Application of a radiophotoluminescent glass dosimeter to nonreference condition dosimetry in the postal dose audit system
Hideyuki MizunoAkifumi FukumuraMai FukahoriSuoh SakataWataru YamashitaNobuhiro TakaseKaori YajimaTetsurou KatayoseKyoko Abe‐SakamaYohsuke KusanoMunefumi ShimboTatsuaki Kanai
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The purpose of this study was to obtain a set of correction factors of the radiophotoluminescent glass dosimeter (RGD) output for field size changes and wedge insertions.Several linear accelerators were used for irradiation of the RGDs. The field sizes were changed from 5 × 5 cm to 25 × 25 cm for 4, 6, 10, and 15 MV x-ray beams. The wedge angles were 15°, 30°, 45°, and 60°. In addition to physical wedge irradiation, nonphysical (dynamic/virtual) wedge irradiations were performed.The obtained data were fitted with a single line for each energy, and correction factors were determined. Compared with ionization chamber outputs, the RGD outputs gradually increased with increasing field size, because of the higher RGD response to scattered low-energy photons. The output increase was about 1% per 10 cm increase in field size, with a slight difference dependent on the beam energy. For both physical and nonphysical wedged beam irradiation, there were no systematic trends in the RGD outputs, such as monotonic increase or decrease depending on the wedge angle change if the authors consider the uncertainty, which is approximately 0.6% for each set of measured points. Therefore, no correction factor was needed for all inserted wedges. Based on this work, postal dose audits using RGDs for the nonreference condition were initiated in 2010. The postal dose audit results between 2010 and 2012 were analyzed. The mean difference between the measured and stated doses was within 0.5% for all fields with field sizes between 5 × 5 cm and 25 × 25 cm and with wedge angles from 15° to 60°. The standard deviations (SDs) of the difference distribution were within the estimated uncertainty (1SD) except for the 25 × 25 cm field size data, which were not reliable because of poor statistics (n = 16).A set of RGD output correction factors was determined for field size changes and wedge insertions. The results obtained from recent postal dose audits were analyzed, and the mean differences between the measured and stated doses were within 0.5% for every field size and wedge angle. The SDs of the distribution were within the estimated uncertainty, except for one condition that was not reliable because of poor statistics.Keywords:
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Wedge (geometry)
The purpose of the work is to compare the beam profiles and wedge factors of physical and enhanced dynamic wedge.The study was conducted using the Varian clinac IX linear accelerator with different photon energies 6MV and 15MV.The slab phantom arranged at a fixed depth (5cm depth) above the PTW seven29 ion chamber array.The physical and enhanced dynamic wedges different angles (15 0 , 30 0 , 45 0 and 60 0 ) were examined for the field sizes, 5x5cm 2 , 10x10cm 2 and 15x15cm 2 by delivering 100 monitoring unit (MU).The same procedure is repeated using 0.6 cc farmer type ionization chamber to find out the wedge factors.From the profiles obtained it was understood that as field size increases there seems only a slight difference between physical wedge (PW) and enhanced dynamic wedge (EDW) in the penumbral region of the profile.The percentage variation of measured dose between physical and enhanced dynamic wedge at shoulder region in beam profiles for the measured field sizes lies between 0.03 and 2.2.The standard deviation of measured dose at shoulder region, between different field sizes lies between 0.099 and 0.188.Based on the results of the study, it can be concluded that the Seven29 2D ion chamber array and 0.6cc Farmer type ionisation chamber can be used effectively to obtain the dosimetric characteristics of both EDW and Physical wedge.
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SU‐FF‐T‐220: Evaluation of the Performance of An Implantable Dosimeter Used in 4D‐Gated Radiotherapy
Purpose: To evaluate the performance of an implantable dosimeter for treatment delivery and daily dose verification for 4D‐gated radiotherapy Method and Materials: The DVS® dosimeter from Sicel Technologies, Inc. measured doses delivered to a volume in a Quasar™ Respiratory Motion Phantom. DVS® measurements were compared to the Pinnacle's treatment planning results and to those made by a NIST‐traceable calibrated ion chamber. Each dosimeter measured dose within the phantom's lung equivalent moving insert. The phantom was scanned on a GE Lightspeed RT‐CT scanner using the 4D respiratory gating option. From the scans, a composite CT data set was built and an average GTV was defined within the lung equivalent moving insert. A treatment plan was created using the composite scans. Both the ion chamber and DVS® were used to track GTV dose. A second plan was created that uses a gated‐beam able to deliver radiation to the GTV during the expiration breathing phases only. Results: Both plans were created to deliver 200 cGy per fraction for 5 fractions to the GTV. The first plan included an average GTV that includes all the breathing phases. The DVS® and the ion chamber differed up to 3% from the Pinnacle average GTV plan. Measurements between the two dosimeters differed by 2.5%. Measurements made by the DVS® and the ion chamber for the gated‐ treatment are also comparable to those made using the treatment plan with all breathing phases. Conclusions: The measurements made by the DVS® dosimeter falls within the device's specified accuracy of 3%. The Dose Verification System provides the ability to verify that the prescribed dose is delivered during 4D‐gated treatments on a daily basis. Conflicts of interest: Sicel Technologies, Inc provided the equipment and materials used in the study.
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ObjectiveThe effects of field size and asymmetric field on wedge factors are studied.Method The measurment of wedge factors were performed for 60°autowedge of the BJ- 6B linear accelerator for field size from 4 cm×4 cm up to 18 cm×18 cm and asymmetric field.ResultWedge factors had an obvious dependence on the field size, and although dependence on the asymmetric field.ConclusionIn clinical dose calculation, it is sig- nificant to use relative field size wedge factors. It should be adjusted that effect of field size and asymmetric field on wedge factors.
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We refer to the recent technical note by Thomas (1990) in which the variation in wedge factor with field size was discussed. The linear accelerator used to produce the results in that article was a Dynaray LA 20 (Asea-Brown Boveri), in which the motorized wedge lies between the beam monitor and the movable, secondary collimators (Fig. 1a). We suggest that this wedge factor variation with field size may not occur with all designs of linear accelerator.
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When a wedge filter is used to modify the dose distribution in a beam of megavoltage X-rays, the beam intensity is reduced. The reduction of beam intensity is quantified by a factor called the wedge factor (WF) defined as follows: where measurements are performed in water at the depth of maximum build-up. It is assumed by the authors of several treatment planning systems that the wedge factor doses do not vary with field size. Palta et al (1988) presented measurements for a range of square field sizes with 4 MV and 6MV X rays, showing that there can be considerable variation of wedge factor with field size, especially for a 60° wedge, and that errors in dose of up to 7% can occur by ignoring this variation. This technical note presents the results of similar measurements on 8 MV and 16 MV beams with a 60° wedge. The variation of wedge factor for a range of rectangular field sizes has also been investigated; this was done to see whether the variation follows an equivalent square law, or whether one should follow the practice of one commercial planning system by considering only the dimension in the wedged direction.
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We refer to the letter by Dean and Davis (1991) in which the authors suggested that “no extensive wedge factor versus field size measurements need be done for linear accelerators designed with a wedge mounted below the movable collimators”. They reported measurements on a Varian 2100C linear accelerator showing very little variation of wedge factor with field size, and contrasted these measurements on a Philips SL75/14 (Palta et al, 1988) and on an ABB LA20 (Thomas, 1990), both of which exhibited large increases in wedge factor (WF) with increasing field size.
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The structural principles and technical performances of different ionization chamber dosimeters used in radiotherapy have been briefly introduced in this paper. In the mean time, a comparison of technical performances of four different kinds of ionization chamber dosimeter used in hospitals in China has also been made. The data show their quality meet the required specification specified by IEC731 and JJG912-96, so they can be used as field survey dosimeters in hospitals. Some precautions in using these dosimeters are given here too.
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