摘要:

The development of treatment planning methods in radiation therapy requires dose calculation methods that are both accurate and general enough to provide a dose per unit monitor setting for a broad variety of fields and beam modifiers. The purpose of this work was to develop models for calculation of scatter and transmission for photon beam attenuators such as compensating filters, wedges, and block trays. The attenuation of the beam is calculated using a spectrum of the beam, and a correction factor based on attenuation measurements. Small angle coherent scatter and electron binding effects on scattering cross sections are considered by use of a correction factor. Quality changes in beam penetrability and energy fluence to dose conversion are modeled by use of the calculated primary beam spectrum after passage through the attenuator. The beam spectra are derived by the depth dose effective method, i.e., by minimizing the difference between measured and calculated depth dose distributions, where the calculated distributions are derived by superposing data from a database for monoenergetic photons. The attenuator scatter is integrated over the area viewed from the calculation point of view using first scatter theory. Calculations are simplified by replacing the energy and angular‐dependent cross‐section formulas with the forward scatter constantr20and a set of parametrized correction functions. The set of corrections include functions for the Compton energy loss, scatter attenuation, and secondary bremsstrahlung production. The effect of charged particle contamination is bypassed by avoiding use ofdmaxfor absolute dose calibrations. The results of the model are compared with scatter measurements in air for copper and lead filters and with dose to a water phantom for lead filters for 4 and 18 MV. For attenuated beams, downstream of the buildup region, the calculated results agree with measurements on the 1.5% level. The accuracy was slightly less in situations where the scatter component is very large, as for very large fields with very short filter to detector distances. The implementation of the model into treatment planning systems is discussed.

ISSN:0094-2405    

DOI:10.1118/1.597534

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Bragg curves for proton beams are different in shape if measured with different detectors, such as silicon diodes, extrapolation, and thimble chambers. Calculations with an analytic algorithm are presented which show that such differences are caused, at least in part, by the shapes of the detectors. Similar differences must be expected for all beams of heavy charged particles. Dose conversion factors representing the conversion of a measured dose into dose in water are discussed.

ISSN:0094-2405    

DOI:10.1118/1.597535

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In contrast to the traditional treatment planning process where beam parameters are adjusted in order to produce a desired dose distribution, the inverse treatment planning technique starts from the desired dose distribution and employs mathematical inversion to derive a set of optimal intensity profiles for beams with predefined directions. Crucial to the success of the inverse technique is the mathematical inversion process that yields the optimized intensity modulation. This paper systematically investigates the influence of phantom scatter in the process of designing the optimized intensity modulation and assesses its clinical consequences through the resultant treatment plans. Optimized intensity modulations and the resultant dose distributions were calculated for four prostate patients and for a simple cubic phantom under two conditions: one includes scatter and the other ignores scatter in designing the optimized intensity modulation. The effects of scatter were assessed by comparing the isodose distributions, the dose–volume histograms of the target and of the critical organs, and the tumor control probabilities and the normal tissue complication probabilities of two set prostate treatment plans. The comparison reveals that the dose homogeneity to the target is significantly improved when scatter is included in the optimization process. For prostate patients, the dose to the volume enclosed by the outer rectal contour is concomitantly reduced. Our results suggest that it is important to include the scatter in the iterative design of the optimized intensity modulations.

ISSN:0094-2405    

DOI:10.1118/1.597536

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A computer‐controlled equivalent of the universal wedge was designed by moving one collimator with step‐wise constant velocity to produce the same primary‐dose profile as a 20×20‐cm2conventional 58‐deg wedge. It was used for smaller fields without changing the profile and combined with open beams to alter the wedge angle. Output factors in air and wedge factors in air and water were determined experimentally for the dynamic wedge and compared to predictions based on the assumption that the primary dose is proportional to the number of monitor units that the point of interest is in the open portion of the beam. This model was found to be accurate within about 2% and the deviations caused by head‐scatter changes and collimator transmission after the moving collimator has passed. Measurements of the wedge factor in water indicated that the scatter factors for large wedged beams slightly exceeded those for open beams.

ISSN:0094-2405    

DOI:10.1118/1.597537

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The thermal neutron distribution in slow and fast neutron beams is usually determined using the foil activation method. In this work a small magnesium walled ionization chamber, in which the inner surface of the wall has been coated with10B to increase the sensitivity for thermal neutrons, is used to estimate the thermal neutron component of the beam. After calibration and determination of the directional response in a thermal neutron beam a comparison with foil activation at different depths in water was performed to investigate the reliability of the ionization measurements. The chamber was used in a computer controlled water phantom to measure the depth and lateral distribution of the thermal neutron dose. With this arrangement two‐dimensional scans of the thermal neutrons could be performed quickly and with high accuracy.

ISSN:0094-2405    

DOI:10.1118/1.597630

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method has been developed for measuring photon beam quality correction factorskQusing direct transfer with a water calorimeter.kQvalues were measured for beam qualities varying between 6 and 23 MV for a Capintec PR‐06 and a PTW W30001 cylindrical ionization chamber. Measured values were intercompared with published sets of computedkQvalues and agreement was found to be within measurement uncertainty (1%, one standard deviation).

ISSN:0094-2405    

DOI:10.1118/1.597631

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For precise localization of the center and determination of the dimensions of the radioactive material within the capsule of brachytherapy sources, we have developed a method based on simultaneous pin‐hole autoradiography of two sources. We constructed a variable magnification pin‐hole camera consisting of two telescopically fitted Plexiglas cylinders which can accommodate two radioactive sources on the plate covering the top cylinder. The192Ir pulsed and high dose‐rate sources were studied and an192Ir seed was used as a reference source. The magnification factor was determined from the dimensions of the192Ir seed image, which was geometrically well defined by a separate transmission radiography experiment. The observed position for the center of radioactivity in the PDR and the HDR source capsules are in agreement with the vendor specifications. The distance from the tip of the PDR capsule to its center of radioactivity was found in this way to be 0.79±0.21 mm, which agrees with the position (0.85 mm) of the pellet situated closer to the tip as specified by the vendor. Quantitative verification of the internal source structure using this method enhances the accuracy with which the dose distribution near brachytherapy sources can be predicted by three‐dimensional Monte Carlo calculations.

ISSN:0094-2405    

DOI:10.1118/1.597538

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597439

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We present an imaging method to obtain a map of the spin‐lattice relaxation time. Images were acquired with the same spatial resolution and in the same time as for a regular spin‐echo acquisition. The sequence was based on the simultaneous acquisition of a spin echo and several stimulated echoes with the same intensity except forT1 weighting which increases with the interval between the excitation pulse and the readout pulse.T1 values obtained on phantoms were compared to those from the inversion‐recovery method and show the accuracy (2%) and the precision (5%) of the method.T1 images of the brain of a healthy volunteer are presented and demonstrate the ability of the method to obtainT1 mappinginvivoin 12 min and without susceptibility artifacts.Invivoandinvitroresults were compared to those obtained by a TOMROP sequence in the same acquisition time.

ISSN:0094-2405    

DOI:10.1118/1.597632

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The relative success of manual palpation in the detection of breast cancer would suggest that a method for remote palpation resulting in a measurement of tissue elasticity could provide a diagnostic tool for detecting cancerous lesions deeper within the breast. This presumption is based in part on the excellent contrast between neoplastic and normal tissue due to the large (orders of magnitude) relative variation in the shear elastic modulus. By comparison, the bulk deformational modulus maintains the same value to within 20% for most soft tissues. A specific method of magnetic‐resonance imaging (MRI) which measures tissue displacements has been used in experiments with a phantom containing regions of increased Young's modulus as a demonstration. The spatial modulation of magnetization technique uses the displacement of a spatial grid pattern caused by spin saturation to track regional motion. Mathematical reconstruction of the distribution of elastic moduli is shown for select examples. Any modality, e.g., MRI, ultrasound, etc., which can detect local tissue motion with sufficient spatial resolution can be used and therefore the results presented here should give an indication of the utility of such motion tracking techniques to future measurement of tissue elasticity.

ISSN:0094-2405    

DOI:10.1118/1.597633

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY