Abstract
Nowadays, in most radiotherapy departments, the commercial treatment planning systems (TPS) used to calculate dose distributions needs to be verified; therefore, quick, easy‐to‐use, and low‐cost dose distribution algorithms are desirable to test and verify the performance of the TPS. In this paper, we put forth an analytical method to calculate the phantom scatter contribution and depth dose on the central axis based on the equivalent square concept. Then, this method was generalized to calculate the profiles at any depth and for several field shapes — regular or irregular fields—under symmetry and asymmetry photon beam conditions. Varian 2100 C/D and Siemens Primus Plus linacs with 6 and 18 MV photon beam were used for irradiations. Percentage depth doses (PDDs) were measured for a large number of square fields for both energies and for 45° wedge, which were employed to obtain the profiles in any depth. To assess the accuracy of the calculated profiles, several profile measurements were carried out for some treatment fields. The calculated and measured profiles were compared by gamma‐index calculation. All γ–index calculations were based on a 3% dose criterion and a 3 mm dose‐to‐agreement (DTA) acceptance criterion. The γ values were less than 1 at most points. However, the maximum γ observed was about 1.10 in the penumbra region in most fields and in the central area for the asymmetric fields. This analytical approach provides a generally quick and fairly accurate algorithm to calculate dose distribution for some treatment fields in conventional radiotherapy.PACS number: 87.10.Ca
Highlights
For a given energy spectrum incident on a homogeneous medium and assuming lateral electronic equilibrium, the primary component of central axis depth dose for any field shape will be the same, and only differences in the scatter component will affect the final shape of the central axis depth dose; the concept of equivalent field size based on the separation of the primary and scatter radiation was proposed by Day and Aird.[4]. For regular fields, tabulated data was presented by Day and Aird, or some empirical formulas by fitting were carried out, such as the equal area-to-perimeter ratio (A/P).(5)
This paper introduces an analytical method to calculate the equivalent field in regard to the central axis, first
Beam profile and Percentage depth doses (PDDs) are the parameters used to verify the dose calculation algorithms of treatment planning systems (TPS); a patient-independent model calculating beam profile and PDD can be used to minimize the number of measurements for verification processes.[3]
Summary
For a given energy spectrum incident on a homogeneous medium and assuming lateral electronic equilibrium, the primary component of central axis depth dose for any field shape will be the same, and only differences in the scatter component will affect the final shape of the central axis depth dose; the concept of equivalent field size based on the separation of the primary and scatter radiation was proposed by Day and Aird.[4]. Fermi-Dirac distribution function suggested by Kornelson and Young[6] and Wood-Saxon term applied by Pal et al[7] represent the off-axis ratio (OAR) in the SAD and SSD techniques, respectively. These methods need data fitting at several depths. A second method, proposed by Kwa et al,(12) applies to the situation where only one of the independent jaws is closed down to form an asymmetric field of smaller width or length than the original symmetric field This method uses the original symmetric field profile corrected point-by-point by a correction factor
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