Intercomparison of methods for image quality characterization. I. Modulation transfer function

The modulation transfer function (MTF) and the noise power spectrum (NPS) are widely recognized as the most relevant metrics of resolution and noise performance in radiographic imaging. These quantities have commonly been measured using various techniques, the specifics of which can have a bearing on the accuracy of the results. As a part of a study aimed at comparing the relative performance of different techniques, in this paper we report on a comparison of two established MTF measurement techniques: one using a slit test device [Dobbins et al., Med. Phys. 22, 1581-1593 (1995)] and another using a translucent edge test device [Samei et al., Med. Phys. 25, 102-113 (1998)], with one another and with a third technique using an opaque edge test device recommended by a new international standard (IEC 62220-1, 2003). The study further aimed to substantiate the influence of various acquisition and processing parameters on the estimated MTF. The slit test device was made of 2 mm thick Pb slabs with a 12.5 microm opening. The translucent edge test device was made of a laminated and polished Pt(0.9)Ir(0.1). alloy foil of 0.1 mm thickness. The opaque edge test device was made of a 2 mm thick W slab. All test devices were imaged on a representative indirect flat-panel digital radiographic system using three published beam qualities: 70 kV with 0.5 mm Cu filtration, 70 kV with 19 mm Al filtration, and 74 kV with 21 mm Al filtration (IEC-RQA5). The latter technique was also evaluated in conjunction with two external beam-limiting apertures (per IEC 62220-1), and with the tube collimator limiting the beam to the same area achieved with the apertures. The presampled MTFs were deduced from the acquired images by Fourier analysis techniques, and the results analyzed for relative values and the influence of impacting parameters. The findings indicated that the measurement technique has a notable impact on the resulting MTF estimate, with estimates from the overall IEC method 4.0% +/- 0.2% lower than that of Dobbins et al. and 0.7% +/- 0.4% higher than that of Samei et al. averaged over the zero to cutoff frequency range. Over the same frequency range, keeping beam quality and limitation constant, the average MTF estimate obtained with the edge techniques differed by up to 5.2% +/- 0.2% from that of the slit, with the opaque edge providing lower MTF estimates at lower frequencies than those obtained with the translucent edge or slit. The beam quality impacted the average estimated MTF by as much as 3.7% +/- 0.9% while the use of beam limiting devices alone increased the average estimated MTF by as much as 7.0% +/- 0.9%. While the slit method is inherently very sensitive to misalignment, both edge techniques were found to tolerate misalignments by as much as 6 cm. The results suggest the use of the opaque edge test device and the tube internal collimator for beam limitation in order to achieve an MTF result most reflective of the overall performance of the imaging system and least susceptible to misalignment and scattered radiation. Careful attention to influencing factors is warranted to achieve accurate results.     

Influence of cassette type on the DQE of CR systems

In our recent paper by Monnin et al. [Med. Phys. 33, 411-420 (2006)], an objective analysis of the relative performance of a computed radiography (CR) system using both standard single-side (ST-VI) and prototype dual-side read (ST-BD) plates was reported. The presampled modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) for the systems were determined at three different beam qualities representative of paediatric chest radiography, at an entrance detector air kerma of 5 microGy. Experiments demonstrated that, compared to the standard single-side read system, the MTF for the dual-side read system was slightly reduced, but a significant decrease in image noise resulted in a marked increase in DQE (+40%) in the low spatial frequency range. However, the DQE improvement for the ST-BD plate decreased with increasing spatial frequency, and, at spatial frequencies above 2.2 mm(-1), the DQE of the dual-side read system was lower than that of the single-side one.     

A systematic review of the precision and accuracy of dose measurements in photon radiotherapy using polymer and Fricke MRI gel dosimetry

The purpose of this work is to undertake a critical appraisal of the evidence in the published literature concerning the basic parameters of accuracy and precision associated with the use of Fricke and polymer gels (in conjunction with MR imaging) as radiation dosimeters in photon radiotherapy, condensing and analysing the body of published information (to the end of April 2002). A systematic review was undertaken addressing specific issues of precision and accuracy asking defined questions of the published literature. Accuracy and precision in relation to gel dosimetry were defined. Information was obtained from published, peer-reviewed journals. A defined search strategy utilizing MeSH headings and keywords, with extensive use of cross-referencing, identified 115 references dealing with gel dosimetry. Exclusion criteria were used to select only data from publications which would give unequivocal evidence. For accuracy, results had to be compared with an ionization chamber as gold standard and all gel samples had to be manufactured in the same batch. For precision, in addition to gels being from the same batch, samples must all have been irradiated at the same time and scanned simultaneously (or within a short time frame). Many results were found demonstrating 'dose mapping' examples using gels. However, there were very few publications containing firm evidence of precision and accuracy. There was no evidence which fulfilled our criteria about accuracy or precision using Fricke gels. For polymer gels only one paper was found for accuracy (4% (Low et al 1999 Med. Phys. 26 1542-51)) and precision (1.7% (Baldock et al 1998 Phys. Med. Biol. 43695-702)); however, both were carried out at only one dose level. If the exclusion criteria were relaxed to include accuracy results comparing gel to a non gold standard dosimeter (e.g. TLD), results give a median accuracy of 10% (range 8-23.5%) for polymer gel (Cosgrove et al 2000 Phys. Med. Biol. 45 1195-210, De Deene et al 1998 Radiother: Oncol. 48 283-91, Farajollahi et al 2000 Br. J. Radiol. 72 1085-92, McJury et al 1999b Phys. Med. Biol. 44 2431-44, Murphy et al 2000b Phys. Med. Biol. 45 835-45, Oldham et al 2001 Med. Phys. 28 1436-45) and 5% for Fricke gel (Chan and Ayyangar 1995b Med. Phys. 22 1171-5). Evidence also points to accuracy worsening at lower dose levels for both gels. The precision data should be viewed with caution as repeated MR measurements were not performed with the same samples. The only precision data for Fricke gels was 1.5% (Johansson Back et al 1998 Phys. Med. Biol. 43 261-76), but for zero dose. In conclusion, despite the amount of published data, sparse research has been undertaken which provides clear evidence of the accuracy and precision for both gels. That which has been published has used higher doses than would be routine in radiotherapy. The basic radiation dosimeter qualities of accuracy and precision have yet to be fully quantified for polymer and Fricke gels at clinically relevant dose levels.     

The effect of intrinsic attenuation correction methods on the stationarity of the 3-D modulation transfer function of SPECT.

The application of stationary restoration techniques to SPECT images assumes that the modulation transfer function (MTF) of the imaging system is shift invariant. It was hypothesized that using intrinsic attenuation correction (i.e., methods which explicitly invert the exponential radon transform) would yield a three-dimensional (3-D) MTF which varies less with position within the transverse slices than the combined conjugate view two-dimensional (2-D) MTF varies with depth. Thus the assumption of shift invariance would become less of an approximation for 3-D post- than for 2-D pre-reconstruction restoration filtering. SPECT acquisitions were obtained from point sources located at various positions in three differently shaped, water-filled phantoms. The data were reconstructed with intrinsic attenuation correction, and 3-D MTFs were calculated. Four different intrinsic attenuation correction methods were compared: (1) exponentially weighted backprojection, (2) a modified exponentially weighted backprojection as described by Tanaka et al. [Phys. Med. Biol. 29, 1489-1500 (1984)], (3) a Fourier domain technique as described by Bellini et al. [IEEE Trans. ASSP 27, 213-218 (1979)], and (4) the circular harmonic transform (CHT) method as described by Hawkins et al. [IEEE Trans. Med. Imag. 7, 135-148 (1988)]. The dependence of the 3-D MTF obtained with these methods, on point source location within an attenuator, and on shape of the attenuator, was studied. These 3-D MTFs were compared to: (1) those MTFs obtained with no attenuation correction, and (2) the depth dependence of the arithmetic mean combined conjugate view 2-D MTFs.(ABSTRACT TRUNCATED AT 250 WORDS)     

An experimental comparison of detector performance for direct and indirect digital radiography systems

Current flat-panel detectors either directly convert x-ray energy to electronic charge or use indirect conversion with an intermediate optical process. The purpose of this work was to compare direct and indirect detectors in terms of their modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Measurements were made on three flat-panel detectors, Hologic Direct-Ray DR-1000 (DRC), GE Revolution XQ/i (XQ/i), and Philips Digital Diagnost (DiDi) using the IEC-defined RQA5 (approximately 74 kVp, 21 mm Al) and RQA9 (approximately 120 kVp, 40 mm Al) radiographic techniques. The presampled MTFs of the systems were measured using an edge method [Samei et al., Med. Phys. 25, 102 (1998)]. The NPS of the systems were determined for a range of exposure levels by two-dimensional (2D) Fourier analysis of uniformly exposed radiographs [Flynn and Samei, Med. Phys. 26, 1612 (1999)]. The DQEs were assessed from the measured MTF, NPS, exposure, and estimated ideal signal-to-noise ratios. For the direct system, the MTF was found to be significantly higher than that for the indirect systems and very close to an ideal function associated with the detector pixel size. The NPS for the direct system was found to be constant in relation to frequency. For the XQ/i and DRC systems, the DQE results reflected expected differences based on the absorption efficiency of the different detector materials. Using RQA5, the measured DQE values in the diagonal (and axial) direction(s) at spatial frequencies of 0.15 mm(-1) and 2.5 mm(-1) were 64% (64%) and 20% (15%) for the XQ/i system, and 38% (38%) and 20% (20%) for the DRC, respectively. The DQE results of the DiDi system were difficult to interpret due to additional preprocessing steps in that system.     

Ir-192 HDR transit dose and radial dose function determination using alanine/EPR dosimetry

Source positioning close to the tumour in high dose rate (HDR) brachytherapy is not instantaneous. An increment of dose will be delivered during the movement of the source in the trajectory to its static position. This increment is the transit dose, often not taken into account in brachytherapeutic treatment planning. The transit dose depends on the prescribed dose, number of treatment fractions, velocity and activity of the source. Combining all these factors, the transit dose can be 5% higher than the prescribed absorbed dose value (Sang-Hyun and Muller-Runkel, 1994 Phys. Med. Biol. 39 1181-8, Nath et al 1995 Med. Phys. 22 209-34). However, it cannot exceed this percentage (Nath et al 1995). In this work, we use the alanine-EPR (electron paramagnetic resonance) dosimetric system using analysis of the first derivative of the signal. The transit dose was evaluated for an HDR system and is consistent with that already presented for TLD dosimeters (Bastin et al 1993 Int. J. Radiat. Oncol. Biol. Phys. 26 695-702). Also using the same dosimetric system, the radial dose function, used to evaluate the geometric dose degradation around the source, was determined and its behaviour agrees better with those obtained by Monte Carlo simulations (Nath et al 1995, Williamson and Nath 1991 Med. Phys. 18 434-48, Ballester et al 1997 Med. Phys. 24 1221-8, Ballester et al 2001 Phys. Med. Biol. 46 N79-90) than with TLD measurements (Nath et al 1990 Med. Phys. 17 1032-40).     

An EPID response calculation algorithm using spatial beam characteristics of primary, head scattered and MLC transmitted radiation

We have developed an independent algorithm for the prediction of electronic portal imaging device (EPID) response. The algorithm uses a set of images [open beam, closed multileaf collimator (MLC), various fence and modified sweeping gap patterns] to separately characterize the primary and head-scatter contributions to EPID response. It also characterizes the relevant dosimetric properties of the MLC: Transmission, dosimetric gap, MLC scatter [P. Zygmansky et al., J. Appl. Clin. Med. Phys. 8(4) (2007)], inter-leaf leakage, and tongue and groove [F. Lorenz et al., Phys. Med. Biol. 52, 5985-5999 (2007)]. The primary radiation is modeled with a single Gaussian distribution defined at the target position, while the head-scatter radiation is modeled with a triple Gaussian distribution defined downstream of the target. The distances between the target and the head-scatter source, jaws, and MLC are model parameters. The scatter associated with the EPID is implicit in the model. Open beam images are predicted to within 1% of the maximum value across the image. Other MLC test patterns and intensity-modulated radiation therapy fluences are predicted to within 1.5% of the maximum value. The presented method was applied to the Varian aS500 EPID but is designed to work with any planar detector with sufficient spatial resolution.     

A solution to the long-object problem in helical cone-beam tomography

This paper presents a new algorithm for the long-object problem in helical cone-beam (CB) computerized tomography (CT). This problem consists in reconstructing a region-of-interest (ROI) bounded by two given transaxial slices, using axially truncated CB projections corresponding to a helix segment long enough to cover the ROI, but not long enough to cover the whole axial extent of the object. The new algorithm is based on a previously published method, referred to as CB-FBP (Kudo et al 1998 Phys. Med. Biol. 43 2885-909), which is suitable for quasi-exact reconstruction when the helix extends well beyond the support of the object. We first show that the CB-FBP algorithm simplifies dramatically, and furthermore constitutes a solution to the long-object problem, when the object under study has line integrals which vanish along all PI-lines. (A PI line is a line which connects two points of the helix separated by less than one pitch.) Exploiting a geometric property of the helix, we then show how the image can be expressed as the sum of two images, where the first image can be reconstructed from the measured CB projections by a simple backprojection procedure, and the second image has zero PI-line integrals and hence can be reconstructed using the simplified CB-FBP algorithm. The resulting method is a quasi-exact solution to the long-object problem, called the ZB method. We present its implementation and illustrate its performance using simulated CB data of the 3D Shepp phantom and of a more challenging head-like phantom.     

Multiple template-based fluoroscopic tracking of lung tumor mass without implanted fiducial markers

Precise lung tumor localization in real time is particularly important for some motion management techniques, such as respiratory gating or beam tracking with a dynamic multi-leaf collimator, due to the reduced clinical tumor volume (CTV) to planning target volume (PTV) margin and/or the escalated dose. There might be large uncertainties in deriving tumor position from external respiratory surrogates. While tracking implanted fiducial markers has sufficient accuracy, this procedure may not be widely accepted due to the risk of pneumothorax. Previously, we have developed a technique to generate gating signals from fluoroscopic images without implanted fiducial markers using a template matching method (Berbeco et al 2005 Phys. Med. Biol. 50 4481-90, Cui et al 2007 Phys. Med. Biol. 52 741-55). In this paper, we present an extension of this method to multiple-template matching for directly tracking the lung tumor mass in fluoroscopy video. The basic idea is as follows: (i) during the patient setup session, a pair of orthogonal fluoroscopic image sequences are taken and processed off-line to generate a set of reference templates that correspond to different breathing phases and tumor positions; (ii) during treatment delivery, fluoroscopic images are continuously acquired and processed; (iii) the similarity between each reference template and the processed incoming image is calculated; (iv) the tumor position in the incoming image is then estimated by combining the tumor centroid coordinates in reference templates with proper weights based on the measured similarities. With different handling of image processing and similarity calculation, two such multiple-template tracking techniques have been developed: one based on motion-enhanced templates and Pearson's correlation score while the other based on eigen templates and mean-squared error. The developed techniques have been tested on six sequences of fluoroscopic images from six lung cancer patients against the reference tumor positions manually determined by a radiation oncologist. The tumor centroid coordinates automatically detected using both methods agree well with the manually marked reference locations. The eigenspace tracking method performs slightly better than the motion-enhanced method, with average localization errors less than 2 pixels (1 mm) and the error at a 95% confidence level of about 2-4 pixels (1-2 mm). This work demonstrates the feasibility of direct tracking of a lung tumor mass in fluoroscopic images without implanted fiducial markers using multiple reference templates.     

Characteristic features of a high-energy x-ray spectra estimation method based on the Waggener iterative perturbation principle

We have redeveloped a high-energy x-ray spectra estimation method reported by Iwasaki et al. [A. Iwasaki, H. Matsutani, M. Kubota, A. Fujimori, K. Suzaki, and Y. Abe, Radiat. Phys. Chem. 67, 81-91 (2003)]. The method is based on the iterative perturbation principle to minimize differences between measured and calculated transmission curves, originally proposed by Waggener et al. [R. G. Waggener, M. M. Blough, J. A. Terry, D. Chen, N. E. Lee, S. Zhang, and W. D. McDavid, Med. Phys. 26, 1269-1278 (1999)]. The method can estimate spectra applicable for media at least from water to lead using only about ten energy bins. Estimating spectra of 4-15 MV x-ray beams from a linear accelerator, we describe characteristic features of the method with regard to parameters including the prespectrum, number of transmission measurements, number of energy bins, energy bin widths, and artifactual bipeaked spectrum production.     

Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the MED3631-A/M125I source

Brachytherapy dosimetry parameters for MED3631-A/M 125I sources have been determined in accordance with the AAPM Task Group No. 43 (TG-43) dosimetry protocol. These data were calculated using the *F8 tally from the MCNP4B2 Monte Carlo radiation transport code with the DLC-189 cross-section libraries. Due to motion of the 125I resin beads and gold-copper markers within the capsule, parameters such as the geometry function, radial dose function, dose rate constant, and anisotropy function were examined with the beads and markers having either "realistic" or "ideal" positions; the realistic position was a weighted combination of "vertical" and "diagonal" capsule orientations. The dose rate constants for the realistic and ideal geometries, lambda99std(realistic) and lambda99std(ideal) were 1.066 and 1.067 cGy h(-1) U(-1), respectively, which were within uncertainties of measured values by Wallace and Fan [Med. Phys. 26, 1925-1931 (1999)] and Li et al. [Med. Phys. 27, 1275-1280 (2000)], 1.06 and 1.067 cGy h(-1) U(-1), respectively. The calculated reference dose rate at r0= 1 and theta0= 90 degrees for the realistic source geometry was 0.7% less than for the ideal source geometry. The anisotropy constants, phian(realistic), for the realistic and ideal geometries were 0.948 and 0.965, respectively. phian(realistic) matched that (0.941) measured by Wallace and Fan, and phian(ideal) was significantly different from that (0.948) calculated by Wierzbicki et al. [Med. Phys. 25, 2197-2199 (1998)] for an ideal MED3631-A/S 125I source.     

Calculation of x-ray spectra emerging from an x-ray tube. Part II. X-ray production and filtration in x-ray targets

A new approach to the calculation of the x-ray spectrum emerging from an x-ray tube is proposed. Theoretical results for the bremsstrahlung cross section appearing in the literature are summarized. Four different treatments of electron penetration, based on the work presented in Part I, are then used to generate bremsstrahlung spectra. These spectra are compared to experimental data at 50, 80 and 100 kVp tube potentials. The most sophisticated treatment of electron penetration was required to obtain good agreement. With this treatment both the National Institute of Standards and Technology bremsstrahlung cross sections, based on accurate partial wave calculations, and the Bethe-Heitler cross section [H. A. Bethe and W. Heitler, Proc R. Soc. London, Ser. A. 146, 83-112 (1934)] corrected by a modified Elwert factor [G. Elwert, Ann. Phys. (Leipzig) 426, 178-208 (1939)], provided good agreement to measured data. An approximate treatment of the characteristic spectrum is suggested. The dependencies of the bremsstrahlung and characteristic outputs of an x-ray tube on tube potential are compared to experimentally derived data for 70-140 kVp potentials. Agreement is to within a few percent of the total output over the entire range. The spectral predictions of the semiempirical models of Birch and Marshall [R. Birch and M. Marshall, Phys. Med. Biol. 24, 505-513 (1979)] (IPEM Report 78) and of Tucker et al. [D. M. Tucker, G. T. Barnes, and D. P. Chakraborty, Med. Phys. 18, 211-218 (1991).] are also assessed. The predictions of Tucker et al. are very close to the model developed here. The predictions of IPEM Report 78 are similar, but consistently harder for the range of tube potentials examined (50-100 kV). Unlike the semiempirical models, the model proposed here requires the introduction of no empirical and unphysical parameters in the differential bremsstrahlung cross section, bar an overall normalization factor which is close to unity.     

A simple derivation and analysis of a helical cone beam tomographic algorithm for long object imaging via a novel definition of region of interest

We derive and analyse a simple algorithm first proposed by Kudo et al (2001 Proc. 2001 Meeting on Fully 3D Image Reconstruction in Radiology and Nuclear Medicine (Pacific Grove, CA) pp 7-10) for long object imaging from truncated helical cone beam data via a novel definition of region of interest (ROI). Our approach is based on the theory of short object imaging by Kudo et al (1998 Phys. Med. Biol. 43 2885-909). One of the key findings in their work is that filtering of the truncated projection can be divided into two parts: one, finite in the axial direction, results from ramp filtering the data within the Tam window. The other, infinite in the z direction, results from unbounded filtering of ray sums over PI lines only. We show that for an ROI defined by PI lines emanating from the initial and final source positions on a helical segment, the boundary data which would otherwise contaminate the reconstruction of the ROI can be completely excluded. This novel definition of the ROI leads to a simple algorithm for long object imaging. The overscan of the algorithm is analytically calculated and it is the same as that of the zero boundary method. The reconstructed ROI can be divided into two regions: one is minimally contaminated by the portion outside the ROI, while the other is reconstructed free of contamination. We validate the algorithm with a 3D Shepp-Logan phantom and a disc phantom.     

An improved MLC segmentation algorithm and software for step-and-shoot IMRT delivery without tongue-and-groove error

We present an improved multileaf collimator (MLC) segmentation algorithm, denoted by SLS(NOTG) (static leaf sequencing with no tongue-and-groove error), for step-and-shoot intensity-modulated radiation therapy (IMRT) delivery. SLS(NOTG) is an improvement over the MLC segmentation algorithm called SLS that was developed by Luan et al. [Med. Phys. 31(4), 695-707 (2004)], which did not consider tongue-and-groove error corrections. The aims of SLS(NOTG) are (1) shortening the treatment times of IMRT plans by minimizing their numbers of segments and (2) minimizing the tongue-and-groove errors of the computed IMRT plans. The input to SLS(NOTG) is intensity maps (IMs) produced by current planning systems, and its output is (modified) optimized leaf sequences without tongue-and-groove error. Like the previous SLS algorithm [Luan et al., Med. Phys. 31(4), 695-707 (2004)], SLS(NOTG) is also based on graph algorithmic techniques in computer science. It models the MLC segmentation problem as a weighted minimum-cost path problem, where the weight of the path is the number of segments and the cost of the path is the amount of tongue-and-groove error. Our comparisons of SLS(NOTG) with CORVUS indicated that for the same intensity maps, the numbers of segments computed by SLS(NOTG) are up to 50% less than those by CORVUS 5.0 on the Elekta LINAC system. Our clinical verifications have shown that the dose distributions of the SLS(NOTG) plans do not have tongue-and-groove error and match those of the corresponding CORVUS plans, thus confirming the correctness of SLS(NOTG). Comparing with existing segmentation methods, SLS(NOTG) also has two additional advantages: (1) SLS(NOTG) can compute leaf sequences whose tongue-and-groove error is minimized subject to a constraint on the maximum allowed number of segments, which may be desirable in clinical situations where a treatment with the complete correction of tongue-and-groove error takes too much time, and (2) SLS(NOTG) can be used to minimize a more general type of error called the tongue-or-groove error.     

On the assay of brachytherapy sources

In many of brachytherapy procedures, a large amount of radioactive sources are used to deliver desired doses to the target volume. It is both the federal regulation recommendation (U.S. Nuclear Regulatory Commission, 10 CFR 35.432) and recommendations of the American Association of Physicists in Medicine (AAPM) [Kutcher et al., Med. Phys. 21, 581-618 (1994); Nath et al., Med. Phys. 24, 1557-1598 (1997)] that users independently verify the sources' strength. Though the reports of AAPM Task Group 40 [Kutcher et al., Med. Phys. 21, 581-618 (1994)] and 56 [Nath et al., Med. Phys. 24, 1557-1598 (1997)] have made specific recommendations on the assay of brachytherapy sources, the relevant statistical significance of the recommendations remain unanswered. In this study, statistical theories were used and a method was presented to quantify the assay process of brachytherapy sources and to evaluate the recommendations. The results showed that the quality of a source assay process was dependent on the measured source strength deviation and number of assayed sources. Its dependence on the total number of sources becomes statistically insignificant if the total number is large enough. It was concluded that the assay process can be determined by the obtained assay information, instead of a preset percentage of total sources. It was further found that the use of manufacturer's stated strength value may possibly lead to bigger uncertainty in source strength accuracy, unless the manufacturer's stated strength is the measured mean value of all the ordered sources.     

Dosimetry for 252Cf neutron emitting brachytherapy sources: protocol, measurements, and calculations.

The mixed-field dosimetry for 252Cf Applicator Tube (AT) type medical sources available from Oak Ridge National Laboratory (ORNL) has been characterized using ionization chambers, a GM counter, and Monte Carlo methods. Unlike the AAPM Task Group No. 43 (TG-43), specification of dose to muscle instead of water is recommended for clinical dosimetry of 252Cf medical sources. A dosimetry protocol similar to ICRU 45 was formulated with parameters determined specifically for 252Cf brachytherapy. Comparisons of experimental and calculative dosimetry results with Colvett et al. [Phys. Med. Biol. 17, 356-364 (1972)] and Krishnaswamy [Phys. Med. Biol. 17, 56-63 (1972)] were performed, and correction factors were determined to compare the different dosimetry formalisms. Using a Maxwellian model for the 252Cf neutron energy spectrum, kerma relative to muscle was determined for a variety of materials, and compared with relative kermas for external neutron beams of three different energies. Neutron isodose distributions and data necessary for clinical implementation of 252Cf AT sources are also presented.     

Calculations of dose distributions using a neural network model

The main goal of external beam radiotherapy is the treatment of tumours, while sparing, as much as possible, surrounding healthy tissues. In order to master and optimize the dose distribution within the patient, dosimetric planning has to be carried out. Thus, for determining the most accurate dose distribution during treatment planning, a compromise must be found between the precision and the speed of calculation. Current techniques, using analytic methods, models and databases, are rapid but lack precision. Enhanced precision can be achieved by using calculation codes based, for example, on Monte Carlo methods. However, in spite of all efforts to optimize speed (methods and computer improvements), Monte Carlo based methods remain painfully slow. A newer way to handle all of these problems is to use a new approach in dosimetric calculation by employing neural networks. Neural networks (Wu and Zhu 2000 Phys. Med. Biol. 45 913-22) provide the advantages of those various approaches while avoiding their main inconveniences, i.e., time-consumption calculations. This permits us to obtain quick and accurate results during clinical treatment planning. Currently, results obtained for a single depth-dose calculation using a Monte Carlo based code (such as BEAM (Rogers et al 2003 NRCC Report PIRS-0509(A) rev G)) require hours of computing. By contrast, the practical use of neural networks (Mathieu et al 2003 Proceedings Journees Scientifiques Francophones, SFRP) provides almost instant results and quite low errors (less than 2%) for a two-dimensional dosimetric map.     

Experimental validation of a versatile system of CT dosimetry using a conventional ion chamber: beyond CTDI100

This article is an experimental demonstration and authentication of a new method of computed tomography dosimetry [R. L. Dixon, Med. Phys. 30, 1272-1280 (2003)], which utilizes a short, conventional ion chamber rather than a pencil chamber, and which is more versatile than the latter. The value of CTDI100 correctly predicts the accumulated dose only for a total scan length L equal to 100 mm and underestimates the limiting equilibrium dose approached for longer, clinically relevant body scan lengths [R. L. Dixon, Med. Phys. 30, 1272-1280 (2003); K. D. Nakonechny, B. G. Fallone, and S. Rathee, Med. Phys. 32, 98-109 (2005); S. Mori, M. Endo, K. Nishizawa, T. Tsunoo, T. Aoyama, H. Fujiwara, and K. Murase, Med. Phys. 32, 1061-1069 (2005); R. L. Dixon, M. T. Munley, and E. Bayram, Med. Phys. 32, 3712-3728 (2005); R. L. Dixon, Med. Phys. 33, 3973-3976 (2006)]. Dixon [Med. Phys. 30, 1272-1280 (2003)] originally proposed an alternative using a short ion chamber and a helical scan acquisition to collect the same integral for any scan length L (and not limited 100 mm). The primary purpose of this work is to demonstrate experimentally the implementation, robustness, and versatility of this small ion chamber method in measuring the accumulated dose in the body phantom for any desired scan length L (up to the available phantom length) including the limiting equilibrium dose (symbolically CTDIinfinity), and validation of the method against the pencil chamber methodology. Additionally, a simple and robust method for independently verifying the active length of a pencil chamber is described. The results of measurements made in a 400 mm long, 32 cm diameter polymethylmethacrylate body phantom using a small Farmer-type ion chamber and two pencil chambers of lengths l=100 and 150 mm confirm that the two methodologies provide the same dose values at the corresponding scan lengths L=l. The measured equilibrium doses obtained for GE MDCT scanners at 120 kVp are CTDIinfinity = 1.75 CTDI100 on the central axis and 1.22 CTDI100 on the peripheral axes, illustrating a nontrivial shortfall of CTDI100 in that regard and in good agreement with comparable data [S. Mori, M. Endo, K. Nishizawa, T. Tsunoo, T. Aoyama, H. Fujiwara, and K. Murase, Med. Phys. 32, 1061-1069 (2005); J. M. Boone, Med. Phys. 34, 1364-1371 (2007)].     

An empirical formula for calculating the output factors of electron beams from a therac 20 linear accelerator

A polynomial formula, deduced from the data published by Mills et al. [Med. Phys. 12, 473 (1985)], in predicting the relative electron beam output factors, is presented in this report. This formula contains four parameters. By choosing four measured output factors, from four field sizes normalized at the field size of (10,10), the values of these parameters can be determined. A comparison of the factors predicted with this formula and the values measured by Mills et al. shows that the differences between the field sizes of (4,4) to (30,30) are 0.5% or less in 31 out of 35 field sizes. All the 35 field sizes are within 1% for an electron beam of 20 MeV. With 6-MeV electron beams, the differences are 0.5% or less in 26 out of 35 field sizes, and 1% or less in 31 out of 35 field sizes. Those having differences greater than 1% have either a small field size (5 cm) or a large field size (20 cm). Considering that this formula requires only four accurately measured relative output factors, one can predict the factors of any field within an acceptable accuracy. The calculation is easy with a scientific hand calculator. This formula provides major improvement over the other methods which require many measurements to be taken in order to interpolate with acceptable accuracy.     

On the question of 3D seed reconstruction in prostate brachytherapy: the determination of x-ray source and film locations

Inaccuracy in seed placement during permanent prostate implants may lead to significant dosimetric deviations from the intended plan. In two recent publications (Todor et al 2002 Phys. Med. Biol. 47 2031-48, Todor et al 2003 Phys. Med. Biol. 48 1153-71), methodology was described for identifying intraoperatively the positions of seeds already implanted, thus allowing reoptimization of the treatment plan and correcting for such seed misplacement. Seed reconstruction is performed using fluoroscopic images and an important (and non-trivial) component of this approach is the ability to accurately determine the position of the gantry relative to the treatment volume. We describe the methodology for acquiring this information, based on the known geometry of six markers attached to the ultrasound probe. This method does not require the C-arm unit to be isocentric and films can be taken with the gantry set at any arbitrary position. This is significant because the patient positioning on the operating table (in the lithotomy position) restricts the range of angles at which films can be taken to a quite narrow (typically +/- 10 degrees) interval and, as a general rule, the closer the angles the larger the uncertainty in the seed location reconstruction along the direction from the x-ray source to the film.