Abstract
The aim of this study is to determine the gantry angle and multileaf collimator (MLC) gap error‐detection threshold of a diode helical array with an inserted microionization chamber in order to use this device for the pretreatment quality assurance (QA) of intensity‐modulated radiation therapy (IMRT) treatments. Implications on the dose‐volume histograms (DVHs) of the patient treatments will also be considered for the establishment of a QA protocol with a reasonable tolerance level. Three dynamic IMRT HN (head and neck) and prostate treatments were studied. Random and systematic variations of gantry angle and systematic errors in MLC gap width of the clinical treatments were analyzed in order to establish the detection sensitivity of the array. The associated clinical significance was studied introducing the same errors in the treatment plan based on the patients' computed tomography (CT) and calculating the corresponding DVHs. The Gamma (3%/3 mm) presented a 4% variation in failure rate for a rotation error of 1° for both types of treatment. Both systematic and random errors in gantry rotation angle have little effect on the patients' DVHs. MLC gap width errors of 1 mm and 2 mm in the prostate treatments imply a mean variation in isocenter‐measured absorbed dose of 2.1% and 4.1%, respectively. In the case of HN, these errors entail a change in measured isocenter dose of 4.7% and 8.6%, respectively. The variation observed in the DVHs of the patients was, basically, a global displacement of the curves proportional to the isocenter dose variation caused by the gap width error. According to the array sensitivity to the analyzed errors and its implication in patient DVHs, a tolerance of 95% point passing rate for the gamma criterion 3%/2 mm and an agreement of 2% in isocenter absolute dose have been established as tolerance criteria for our pretreatment IMRT QA protocol.PACS number: 87.56.Fc
Highlights
112 García-Vicente et al.: Sensitivity of a helical array lead to the overlooking of some potential errors as the result of the gantry rotation and the multileaf collimator (MLC) positioning
The aim of this study is to evaluate the error-detection threshold of this array for the pretreatment intensity-modulated radiation therapy (IMRT) quality assurance (QA)
Our study shows that if the positive gap width error affects all pairs of leaves uniformly, the result is an increase of the absorbed dose in PTVs and OARs for both prostate and high degree modulation treatments (HN) treatments (Fig. 6): the shape of the error-free and error-introduced dose-volume histograms (DVHs) curves are very similar
Summary
112 García-Vicente et al.: Sensitivity of a helical array lead to the overlooking of some potential errors as the result of the gantry rotation and the MLC positioning. Systems with detectors’ configuration that allows for three-dimensional dose verifications have recently become available. These systems could be used for step-and-shoot or sliding window IMRT treatments, as well as volumetric-modulated arc therapy (VMAT) and tomotherapy. Delta system (ScandiDos AB, Uppsala, Sweden) was the first commercial 3D array and has already been described by several authors.[4,5] It consists of two two-dimensional diode arrays embedded in a cylindrical phantom. It performs absolute and relative dose measurements, as well as integrated or per control point dose measurements. The usefulness of this device for different types of treatments has already been studied by several authors.[5,6]
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