Dosimetry verification of virtual wedge using a two-dimensional ionisation chamber array / 中华放射肿瘤学杂志

Objective To investigate the virtual wedge (VW) dosimetric parameters with the ionization chamber array Matrixxenvlution.Methods Using Matrixxenvlution and solid water to measure and calculate Siemens accelerator's VW angle and VW factors of different fields and compare the wedge field dose distribution to that of treatment planning system (TPS) by gamma analysis,summarized the measurement results of 50 times.Results The γ pass rate (3 mm/ 3％) of 15° and 30° VW in both direction were (91.47 ± 1.76)％,(92.99 ± 1.54)％ and (93.27 ± 1.24)％,(93.27 ± 1.68)％,respectively,with the increase of filed size and the VW angle,but for 20 cm ×20 cm field and VW 60°,the result was not very good.The largest angle deviation ＜ 2° except small field size and wedge angle,VW factors were approximately equal to 1,the maximum deviation was no more than 0.05,plan dose distribution and the measured dose distribution have good consistency except large field with large wedge angle.Conclusions Matrixxenvlution used in the measurement of VW dosimetric parameters which can obtain all parameters for angle calculation and dose plane analysis with only once positioning,and become more rapid,convenient,economical and practical one of quality assurance tools for VW dose verification.

A Study on Dose Distribution using Virtual Wedge in Breast Cancer / 의학물리

In the radiation therapy for breast cancer patients, wedge shaped compensators are essentially used to achieve appropriate dose distribution because of thickness difference according to breast shapes. Tangential irradiation technique has usually been applied to radiation therapy for breast cancer patients treated with breast conservative surgery. When a primary beam is incident on wedge shaped compensators from medial direction in tangential irradiation technique, low energy scattered radiation is generated and gives additional dose to the breast surface. As a method to reduced additional dose to breast surface, the use of virtual wedge shaped compensator is possible. Eclipse radiation treatment planning (RTP) systems installed at our institution have virtual wedge shaped compensator for radiation therapy treatment planning. The dose distributions of 15, 30, 45, 60 degree physical wedges and virtual wedges were measured and compared. Results showed that there was no significant differences in symmetry of 10 x 10 field among various wedge angles. When the transmission factor was compared, transmission factor increased linearly as the wedge angle increased. These results indicates that the application of virtual wedge in clinical use is appropriate.

Contralateral Breast Dose Reduction Using a Virtual Wedge / 대한방사선종양학회지

PURPOSE: To evaluate the contralateral breast dose using a virtual wedge compared with that using a physical wedge and an open beam in a Siemens linear accelerator. MATERIALS AND METHODS: The contralateral breast dose was measured using diodes placed on a humanoid phantom. Diodes were placed at 5.5 cm (position 1), 9.5 cm (position 2), and 14 cm (position 3) along the medial-lateral line from the medial edge of the treatment field. A 6-MV photon beam was used with tangential irradiation technique at 50 and 230 degrees of gantry angle. Asymmetrically collimated 17 x 10 cm field was used. For the first set of experiment, four treatment set-ups were used, which were an open medial beam with a 30-degree wedged lateral beam (physical and virtual wedges, respectively) and a 15-degree wedged medial beam with a 15-degree wedged lateral beam (physical and virtual wedges, respectively). The second set of experiment consists of setting with medial beam without wedge, a 15-degree wedge, and a 60-degree wedge (physical and virtual wedges, respectively). Identical monitor units were delivered. Each set of experiment was repeated for three times. RESULTS: In the first set of experiment, the contralateral breast dose was the highest at the position 1 and decreased in order of the position 2 and 3. The contralateral breast dose was reduced with open beam on the medial side (2.70+/-1.46%) compared to medial beam with a wedge (both physical and virtual) (3.25+/-1.59%). The differences were larger with a physical wedge (0.99+/-0.18%) than a virtual wedge (0.10+/-0.01%) at all positions. The use of a virtual wedge reduced the contralateral breast dose by 0.12% to 1.20% of the prescribed dose compared to a physical wedge with same technique. In the second experiment, the contralateral breast dose decreased in order of the open beam, the virtual wedge, and the physical wedge at the position 1, and it decreased in order of a physical wedge, an open beam, and a virtual wedge at the position 2 and 3. CONCLUSION: The virtual wedge equipped in a Siemens linear accelerator was found to be useful in reducing dose to the contralateral breast. Our additional finding was that the surface dose distribution from the Siemens accelerator was different from a Varian accelerator.

Comparison of Virtual Wedge versus Physical Wedge Affecting on Dose Distribution of Treated Breast and Adjacent Normal Tissue for Tangential Breast Irradiation / 대한방사선종양학회지

PURPOSE: The ideal breast irradiation method should provide an optimal dose distribution in the treated breast volume and a minimum scatter dose to the nearby normal tissue. Physical wedges have been used to improve the dose distribution in the treated breast, but unfortunately introduce an increased scatter dose outside the treatment field, particularly to the contralateral breast. The typical physical wedge (PW) was compared with the virtual wedge (VW) to determine the difference in the dose distribution affecting on the treated breast and the contralateral breast, lung, heart and surrounding peripheral soft tissue. METHODS AND MATERIALS: The data collected consisted of a measurement taken with solid water, a Humanoid Alderson Rando phantom and patients. The radiation doses at the ipsilateral breast and skin, contralateral breast and skin, surrounding peripheral soft tissue, and ipsilateral lung and heart were compared using the physical wedge and virtual wedge and the radiation dose distribution and DVH of the treated breast were compared. The beam-on time of each treatment technique was also compared. Furthermore, the doses at treated breast skin, contralateral breast skin and skin 1.5 cm away from the field margin were also measured using TLD in 7 patients of tangential breast irradiation and compared the results with phantom measurements. RESULTS: The virtual wedge showed a decreased peripheral dose than those of a typical physical wedge at 15degrees, 30degrees, 45degrees, and 60degrees. According to the TLD measurements with 15degrees and 30degrees virtual wedge, the irradiation dose decreased by 1.35% and 2.55% in the contralateral breast and by 0.87% and 1.9% in the skin of the contralateral breast respectively. Furthermore, the irradiation dose decreased by 2.7% and 6.0% in the ipsilateral lung and by 0.96% and 2.5% in the heart. The VW fields had lower peripheral doses than those of the PW fields by 1.8% and 2.33%. However the skin dose increased by 2.4% and 4.58% in the ipsilateral breast. VW fields, in general, use less monitor units than PW fields and shortened beam-on time about half of PW. The DVH analysis showed that each delivery technique results in comparable dose distribution in treated breast. CONCLUSIONS: A modest dose reduction to the surrounding normal tissue and uniform target homogeneity were observed using the VW technique compare to the PW beam in tangential breast irradiation The VW field is dosimetrically superior to the PW beam and can be an efficient method for minimizing acute, late radiation morbidity and reduce the linear accelerator loading by decreasing the radiation delivery time.

Physical Characteristics Comparison of Virtual Wedge Device With Physical Wedge / 대한방사선종양학회지

PURPOSE: We have compared the characteristics of Siemens virtual wedge device with physical wedges for clinical application. MATERIALS AND METHODS: We investigated the characteristics and physical wedges for various wedge angles (15,30,45,and 60 degrees)using 6- and 15MV photon beams. Wedge factors were measured in water using an ion chamber for various field sizes and depths. In case of virtual wedge device, as upper jaw moves during irradiation, wedge angles were estimated by accumulated doses. These measurements were performed at off-axis points perpendicular to beam central axis in water for a 15Cm x20Cm radiation field size at the depth of 10Cm. Surface doses without and with virtual or physical wedges were measured using a parallel plate ion chamber at surface. Field size was15Cmx20Cm and a polystyrene phantom was used. RESULT: For various field sizes, virtual and physical wedge factors were changed by maximum 2.1%and 3.9%, respectively. For various depths, virtual and physical wedge factors were changed by maximum 1.9% and 2.9%, respectively. No major difference was found between the virtual and physical wedge angles and the difference was within 0.5 degrees. Surface dose with physical wedge was reduced by maximum 20% (x-ray beam : 6 MV, Wedge angle: 45 degrees, SSD; 80 Cm) relative to one with virtual wedge or without wedge. CONCLUSION: comparison of the characteristics of Siemens virtual wedge device with physical wedges was performed.Depth dependence of virtual wedge factor was smaller than of physical wedge factor. Virtual and physical wedge factors were nearly independent of field size. The accuracy of virtual and physical wedge angles was excellent. Surface dose was found to be reduced using physical wedge.

Dosimetry verifications of the physical parameters of virtual wedge on a Siemens accelerator / 中华放射肿瘤学杂志

Objective To verify the wedge angle of virtual wedge and the relation between wedge factor and beam energy, field size, wedge angle and to study the difference in percent depth dose (PDD) of virtual wedge field, hard wedge field and open field.Methods Using wedge angle and wedge factor of 15?,30?,45?and 60? virtual wedge of Siemens Mevatron 6?MV and Primus 8?MV, 18?MV X rays were measured by RFA-plus 3D water phantom and RK finger chamber the PDD of the virtual wedge field, hard wedge field and open field were measured by Kodak XV-2 verifying film and FDM-300 film dosimeter. These PDDs were normalized to Dmax then compared. Results There was good conformation between virtual wedge measured by four point method and set value. The virtual wedge was almost equal to 1,with a maximal variation of 0.031 no matter what the value of beam energy, field size or wedge angle was. Generally, for certain energy and field size, the wedge factor of larger wedge angle was slightly larger than smaller wedge angle. For certain energy and wedge angle, the wedge factor of larger field was also a little larger than smaller field. The PDD of virtual wedge field was similar to that of open field. Conclusions The four point method measurement for virtual wedge angle is good for daily QA. Radiotherapy of virtual wedge field is not only simpler than hard wedge field,but also spares the beam output. The PDD comfarmation between virtual field and open field simplifies radiation treatment planning and increases the accuracy of wedge field therapy.