摘要:

We analyze the influence of sampling technique on the accuracy of estimating irradiated volumes, dose‐volume histograms and tumor control and normal tissue complication probabilities. The sampling techniques we consider are uniform distribution of points on a regular Cartesian grid and random selection of points. For three‐dimensional treatment planning, random sampling leads to a significant reduction in estimation error and/or in the number of calculation points necessary to achieve a required accuracy. We discuss advantages and drawbacks of random sampling, as compared to sampling on a regular grid. It is suggested that, in practical situations, at least 50 times fewer randomly sampled points per organ/volume of interest are needed for fast estimation of complication probability with the same accuracy, i.e., not exceeding 5% (within 95% confidence limits) in the worst case.

ISSN:0269-3127    

DOI:10.1118/1.596473

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system differs from previously reported devices in the construction of the detector and in the operation of the TV camera. The signal, noise, and signal‐to‐noise properties of this system have been determined by measuring the modulation transfer function [MTF(f)], the noise power spectra [NPS(f)], and by calculating the detective quantum efficiency [DQE(f)] of the system. The results show: (i) that the spatial resolution of the system is determined largely by the lens of the TV camera and by frame grabber; and (ii) that the noise in the system is dominated by the secondary light quanta, due to the poor light collection efficiency of the optical chain. Despite these physical limitations, a contrast‐detail study shows that the fluoroscopic system is better at detecting large, low contrast objects than portal films. Therefore the system is already a reasonable alternative to portal films and modifications to the metal plate/phosphor detector, lens, TV camera, and frame grabber should improve the performance of the system further.

ISSN:0269-3127    

DOI:10.1118/1.596474

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Bremsstrahlung spectra from thick targets of Al and Pb have been measured absolutely (photons per incident electron) along the beam axis for electrons of 10‐, 15‐, 20‐, 25‐, and 30‐MeV incident energy. The spectra have a 220‐keV low‐energy cutoff. The targets were cylinders with nominal thicknesses of 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20‐cm diameter by 25‐cm‐long cylindrical NaI detector. Measured spectra were corrected for pile‐up, background, detector response, detector efficiency, attenuation in materials between the target and detector and the collimator effect. Spectra were calculated using theegs4Monte Carlo system for simulating the radiation transport. The simulation model included the small amount of material upstream of the target. This material contributed about 40% of the spectrum, but its presence or absence had little effect on the calculated bremsstrahlung yield. The shapes of the measured and calculated spectra were in excellent agreement. The ratio of the total number of photons in each measured spectrum to those in the corresponding calculated spectrum varied from 0.97±0.06 to 1.12±0.06, depending largely on the atomic number of the target. Absolute spectral measurements in the literature agreed with our calculations of spectral shape but showed a range of ±30% in the number of photons per incident electron relative to the calculated values, which is contrary to our result.

ISSN:0269-3127    

DOI:10.1118/1.596560

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Perturbation in dose distributions of point sources of low‐energy electrons at planar interfaces of cortical bone (CB) and red marrow (RM) was investigated experimentally and by Monte Carlo codesegsand thetigerseries. Ultrathin LiF thermoluminescent dosimeters were used to measure the dose distributions of point sources of204Tl and147Pm in RM. When the point sources were at 12 mg/cm2from a planar interface of CB and RM equivalent plastics, dose enhancement ratios in RM averaged over the region 0–12 mg/cm2from the interface were measured to be 1.08±0.03 (SE) and 1.03±0.03 (SE) for204Tl and147Pm, respectively. The Monte Carlo codes predicted 1.05±0.02 and 1.01±0.02 for the two nuclides, respectively. However,egsgave consistently 3% higher dose in the dose scoring region than thetigerseries when point sources of monoenergetic electrons up to 0.75 MeV energy were considered in the homogeneous RM situation or in the CB and RM heterogeneous situation. By means of thetigerseries, it was demonstrated that aluminum, which is normally assumed to be equivalent to CB in radiation dosimetry, leads to an overestimation of backscattering of low‐energy electrons in soft tissue at a CB–soft‐tissue interface by as much as a factor of 2.

ISSN:0269-3127    

DOI:10.1118/1.596554

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The absorbed doses on the central axes of narrow beams (radii 0.07–2.5 cm) of 6‐MV x rays have been studied by experiments and Monte Carlo simulations. The measurements were made in a geometry used for irradiation of intracranial lesions. For radii<1.0 cm the dose on the central axis is progressively reduced due to electron disequilibrium. This leads to measurement artifacts when the detector is too large, as was readily observed with ionization chambers. Radiographic and radiochromic films were used with densitometric evaluation to provide the resolution necessary to measure absorbed doses for the narrowest beams. The contribution by phantom‐scattered photons is significant even at small field sizes, and scatter factors were determined from the experimental results. Photons scattered by the auxiliary collimator did not add appreciably to the dose on the central axis. The data were used to characterize the dose‐to‐kerma ratio as a function of beam radius. Differences between experimental results and those from Monte Carlo calculations were observed.

ISSN:0269-3127    

DOI:10.1118/1.596475

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Compositional analyses of samples of soft tissue and lung substitutes used in the construction of anthropomorphic radiotherapy phantoms were performed for products from Alderson Research Laboratories Inc., Kyoto Scientific Specimens Company/Capintec Inc., Humanoid Systems Inc., Radiology Support Devices Inc., and The Phantom Laboratory Inc. To assess quality control in the manufacture process, the results of these compositional analyses were compared with individual manufacturer's design specifications and electron densities measured by computed tomography analysis of actual phantom slices. A comparison of the manufacturers design specifications against reference tissue parameters fromTissueSubstitutesinRadiationDosimetryandMeasurement(International Commission on Radiation Units and Measurements) indicates marked differences in the basic design of commercially available anthropomorphic phantoms.

ISSN:0269-3127    

DOI:10.1118/1.596552

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Eight methods of photon inhomogeneity correction were examined for their photon transport approximations. The methods were categorized according to the different approaches used to model scatter photon dose contribution. They were the ratio of TAR (RTAR) and the modified Batho power law which utilized only the 1‐D density information along the primary photon path; the equivalent TAR (ETAR) and the FFT convolution methods which incorporated the 3‐D density information of the medium for empirical scatter dose calculation; the differential SAR (DSAR), the delta volume (DV), dose spread array (DSA), and differential pencil beam (DPB) methods which employed explicit 3‐D scatter ray‐trace calculation. Cobalt‐60 measurements in horizontal slab phantoms were used to allow simpler data analysis. RTAR consistently overestimated lung corrections by ∼10%. The scatter ray‐trace approach was not always better as the DSAR calculations were inferior to those using the Batho method. The ray‐tracing DV, DPB, and DSA methods agreed with measurements mostly to within 2%, at the expense of long computation time. The nonscatter ray‐tracing ETAR and FFT convolution calculations were only slightly inferior in the same geometries. These methods improve on the current 1‐D methods and should be seriously considered for fast optimization purposes in practical 3‐D treatment planning.

ISSN:0269-3127    

DOI:10.1118/1.596555

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose rates in water have been determined for the two types of125I seed currently used in brachytherapy. The need for such determinations became evident when water/air ratios measured with a silicon diode were found to be lower than expected. Extensive measurements using lithium fluoride thermoluminescent dosimeters (TLD's) have been performed in a solid water phantom, at distances from 0.1 to 10 cm from the seed center and at angular increments of 10°, 15°, or 30° within a plane through the seed axis. Dose calibration of the TLD's was accomplished by irradiation in air with125I seeds of the same type and of strengths traceable to a calibration at the National Institute of Standards and Technology (NIST). Relative calibration of TLD's was monitored by irradiation, in an oven‐type x‐ray machine, of control dosimeters simultaneously and all dosimeters intercurrently with the125I irradiations. Values obtained for the dose rate constant, i.e., dose rate per unit air‐kerma strength at 1 cm on the transverse axis, were 0.853 and 0.932 cGy h−1U−1(1.08 and 1.18 cGy h−1mCi−1) for the 6711 and 6702 seeds, respectively. Measured data were supplemented with Monte Carlo‐calculated relative dose rate data generated using the MORSE code. These calculations used 100 energy groups from 10 to 35.4 keV and involved energy collection bins ranging from 0.025 to 1.2 cm on an edge. Normalized at 1 cm, transverse axis calculated data are not significantly different from measured data (ours or cited literature) at distances either8 cm. Normalized at different distances along the transverse axis, our off‐axis calculated and measured distributions agree closely at all angles but differ from literature measured distributions at small (≤1 cm) distances and, for small angles, increasingly at larger distances (≥5 cm).

ISSN:0269-3127    

DOI:10.1118/1.596586

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The low‐energy photons of125I deposit energy in tissues primarily by the photoelectric effect, which is strongly dependent on the atomic numberZ. Thus dose distributions of125I sources in media of differentZare not the same. LiF and CaF2thermoluminescent dosimeters were used for relative dose distribution measurements in materials that are equivalent to muscle, breast, and bone. The experimental data are in good agreement with published results for muscle, and in reasonable agreement with Monte Carlo calculations for all the tissues tested. These measured data provide the basis for dose distribution calculations when125I seeds are implanted in or near inhomogeneous tissues.

ISSN:0269-3127    

DOI:10.1118/1.596476

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A dose computation model has been developed for the determination of dose distributions around vaginal plaque applicators containing encapsulated241Am sources. Encapsulated sources of241Am emit primarily 60‐keV photons which have a half‐value layer thickness of 1/8 mm of lead. This makes possible highly effectiveinvivoshielding of normal tissues at risk, by placing thin lead shields at appropriate places on the applicator. However, self‐absorption of photons in the source material itself is intense, requiring bulky sources of about 1 cm diameter. These sources also produce considerable source‐to‐source shielding which must be taken into account in dose calculations. Our dose computation model for a single source employs three‐dimensional integration of dose contributions from volume elements of the source including the effects of absorption and scattering of photons in the source material, titanium encapsulation, and water. An empirical correction to Berger's data on buildup factors of point, isotropic sources is made to account for the effects of anisotropic photon emission by cylindrical241Am sources. The second part of our dose computation model takes into account source‐to‐source shielding effects on both primary and scattered photons for the vaginal plaque geometry. The results of the model have been verified for accuracy by comparisons with extensive dosimetry measurements using lithium fluoride thermoluminescent dosimeters.

ISSN:0269-3127    

DOI:10.1118/1.596587

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY