摘要:

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer‐controlled radiation therapy, its application to produce wedge‐shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer‐controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.

ISSN:0269-3127    

DOI:10.1118/1.594440

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A relation is developed between tolerance dose and field size from an analysis of available clinical data on skin‐tolerance doses for various treatment field sizes in radiotherapy. It is shown that plots of log‐dose against log‐field‐size for various fractionation schedules give isoeffect lines all having approximately the same slope. This suggests a power law relation between tolerance dose and field size. A generalized form of NSD equation, incorporating the field‐size dependence of tolerance dose, is shown to follow naturally when this new relation is combined with Ellis’ NSD relation. Certain discrepancies between the sets of data used in the present analysis are pointed out and it is suggested that more data are needed before the generalized NSD relation can be applied under varying conditions of dose, time, fractionation, and field size.

ISSN:0269-3127    

DOI:10.1118/1.594441

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

As a result of a reported incident which resulted in the unintended overexposure of several hundred patients, teletherapy installations throughout the United States were evaluated to verify the accuracy of teletherapy equipment calibrations. Selected licensees received TLDs for exposure under specified conditions. Site visits were made to each licensee showing an apparent discrepancy greater than ± 5% and intercomparison measurements made. This paper describes results of a sampling of 373 teletherapy licensees and the success of site visits in reducing final discrepancies to less than ± 5%. TLD and site visit measurement techniques, results, comparison with data from other studies, and possible sources of uncertainty in the management of a large‐scale screening program are discussed.

ISSN:0269-3127    

DOI:10.1118/1.594485

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effects of field elongation on the central‐axis output atdmaxand on the depth‐dose curves for the 6‐MV x rays from the Therac 6 linear accelerator are considered. These parameters are independent of the collimator angle for both square and elongated fields. The exchange of collimator pairs results in significant output differences but has a negligible effect on the depth‐dose curves. Both the central‐axis output atdmaxand the depth‐dose curves agree satisfactorily with the predictions of the equivalent‐square technique. An alternative method of greater accuracy is also indicated for the prediction of central‐axis outputs atdmaxfor elongated fields.

ISSN:0269-3127    

DOI:10.1118/1.594508

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

One of the important factors affecting radiographic image quality is the film contrast of the recording system, which may be expressed by the gradient of its H &  D curve. Experimental determinations of H & D curves are subject to relatively small error, which can nevertheless cause large fluctuations and uncertainties in the calculated gradient. To reduce this fluctuation, we wrote a computer program for orthogonal‐polynomial (discrete Legendre polynomials) curve fitting by means of least‐squares estimation of parameters. This procedure facilitates curve smoothing and gradient computation following sensitometric measurements. The program was tested against a large number of measured characteristic curves. A brief description of the program is given, with examples which illustrate its applications.

ISSN:0269-3127    

DOI:10.1118/1.594472

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Therac 6 is new generation of low‐energy linear accelerator. It incorporates a PDP‐11/05 computer for beam control, treatment‐factor input, and beam shutdown in the event of failure of the system. The performance of the unit has not been hindered by computer or software malfunction, and the computer has provided an excellent means for preventive maintenance and repair. Dosimetry parameters are similar to other 6 MV x‐ray beams, and comparison to60Co therapy beams shows that this unit may be more like60Co units in penumbra and absence of off‐axis peaking than other low‐energy accelerators.

ISSN:0269-3127    

DOI:10.1118/1.594442

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594518

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594495

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594496

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594497

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY