摘要:

A model based upon steady‐state diffusion theory which describes the radial dependence of diffuse reflectance of light from tissues is developed. This model incorporates a photon dipole source in order to satisfy the tissue boundary conditions and is suitable for either refractive index matched or mismatched surfaces. The predictions of the model were compared with Monte Carlo simulations as well as experimental measurements made with tissue simulating phantoms. The model describes the reflectance data accurately to radial distances as small as 0.5 mm when compared to Monte Carlo simulations and agrees with experimental measurements to distances as small as 1 mm. A nonlinear least‐squares fitting procedure has been used to determine the tissue optical properties from the radial reflectance data in both phantoms and tissuesinvivo. The optical properties derived for the phantoms are within 5%–10% of those determined by other established techniques. Theinvivovalues are also consistent with those reported by other investigators.

ISSN:0094-2405    

DOI:10.1118/1.596777

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For five commonly used parallel‐plate ion chambers, Monte Carlo calculations are presented of the wall attenuation and scatter correction factors (Kwall=Awall−1orkatt−1) and the correction for nonhomogeneous composition of the chamber (Kcompor related tokm). The chambers are assumed to have 0.5 g/cm2buildup caps made of the predominant material in each chamber. These correction factors are needed if air‐kerma calibration factors are used to deduce the chamber's cavity‐gas calibration factor,Ngas. The scatter from the material around the cavity more than compensates for the attenuation in the front wall and henceKwallvalues are less than unity. The corrections for the nonhomogeneous composition can be significant. Thin collectors or insulators behind the collecting volume can have a major effect because electron backscattering depends strongly on material. The calculations agree well with experimental data and explain previous results that had not been understood. Calculations for different buildup caps predict trends in chamber in‐air responses, which go in the opposite direction to thekmvalues for homogeneous chambers of the same materials. It is argued thatPwall, the wall correction factor for photon‐beam measurements in‐phantom is, to first order, the same correction factor asKcomp, calculated for a buildup cap of the same material as the phantom material and the initial assumption that the wall material is made of the same material. Hence,Pwallcan also be significantly different from unity. Recommended values ofkmkattor the equivalentNgas/NX, andPwall(in60Co beams) are presented for the 5 chambers. These values treat the Exradin P11, Markus and Holt parallel‐plate chambers as homogeneous in construction whereas the values for the NACP and Capintec PS‐033 chambers demonstrate that the chambers are distinctly nonhomogeneous.

ISSN:0094-2405    

DOI:10.1118/1.596915

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The concepts of the equivalent square or circular field have long been used in dose calculations for photon beams. These concepts allow data measured for square or circular fields to be extended to calculate, for example, the percentage depth doses or output factors of rectangular or irregular fields. It has been pointed out in the past that an electron beam equivalent field dimension varies with depth and, thus, will have questionable utility. As the equivalent square and circle have proven to be useful in photon beam dose calculations, the work described in this paper has sought to analyze conditions under which equivalent fields may be useful for electron beam dose calculations. Equivalent square field dimensions and circular field radii are derived using the Fermi–Eyges theory and are compared to a number of approximate equivalent fields that have been applied to electron dose calculations. Calculations are also compared with measurements presented in the literature. It is shown that the accuracy of an electron dose calculation using these approximate equivalent fields diminishes with a decreasing degree of lateral scatter equilibrium at the central axis and only becomes accurate once equilibrium is established. As the central‐axis dose under this latter condition is in any event independent of field shape or size, the equivalent field approach becomes unnecessary. Because of this and other restrictions discussed, it is concluded that the equivalent fields analyzed here should not be used for electron beam dose calculations.

ISSN:0094-2405    

DOI:10.1118/1.596916

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper has improved some of the weak points appearing in a previous article [A. Iwasaki, Med. Phys.17, 203–211 (1990)] dealing with the calculation of 10‐MV x‐ray primary and scatter dose. The main improved points are as follows: (i) A pair of new functional equations expressing the primary dose spread array has been yielded. Consequently, the accuracy of the primary dose calculation both in the aluminum layer and in the soft tissue layer beyond the aluminum has been improved. (ii) A new functional equation expressing the backscatter factor has been developed. It has been utilized in the differential backscatter factor equation. Consequently, the calculated scatter dose spread array has been improved. (iii) A method of calculating the dose due to electron contamination has been introduced. With respect to the primary dose, the primary plus scatter dose, and the primary plus scatter plus electron contamination dose, it has been shown how the depth of maximum dose (dmax) varies with field size.

ISSN:0094-2405    

DOI:10.1118/1.596778

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The physical characteristics of the interactions of megavoltage photons and electrons with matter provide distinct advantages, relative to low‐energy (orthovoltage) x rays, that lead to better radiation dose distributions in patients. Use of these high‐energy radiations has resulted in better patient care, which has been reflected in improved radiation treatment outcome in recent years. But, as the desire for higher energy radiation beams increases, it becomes important to determine whether the physical characteristics that make megavoltage beams beneficial continue to provide a net advantage. It is demonstrated that, in fact, there is an energy range from 4 to 15 MV for photons and 4 to 20 MeV for electrons that is optimally suited for the treatment of cancer in humans. Radiation beams that exceed these maximum energies were found to add no advantage. This is because the costs (price of unit, installation, maintenance, shielding for neutron and photons) are not justified by either improved physical characteristics of the radiation (penetration, skin sparing, dose distribution) or treatment outcome. In fact, for photon beams some physical characteristics result in less desirable dose distributions, less accurate dosimetry, and increased safety problems as the energy increases for example, increasingly diffuse beam edges, loss of electron equilibrium, uncertainty in dose perturbations at interfaces, increased neutron contamination, and potential for higher personnel dose. The special features that make electron beams useful at lower energies, for example, skin sparing and small penetration, are lost at high energies. These physical factors are analyzed together with the economic factors related to radiation therapy patient care using megavoltage beams.

ISSN:0094-2405    

DOI:10.1118/1.596779

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The photon spectral characteristics of a recently designed Iodine‐125 source have been measured. The source has a physical length of 5 mm and a diameter of 0.8 mm. A thin tungsten filament coated with radioactive Iodine‐125 is used as a radiographic marker and is encapsulated in a double wall titanium shell of uniform thickness all around. The photon spectral characteristics, measured with an intrinsic germanium (Ge) detector coupled to a multichannel analyzer, reveal that the seed emits the 27.4‐keVKαand 31.4‐keVKβx rays and 35.5‐keV γ photons from the decay of Iodine‐125. Because of their low energy, the tungsten x rays are not observed in the spectrum. The anisotropy of the radiation fluence for each of the above‐mentioned photon energies was measured in planes containing the seed short and long axes. The 4π‐averaged anisotropy factor for the total radiation fluence, i.e., sum of the above three photon energies is 0.92. The photon intensity radiated along the seed long axis is approximately equal to the intensity in the seed transverse direction due to the absence of end welds. The new Iodine‐125 source is characterized by good radiographic visualization, greater structural strength due to double wall encapsulation design, and emission of more isotropic Iodine‐125 photon spectrum.

ISSN:0094-2405    

DOI:10.1118/1.596780

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this paper a method of computer‐aided optimization of 3‐D conformal treatment plans is presented which incorporates models to predict the clinical consequences of resulting dose distributions. Even though these models are simplistic, it is submitted that their intelligent use leads to treatment plans which indicate lower normal tissue complications and higher tumor control. Dose distribution data, biological models, and observed normal tissue and tumor response data are used to compute tumor control and normal tissue complication probabilities for each of the critical normal structures encountered in a treatment plan. These quantities are combined into a single score using an objective function which incorporates the importance of each end point as assessed by the physician. Using the “simulated annealing” method of optimization, the beam weights are adjusted to maximize the score. Additional constraints are applied to ensure consistency of the results of optimization with the judgment of the physician. These optimization methods have been applied to conformal treatment plans consisting of multiple fixed fields with conformal field shaping. The results indicate that the methods presented have considerable potential.

ISSN:0094-2405    

DOI:10.1118/1.596781

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Three‐dimensional conformal radiation treatments are highly complex and may comprise a large number of coplanar and noncoplanar beams, virtually all of which are shaped and may also employ arbitrary intensity modulation. The delivery of such treatments with conventional means is highly labor intensive and limited to a relatively small number of fields. A generalized model for a system that can deliver 3‐D conformal treatments rapidly and safely using a computer‐controlled treatment machine equipped with a multileaf collimator is presented. The model emphasizes the separation of tasks between an external computer programmed by the user institution and a control computer proprietary to the treatment machine manufacturer. The treatment scheme that is employed consists of a sequence of fixed fields, called segments, which are delivered in succession without human intervention. Settings for all segments, derived from a 3‐D conformal treatment planning system, are downloaded by the external computer into the control computer of the treatment machine. During patient setup, the control computer enables the operator to step through computer‐controlled setup of the segments without radiation to ensure the absence of collisions, and to allow the adjustment of setup parameters if necessary. The external computer verifies the treatment setup and permits the control computer to carry out the treatment segment by segment. Safety aspects of the model and anomalous situations which may arise are discussed.

ISSN:0094-2405    

DOI:10.1118/1.596782

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An algorithm based on the method of simulated annealing is presented for optimizing brachytherapy dose distributions. The algorithm accommodates either static configurations of multiple sources or single stepping sources, hence in principle can be used to optimize both low‐ and high‐dose rate treatments delivered with remote afterloading equipment. Required inputs include the specification of target dose rates and dose rate limits, expressed in absolute or relative terms, at operator selected points near the treatment site. The influence of the dose rate limits can be adjusted continuously through the use of one or more penalty factors. The algorithm generates a set of integer weights, one for each available source position, which are interpreted in terms of configuration occupancy numbers for static source arrangements and relative dwell times for stepping sources. Application is made to several variations of a hypothetical low‐dose rate vaginal vault planning problem involving one rectal and six applicator calculation points. The algorithm's performance for different source strengths, annealing schedules, target dose rates, dose rate limits, and values of a single penalty factor λ was examined. With a simple annealing schedule and value of λ=25, the algorithm found solutions of high quality for all problem variants. The CPU time required for optimization on a Vax 11/750 computer ranged from 2 min for a single configuration to 25 min for a solution consisting of four configurations. These results support the use of simulated annealing for clinical planning of low dose rate vaginal treatments, and encourage investigation of other applications in brachytherapy.

ISSN:0094-2405    

DOI:10.1118/1.596783

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A three‐dimensional dose calculation algorithm is described for stereotactic radiosurgery using multiple noncoplanar beam arcs. Precalculated dose libraries of 20‐deg arc segments, or mini arcs, are stored in computer memory which permits rapid calculation of complete, high resolution, three‐dimensional isodose distributions and dose volume histograms. Three‐dimensional patient contours and target volumes are obtained from CT scans and angiographic x rays. Rapid dose calculations are made possible by the use of arc libraries and an improved algorithm for mapping beam doses to the dose calculation grid. This permits more flexibility in designing optimum treatment plans, as five–six complete plans can be generated in less than 1 h. Thus many possible treatment options can be tested in the 3–4‐h time period typically available in stereotactic procedures between CT scanning and treatment.

ISSN:0094-2405    

DOI:10.1118/1.596784

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY