摘要:

A computer‐driven milling machine is being installed at the Mallinckrodt Institute of Radiology to fabricate photon compensating filters for conventional compensation and beam‐intensity modulation. Commissioning and quality assurance procedures are being developed for the design, fabrication, and delivery systems prior to using the milled filters in the clinic. The portion of the quality assurance procedures governing the filter fabrication using a computer‐driven milling machine includes, in part, a comparison of designed and fabricated filters. Test filters include geometrically regular filters, such as flat surfaces and steps. The verification of these shapes is accomplished using spatial measurements. However, to test the mill's ability to generate complex curved surfaces, filters with more complicated surfaces are generated. These filters do not lend themselves to precise verification using physical measurement of the thickness distribution. A method has been developed to verify the thickness distributions of these complex filters by irradiating the filters with high‐energy x rays and comparing the scattered and transmitted fluence to the fluence calculated using the intended filter shape. The fluence measurement is made using a calibrated commercial liquid ionization chamber electronic portal imaging device. The calculated fluence is separated into transmitted primary and scattered fluences and is determined using a convolution of a distributed radiation source kernel with an exponential filter attenuation function. The attenuation coefficient is measured for the filter material (Lipowitz metal) and fit to a second‐order polynomial in filter thickness and off‐axis distance. The distributed source kernel is measured using a split‐field technique and fit to a sum of three two‐dimensional Gaussian distributions. The scattered radiation is modeled by the Klein–Nishina cross section. The algorithm is tested by comparing calculated fluences with measured fluences for a series of machined filters: an open field, flat filters of 9.6‐, 15.8‐, and 31.6‐mm thickness, split‐field filters, and a pyramid‐shaped filter. In each case, the algorithm calculates the fluence to within 3% of the measured values over the entire irradiated field size to within 1.5 cm of the collimated field edges.

ISSN:0269-3127    

DOI:10.1118/1.597856

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Tumor treatment with charged particle beams is a quickly developing field aimed to translate the potential advantages offered by the superior physical dose distribution and relative biological effectiveness of heavy charged particles into a real improvement of tumor therapy. To this purpose the new proton and light‐ion radiation therapy facilities must be designed according to strict clinical specifications to provide a reliable and effective tool against cancer. This paper provides the performance specifications of the accelerator and of the beam transport and delivery systems of the Italian Hadrontherapy Centre, which should be satisfied to meet the clinical specifications. A discussion is given on the requirements on energy range, energy variability, beam intensity, lateral penumbra, distal dose falloff, source‐to‐surface distance, time structure of the extracted beam, raster scanning system specifications, and beam abort time. Though the physical specifications are given for a particular accelerator, they can be used as a general guideline for the design of future biomedical particle accelerator facilities.

ISSN:0269-3127    

DOI:10.1118/1.597735

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Neodymium Iron Boron (NdFeB) rare earth permanent magnets have unique properties that enable them to fit easily onto the accessory mount of a clinical linear accelerator to partially sweep away electron contamination produced by the treatment head and block trays and thus increase skin sparing. Using such magnets the central axis entrance surface dose has been reduced by 11% for a 20×30 cm field size from 32% to 21% of maximum dose by the magnetic device. A reduction of 14% from 32% to 18% was seen for a 20×20 cm field size with a 6 mm perspex block tray positioned above the magnet. The magnetic device is light weight and thus clinically usable.

ISSN:0269-3127    

DOI:10.1118/1.597818

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two competing methods for assigning intensities to radiation treatment beams were tested. One method was derived from mixed integer programming and the other was based on simulated annealing. The methods faced a common objective and identical constraints. The goal was to maximize the minimum tumor dose while keeping the dose in required fractions of normal organ volumes below a threshold for damage. The minimum tumor doses of the two methods were compared when all the dose‐volume constraints were satisfied. A mixed integer linear program gave a minimum tumor dose that was at least 1.8 Gy higher than that given by simulated annealing in 7 of 19 trials. The difference was ≥5.4 Gy in 4 of 19 trials. In no case was the mixed integer solution one fraction size (1.8 Gy) worse than that of simulated annealing. The better solution provided by the mixed integer program allows tumor dose to increase without violating the dose‐volume limits of normal tissues.

ISSN:0269-3127    

DOI:10.1118/1.597857

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A genetic algorithm for generating beam weights is described. The algorithm improves an objective measure of the dose distribution while respecting dose volume constraints placed on critical structures. The algorithm was used to select beam weights for treatment of abdominal tumors. Weights were selected for up to 36 beams. Dose volume limits were placed on normal organs and a dose inhomogeneity limit was placed on tumor. Volumes were represented as sets of several hundred discrete points. The algorithm searched for the beam weights that would make the minimum tumor dose as high as the constraints would allow. The results were checked using dose volume histograms with standard sized grids. Nineteen trials were created using six patient cases by changing the required field margin or allowed beam position in each case. The sampling of points was sufficiently dense to yield solutions that strictly satisfied the constraints when the prescribed dose was renormalized by a factor of less than 6%. The genetic algorithm supplied solutions in 49 min on average, and in a maximum time of 87 min. The randomized search does not guarantee optimality, but high tumor doses were obtained. An example is shown for which the solution of the genetic algorithm gave a minimum tumor dose 7 Gy higher than the solution given by a simulated annealing algorithm under the same set of constraints. The genetic algorithm can be generalized to admit nonlinear functions of the beam intensities in the objective or in the constraints. These can include tumor control and normal tissue complication probabilities. The genetic algorithm is an attractive procedure for assigning beam weights in multifield plans. It improves the dose distribution while respecting specified rules for tissue tolerance.

ISSN:0269-3127    

DOI:10.1118/1.597858

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An analytic representation proposed for the relative intensity distribution of the extra‐focal source in a two‐component x‐ray source model serves as the basis for calculation. From this representation, a closed‐form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra‐focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multi‐leaf collimators (MLCs). For 6‐ and 15‐MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

ISSN:0269-3127    

DOI:10.1118/1.597827

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1996.tb01019.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1996.tb01020.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597860

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597861

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY