Abstract
We used the two available calculation algorithms of the Varian Eclipse 7.3 three‐dimensional (3D) treatment planning system (TPS), the anisotropic analytic algorithm (AAA) and pencil‐beam convolution (PBC), to compare measured and calculated two‐dimensional enhanced dynamic wedge (2D EDW) dose distributions, plus implementation of the dynamic wedge into the TPS. Measurements were carried out for a 6‐MV photon beam produced with a Clinac 2300C/D linear accelerator equipped with EDW, using ionization chambers for beam axis measurements and films for dose distributions. Using both algorithms, the calculations were performed by the TPS for symmetric square fields in a perpendicular configuration. Accuracy of the TPS was evaluated using a gamma index, allowing 3% dose variation and 3 mm distance to agreement (DTA) as the individual acceptance criteria. Beam axis wedge factors and percentage depth dose calculation were within 1% deviation between calculated and measured values. In the non‐wedged direction, profiles exhibit variations lower than 2% of dose or 2 mm DTA. In the wedge direction, both algorithms reproduced the measured profiles within the acceptance criteria up to 30 degrees EDW. With larger wedge angles, the difference increased to 3%. The gamma distribution showed that, for field sizes of 10×10 cm or larger, using an EDW of 45 or 60 degrees, the field corners and the high‐dose region of the distribution are not well modeled by PBC. For a 20×20 cm field, using a 60‐degree EDW and PBC for calculation, the percentage of pixels that do not reach the acceptance criteria is 28.5%; but, using the AAA for the same conditions, this percentage is only 0.48% of the total distribution. Therefore, PBC is not reliable for planning a treatment when using a 60‐degree EDW for large field sizes. In all the cases, AAA models wedged dose distributions more accurately than PBC did.PACS numbers: 87.53.Bn, 87.53.Dq, 87.53.Kn
Highlights
Wedge-shaped isodoses are required in many clinical situations
Evaluation of calculations We evaluated the treatment planning system (TPS) by comparing measurements and calculations performed under the same conditions, based on a phantom created by the TPS with a homogeneous density of 1 g/cm3, emulating a plastic water phantom with density of 1.013 g/cm3
It was possible to verify for the open fields a better adjustment of the penumbra region when using the analytic algorithm (AAA) than when using the pencil-beam convolution (PBC)
Summary
Wedge-shaped isodoses are required in many clinical situations. Sloping distributions can be obtained by inserting a physical wedge in the beam. Physical wedges have several unfavorable functional and dosimetric characteristics[1] such as beam hardening, fixed wedge angles, and limited field size. 48 Caprile et al.: Comparison between measured and calculated. With the advent of computer control, the simulation of wedge filters by movement of the collimator jaw during the irradiation process becomes possible.[2] The dynamic wedge (DW) has considerable advantages over the physical wedge. The use of DW requires accurate configuration of the treatment planning system (TPS). Implementation of DW requires measurement of percentage depth doses (PDDs), beam profiles, and wedge factors (WFs). Measurements must take into account the need for integration of the dose at the measurement point for the wedged fields during the entire exposure. The procedure has been described in previous works.[3,4,5,6]
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