Feasibility of Brain Atrophy Measurement in Clinical Routine without Prior Standardization of the MRI Protocol: Results from MS-MRIUS, a Longitudinal Observational, Multicenter Real-World Outcome Study in Patients with Relapsing-Remitting MS

BACKGROUND AND PURPOSE

Feasibility of brain atrophy measurement in patients with MS in clinical routine, without prior standardization of the MRI protocol, is unknown. Our aim was to investigate the feasibility of brain atrophy measurement in patients with MS in clinical routine.

MATERIALS AND METHODS

Multiple Sclerosis and Clinical Outcome and MR Imaging in the United States (MS-MRIUS) is a multicenter (33 sites), retrospective study that included patients with relapsing-remitting MS who began treatment with fingolimod. Brain MR imaging examinations previously acquired at the baseline and follow-up periods on 1.5T or 3T scanners with no prior standardization were used, to resemble a real-world situation. Brain atrophy outcomes included the percentage brain volume change measured by structural image evaluation with normalization of atrophy on 2D-T1-weighted imaging and 3D-T1WI and the percentage lateral ventricle volume change, measured by VIENA on 2D-T1WI and 3D-T1WI and NeuroSTREAM on T2-fluid-attenuated inversion recovery examinations.

RESULTS

A total of 590 patients, followed for 16 months, were included. There were 585 (99.2%) T2-FLAIR, 425 (72%) 2D-T1WI, and 166 (28.2%) 3D-T1WI longitudinal pairs of examinations available. Excluding MR imaging examinations with scanner changes, the analyses were available on 388 (65.8%) patients on T2-FLAIR for the percentage lateral ventricle volume change, 259 and 257 (43.9% and 43.6%, respectively) on 2D-T1WI for the percentage brain volume change and the percentage lateral ventricle volume change, and 110 (18.6%) on 3D-T1WI for the percentage brain volume change and percentage lateral ventricle volume change. The median annualized percentage brain volume change was -0.31% on 2D-T1WI and -0.38% on 3D-T1WI. The median annualized percentage lateral ventricle volume change was 0.95% on 2D-T1WI, 1.47% on 3D-T1WI, and 0.90% on T2-FLAIR.

CONCLUSIONS

Brain atrophy was more readily assessed by estimating the percentage lateral ventricle volume change on T2-FLAIR compared with the percentage brain volume change or percentage lateral ventricle volume change using 2D- or 3D-T1WI in this observational retrospective study. Although measurement of the percentage brain volume change on 3D-T1WI remains the criterion standard and should be encouraged in future prospective studies, T2-FLAIR-derived percentage lateral ventricle volume change may be a more feasible surrogate when historical or other practical constraints limit the availability of percentage brain volume change on 3D-T1WI.

Patient-reported outcomes in multiple sclerosis: a systematic comparison of available measures

Multiple patient-reported outcomes (PROs) are currently being used in multiple sclerosis (MS) but their application is inconsistent and guidance on the appropriateness of each tool is lacking. The objective of our study was to identify MS-specific PROs and systematically to assess the development process and the reliability and validity of various instruments. A systematic literature search was conducted on multiple data sources, including MEDLINE, Embase (using the Ovid platform) and Google Scholar, from 1996 to March 2015. Search terms included combinations of MS, PROs and quality of life. Randomized controlled trials or observational studies conducted on patients with MS and published in English were included. In addition, the PROQOLID database was explored. The MS-specific PROs were systematically assessed using the Evaluating the Measurement of Patient-Reported Outcomes tool. In total, 8094 articles were screened and 405 PROs were identified from 1102 relevant articles. PROs were classified into MS-specific (n = 82) and non-MS-specific (n = 323). The results for the eight PROs that are most commonly used in MS clinical trials are presented here. For these eight PROs, the overall summary scores ranged between 50.1 and 68.7. The Multiple Sclerosis Impact Scale-29 had the best overall mean score (68.7), followed by the Leeds Multiple Sclerosis Quality of Life (67.0). This is the first study to provide a standardized assessment of all PROs for MS. There is a lack of data on content validity for PROs used in MS research, which indicates the need for a robust instrument in MS developed according to the US Food and Drug Administration guidelines.

Nanodosimetry in a clinical neutron therapy beam using the variance-covariance method and Monte Carlo simulations

Nanodosimetric single-event distributions or their mean values may contribute to a better understanding of how radiation induced biological damages are produced. They may also provide means for radiation quality characterization in therapy beams. Experimental nanodosimetry is however technically challenging and Monte Carlo simulations are valuable as a complementary tool for such investigations. The dose-mean lineal energy was determined in a therapeutic p(65)+Be neutron beam and in a (60)Co gamma beam using low-pressure gas detectors and the variance-covariance method. The neutron beam was simulated using the condensed history Monte Carlo codes MCNPX and SHIELD-HIT. The dose-mean lineal energy was calculated using the simulated dose and fluence spectra together with published data from track-structure simulations. A comparison between simulated and measured results revealed some systematic differences and different dependencies on the simulated object size. The results show that both experimental and theoretical approaches are needed for an accurate dosimetry in the nanometer region. In line with previously reported results, the dose-mean lineal energy determined at 10 nm was shown to be related to clinical RBE values in the neutron beam and in a simulated 175 MeV proton beam as well.

An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al2O3:C

Although the radioluminescence (RL) signal from optical fibre Al(2)O(3):C dosemeters used in medical applications is essentially proportional to dose rate, the crystals used so far are imperfect in the sense that their RL sensitivity changes with accumulated dose. A computational algorithm has been developed that corrects for these sensitivity changes. We further report on a new system that effectively separates the RL signal generated in the crystal from fluorescence and Cerenkov emission generated in the optical fibre cable using a gating technique in connection with pulsed linear accelerator radiation beams. The dosimetry system has been used for dose measurements in a phantom during an intensity-modulated radiation therapy (IMRT) treatment with 6 MV photons. The RL measurement results are in excellent agreement (i.e. within 1%) with both the OSL results and the dose delivered according to the treatment planning system. RL signals from Al(2)O(3):C can be used for real-time dose rate measurements with a time resolution of approximately 0.1 s and a spatial resolution only limited by the size of the detector (<0.5 mm).

The influence of RBE variations in a clinical proton treatment plan for a hypopharynx cancer

Currently, most clinical range-modulated proton beams are assumed to have a fixed overall relative biological effectiveness (RBE) of 1.1. However, it is well known that the RBE increases with depth in the spread-out Bragg peak (SOBP) and becomes about 10% higher than mid-SOBP RBE at 2 mm from the distal edge (Paganetti 2003 Technol. Cancer Res. Treat. 2 413-26) and can reach values of 1.3-1.4 in vitro at the distal edge (Robertson et al 1975 Cancer 35 1664-77, Courdi et al 1994 Br. J. Radiol. 67 800-4). We present a fast method for applying a variable RBE correction with linear energy transfer (LET) dependent tissue-specific parameters based on the alpharef/betaref ratios suitable for implementation in a treatment planning system. The influence of applying this variable RBE correction on a clinical multiple beam proton dose plan is presented here. The treatment plan is evaluated by RBE weighted dose volume histograms (DVHs) and the calculation of tumour control probability (TCP) and normal tissue complication probability (NTCP) values. The variable RBE correction yields DVHs for the clinical target volumes (CTVs), a primary advanced hypopharynx cancer and subclinical disease in the lymph nodes, that are slightly higher than those achieved by multiplying the absorbed dose with RBE=1.1. Although, more importantly, the RBE weighted DVH for an organ at risk, the spinal cord is considerably increased for the variable RBE. As the spinal cord in this particular case is located 8 mm behind the planning target volume (PTV) and hence receives only low total doses, the NTCP values are zero in spite of the significant increase in the RBE weighted DVHs for the variable RBE. However, high NTCP values for the non-target normal tissue were obtained when applying the variable RBE correction. As RBE variations tend to be smaller for in vivo systems, this study-based on in vitro data since human tissue RBE values are scarce and have large uncertainties-can be interpreted as showing the upper limits of the possible effects of utilizing a variable RBE correction. In conclusion, the results obtained here still indicate a significant difference in introducing a variable RBE compared to applying a generic RBE of 1.1, suggesting it is worth considering such a correction in clinical proton therapy planning, especially when risk organs are located immediately behind the target volume.

In vivoabsorbed dose measurements in mammography using a new real-time luminescence technique

A dosimetry system based on radioluminescence (RL) and optically stimulated luminescence (OSL) from carbon doped aluminium oxide (Al2O3:C) crystals was developed for in vivo absorbed dose measurements in mammography. A small cylindrical crystal of Al2O3:C (diameter 0.48 mm and length 2 mm) was coupled to the end of a 1 mm diameter optical fibre cable. Owing to their small size and characteristic shape, these probes can be placed on the body surface in the field of view during the examination, without compromising the reading of the mammogram. Our new technique was tested with a mammography unit (Siemens Mammomat 3000) and screen-film technique over a range of clinically relevant X-ray energies. The results were compared with those obtained from an ionization chamber usually used for the determination of absorbed dose in mammography. The reproducibility of measurements was around 3% (1 standard deviation) at 4.5 mGy for both RL and OSL data. The dose response was found to be linear between 4.5 mGy and 30 mGy. The energy dependence of the system is around 18% between 23 kV and 35 kV. In vivo measurements were performed during three patient examinations. It was shown that entrance and exit doses could be measured. The presence of the small probes did not significantly interfere with the diagnostic quality of the images. Entrance doses estimated by RL/OSL results agreed within 3% with entrance surface dose values calculated from the ionization chamber measurements. These results indicate a considerable potential for use in routine control and in vivo dose measurements in mammography.

Commissioning of an NRC-type sealed water calorimeter at METAS using 60Co gamma-rays

As part of a collaborative project between the National Research Council of Canada (NRC) and the Swiss Federal Office of Metrology and Accreditation (METAS), a sealed water calorimeter was built at NRC and transferred to METAS. The calorimeter is operated at 4 degrees C and uses two thermistor probes in a sealed glass vessel containing high-purity water to measure the radiation-induced temperature rise. The various correction factors have been evaluated and the estimated standard uncertainty on the absorbed dose to water is 0.41%. An extensive set of measurements using 60Co gamma-rays was carried out at NRC and two ionization chambers were calibrated against the absorbed dose determined calorimetrically. The chambers were also calibrated against the NRC standard for absorbed dose. After transferring the calorimeter to METAS, a similar set of measurements was carried out using their 60Co beam and the same two ionization chambers were calibrated against the absorbed dose to water established at METAS. The discrepancy between the three sets of calibration coefficients was smaller than the estimated standard uncertainty of 0.47% on the ratio of any pair of calibration coefficients.

A Monte Carlo study of the energy dependence of Al2O3:C crystals for real-time in vivo dosimetry in mammography

In a previous experimental study, a novel method for in vivo dosimetry has been investigated, based on radioluminescence (RL) and optically stimulated luminescence (OSL). However, because of the large difference in atomic composition between the detector material and the breast tissue, relatively large energy dependence in low-energy X-ray beams can be expected. In the present work, the energy dependence of Al2O3:C crystals was modelled with the Monte Carlo code EGSnrc using three types of X-ray spectra. The results obtained (5.6-7.3%) agree with a previously determined experimental result (9%) within the combined standard uncertainty of the two methods. The influence of the size of the crystal on the energy dependence was investigated together with the effect of varying the thickness of the surrounding light-protective material. The results obtained indicate a minor effect owing to the thickness of the light-protective material, and a somewhat larger effect from reducing the diameter of the crystal. The outcome of this study can be used to improve the future design of the RL/OSL dosimetry system for use in mammography.

Clinical evaluation of a new set of image quality criteria for mammography

The European Commission (EC) quality criteria for screen-film mammography are used as a tool to assess image quality. A new set of criteria was developed and initially tested in a previous study. In the present study, these criteria are further evaluated using screen-film mammograms that have been digitised, manipulated to simulate different image quality levels and reprinted on film. Expert radiologists have evaluated these manipulated images using both the original (EC) and the new criteria. A comparison of three different simulated dose levels reveals that the new criteria yield a larger separation of image criteria scores than the old ones. These results indicate that the new set of image quality criteria has a higher discriminative power than the old set and thus seems to be more suitable for evaluation of image quality in mammography.

Can the average glandular dose in routine digital mammography screening be reduced? A pilot study using revised image quality criteria

There is a need for tools that in a simple way can be used for the evaluation of image quality related to clinical requirements in mammography. The aim of this work was to adjust the present European image quality criteria to be relevant also for digital mammography images, and to use as simple and as few criteria as possible. A pilot evaluation of the new set of criteria was made with mammograms of 28 women from a General Electric Senographe 2000D full-field digital mammography system. One breast was exposed using the standard automatic exposure mode, the other using about half of that absorbed dose. Three experienced radiologists evaluated the images using visual grading analysis technique. The results indicate that the new quality criteria can be used for the evaluation of image quality related to clinical requirements in digital mammography in a simple way. The results also suggest that absorbed doses for the mammography system used may be substantially reduced.

Increasing stroke incidence in Sweden between 1989 and 2000 among persons aged 30 to 65 years: evidence from the Swedish Hospital Discharge Register

BACKGROUND AND PURPOSE

Stroke mortality is decreasing in Sweden, as is the case in other Western European countries. However, both decreases and increases have been reported in Sweden for persons younger than age 65 years. The aim of this study was to compare the incidence of stroke in Sweden between the periods 1989 and 1991 and 1998 and 2000 in persons aged 30 to 65 years.

METHODS

All first-ever stroke patients aged 30 to 65 years in the Swedish Hospital Discharge Register between 1989 and 2000 were included.

RESULTS

The age-standardized, 3-year average incidence increased by 19%, from 98.9 to 118.0 per 100 000 among men, and by 33%, from 48.4 to 64.4 among women, between 1989 and 1991 and 1998 and 2000. The largest increase was seen among those younger than 60 years. On a county level, the change in age-standardized stroke incidence varied from small decreases (-3%) to large increases (82%).

CONCLUSIONS

Stroke incidence increased in Sweden for both men and women between 1989 and 2000. The increase was larger among women. This calls for action when it comes to studying risk factors and planning for prevention and health promotion and indicates the need for gender-specific studies.

Health promotion and rehabilitation: a case study

BACKGROUND

Since the number of people in Sweden on long-term sick leave has rapidly increased since 1996, new non-biomedical models of occupational rehabilitation are at stake. A group of seven women who had finished medical treatment and rehabilitation but were still on sick leave or temporary disability pension for several years, worked in a problem-based rehabilitation group for 6 months. Focus for the group was on a process of change towards health and work ability.

PURPOSE

The aim of this case study was to improve understanding of effects of a problem-based rehabilitation model (PBR) on health promoting processes amongst a group of women on long-term sick leave.

METHOD

Data source was a focus group interview. The analysis follows the guidelines of qualitative analysis that emerges from grounded theory.

RESULTS

The pedagogical model of PBR enhanced the participant's internal resources such as self-confidence and ability to act in a social setting. External resources such as social support were improved. An individual follow-up was conducted 2 years after the rehabilitation process and four out of seven women had returned to work.

CONCLUSION

Among this group of women PBR launched health-promoting processes. When the more medically oriented treatment is finished or is not able to contribute further to the individual's recovery, other aspects of the individuals abilities and health resources will be focused upon.

Comparison of dosimetry recommendations for clinical proton beams

The formalism and data in the two most recent dosimetry recommendations for clinical proton beams, ICRU Report 59 and the forthcoming IAEA Code of Practice, are compared. Chamber calibrations in terms of air kerma and absorbed dose to water are considered, including five different cylindrical ionization chamber types commonly used in proton beam dosimetry. The methodology for both types of calibration for ionization chambers is described in ICRU Report 59. The procedure based on air kerma calibrations is compared with an alternative formalism based on IAEA Codes of Practice (TRS-277, TRS-381), modified for proton beams. The new IAEA Code of Practice is exclusively based on calibrations in terms of absorbed dose to water and a direct comparison with ICRU Report 59 recommendations is made. Common to the two formalisms are the fundamental quantities Wair and w(air) and their atmospheric conditions of applicability. The difference in the recommended values of the ratio w(air)/Wair (protons to 60Co) is as large as 2.3%. The use of Wair and w(air) values for dry air (IAEA) and for ambient air (ICRU) is a contribution to the discrepancy, and the ICRU usage is questioned. For air kerma based chamber calibrations, ICRU Report 59 does not take into account the effect of different compositions of the build-up cap and chamber wall on the calibration beam quality. For the chamber types included in the study, this introduces discrepancies of up to 1.1%. Combined with differences in the recommended basic data, discrepancies in absorbed dose determination in proton beams of up to 2.1% are found. For the absorbed dose to water based formalism, differences in the formalism, notably the omission of perturbation factors for 60Co in ICRU 59, and data yield discrepancies in calculated kQ factors, and in absorbed dose determinations, between -1.5% and +2.6%, depending on the chamber type and the proton beam quality.

General characteristics of the use of silicon diode detectors for clinical dosimetry in proton beams

The properties of silicon diode detectors, used for dosimetry in clinical proton beams, were investigated with special regard to the measurement of relative dose distributions in water. Different types of silicon diode detector were studied, and the resulting distributions of detector signal versus depth in the water phantom were compared with the corresponding distributions obtained with a plane-parallel NACP ionization chamber. The measurements were performed in a proton beam with an initial energy of 173 MeV. It is shown that the Hi-p silicon detector gives a signal which is proportional to the ionization density in the silicon crystal in all parts of the Bragg curve, and for all levels of accumulated dose to the detector. This is in contrast to detectors based on n-type silicon, or on low resistivity p-type silicon. After pre-irradiation, these latter detectors show a stopping-power dependent recombination, yielding an increase in the detector signal per unit dose with increasing LET. This effect leads to an over-response in the Bragg peak, which increases gradually with the accumulated detector dose. Using the Hi-p silicon diode detector, the depth ionization distribution was found to be equal to the distribution obtained with the plane-parallel NACP ionization chamber at all pre-irradiation levels, within the experimental accuracy. This implies that the quotient between the ionization in the detector and the absorbed dose to the surrounding water is equal for these detectors.

Development of a compact proton scanning system in Uppsala with a moveable second magnet

A scanned proton beam yields dose distributions that in most cases are superior to passively scattered proton beams and to other external radiation treatment modalities. The present paper gives a description of the scanning system that has been developed at the Svedberg Laboratory (TSL) in Uppsala. The scanning technique and the technical design are described. The solution with a small pole gap of the magnets and a moveable second magnet results in a very compact scanning head, which can therefore be incorporated in a gantry of relatively limited size. A prototype was constructed that has been used to realize various dose distributions with a scanned beam of 180 MeV protons at TSL.

Ferrous sulphate gel dosimetry and MRI for proton beam dose measurements

Ferrous sulphate gel dosimetry has the potential for measurement of absorbed dose distributions in proton therapy. The chemical properties of the gel are altered according to the radiation dose and these changes can be evaluated in three dimensions using MRI. The purpose of this work was to investigate the properties of a ferrous gel used with clinical proton beams. The gel was irradiated with both monoenergetic and range-modulated proton beams. It was then evaluated using MRI. The depth dose by means of the 1/T1 distribution was studied and compared with data from a plane-parallel plate ionization chamber. 1/T1 was shown to be proportional to the dose at a mean proton energy of approximately 90 MeV. The dose response was no different from that obtained using photon beams. However, on normalization at the entrance, the relative 1/T1 at the Bragg peak was 15-20% lower than the corresponding ionization chamber data for the monoenergetic proton beam. Better agreement was found for the modulated beam, but with significant differences close to the distal edge of the 1/T1 distribution. The change in sensitivity with depth was explained by means of a linear energy transfer dependence. This property was further studied using Monte Carlo methods.

Fast 2D phantom dosimetry for scanning proton beams

A quality control system especially designed for dosimetry in scanning proton beams has been designed and tested. The system consists of a scintillating screen (Gd2O2S:Tb), mounted at the beam-exit side of a phantom, and observed by a low noise CCD camera with a long integration time. The purpose of the instrument is to make a fast and accurate two-dimensional image of the dose distribution at the screen position in the phantom. The linearity of the signal with the dose, the noise in the signal, the influence of the ionization density on the signal, and the influence of the field size on the signal have been investigated. The spatial resolution is 1.3 mm (1 s.d.), which is sufficiently smaller than typical penumbras in dose distributions. The measured yield depends linearly on the dose and agrees within 5% with the calculations. In the images a signal to noise ration (signal/1 s.d.) of 10(2) has been found, which is in the same order of magnitude as expected from the calculations. At locations in the dose distribution possessing a strong contribution of high ionization densities (i.e., in the Bragg peak), we found some quenching of the light output, which can be described well by existing models if the beam characteristics are known. For clinically used beam characteristics such as a Spread Out Bragg peak, there is at most 8% deviation from the NACP ionization chamber measurements. The conclusion is that this instrument is a useful tool for quick and reliable quality control of proton beams. The long integration-time capabilities of the system make it worthwhile to investigate its applicability in scanning proton beams and other dynamic treatment modalities.

Radiological videoconferencing in Sweden

For the last four years there has been a videoconferencing link between the Karlskrona hospital and the university hospital of Lund. Because of a need to transmit angiographic films, its use has been gradually expanded to become an everyday routine, used for both elective and emergency examinations. During 1994 and 1995, 1121 cases were examined over the video link. Apart from 156 normal cases all were discussed with surgeons, most at 96 weekly conferences but approximately 50 at conferences taking place as soon as possible after the examination. Assuming that an average of three doctors attended the conferences before the video link was available, this amounts to a saving of 3 x 8 h in travelling time per videoconference. The average salary was 300 SKr/h and train tickets cost a further 400 SKr per person. The cost of the ISDN connection was approximately 1800 SKr/conference. The net gain was thus about 6500 SKr per videoconference, without taking into account the work that could be done with the doctors still in Karlskrona after the conference.

Teleradiology by two different concepts. Technical note

Two different teleradiology concepts are described. Their advantages, disadvantages and costs are discussed.

Monte Carlo calculated stopping-power ratios, water/air, for clinical proton dosimetry (50-250 MeV)

Calculations of stopping power ratios, water to air, for the determination of absorbed dose to water in clinical proton beams using ionization chamber measurements have been undertaken using the Monte Carlo method. A computer code to simulate the transport of protons in water (PETRA) has been used to calculate sw.air-data under different degrees of complexity, ranging from values based on primary protons only to data including secondary electrons and high-energy secondary protons produced in nonelastic nuclear collisions. All numerical data are based on ICRU 49 proton stopping powers. Calculations using primary protons have been compared to the simple continuous slowing-down approximation (c.s.d.a.) analytical technique used in proton dosimetry protocols, not finding significant differences that justify elaborate Monte Carlo simulations except beyond the mean range of the protons (the far side of the Bragg peak). The influence of nuclear nonelastic processes, through the detailed generation and transport of secondary protons, on the calculated stopping-power ratios has been found to be negligible. The effect of alpha particles has also been analysed, finding differences smaller than 0.1% from the results excluding them. Discrepancies of up to 0.6% in the plateau region have been found, however, when the production and transport of secondary electrons are taken into account. The large influence of nonelastic nuclear interactions on proton depth-dose distributions shows that the removal of primary protons from the incident beam decreases the peak-to-plateau ratio by a large factor, up to 40% at 250 MeV. It is therefore emphasized that nonelastic nuclear reactions should be included in Monte Carlo simulations of proton beam depth-dose distributions.

Proton dosimetry intercomparison

BACKGROUND AND PURPOSE

Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols.

MATERIALS AND METHODS

Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV).

RESULTS

The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%.

CONCLUSION

The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Phantom study of chest radiography with storage phosphor, selenium, and film-screen systems

PURPOSE

Chest radiographs from 3 digital systems--2 based on luminescent phosphors and one on selenium--and a conventional film-screen system were evaluated and compared.

MATERIAL AND METHODS

Computed radiography (CR) has for the past years been dominated by a single manufacturer, but now several systems have been marketed. Using a chest phantom and, as test objects, 2 simulated tumours for the lung and mediastinum, respectively, and one object simulating pulmonary lines, a total of 400 exposures were made, 100 on each system. The test objects were placed randomly with a ratio of presence/absence of each object of about 50. Six radiologists participated, 2 residents and 4 staff members. A receiver operating characteristics (ROC) analysis was performed with construction of curves, and the difference between the curves was estimated with a 2-tailed paired t-test.

RESULTS AND CONCLUSION

The selenium-based system performed significantly better for pulmonary line detection than all the other systems, and better than one storage phosphor system for the lung "tumour" (p < 0.05), while one storage phosphor system was slightly better than the other in diagnosing all 3 test objects. The score for the film-screen system was only average.

Faraday cup dosimetry in a proton therapy beam without collimation

A Faraday cup in a proton beam can give an accurate measurement of the number of protons collected by the cup. It is shown that the collection efficiency with a proper design can be close to unity. To be able to calibrate an ionization chamber from such a measurement, as is recommended in some dosimetry protocols, the energy spectrum of the proton beam must be accurately known. This is normally not the case when the lateral beam extension is defined by collimators. Therefore a method for relating an ionization chamber measurement in an uncollimated beam to the total number of protons in the beam has been developed and is described together with experimental results from calibrating an ionization chamber using this method in the therapeutic beam in Uppsala. This method is applicable to ionization chambers of any shape and the accuracy is estimated to be 1.6% (1 SD).

Ionization chamber dosimetry of proton beams using cylindrical and plane parallel chambers. Nw versus Nk ion chamber calibrations

Determinations of the absorbed dose in a 170 MeV proton beam have been performed using seven ionization chambers of different types: five cylindrical (two FWT IC-18 and three NE-2571, of which one was modified to have the central electrode made of graphite) and two plane parallel (NACP-02 and Roos FK-6). The ionization was converted into absorbed dose in the proton beam according to the generalization of the formalism provided by the IAEA Code of Practice (TRS 277), which enables the use of the same equations for all kinds of beam used in radiotherapy. The absorbed dose obtained with the two IC-18 chambers, a chamber type commonly used as a reference in proton beams, was up to 1.5% lower than that obtained with the Farmer NE-2571 chamber, which was used as the reference in this work when calibration factors in terms of NK were used. To investigate this difference, experimental ND factors for six chambers (the two IC-18 chambers, the NACP-02, the FK-6 and two of the NE-2571 chambers) were determined in a high-energy electron beam. The procedure commonly recommended for plane parallel ion chambers was used for all the chambers, using the same reference chamber, a Farmer NE-2571. In the 170 MeV proton beam all the ND factors yielded consistent absorbed dose determinations within the estimated experimental uncertainties. This finding calls into question the value of the product kattkm for the IC-18 chamber given by the IAEA Code of Practice used in this comparison, and points at possible chamber to chamber variations that theoretical kattkm factors cannot predict. The investigations enabled the determination of the Pwall(60Co) factor of the Roos FK-6 plane parallel chamber, yielding 1.003 +/- 0.5%, and a correction for the effect of the aluminium central electrode of NE-2571 chambers in proton beams, equal to 1.003 +/- 0.4%. Two of the chambers (the plane parallel FK-6 and the modified cylindrical NE-2571) were provided with calibration factors in terms of absorbed dose to water, Nw, at the quality of 60Co by the Primary Standard Dosimetry Laboratory in Germany (PTB). Using the Nw formalism excellent agreement was found with the determination based on the experimental ND, giving support to the implementation of the NW procedure in therapeutic proton beams.

Xenopus peroxisome proliferator activated receptors: genomic organization, response element recognition, heterodimer formation with retinoid X receptor and activation by fatty acids

Peroxisome proliferator activated receptors are ligand activated transcription factors belonging to the nuclear hormone receptor superfamily. Three cDNAs encoding such receptors have been isolated from Xenopus laevis (xPPAR alpha, beta, and gamma). Furthermore, the gene coding for xPPAR beta has been cloned, thus being the first member of this subfamily whose genomic organization has been solved. Functionally, xPPAR alpha as well as its mouse and rat homologs are thought to play an important role in lipid metabolism due to their ability to activate transcription of a reporter gene through the promoter of the acyl-CoA oxidase (ACO) gene. ACO catalyzes the rate limiting step in the peroxisomal beta-oxidation of fatty acids. Activation is achieved by the binding of xPPAR alpha on a regulatory element (DR1) found in the promoter region of this gene, xPPAR beta and gamma are also able to recognize the same type of element and are, as PPAR alpha, able to form heterodimers with retinoid X receptor. All three xPPARs appear to be activated by synthetic peroxisome proliferators as well as by naturally occurring fatty acids, suggesting that a common mode of action exists for all the members of this subfamily of nuclear hormone receptors.

Constraints of the multiple-scattering theory of Molière in Monte Carlo simulations of the transport of charged particles

The breakdown of Molière's multiple-scattering theory for short pathlengths occurring during Monte Carlo simulations with charged particles is demonstrated. It has been found that in certain conditions where the theory is assumed to be valid, significant distortions of the angular distribution occur that make the sampling of the polar angle questionable in numerous steps of Monte Carlo simulations. The limits of the theory have been investigated, both using a large number of terms in the Molière's series and using steps of Molière's theory where 1/B expansions are not involved. At B = 4.5 the commonly accepted 3-term series expansion yields differences up to +/- 6% compared with the evaluation of the complete Molière angular distribution, and up to 7 terms in the series are needed in order to achieve agreement within +/- 2%. One percent agreement requires B = 10. Numerical values of the full distribution are given in terms of Molière's parameters B and reduced angle theta. By using the general dependence of the distribution results are valid for both electron and proton Monte Carlo simulations in any material.

Peroxisome proliferator-activated receptors and lipid metabolism

PPARs are nuclear hormone receptors which, like the retinoid, thyroid hormone, vitamin D, and steroid hormone receptors, are ligand-activated transcription factors mediating the hormonal control of gene expression. Two lines of evidence indicate that PPARs have an important function in fatty acid metabolism. First, PPARs are activated by hypolipidemic drugs and physiological concentrations of fatty acids, and second, PPARs control the peroxisomal beta-oxidation pathway of fatty acids through transcriptional induction of the gene encoding the acyl-CoA oxidase (ACO), which is the rate-limiting enzyme of the pathway. Furthermore, the PPAR signaling pathway appears to converge with the 9-cis retinoic acid receptor (RXR) signaling pathway in the regulation of the ACO gene because heterodimerization between PPAR and RXR is essential for in vitro binding to the PPRE and because the strongest stimulation of this gene is observed when both receptors are exposed simultaneously to their activators. Thus, it appears that PPARs are involved in the 9-cis retinoic acid signaling pathway and that they play a pivotal role in the hormonal control of lipid metabolism.

Fatty acids and retinoids control lipid metabolism through activation of peroxisome proliferator-activated receptor-retinoid X receptor heterodimers

The nuclear hormone receptors called PPARs (peroxisome proliferator-activated receptors alpha, beta, and gamma) regulate the peroxisomal beta-oxidation of fatty acids by induction of the acyl-CoA oxidase gene that encodes the rate-limiting enzyme of the pathway. Gel retardation and cotransfection assays revealed that PPAR alpha heterodimerizes with retinoid X receptor beta (RXR beta; RXR is the receptor for 9-cis-retinoic acid) and that the two receptors cooperate for the activation of the acyl-CoA oxidase gene promoter. The strongest stimulation of this promoter was obtained when both receptors were exposed simultaneously to their cognate activators. Furthermore, we show that natural fatty acids, and especially polyunsaturated fatty acids, activate PPARs as potently as does the hypolipidemic drug Wy 14,643, the most effective activator known so far. Moreover, we discovered that the synthetic arachidonic acid analogue 5,8,11,14-eicosatetraynoic acid is 100 times more effective than Wy 14,643 in the activation of PPAR alpha. In conclusion, our data demonstrate a convergence of the PPAR and RXR signaling pathways in the regulation of the peroxisomal beta-oxidation of fatty acids by fatty acids and retinoids.

Mutational analysis of active site residues of human adenosine deaminase

Adenosine deaminase was overexpressed in a baculovirus system. The pure recombinant and native enzymes were identical in size, Zn2+ content, and activity. Five amino acids, in proximity to the active site, were replaced by mutagenesis. The altered enzymes were purified to homogeneity and compared to wild-type adenosine deaminase with respect to zinc content, enzymatic activity, and kinetic parameters. All but one of the alterations produced significant activity perturbations. Replacement of Cys262 produced a protein that retained at least 30-40% of wild-type activity. In contrast, replacements of His17, His214, His238, and Glu217 resulted in dramatic losses of enzyme activity. None of these mutants exhibited large variations in Km. The proteins produced from alterations of amino acids implicated in metal coordination were slightly activated by inclusion of Zn2+ throughout purification. These experiments confirm that in the active enzyme Zn2+ plays a critical role in catalysis, that a histidine or glutamate residue plays a mechanistic role in the hydrolytic deamination step, and that cysteine is not involved in the catalytic mechanism of adenosine deaminase. These data support the roles for these amino acid residues suggested from the x-ray structure of murine adenosine deaminase (Wilson, D. K., Rudolf, F. B., and Quicho, F. A. (1991) Science 252, 1278-1284).