摘要:

An iterative pencil beam algorithm for optimization of multidimensional radiation therapy dose plans has been developed. The algorithm allows the use of both physical and radiobiological treatment objective functions and allows arbitrary sampling such as straight Cartesian grids with linear or nonlinear sampling functions or random sampling. The algorithm can account for and optimally combine almost all the degrees of freedom at an advanced radiotherapy clinic, such as different beam modalities and spectra, beam directions, beam fluence distributions, and time–dose fractionations. The algorithm allows for external charged and neutral beams as well as intracavitary and interstitial sources to be optimally combined. A quantity termed the generalized fluence vector is introduced, combining fluences and energy fluences from external beams as well as the radiation source densities of intracavity and interstitial sources or external source distributions. The positivity constraint on the generalized fluence can therefore be applied directly during the optimization procedure. The convergence properties and the required iteration time of the algorithm are discussed. Several examples with combinations of photon and electron beams of different energies and directions of incidence are presented. The optimization has been made with the treatment objective to maximize the probability of achieving tumor control without causing severe complications in healthy normal tissues.

ISSN:0094-2405    

DOI:10.1118/1.597302

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A computed tomography (CT) based system that compensates for patient surface contour and internal tissue inhomogeneity was implemented in our clinic. The compensators are fabricated with a mixture of tin granules and bee's wax. The tin/wax mixture was optimized for tin granule size and tin granule to wax ratio. The narrow beam attenuation coefficients were measured for 4‐, 6‐, 10‐, and 24‐MV photon beams. The compensator design and fabrication methodology were verified by measuring the dose distribution for a known surface contour irradiated with a compensated beam and for a known inhomogeneity that was submerged in a water phantom and irradiated with a compensated beam. For the surface contour, the uncompensated isodose levels varied by as much as 10% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.3%. Measured and calculated doses for this surface contour were found to differ by less than 3.4%. For the inhomogeneity, the uncompensated isodose levels varied by 27% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.5%. Measured and calculated doses for the known inhomogeneity were found to differ by less than 2%. Measurements of depth‐dose curves indicate that the presence of the compensator in the beam does not significantly increase the surface dose. Twenty‐six compensators have now been fabricated for clinical cases. In these patients, dose variations as great as 19% occurred in the plane of compensation prior to placing the compensator in the beam. Measured and calculated dose profiles with the compensators in place have been found to agree within 2.3%.

ISSN:0094-2405    

DOI:10.1118/1.597303

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

To calculate electron beam dose distributions accurately, numerical methods of electron transport calculations must account for the statistical variation (or “straggling”) in electron energy loss. This paper shows that the various energy straggling theories that are applicable to short path lengths all derive from a single statistical model, known as thecompoundPoissonprocess. This model in turn relies on three assumptions: (1) the number of energy‐loss events in a given path length is Poisson distributed; (2) events are mutually independent; and (3) each event has the same probability distribution for energy loss (i.e., the same energy‐loss cross section). Applying the principles of the compound Poisson process and using fast Fourier transforms, a new method for calculating energy‐loss spectra is developed. The spectra calculated using this method for 10, 20, and 30 MeV electrons incident on graphite and aluminum absorbers agreed with Monte Carlo simulations (egs4) within 1% in the spectral peak. Also, stopping powers derived from the calculated spectra agreed within 1.2%, with stopping powers tabulated by the International Commission on Radiation Units and Measurements. Several numerical transport methods “propagate” the electron distribution (in position, direction, and energy) over small discrete increments of path length. Thus the propagation of our calculated spectra over multiple path length increments is investigated. For a low atomic number absorber (graphite in this case), calculated spectra agreed withegs4 Monte Carlo simulations over the full electron range, provided the path length increments were sufficiently small (less than 0.5 g/cm2). It is concluded from these results that numerical methods of electron transport should restrict the size of path length increments to less than 0.5 g/cm2if energy straggling is to be modeled accurately.

ISSN:0094-2405    

DOI:10.1118/1.597383

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The measurement of absorbed dose as well as dose distributions (profiles and isodose curves) for small radiation fields (as encountered in stereotactic surgery) has been difficult due to the usual large detector size or densitometer aperture (≳1 mm) relative to the radiation field (as small as 4 mm). The radiochromic direct‐imaging film, when read with a scanning laser microdensitometer (laser beam diameter 0.1 mm), overcomes this difficulty and has advantages over conventional film in providing improved precision, better tissue equivalence, greater dynamic range, higher spatial resolution, and room light handling. As a demonstration of suitability, the calibrated radiochromic film has been used to measure the dose characteristics for the 18‐, 14‐, 8‐, and 4‐mm fields from the gamma‐ray stereotactic surgery units at Mayo Clinic and the University of Pittsburgh. Intercomparisons of radiochromic film with conventional methods of dosimetry and vendor‐supplied computational dose planning system values indicate agreement to within ±2%. The dose, dose profiles, and isodose curves obtained with radiochromic film can provide high‐spatial‐resolution information of value for acceptance testing and quality control of dose measurement and/or calculation.

ISSN:0094-2405    

DOI:10.1118/1.597384

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Standard silver‐based films are usually too sensitive to be used as direct indicators of dose in dynamic radiosurgery because of optical saturation. This paper describes the use of a new radiochromic film to measure 6‐MV radiosurgery doses and dose distributions in a head phantom. Dose calibration of the radiochromic film was performed in the range of 2.3–50.2 Gy using light of 632‐ and 530‐nm wavelengths. Radiosurgery dose distributions were measured using the radiochromic film in a head phantom undergoing the same treatment as a patient, and were compared with the planned distributions. For an example case (nominal 2.0‐cm‐diam cone), film measurement verified the calculated dose distribution in one plane. The simple measurement technique described led to experimental uncertainties of ±0.1 cm for the 90% and 50% isodose lines, ±0.3 cm for the 20% line, and ±0.5 cm for the 10% line. Isocenter dose was measured with an uncertainty of ±3%. Refinements to the technique should allow more precise measurements. It is concluded that the radiochromic film, with some limitations, is a convenient and useful tool for dynamic radiosurgery quality assurance.

ISSN:0094-2405    

DOI:10.1118/1.597385

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A computer‐assisted method for selecting beam orientation in a one‐step procedure is presented. Inverse and adjoint techniques are developed to obtain the best beam directions. Both methods rely on determining the “path of least resistance” to radiation from the tumor location to the surface of the section. The effectiveness of beam directions is then determined by monitoring the dose distribution along the section boundary. The inverse and adjoint calculations are performed for three tumor cases using a two‐dimensional discrete ordinates transport code. The proposed treatment plans from these calculations are verified against typical treatment plans. The new techniques improved the dose distribution in the treated section. The inverse calculations are useful in sections involving low‐density tissues. The adjoint technique can effectively deal with multiple target volumes and/or sections with complex geometry. The proposed method is potentially useful in selecting beam orientations for three‐dimensional planning systems and in determining beam intensities in rotational and conformal therapy.

ISSN:0094-2405    

DOI:10.1118/1.597304

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

X‐ray fields that are shifted away from the conventional central axis exhibit alterations in output, beam profile, and beam energy. An extension of a conventional dose calculation algorithm to correct for these perturbations is presented. This algorithm employs the product of three functions describing the dose at the center of the field atdmax, the relative off‐center dose atdmax, and the relative dose at depth along a fanline passing through the point of interest. The nature of the problem is characterized, a basic data set necessary to support the algorithm is described and an abridged set of data that may assist in the development of an independent collimator dose calculation capability is presented. The appropriateness of this technique for both conventional and dynamically collimated fields is illustrated.

ISSN:0094-2405    

DOI:10.1118/1.597386

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The depth of dose maximum,dmax, of megavoltage x‐ray beams was studied as a function of beam energy and field size for 6‐, 10‐, and 18‐MV x‐ray beams and field sizes ranging from 1×1 to 30×30 cm2. For a given beam energy,dmaxincreases rapidly with increasing field size at small fields, reaches a maximum around 5×5 cm2and then gradually decreases with increasing field size for large fields. Monte Carlo simulations combined with measurements verified that the effect observed at small field sizes is caused by in‐phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between thedmaxbehavior of flattened beams to that of unflattened beams indicates that thedmaxdecrease at large fields for flattened beams is caused mainly by contamination electrons which are produced in the flattening filter and further scattered by collimator jaws and air.

ISSN:0094-2405    

DOI:10.1118/1.597305

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The detective quantum efficiency (DQE) is a system parameter that can be used to accurately describe image noise transfer characteristics through many imaging systems. A simpler approach used by some investigators, particularly when evaluating new ideas and system designs, is to describe the system as a series of cascaded stages. Each stage may correspond to either an increase in the number of quanta (e.g., conversion from x‐ray to optical quanta in a radiographic screen), or a loss (a detection or coupling probability). The number of secondary quanta at each stage per incident primary quantum is given by the product of all preceding gains, and can be displayed graphically for convenient interpretation. The stage with the fewest quanta is called the “quantum sink,” limiting the pixel signal‐to‐noise ratio to less than the square root of the number of quanta per pixel. This conventional zero‐spatial‐frequency “quantum accounting diagram” (QAD), however, neglects the spatial spreading of secondary quanta and can seriously underestimate image noise. It is shown that this problem is avoided with the introduction of a spatial‐frequency dependent QAD, expressed as the product of the gains and squared modulation‐transfer functions (MTF) of each stage. A generalized expression is developed for the DQE of a cascaded imaging system that is dependent only on the gain, gain Poisson excess (related to the variance), and MTF, of each stage. A direct relationship is then shown to exist between the DQE and values in the QAD. The QAD of a hypothetical system consisting of a charge‐coupled device camera and a scintillating screen is evaluated as an illustrative example. The conventional zero‐frequency analysis suggests two quantum sinks occur with approximately equal importance: one in the number of x rays, and one in the number of optical quanta. The spatial‐frequency dependent analysis, however, shows the optical quantum sink becomes severe and dominates at nonzero frequencies. The necessary increase in gain or optical numerical aperture required to prevent the optical quantum sink for spatial frequencies of interest is determined from the QAD analysis. The visual impact of this nonzero spatial‐frequency quantum sink is shown in images generated using a Monte Carlo simulation of the cascading process.

ISSN:0094-2405    

DOI:10.1118/1.597401

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The primary advantage of helical computerized tomography (CT) is the capability of scanning a complete anatomical volume in a single breath hold. Due to the table motion and subsequent interpolation process, the slice sensitivity profile (SSP) in helical CT is worse than the response function of the detector array. In this paper, image longitudinal resolution in volumetric x‐ray CT is analytically characterized, and a comparison made between conventional and helical CT. First, the SSPs are derived for both conventional and helical CT with the half‐scan interpolation method under the condition that the table increment and detector collimation are the same. Then, the corresponding transfer functions are obtained for bandwidth determination, which directly describe the spatial resolution. Both one‐tenth‐cutoff and mean‐square‐root measures are used to quantify the bandwidth. Although it appears that broadening the SSP in helical CT could adversely affect longitudinal resolution, it is proved that for a given x‐ray dose, helical CT allows substantially better longitudinal resolution than conventional CT due to its inherent retrospective reconstruction capability. To make full use of the potential of helical CT scan data, it is recommended that about five slices be reconstructed per table increment. Helical CT is superior in applications requiring a high longitudinal resolution.

ISSN:0094-2405    

DOI:10.1118/1.597306

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY