ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1996.tb01033.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The definition of tissue–air ratio (TAR) is based on the concept of primary dose. To determine TAR, both in‐phantom and in‐air ionization measurements are utilized. To convert ionization in the phantom into dose and that in air into primary dose, correction factors must be applied to chamber readings in both geometries. Due to difficulties in selecting proper correction factors, TAR is subject to systematic error. The error comes from two sources of uncertainty: (1) Primary dose cannot be measured. Therefore approximate methods, such as in‐air ionization measurements, are used. (2) Detectors of ionization are of finite dimensions and they are inhomogeneous. In this study, analytical expression for a systematic error is derived. Because in this derivation systematic error is an accumulative error, it is no longer necessary to convert ionization, both in air and in phantom, into a dose when calculating TARs. A method of determining systematic error is described. This method is based on the ability to produce accurate zero‐field data in photon beams by means of a linear extrapolation technique. Using60Co γ radiation in water as an example, it is shown how to generate TAR data free of systematic error. A possibility of determining TARs for therapeutic x rays is discussed.

ISSN:0094-2405    

DOI:10.1118/1.597864

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A dynamic multileaf collimator (D‐MLC) was used to investigate the feasibility of producing missing‐tissue compensators. The modulation of the x‐ray field in two dimensions produced by conventional physical compensators was mimicked by delivering a sequence of D‐MLC‐shaped subfields. A method is introduced to calculate monitor units (MU) for dynamically compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculations. In this investigation, the tissue deficit at the surface of an anatomical phantom was measured using a Moiré camera. The tissue deficit data were used to generate a series of D‐MLC subfields that, delivered in sequence, provided the compensated treatment. Film was used to integrate the dose delivered to a specified depth of compensation. Isodose distributions were measured for uncompensated fields, fields compensated with a conventional lead or plastic filter, and fields compensated with the D‐MLC. A comparison of the dose distributions shows the compensation achieved with the dynamic compensating filter is comparable to that achieved using conventional physical compensating filters.

ISSN:0094-2405    

DOI:10.1118/1.597675

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The collimator scatter factorSchas generally been measured in air using an ionization chamber inside a buildup cap or mini‐phantom. Here,Scwas measured in phantom at 10 cm depth for 6 and 15 MV photons with square collimator settings of 2.5–40.0 cm. The results were consistent with in air measurements with a mini‐phantom to within 0.4%. In the measurements, a series of Cerrobend field shaping blocks were used to define the field size in the phantom while the collimator settings were varied from the field size in the phantom to twice that value. Corrections of up to 2% for scattered radiation from the added Cerrobend field shaping blocks were necessary. Since a buildup cap or mini‐phantom is not used, the smallest field size that can be measured is limited only by the size of the detector and the measurement is performed with full scatter resembling the treatment condition of a patient.

ISSN:0094-2405    

DOI:10.1118/1.597676

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The output factor for the dynamic wedge, unlike that for the physical wedge, is a complex function of the field dimension along the moving jaw and wedge angle. The large change in output (varying as much as 40% for 45° and 60° wedge angles) can be attributed clearly to the segmented treatment tables (STTs), which specify cumulative monitor unit weighting as a function of jaw position,y. We found that the output factor (in air or water) on the central axis for the dynamic wedge can be characterized by multiplying the output factor (in air or water) for an open field by a normalization factor, which is determined from the STTs, thus indicating that collimator scatter is similar for both the dynamic wedge field and the open field. The introduction of the normalization factor decreases the commissioning time for dynamic wedges significantly and is useful for quality assurance.

ISSN:0094-2405    

DOI:10.1118/1.597677

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A commercial linear accelerator with a factory‐fitted multileaf collimator (MLC) was commissioned for clinical use. Measurements made of dosimetric parameters included central axis depth‐dose, field‐size factors, wedge factors, penumbra, and leaf leakage for the 6‐MV and 15‐MV photon beams available on this accelerator. The depth‐dose characteristics, output factors, and transmission factors were similar to those reported in the literature for a machine by the same manufacturer with a standard treatment head. Because of scalloping, the effective penumbra for the MLC was 3 to 4 mm wider than that for the conventional collimator jaws. The output for the fields shaped by the MLC was generally lower than that for similar fields shaped with Lipowitz's metal (Cerrobend). The magnitude of the difference was field‐size dependent and ranged from 0.5% to 4.5% for open shaped fields, increasing to 1% to 5% in the presence of wedges. Further analysis of this observation has shown it to be primarily due to differences in the scattered radiation from the collimator head.

ISSN:0094-2405    

DOI:10.1118/1.597678

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

With all the advantages of film dosimetry in the megavoltage energy range, the use of film as a dosimeter is still limited due to the various difficulties associated with films such as energy dependence, film orientation, and sensitometric nonlinearity. Recently, therapy verification and localization films (CEA TVS and TLF films) from a Swedish manufacturer have become available in vacuum‐sealed water‐proof packaging in the US. The packaging renders the CEA films useful in a water phantom and ideal for photon and electron dosimetry. A systematic study has been carried out to investigate the potential of dosimetric application of the new films for high energy photon and electron beams. For the TVS films, the characteristic curve is generally energy independent but appears to be dependent on the source of the radiation, i.e., whether it is gamma rays or bremsstrahlung x rays. Compared to Kodak Readypack XV films, the CEA TVS film is linear in optical density over a much larger range of radiation dose. The inter‐ and intra‐variation of the TVS films is less than 2%. For electrons, the characteristic curve is linear over a similar density range as photons but exhibit a slight energy dependence. TVS film is slightly directional dependent on the incident radiation for both photons and electrons. The perpendicular orientation results in higher optical density than the parallel orientation. The differences are within ±2% except in the buildup region for photons and in the exponential fall‐off region of the electron beams where differences up to 4% are noted. For the CEA TLF film which is about three times faster than the TVS film, the characteristic curve is reasonably linear over the dose range of 0–15 cGy and energy independent within the experimental uncertainty (±5%). Percent depth dose and isodose measurements with the TVS films are in good agreement with ion chamber results.

ISSN:0094-2405    

DOI:10.1118/1.597865

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597684

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A quality control (QC) test suitable for routine daily use has been developed for video based electronic portal imaging devices. It provides an objective and quantitative test for acceptable image quality on the basis of the high contrast spatial resolution and the contrast‐to‐noise ratio (CNR). The test uses a phantom consisting of five sets of high‐contrast rectangular bar patterns with spatial frequencies of 0.1, 0.2, 0.25, 0.4, and 0.75 lp/mm. Data obtained during a one month calibration period were used to determine a critical frequencyfcfor the relative square wave modulation transfer function and a critical contrast‐to‐noise ratio (CNRc). Subsequent measurements indicating significant deviations from these critical values result in warning messages to the operator indicating potential problems in system performance. Measurements over a period of two years show that the QC test provides a sensitive indication of imaging performance.

ISSN:0094-2405    

DOI:10.1118/1.597866

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Projected light fields are used on treatment simulators and teletherapy treatment units to delineate the size and position of the radiation beam. Any discrepancy between these fields will lead to a systematic field placement error, with possibly serious implications with regard to the accuracy of the delivered dose distribution in the patient. Conventionally, film has been used for regular quality control tests of light and radiation field congruence, but this is a time consuming method and is not suitable for daily checks. A new method is described that uses a specially designed test phantom, a video‐based electronic portal imaging device, and a personal computer to test for radiation and light field congruence on treatment accelerators. This method consists of aligning the test phantom in the light field of a treatment linac and acquiring an electronic portal image. A computer program then automatically analyzes the image and determines the degree of congruence between the two fields. The final result of the test is ago,warning, ornogodecision depending on the extent of misalignment between the light and radiation fields. Two algorithms were tested for reproducibility (<0.4 mm), sensitivity to noise (<0.2 mm), and positional accuracy (<0.4 mm) and are shown to give results comparable to the conventional film method. Daily testing of field congruence over a period of 84 days demonstrated differences in the results determined by the two algorithms of less than 0.1±0.2 mm (standard deviation) at 6 MV and 0.22±0.13 mm at 23 MV. Routine testing is possible as the effort and time required are minimal, and the test can be performed during daily routine start‐up procedures.

ISSN:0094-2405    

DOI:10.1118/1.597867

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY