A modified sector-integration method has been developed that predicts electron beam output factor at any point on the beam central axis, for a given source to surface distance (SSD), as a function of the geometry of the irradiated field. The main concept of this method is that with the arbitrary field shape divided into small sectors, the individual contributions from each sector can be calculated based on the sector radius, using a dataset consisting of circular inserts of standard radii. A computer program was developed based on this algorithm. The program interfaces to a digital camera that is used to capture the shape of the electron insert. We compared the calculated and the measured output factors and per cent depth doses (PDDs) at different SSDs for various rectangular inserts and a typical irregularly shaped insert used in our clinic. To determine the geometric limitations of this algorithm, a series of rectangular inserts were designed with the long-to-short axis ratio between 1:1 and 7:1. The agreement between calculation and measurement for the electron output and PDD was generally within 2% (or 2 mm) for energies from 6 to 20 MeV.
A modified sector-integration method is presented that can predict the output factors of irregular shaped electron fields even in the case of extended source to surface distance (SSD). The model takes as input measured output factors for circular inserts of various radii. These circular fields were measured at SSDs of 100, 105 and 110 cm to determine the effective source distance as a function of radius (ESD(r)). For an arbitrary electron field at any SSD, the shape is divided into small sectors, and the contribution calculated from the radius and ESD(r). The calculated output factors were verified by direct measurements of various types of electron fields mainly based on clinical use. The energies modelled were 8, 10 and 12 MeV for applicator sizes of 10 cm x 10 cm and 14 cm x 14 cm (defined at 95 cm). The calculated values agreed with the measured data within 1% for the various rectangular cutouts including extended source to surface distance. We retrospectively modelled 97 patient inserts of irregular shape, and found agreement within 2% of measured values.
Purpose: To investigate the response of electronic portal imaging device (EPID) in integration mode for intensity modulated beams including step‐ and‐shoot (SS) and sliding window (SW) deliveries. Methods: We evaluated EPID dose response measurements of open, SS, and SW irradiations. We designed three beams using 6MV x‐rays with 10×10 cm2 field size. For the SS irradiation, two MLC leaves with 0.5 cm opening outside 10 × 10 cm2 were discontinuously moved by 10 cm during each segmental irradiation among ten segments of the entire delivery. For the SW beam, the same MLC leaves were continuously moved by 10 cm ten times during the delivery. The employment of the same open beam area ensures the same dose irradiation by the three. Monitor units were varied. We additionally investigated the dose linearity of EPID response. We used EPID‐ADU1000 operated with IAS3 system for the integrated acquisition. Results: The SS delivery showed a deviation greater than 2% from the open beam, if a segment employed less than 10 MUs. If it employed greater than 10 MUs, then a deviation less than 1% was observed. The SW delivery showed a maximum deviation of 1.4% at the lowest MU. The open beam irradiation showed linearity within 0.8%, the SW irradiation 1.2%, and the SS irradiation 1.7%. Conclusions: The SS delivery is associated with a greater error than the SW delivery due to beam instability at the start of acquisition and reading loss during MLC movement. Using large monitor units greater than 10 MUs/segment can minimize the deviation for the SS delivery from open beam. The linearity of EPID response to dose was better with the SW delivery than the SS delivery. This study will help understand dosimetric response of EPID for the SS delivery.
This study was in part supported by Varian Medical Systems, Inc.
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