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Comparison of measured and computed portal dose for IMRT treatments
Abstract
A new 2D array Seven 29TM model (PTW, Freiburg), equipped with 729 vented plane-parallel ion-chambers, projected for pre-treatment verification of radiotherapy plans has been used as a detector for the transmitted or portal dose measurements below a Rando phantom. The appealing dosimetric qualities of the 2D-array make it attractive for measuring transmitted dose maps from step and shoot IMRT. It is well known that for step and shoot IMRT beams that use a small number of monitor units per sequence, the early and recent electronic portal imaging devices present a different response at x-ray start-up that affects the accuracy of the measured transmitted dose.
The comparison of portal doses measured to ones calculated by a commercial treatment planning system can verify correct dose delivery during the treatment. This direct validation was tested irradiating a simulated head tumor in a Rando anthropomorphic phantom by step and shoot IMRT beams. The absolute transmitted doses on a plane, orthogonal to the beam central axis below the phantom were measured by the 2D-array calibrated in terms of dose to water and compared with the computed portal dose extracted by a home-made software.
In a previous paper the comparison between the IMRT portal doses, computed by a commercial TPS, and measured by a linear array that supplied a 1 mm spatial dose resolution, was carried out. The gamma index analysis supplied an agreement of more than 95% of the dose-point with acceptance criteria, in terms of dose-difference, DDmax , and distance-agreement, Ddmax , equal to 4% and 4mm, respectively.
In this paper we have verified the possible use of the PTW 2D-array for measurements of the transmitted doses during several fractions of the head-neck tumor radiotherapy. There could be two advantages in the use of this 2D array as a portal dose device for the IMRT quality assurance program. First is the ability to perform absolute dose comparisons for hundreds of measurement positions to verify the correct dose delivery in several fractions of the therapy, and second is the efficiency in time to detect this kind of dose distributions within FOV area of the CT scanner.
The comparison of portal doses measured to ones calculated by a commercial treatment planning system can verify correct dose delivery during the treatment. This direct validation was tested irradiating a simulated head tumor in a Rando anthropomorphic phantom by step and shoot IMRT beams. The absolute transmitted doses on a plane, orthogonal to the beam central axis below the phantom were measured by the 2D-array calibrated in terms of dose to water and compared with the computed portal dose extracted by a home-made software.
In a previous paper the comparison between the IMRT portal doses, computed by a commercial TPS, and measured by a linear array that supplied a 1 mm spatial dose resolution, was carried out. The gamma index analysis supplied an agreement of more than 95% of the dose-point with acceptance criteria, in terms of dose-difference, DDmax , and distance-agreement, Ddmax , equal to 4% and 4mm, respectively.
In this paper we have verified the possible use of the PTW 2D-array for measurements of the transmitted doses during several fractions of the head-neck tumor radiotherapy. There could be two advantages in the use of this 2D array as a portal dose device for the IMRT quality assurance program. First is the ability to perform absolute dose comparisons for hundreds of measurement positions to verify the correct dose delivery in several fractions of the therapy, and second is the efficiency in time to detect this kind of dose distributions within FOV area of the CT scanner.
Keywords
Portal dose, IMRT, Radiotherapy