DR) systems, the relevant data available in the DICOM header were used. All data is automatically added to a database and processed. The data were used to calculate the mean glandular dose for every image and for different thicknesses of polymethyl methacrylate phantoms using available conversion factors. Second-degree polynomials were fitted to the patient dose data and a reference dose curve was constructed for a range of thicknesses instead of a dose reference level at a single point. The dose reference curve rises from 1.57 mGy for a thickness of 30 mm to 2.50 mGy for 55 mm and 3.83 mGy for 75 mm. The results show centres that exceed this curve lie only in the lower or higher range of thicknesses and would remain undetected using a dose reference value in a single point. This gives better information to radiographers on where there is room for improvement of the dose levels in their system.

Method for determination of the mean fraction of glandular tissue in individual female breasts using mammography

The nationwide breast cancer screening programme using mammography has been in full operation in the Netherlands since 1997. Quality control of the screening programme has been assigned to the National Expert and Training Centre for Breast Cancer Screening. Limits are set to the mean glandular dose and the centre monitors these for all facilities engaged in the screening programme. This procedure is restricted to the determination of the entrance dose on a 5 cm thick polymethylmethacrylate (PMMA) phantom. The mean glandular dose for a compressed breast is estimated from these data. Individual breasts may deviate largely from this 5 cm PMMA breast model. Not only may the compressed breast size vary from 2 to 10 cm, but breast composition varies also. The mean glandular dose is dependent on the fraction of glandular tissue (glandularity) of the breast. To estimate the risk related to individual mammograms requires the development of a method for determination of the glandularity of individual breasts. A method has been developed to derive the glandularity using the attenuation of mammography x-rays in the breast. The method was applied to a series of mammograms at a screening unit. The results, i.e., a glandularity of 93% within the range of 0 to 1, were comparable with data in the literature. The glandularity as a function of compressed breast thickness is similar to results from other investigators using differing methods.

General ion recombination for ionization chambers used under irradiation conditions relevant for diagnostic radiology.

General ion recombination has been studied under irradiation conditions relevant for diagnostic radiology and with four different ionization chambers. When the exposure time is appreciably shorter than the ion transit time, the exposure can be designated as pulsed irradiation. On the contrary, for relatively long irradiation times, the term continuous irradiation can be applied. Recombination was estimated by measuring the collected charge at various collecting potentials of the ionization chamber. This is a well-known method in radiotherapy, but unfortunately it cannot be used in diagnostic radiology with typical exposure meters, since they do not offer the option of varying the collecting potential. For exposures with diagnostic x-ray units, an alternative approach is to vary the exposure or exposure rate over a wide range at a constant collecting potential. Experimental and theoretical estimates of ion recombination did not yield similar values. This might be due to several causes, such as differences between the actual and the nominal dimensions and volumes of the ionization chambers, due to errors and uncertainties in the physical parameters used in the theoretical models or due to deviations of the shape of the ionization chambers from the perfect cylindrical or parallel plate geometry. For better accuracy, corrections for recombination losses should therefore be based on experimental verification rather than on theoretical models.

Characteristics of Mg/Ar ionisation chambers used as gamma-ray dosemeters in mixed neutron-photon fields

Neutron fields are always accompanied by photons. Because of the difference in relative biological effectiveness of these two radiation components, it is necessary to determine neutron and photon absorbed dose in the mixed field separately. The use of Mg/Ar or Al/Ar ionisation chambers for determination of the photon component of the total absorbed dose could provide specific advantages related to the energies of the photons in the mixed field. Therefore information is presented to enable the greatest accuracy to be obtained in the use of these chambers. The gas flow characteristics of an Mg/Ar ionisation chamber show a complex behaviour most likely due to the Jesse effect in Ar. The dependence of the reading on the wall thickness of the chamber might be influenced by the irradiation geometry. The presence in the beam of charged particles created in hydrogen-rich materials could be important, as well as the contribution of low-energy photons produced by charged particles in the target construction. In Ar, the phenomenon that only positive ions and electrons are produced by ionising particles determines the ion collection. A polarity effect is often observed. The relative neutron sensitivities of different chambers of the same design irradiated under similar conditions show considerable variations.

Calculated energy response correction factors for LiF thermoluminescent dosemeters employed in the seventh EULEP dosimetry intercomparison.

Several dosimetry intercomparisons for whole body irradiation of mice have been organized by the European Late Effects Project Group (EULEP). These studies were performed employing a mouse phantom loaded with LiF thermoluminescent dosemeters (TLDs). In-phantom, the energy response of the LiF TLDs differs from free-in-air, due to spectral differences caused by attenuation and scatter of x-rays. From previous studies, energy response correction factors in-phantom relative to free-in-air were available for full scatter conditions. In the more recent intercomparisons, however, full scatter conditions were not always employed by the participants. Therefore, Monte Carlo calculations of radiation transport were performed to verify the LiF TLD energy response correction factors in-phantom relative to free-in-air for full scatter conditions and to obtain energy response correction factors for geometries where full scatter conditions are not met. For incident x-rays with HVLs in the 1 to 3.5 mm Cu range, the energy response correction factor in-phantom deviates by 2 to 4 per cent from that measured free-in-air. This is in reasonable agreement with previously published results. The energy response correction factors obtained from the present study refer to a calibration in terms of muscle tissue dose in-phantom using 60Co gamma rays. For geometries where full scatter conditions are not fulfilled, the energy response correction factors are different by up to about 3 per cent at maximum from that at full scatter conditions. The dependence of the energy response correction factor as a function of the position in-phantom is small, i.e. about 1 per cent at maximum between central and top or bottom positions.

Dosemeter readings and effective dose to the cardiologist with protective clothing in a simulated interventional procedure.

A personal dosemeter issued for individual monitoring is calibrated in terms of personal dose equivalent, usually H(P)(10). In general it yields a reasonable estimate of effective dose (E) when the exposed person does not wear protective clothing. In interventional cardiology, however, a lead equivalent apron is worn and often a thyroid collar. A correction factor will then be necessary to convert a dosemeter reading to E. To explore this factor an interventional cardiology procedure is simulated based on exposure conditions typical for a modern hospital in the BENELUX area. The dose to the cardiologist is investigated using Monte Carlo simulation of radiation transport. It is concluded that a personal dosemeter may best be worn outside the apron at a central position high on the chest for least dependence on the beam direction. It will overestimate E by roughly a factor of 20 (apron and thyroid collar of 0.25 mm Pb).