Large-area Ionization Chamber Research Articles

Uniformity calibration for large area XY strip parallel plate ionization chamber

Demand for large area parallel plate ionization chamber (PPIC) or large area ionization chamber (LAIC) has risen in recent years due to several advantages of the large effective area in monitoring therapeutic radiation beams. PPICs are designed for the measurements of beam profile and dosimetry in radiation therapy quality assurance (QA) procedures. Objective. Heterogeneous responses over the large sensitive area pose an undeniable concern for the straightforward applications of PPICs in clinical dosimetry. Uniformity calibration for the detector response is thus essential for the accurate performance of each PPIC unit. Approach. A large area XY strip PPIC, characterized by a large effective area of 345.44 × 345.44 mm2 and 256 readout channels, was investigated in this study. A new systematic uniformity calibration is developed to improve the lateral response of the PPIC over the measurements for both narrow beams and large square field beams. A 2D response map of the PPIC was obtained by a spot-scanning method using a compact x-ray tube (mini x-ray). The mini x-ray, providing stable radiation (uncertainty <0.1%), was moved with a step size of 20 mm in 2 dimensions across the entire PPIC surface to collect a complete spot scan. Different uniformity calibration methods were introduced for the measurement of the PPIC by adopting the information from the detector 2D response map. Main results. Deviation of the detector response, before calibration, was observed to reach about 7% for the testing PPIC unit which is much higher than the recommended uniformity response of 1% (IAEA TRS-398). The uniformity response of the PPIC improved significantly to less than 1% across the detector surface after calibration. Significance. The proposed methods enable the practical application of PPIC in routine clinical dosimetry and can be reliably adopted by any radiation facility to perform daily and monthly QA.

Dose-area product ratio in external small-beam radiotherapy: beam shape, size and energy dependencies in clinical photon beams

In small-field radiotherapy (RT), a significant challenge is to define the amount of radiation dose absorbed in the patient where the quality of the beam has to be measured with high accuracy. The properties of a proposed new beam quality specifier, namely the dose-area-product ratio at 20 and 10 cm depths in water or DAPR20,10, were studied to yield more information on its feasibility over the conventional quality specifier tissue-phantom ratio or TPR20,10. The DAPR20,10 may be measured with a large-area ionization chamber (LAC) instead of small volume chambers or semi-conductors where detector, beam and water phantom positioning and beam perturbations introduce uncertainties. The effects of beam shape, size and energy on the DAPR20,10 were studied and it was shown that the DAPR20,10 increases with increasing beam energy similarly to TPR20,10 but in contrast exhibits a small beam size and shape dependence. The beam profile outside the beam limiting devices has been shown to have a large contribution to the DAPR20,10. There is potential in large area chambers to be used in DAPR measurement and its use in dosimetry of small-beam RT for beam quality measurements.

A GEMPix-based integrated system for measurements of 3D dose distributions in water for carbon ion scanning beam radiotherapy.

PurposeCommercially available systems for ion beam reference dosimetry in water are mainly based on ionization chambers. In those systems, a large number of small detectors are typically arranged in a two‐dimensional (2D) array or matrix to achieve high spatial resolution (order of several millimeters) and large field coverage at the same time. The goal of this work was to investigate the reliability of a detector of superior spatial resolution to perform three‐dimensional (3D) ionization measurements in carbon ion pencil beams.MethodsThe GEMPix is a small gaseous detector with a highly pixelated readout, consisting of a drift region (with 2.8 cm3 × 2.8 cm3 × 0.3 cm3 volume), three gas electron multipliers (GEMs) for signal amplification and four Timepix ASICs with 55 µm pixel pitch and a total of 262,144 pixels. An integrated system was designed and built, which consists of a commercial water phantom with a three‐axis motorized arm, a reference large‐area ionization chamber for signal normalization to the beam output and the GEMPix itself. Measurements at different depths in water have been performed at the Italian National Centre for Oncological Hadrontherapy (CNAO) with three carbon ion beam energies. Lateral beam profiles measured with the GEMPix at the shallowest depth were compared to those measured with radiochromic EBT3 films in air in the position of the reference ionization chamber. The Timepix readout was calibrated in energy by using one independent depth scan with carbon ions of 150 mm range. Bragg peak curves were also simulated using the Monte Carlo FLUKA code as a reference.ResultsBeam profiles measured with the GEMPix were smooth and showed similar shape and full width at half maximum when compared to those measured with radiochromic EBT3 films. Smooth, reproducible Bragg curves were obtained with statistical uncertainties of about 2%, matching FLUKA simulations of the Bragg curves within 15% for most data points. This difference is partially explained for the measurement with carbon ions of 150 mm range by a saturation effect in the GEMs. The high granularity of the readout allowed to produce 2D images of the deposited dose at different depths, as well as 3D data distributions.ConclusionsThis paper demonstrates the capability of the GEMPix detector to measure the 3D dose distribution of carbon ions in water for a clinical pencil beam reliably. In the future, the detector area will be increased to cover fields of scanned beams. Measurements at higher beam intensities and with protons are planned.

Sensitivity of the IQM transmission detector to errors of VMAT plans.

The integral quality monitor (IQM) transmission detector is a wedge-shaped large area ionization chamber that reports a position-weighted dose area product for each control point of an IMRT or VMAT plan. In this study, the accuracy of the signal prediction is verified for the Synergy Agility MLC. Tolerance criteria for VMAT plan verification with the IQM were obtained from the observed sensitivity for the detection of incorrectly delivered plans. The predicted IQM signal was compared to the measured signal recorded for a set of 30 VMAT plans for each beam quality of 6 and 10MV. The system's capability to detect incorrectly delivered plans was tested by measuring altered plans containing small, random deviations. In addition, the observed deviations were related to measurements performed with a second QA phantom. The cumulative IQM signal per arc deviated from the respective calculation on average by -0.48% (6MV) and +0.21% (10MV) with a standard deviation of 1.08% in both cases, suggesting a 2% warning and 3% action threshold as plan acceptance criteria. This choice was confirmed by the optimum threshold of 2.5% obtained via receiver operating characteristic (ROC) analysis. Reproducibility of individual control points in multiply measured plans was low (on average 7% for 1SD) and thus, segment-by-segment comparison was impractical. A suitable criterion to resolve the angular distribution of the plan was identified by binning three to five control points as a running average. While the correlation between IQM signal deviations and gamma passing rates obtained with the ArcCHECK phantom was low for clinical plans, it was apparent for erroneous plans. Binning led to even higher sensitivity to errors. The IQM was able to detect induced errors at least as reliable as the standard phantom and showed the potential to be used in pretreatment plan verification to ensure the correct plan transfer and delivery. However, there is no direct correlation between the IQM signal deviation and DVH metrics, so the IQM should be primarily used to screen for errors. Finer diagnostics should then be carried out using a different phantom.

Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry

Large area ionization chambers (LAICs) can be used to measure output factors of narrow beams. Dose area product measurements are proposed as an alternative to central-axis point dose measurements. Using such detectors requires detailed information on the uniformity of the response along the sensitive area. Eight LAICs were investigated in this study: four of type PTW-34070 (LAICThick) and four of type PTW-34080 (LAICThin). Measurements were performed in an x-ray unit using peak voltages of 100–200 kVp and a collimated beam of 3.1 mm (FWHM). The LAICs were moved with a step size of 5 mm to measure the chamber response at lateral positions. To account for beam positions where only a fraction of the beam impinged within the sensitive area of the LAICs, a corrected response was calculated which was the basis to calculate the relative response. The impact of a heterogeneous LAIC response, based on the obtained response maps was henceforth investigated for proton pencil beams and small field photon beams. A pronounced heterogeneity of the responses was observed in the investigated LAICs. The response of LAICThick generally decreased with increasing radius, resulting in a response correction of up to 5%. This correction was more pronounced and more diverse (up to 10%) for LAICThin. Considering a proton pencil beam the systematic offset for reference dosimetry was 2.4–4.1% for LAICThick and −9.5 to 9.4% for LAICThin. For relative dosimetry (e.g. integral depth-dose curves) systematic response variation by 0.8–1.9% were found. For a decreasing photon field size the systematic offset for absolute dose measurements showed a 2.5–4.5% overestimation of the response for 6 × 6 mm2 field sizes for LAICThick. For LAICThin the response varied even over a range of 20%. This study highlights the need for chamber-dependent response maps when using LAICs for absolute and relative dosimetry with proton pencil beams or small photon beams.

Error detection capability of a novel transmission detector: a validation study for online VMAT monitoring

The purpose of this study was to characterize a new single large-area ionization chamber, the integral quality monitor system (iRT, Germany), for online and real-time beam monitoring.Signal stability, monitor unit (MU) linearity and dose rate dependence were investigated for static and arc deliveries and compared to independent ionization chamber measurements. The dose verification capability of the transmission detector system was evaluated by comparing calculated and measured detector signals for 15 volumetric modulated arc therapy plans. The error detection sensitivity was tested by introducing MLC position and linac output errors. Deviations in dose distributions between the original and error-induced plans were compared in terms of detector signal deviation, dose-volume histogram (DVH) metrics and 2D γ-evaluation (2%/2 mm and 3%/3 mm).The detector signal is linearly dependent on linac output and shows negligible (<0.4%) dose rate dependence up to 460 MU min−1. Signal stability is within 1% for cumulative detector output; substantial variations were observed for the segment-by-segment signal.Calculated versus measured cumulative signal deviations ranged from −0.16%–2.25%. DVH, mean 2D γ-value and detector signal evaluations showed increasing deviations with regard to the respective reference with growing MLC and dose output errors; good correlation between DVH metrics and detector signal deviation was found (e.g. PTV Dmean: R2 = 0.97). Positional MLC errors of 1 mm and errors in linac output of 2% were identified with the transmission detector system.The extensive tests performed in this investigation show that the new transmission detector provides a stable and sensitive cumulative signal output and is suitable for beam monitoring during patient treatment.

A new approach in patient QA: Beam delivery check and in-vivo dosimetry during radiotherapy treatment

Introduction The development of complex radiotherapy techniques, together with the huge workload of linacs, requires comprehensive QA programs and the development of detectors and softwares allowing for reliable checks of the treatment delivery. Purpose The aim of this work was to study the impact in patient QA of the use of the Integral Quality Monitoring (IQM) device (iRTSystems) and the portal imaging with the software SoftDiso (Best Medical) to verify the safe and accurate delivery of the prescribed dose. Materials & methods The Anderson-Rando phantom was modified to mimic a female torso by adding two silicon gel breast implants. Standard conformal breast plans were calculated and the large area ionization chamber IQM was mounted on the accelerator gantry. PI acquired during treatments were used for in-vivo dose measurements using SoftDiso. Small errors induced on plans simulated deviation due to delivery problems and/or wrong positioning of the phantom. Results Tests showed good performances of the combined use of the devices to detect errors, which could not be highlighted during the pre-treatment QA. There’s correlation between two systems when errors due to small changes in delivered MU are induced. IQM can detect errors when field dimensions are changed of few millimetres while SoftDiso cannot, but is able to detect discrepancy on dose reconstruction with a precision of 5%. Conclusions In the clinical practice these devices can play an important role in meeting the needs of modern and upcoming radiotherapy QA, and it allows checking the correct functioning of all components in the radiotherapy chain.

A procedure to determine the planar integral spot dose values of proton pencil beam spots

Planar integral spot dose (PISD) of proton pencil beam spots (PPBSs) is a required input parameter for beam modeling in some treatment planning systems used in proton therapy clinics. The measurement of PISD by using commercially available large area ionization chambers, like the PTW Bragg peak chamber (BPC), can have large uncertainties due to the size limitation of these chambers. This paper reports the results of our study of a novel method to determine PISD values from the measured lateral dose profiles and peak dose of the PPBS. The PISDs of 72.5, 89.6, 146.9, 181.1, and 221.8 MeV energy PPBSs were determined by area integration of their planar dose distributions at different depths in water. The lateral relative dose profiles of the PPBSs at selected depths were measured by using small volume ion chambers and were investigated for their angular anisotropies using Kodak XV films. The peak spot dose along the beam's central axis (D(0)) was determined by placing a small volume ion chamber at the center of a broad field created by the superposition of spots at different locations. This method allows eliminating positioning uncertainties and the detector size effect that could occur when measuring it in single PPBS. The PISD was then calculated by integrating the measured lateral relative dose profiles for two different upper limits of integration and then multiplying it with corresponding D(0). The first limit of integration was set to radius of the BPC, namely 4.08 cm, giving PISD(RBPC). The second limit was set to a value of the radial distance where the profile dose falls below 0.1% of the peak giving the PISD(full). The calculated values of PISD(RBPC) obtained from area integration method were compared with the BPC measured values. Long tail dose correction factors (LTDCFs) were determined from the ratio of PISD(full)∕PISD(RBPC) at different depths for PPBSs of different energies. The spot profiles were found to have angular anisotropy. This anisotropy in PPBS dose distribution could be accounted in a reasonable approximate manner by taking the average of PISD values obtained using the in-line and cross-line profiles. The PISD(RBPC) values fall within 3.5% of those measured by BPC. Due to inherent dosimetry challenges associated with PPBS dosimetry, which can lead to large experimental uncertainties, such an agreement is considered to be satisfactory for validation purposes. The PISD(full) values show differences ranging from 1 to 11% from BPC measured values, which are mainly due to the size limitation of the BPC to account for the dose in the long tail regions of the spots extending beyond its 4.08 cm radius. The dose in long tail regions occur both for high energy beams such as 221.8 MeV PPBS due to the contributions of nuclear interactions products in the medium, and for low energy PPBS because of their larger spot sizes. The calculated LTDCF values agree within 1% with those determined by the Monte Carlo (MC) simulations. The area integration method to compute the PISD from PPBS lateral dose profiles is found to be useful both to determine the correction factors for the values measured by the BPC and to validate the results from MC simulations.

SU-DD-A1-06: Improving Patient Safety through a Simple Independent On-Line Beam Monitoring System

Purpose: To investigate the effectiveness of an independent on-line monitor for validation of beam delivery. Method and Materials: An on-line beam monitoring system consisting of a spatially sensitive large area ionization chamber mounted below the MLC, has been tested to validate treatment beams. The accuracy of a beam delivery is ensured by comparing the measured ion chamber signals with the pre calculated expected values. The system performance has been evaluated by phantom treatments of 20 patients' IMRT plans for various anatomic sites, delivered in multiple fractions over a period of six months. The performance of the system has been analyzed in terms of reproducibility, dosimetry and sensitivity. The sensitivity of the system has been evaluated by introducing a number of probable errors, including errors in MLC leaves and jaw positions. The uniqueness of signals per field, in terms of a temporal signature has been investigated by calculating a statistical index between IMRT fields taken from plans of same anatomic site. Results: The system response was reproducible to within 0.4%. The agreement between the calculation and measured signals for prostate IMRT field segments were within 1% on average, with a maximum deviation of 5%. The system response for a small geometrical segment changed by 2% for a leaf bank calibration error of 1 mm or a single leaf positioning error of 3 mm. Incorrect fields and incorrect use of wedge filters were easily identified. To date all the fields have been found to have unique temporal signature. Conclusion: An independent simple on-line beam monitoring system has been tested for validation of beam delivery. The results of a clinical simulation study demonstrated that the system is capable of validating daily beam delivery and capturing probable errors. The system could potentially improve quality and patient safety in radiation therapy

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

TU‐D‐M100F‐01: A Novel Quality Assurance Monitor for Real‐Time Verification of IMRT and IGART

Purpose: To develop an independent treatment verification method which can validate radiation field delivery in real‐time throughout the treatment course. The system is designed to capture common treatment delivery errors, and is intended to eliminate the need for pre‐treatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT) and enable the implementation of image guided adaptive radiation therapy. Method and Materials: A monitoring system, termed Integral Quality Monitor (IQM), has been developed that utilizes an area integrated energy fluence monitoring sensor (AIMS) positioned after the final beam shaping device (i.e. multileaf collimator (MLC)) and a signal prediction algorithm, IQM_Calc. The AIMS consists of a novel large area ionization chamber with a gradient oriented along the direction of the MLC motion. The measured signal from the AIMS can be compared in real‐time with the IQM_Calc predicted values. A prototype AIMS has been built with 2 mm thick Aluminum plates, an area of 22 cm × 22 cm and continuously varying electrode separation of 2 to 22 mm. The IQM_Calc uses a modified sector integration of MLC defined apertures and accounts for MLC characteristics such as: rounded leaf ends, transmission, and relative output factor. Testing of the IQM system was performed on Varian and Elekta linear accelerators. Results: Initial results for prostate IMRT fields show an average agreement of 2% between the measured IQM signals and the IQM_Calc results. For a 3 mm simulated MLC leaf positioning error, the signal of a prostate IMRT field changed by 2%. Conclusion: It is demonstrated that the prototype IQM system has the capability of verifying the accuracy of treatment delivery in real‐time. The system is also capable of capturing common treatment errors. The IQM system has the potential of playing an important role in the challenging QA environment of modern radiation therapy.

The dose-area product, a new parameter for the dosimetry of narrow photon beams

In the dosimetry of narrow photon fields with side lengths of the order of 1 cm, the traditional parametrisation via the absolute dose on the beam axis and the relative lateral dose distribution has to deal with the difficulty to find sufficiently small detectors and to adjust them accurately on the narrow-beam axis. This can be avoided by reconsidering the parametrisation, using as normalization factor the surface integral of the dose in the plane perpendicular to the beam axis, abbreviated as the “dose-area product” (DAP). We investigated and confirmed the ability of a large-area parallel-plate ionisation chamber, with a sensitive volume shaped as a flat cylinder of 81.6 mm diameter and 2 mm thickness, to perform the integration over the full lateral dose profile of narrow photon beams with side lengths up to 5 cm. The lateral adjustment of this large-area detector relative to a narrow photon beam is not critical. The large-area ionisation chamber was calibrated in terms of the DAP by reference to a 0.3 cm3 ionisation chamber. A field-size dependent “modified output factor” was defined as the ratio of the DAP, measured at 5 cm phantom depth for 100 cm SSD, and the monitor reading. A prominent phenomenon of narrow photon fields is the field-size and source-distance independence of the relative axial profile of the DAP as function of the thickness of a pre-absorber or of the depth in a phantom. For narrow-beam treatment planning in IMRT, the DAP is combined with the energy- and field size-dependent relative lateral dose distribution which is represented, for example, by a Gaussian convolution kernel. Another useful feature of the DAP is the possibility of its direct control during patient irradiation by means of an on-line monitor with spatial resolution, arranged in the accessory holder.

A large-area ionization chamber for portal image calibration

Methods of removing the effects of linear accelerator (linac) output fluctuation from electronic portal images are described and compared. The output of the linac is measured using a specially constructed large-area ionization chamber during imaging and recorded with the image. The use of a dose-rate signal directly from the linac monitor chamber is discussed. Various versions of a quadratic thickness calibration scheme are tested, incorporating linac output data measured by the ionization chamber. Experimental results are presented showing that the incorporation of data from the ionization chamber gives improved absolute calibration accuracy and flatness. Immediately after calibration, the mean systematic thickness error in calibration of a uniform 136.8 mm water-equivalent slab was shown to be no more than 0.6 mm with a thickness variation within each image also of no more than mm. This was true even when imaging with an unstable linac beam giving mean thickness errors between images of 8.8 mm and variations within each image of mm without the ionization chamber correction. Up to one month after calibration, use of the ionization chamber to remove short-term linac fluctuations is shown to still keep mean thickness errors to less than 1.6 mm with variations within each image of no more than mm.

Radiation protection in interventional radiology.

Introduction Furthermore, this information is usually displayed at the operator’s console where it cannot be seen In recent years there has been an increase in by the interventionalist undertaking the examinthe number, type and complexity of interventional ation. Unfortunately, total elapsed fluoroscopy radiology procedures. The impetus behind this time does not correlate very well with maximum rapid and continuing expansion has been the desire skin entrance dose. The latter quantity being for improved, more cost-effective medicine, in dependent on the automatic dose-rate control setwhich the patient can look forward to an improved ting selected, patient’s size, focus–skin distance and prognosis. Often patients having interventional the period of time that the area of skin was radiology procedures are treated as either outirradiated. Equipment developments are required patients or day cases, whereas the alternative surgito enable the interventionalist to be provided with cal technique would require hospitalization. This an on-line display which provides a better indiexpansion in interventional radiology has occurred cation of the potential onset of deterministic effects. in many countries worldwide, irrespective of the Extended fluoroscopy times, sometimes coupled type of system for health care delivery, because of with higher than average fluoroscopy currents can the many benefits. lead to an increased risk of non-deterministic In the main, interventional procedures are characeffects such as leukaemia in which the probability terized by having extended fluoroscopy times, and of the effect is proportional to the dose. The risk sometimes requiring many radiographic images to of inducing a hypothetical cancer at some time in be taken. As a consequence, developments in interthe future from interventional radiology correlates ventional radiology have a number of profound reasonably well with dose–area product. radiation protection implications for both patients Dose–area product is a quantity which may be and staff. In particular, the application of the conmeasured using a large area ionization chamber cept of justification to these procedures is different placed at the output port of the X-ray tube or than to other radiological examinations, as both inferred from a knowledge of X-ray technique deterministic and non-deterministic effects on the factors and the field size. Measurement of dose– patient have to be considered. area product is also recommended in the National Patient Dosimetry Protocol [1]. Patient dose The maximum skin entrance dose can be deduced from the dose–area product if the field For the patient, long fluoroscopy times lead to size and focus–skin distance are known. higher radiation doses, sometimes limited to a Instrumentation that is able to accomplish this is small area of the patient’s skin surface. This in in an early developmental stage. The display of turn leads to the potential for deterministic effects, total dose–area product and the estimated maxisuch as skin erythema, to be seen in patients who mum skin entrance dose on the interventionalist’s have had interventional radiology procedures. television monitor would provide information Physicians treating patients who have had an about the two types of radiation effect. interventional radiology procedure need to be It is also important to minimize patient doses aware of this possibility. whenever possible. One method of trying to achieve The potential induction of deterministic effects patient dose reductions would be to provide the in the skin is compounded by the lack of approinterventionalist with online dose rate information. priate patient dosimetry information available to The interventionalist could then select techniques the interventionalist during the procedure. At prewhich resulted in a lower patient dose, if desired. sent, fluoroscopy units used for interventional radiology are only required to be fitted with an Staff dose indication of total elapsed fluoroscopy time.

Charge-changing nuclear reactions of relativistic light-ion beams (5 ≤ Z ≤ 10) passing through thick absorbers

Light-ion beams of 20Ne, 19Ne, 18F, 16O, 15O, 14N, 12C and 10B in the energy range between 200 and 670 MeV/u were fragmented in thick targets of water, carbon, lucite, polyethylene, and aluminum. The nuclear charge composition of the fragmented beam was measured via energy loss in a large-area ionisation chamber. Using a water absorber of variable thickness up to 25 cm the elemental fragment yields were measured down to Z = 5. Their build-up and decay characteristics are described by a system of differential equations. From an analysis of the depth distributions of the surviving projectiles and the lower- Z projectile fragments, both total and partial charge-changing cross sections were obtained.

An International Intercomparison of Dose-Area Product Meters

There has been an increase in the complexity of clinical examinations and treatments using fluoroscopy; consequently, it is important to measure radiation dose levels. A large area ionisation chamber linked to an electrometer is particularly suited to these measurements in fluoroscopy in which the technique factors, field size and projection direction change throughout the examination. An international study into radiation doses in fluoroscopy and associated digital imaging techniques is being undertaken using, in the main, dose-area product meters. A number of dose-area product meters have been compared against a dosemeter with a calibration traceable to a national standards laboratory. In some instances the dose-area product meters were also compared against a dosemeter used in the extended intercomparison of dosemeters used for quality control in diagnostic radiology.

Strahlenschutzsituation und Qualitätssicherung bei der Knochendensitometrie mit Röntgenstrahlung

During a relatively short period the use of radiological bone densitometers has been increasingly introduced into clinical routine. Patient entrance dose measured with a large area ionization chamber was found to be small compared with other radiological examinations. E.g. for standard examinations of lumbar spine and femur dose equivalents of 2.9 μSv in the incident plane and effective dose equivalents of 0.09 μSv respectively 0.05 μSv have been obtained. Accordingly, the delineation of a controlled area around the densitometer was not necessary. Essential instruments of quality assurance are test phantoms and radiographic films for the verification of constant operation of the unit.

Nuclear fragmentation in 35Cl-induced reactions at 11 MeV/u bombarding energy

The reactions 35Cl + 58Ni, 40Ca, 27Al are studied at 385 MeV bombarding energy. The experiment employs 6 large-area ionization chambers which are set up to detect fragments with nuclear charge Z ≧ 4 in coincidence multiplicity m ≦ 6. The highest multiplicity observed is m = 4; a detailed analysis of the data is performed for m = 2 and m = 3 events. The 58Ni data contain reactions with 2 fragments ( σ = 1350±500 mb) and 3 fragmentes ( σ = 175±90 mb) in the exit channel. The three fragments appear to have their origin in two different processes: a sequential-fragmentation process and a fast break-up process. A similar conclusion is reached for the 27Al data, whereas in the 40Ca data only the sequential fragmentation could be identified.

Light-particle emission in the reaction 32S + 27A1 AT 135 and 190 MeV

The properties oflight particles emitted by the 32S + 27Al reaction at 135and 190 MeV bombarding energies were studied by means of a coincidence spectrometer. The spectrometer consisted of two large-area ionization chambers which measured the energy, momentum, mass and nuclear charge of the heavy reaction products. By requiring the conservation of those quantities the energy, momentum, mass and charge deficits were determined which are representative of the unobserved light particles. The analysis of the momentum deficit in the event plane did not yield an indication for a fast, direct process of light-particle emission. The alternative analysis in the fragments rest system confirmed the statistical nature of the emission process. The out-of-plane angular correlations were used to determine the spins of the particle-emitting fragments.

Correlations between slow and fast fragments in relativistic nuclear collisions

Heavy low-energy fragments with Z > or = 4 have been measured in relativistic nuclear collisions, using a large area ionization chamber telescope. For all events, correlated slow fragments were looked for in five solid state counters and correlated fragments (E>25 MeV/u) were measured in 80 scintillators coupled to photomultipliers. Fragment spectra were taken in the energy range of 5--150 MeV E/sub tot/. p/sub parallel/ was extracted for fissionlike events. Other nonbinary fragments showed an in-plane correlation with fast charged particles. Lighter particles (4 < or = Z < or = 12) were found to be associated with a high charged-particle multiplicity. Conflicts with previous views on high-energy proton-nucleus data are pointed out and a qualitative comparison to hydrodynamical effects is tried.