摘要:

A theory is presented on dose calculations in inhomogeneous media that takes advantage of fast Fourier transform (FFT) convolution for practical three‐dimensional treatment planning using photon beams. While the initial work of Boyer and Mok [Med. Phys.13, 503–509 (1986)] provided a theory which is based on first principles, it failed to give satisfactory predictions inside inhomogeneities. Subsequently, Zhu and Boyer [Phys. Med. Biol.35, 351–368 (1990)] showed that their formulas agreed well with measured data, but these formulas were empirically altered from Boyer and Mok's. In this work, Boyer and Mok's first‐order theory is extended to include second‐order inhomogeneity effects. A newcorrectiondose formula is derived which corrects the first scattered dose due to the presence of inhomogeneities. This correction dose formula works better than Zhu and Boyer's empirical correction dose formula. Furthermore, theprimarydose formula used by Zhu and Boyer, which was empirically modified from Boyer and Mok's, is justified theoretically. Clear statements are made about the assumptions and the approximations that enter into the derivation which in turn uncover the limitations of this FFT convolution dose calculation.

ISSN:0269-3127    

DOI:10.1118/1.597883

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose–response relationship, as well as to avoid critical organ toxicity. Given a spatial distribution of cumulated activity, an absorbed dose distribution that accounts for the effects of attenuation and scatter can be obtained using a Monte Carlo method that simulates particle transport across the various densities and atomic numbers encountered in the human body. Patient‐specific information can be obtained from CT and SPECT or PET imaging. Since the data from these imaging modalities is discrete, it is necessary to develop a technique to efficiently transport particles across discrete media. The Monte Carlo‐based algorithm presented in this article produces accurate absorbed dose distributions due to patient‐specific density and radionuclide activity distributions. The method was verified by creating CT and SPECT arrays for the Medical Internal Radionuclide Dose (MIRD) Committee's Standard Man phantom, and reproducing the spatially averaged specific absorbed fractions reported in MIRD Pamphlet 5. The algorithm was used to investigate the implications of replacing a mean absorbed dose with a distribution, and of neglecting atomic number and density variations for various patient geometries and energies. For example, the I‐131 specific absorbed fraction for spleen to liver is the same as for liver to spleen, yet the distributions were different. Furthermore, neglecting atomic number variations across the vertebral bone led to an overestimation of I‐125 absorbed dose by an order of magnitude, while no error was observed for I‐131.

ISSN:0269-3127    

DOI:10.1118/1.597882

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The scattered‐photon part of pencil‐beam dose kernels for high‐energy x‐ray beams can be derived experimentally by differentiating the broad‐beam scatter‐to‐primary dose ratio as a function of radius. Formally, this requires a uniform and parallel beam, and the procedure is complicated by the nonideal, actual beam conditions: the primary dose profile is not uniform, the beam quality is not constant, and the distance to the source is not infinite. The experimentally determined scatter‐to‐primary ratios can be corrected for these effects before they are differentiated to give the pencil‐beam kernels. The correction factors were calculated and shown to reach as much as 5% of the true scatter‐to‐primary ratio. The effect on the pencil beam was evaluated and corrected pencil beams were determined.

ISSN:0269-3127    

DOI:10.1118/1.597884

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this study we investigate a method for positioning the margin required around the clinical target volume (CTV) to account for the random geometrical treatment uncertainties during conformal radiotherapy. These uncertainties are introduced by patient setup errors and CTV motion within the patient. Three‐dimensional dose distributions are calculated for two four‐field box techniques and a three‐field technique, using rectangular fields. In addition, dose calculations are performed for four prostate cases, treated with a three‐field conformal technique. The effects of random rotational and translational deviations on the delivered dose are described as a convolution of the “static” dose with the distribution of the deviations. For the rectangular field techniques, these convolutions are performed with a range of standard deviations (SDs) of the distribution of random translations (0–7 mm in the three directions) and rotations (0°–5° around the main axes). Two centers of rotation are considered: the isocenter and a position that is 3.5 cm shifted with respect to the isocenter. For the prostate cases, the random deviations are estimated by combining the results from organ motion and setup accuracy studies. The required margin is defined as the change in the position of the static 95% isodose surface by the convolution and it is approximated by a morphological erosion operator, applied to the static 95% isodose surface. When the center of rotation coincides with the isocenter the change in the position of the static 95% isodose surface can accurately be described by an erosion operator. For the rectangular field techniques, the margin is equal to about 0.7 SD of the distribution of translations, independent of the distribution of rotations. When the center of rotation does not coincide with the isocenter and rotations are considerable, the margin is strongly place dependent, and the accuracy of the approximation by an erosion operator is much lower. In conclusion, margins for random uncertainties can be approximated by a dilation operator (inverse of an erosion operator) when the center of rotational deviations coincides with the isocenter. The size of the margin is about 0.7 SD of the distribution of translations. When rotational deviations are present and the center of rotation does not coincide with the isocenter, the margin can become strongly place dependent and the convolution computation should be incorporated in the planning system.

ISSN:0269-3127    

DOI:10.1118/1.597745

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In radiation therapy, there is often a need to treat irregular surfaces with electron and photon beams. These surfaces require smoothing to achieve uniform doses at depth and proper buildup of dose at the surface. The surface smoothing and dose buildup is achieved by applying bolus. To deliver a known dose, produce a known central axis depth dose, and beam flatness for successful treatment, it is necessary that water or tissue equivalent bolus material is used. This material must also be able to fill extremely irregular voids. Several moldable materials, currently or formerly used in dental clinics, were evaluated for adequacy as tissue equivalent bolus. Availability was also considered during evaluation. Polyflex®, a hydrocolloid, was found to be near water equivalent for electron and photon beams. It was also inexpensive, readily available, and held up well over time.

ISSN:0269-3127    

DOI:10.1118/1.597820

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A miniature, interstitial x‐ray generator has recently been developed and is currently undergoing clinical trials for the treatment of brain tumors. The maximum photon energy from this x‐ray tube is 50 keV, although most of the initial testing has been carried out at 40 keV. Dose rates of up to 2 Gy/min in a water phantom at a distance of 10 mm from the tube tip are produced. In this paper we describe the modeling and simulation of x‐ray production from this device using the ITS 3.0 Monte Carlo code. Verification of the simulation of x‐ray production in the device was carried out by comparing predictions of spatial photon distribution, energy spectrum, and dose versus depth in water with experimentally obtained measurements. Agreement between the simulated results and experimental measurements was fairly good when comparing the angular distribution of photons emitted from the x‐ray tube and very good when comparing dose rate versus depth in a water phantom. Discrepancies observed when comparing the calculated and measured estimates of characteristic line radiation were reduced by incorporation of a modification to the ITS code. Possible causes of the remaining discrepancy in bremsstrahlung intensity are discussed.

ISSN:0269-3127    

DOI:10.1118/1.597885

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this study is to develop a mathematical model and calculate photon‐absorbed fractions in a homogeneous nonradioactive cylinder placed inside off‐center and outside a cylindrical homogeneous distribution of activity. In the second case, both the radioactive cylinder and the nonradioactive one are placed in a tissue‐equivalent nonradioactive medium. The values of the photon‐absorbed fractions are investigated for various geometrical configurations using water as the material filling the cylinders and the medium in between and an isotope commonly used in Nuclear Medicine,99mTc. The calculations for off‐center cylinders allows for modeling inhomogeneous distributions of activity within a tumor by placing several “cold” cylinders of various sizes in a radioactive finite cylinder. This three‐dimensional model calculates photon‐absorbed fractions for inhomogeneous activity distributions that can be used in quantitative nuclear medicine for self‐absorption correction, thus introducing a more realistic correction than the one‐dimensional corrections. These calculations are also used to model the response of a cylindrical TLD (thermoluminiscent dosimeter) placed inside a homogeneous radioactive cylinder and outside the homogeneous radioactive cylinder, in an absorbing nonradioactive surrounding medium. The purpose of these calculations is to evaluate the photon‐absorbed fraction in the TLD as an instrument of measuring the time‐integrated activity of a homogeneous radioactive source versus an inhomogeneous one. The dependence of the TLD‐absorbed fraction on the position of the TLD with respect to the radioactive cylinder is investigated.

ISSN:0269-3127    

DOI:10.1118/1.597886

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597887

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597888

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597889

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY