ABSTRACT
AIM: To validate the Acuros®XB (AXB) dose calculation algorithm for a 6â¯MV beam from the Varian TrueBeam treatment units. BACKGROUND: Currently Anisotropic Analytic Algorithm (AAA) is clinically used on authors' department but AXB could replace it for VMAT treatments in regions where inhomogeneities and free air are present. MATERIALS AND METHODS: Two steps are followed in the validation process of a new dose calculation algorithm. The first is to check the accuracy of algorithm for a homogenous phantom and regular fields. Multiple fields of increasing complexity have been acquired using a Mapcheck diode array. The accuracy of the algorithm was evaluated using the gamma analysis method. The second is to validate the algorithm in the presence of heterogeneous media. Planar absolute dose was measured with GafChromic®EBT2 film and was compared with the dose calculated by AXB. Gamma analysis was performed between Mapcheck measurements and AXB dose calculations, at a range of clinical source-surface distance. RESULTS: For SSDs ranging from 80 to 100â¯cm, the results show a minimum pass rate of 95% between AXB and Mapcheck acquisition. For open 6â¯MV photon beam interacting with a phantom with an air gap, the agreement after the air gap between AXB and GafChromic®EBT2 is less than 1% in the 3â¯×â¯3cm2 field and less than 2% in the 10â¯×â¯10â¯cm2 field. CONCLUSIONS: AXB has advanced modelling of lateral electron transport that enables a more accurate dose calculation in heterogeneous regions and, compared with AAA, improves accuracy between different density interfaces. This will be of particular benefit for head/neck treatments.
ABSTRACT
The characteristics of an Elekta amorphous silicon (a-Si) electronic portal imaging device (EPID) in response to a 6 MV photon beam generated without a flattening filter, an unflattened beam, have been determined. The characteristics were then compared to those for a conventional photon beam generated with a flattening filter in the beam, a flattened beam, in order to determine the suitability of an a-Si EPID for transit dosimetry. The response of the EPID to the unflattened beam increased by 7.3% compared to the flattened beam, and copper buildup of 3 mm reduces the variation in the EPID response over air gaps ranging from 60 to 40 cm to within 2.5%. The scattering properties of the EPID with changing field size for the unflattened beam agree with those measured for a flattened beam to within 2%. Due to the minimal variation in the energy spectrum of the unflattened beam with the distance from the central axis, it was expected and experimentally found that the profile shape of the unflattened beam changes minimally with increasing phantom thickness. For an unflattened beam, EPID measured profiles with and without a phantom in the beam agree to within 2% using confidence limits. The difference between EPID and ionization chamber profiles measured at a depth of 5 cm in water is reduced compared to a flattened beam and remains unchanged with increasing phantom thickness. A difference of 4% was found between EPID profiles and the corresponding profiles measured with an ionization chamber measured in water over a range of phantom thickness. A calibration procedure was developed to convert EPID images to the equivalent absolute dose in water, at the EPID plane. A gamma evaluation was performed comparing the calibrated EPID images to dose measured with an ionization chamber array for rectangular fields and an IMRT segment. The fields were situated on axis and at 5 cm off axis with and without a 25 cm thick phantom in the beam. The gamma evaluation criteria of 3% and 3 mm were met within the field, for all fields examined. This study concludes that a-Si EPIDs are suitable dosimeters for IMRT treatments using unflattened photon beams with the advantage that the characteristics of the unflattened beam result in a reduction in the number of measurements necessary to calibrate an a-Si EPID as a transit dosimeter.
