More than 200 power arc tests were carried out with original arrangements of overhead line insulators and arcing fittings for 20 kV, 110 kV and 220 kV. With an adequately modelled network, short‐circuit currents in the range of 2 kA to 12.5 kA can be realized. Analysis of the measurements shows that a typical arc voltage time function can be attributed to each insulator type and arrangement. Using the measured variables, which were digitally recorded and stored on optical media, an existing numerical arc model has been enhanced, which can be applied in an electromagnetic transients program to reproduce the dynamic and random behaviour of power arcs for any insulator arrangement, current and system voltage.

Purpose:To demonstrate a new quality assurance (QA) technique for single isocenter volumetric modulated arc therapy (VMAT) plans for stereotactic radiosurgery of multiple brain metastases and to compare quantitative QA results using this method with results using a cylindrical diode array and full field coronal films.Methods:This study employs a phantom built of 28 thin slabs to accommodate radiochromic film insertions near all planning target volume (PTV) locations, providing target specific data for individual PTVs. Single isocenter VMAT plans were designed for two patients with 5 and 11 intra‐cranial metastases. Both plans were modified three times to create plans that overdosed a target by 10%, and underdosed a target by 10 % and 20%. Gamma analyses were performed for full field coronal films, the diode array measurements and for individual PTVs with the proposed film stack method using square ROIs centered on the PTVs, measuring twice the length of the PTV's equivalent sphere diameter. Passing gamma criteria was 95% of points passing at 5%/1mm and a threshold of 10% of the prescription dose.Results:Of the 6 introduced errors, 1 was detected by the diode array gamma test (93.1% pass rate). Pass rates for the other 5 ranged from 95.4–98.8%. None of the errors were detected using the full field film gamma test (pass rates: 98.7–99.9%). Using the proposed film stack method, 5 of 6 errors were detected by failed gamma tests (pass rates: 51, 53, 71, 87.3, 93.9 and 97.8%). The two high pass rates were due to attempts to underdose a PTV that measured too high in the original plan.Conclusion:The results suggest that current methods may be lacking quantitative value and rely heavily on challenging qualitative evaluation, while the proposed method offers a much needed practical method for meaningful quantitative results.

PurposeTo determine if the treatment planning computed tomography scan (CT) from an initial intracranial stereotactic radiosurgery (SRS) treatment can be used for repeat courses of SRS.Methods and materialsTwenty‐five patients with 40 brain metastases that received multiple courses of SRS were retrospectively studied. Magnetic resonance scans from repeat SRS (rMR) courses were registered to CT scans from the initial SRS (iCT) and repeat SRS (rCT). The CT scans were then registered to find the displacement of the rMR between iCT and rCT registrations. The distance from each target to proximal skull surface was measured in 16 directions on each CT scan after registration. The mutual information (MI) coefficients from the registration process were used to evaluate image set similarity. Targets and plans from the rCTs were transferred to the iCTs, and doses were recalculated on the iCT for repeat plans. The two dose distributions were compared through 3D gamma analysis.ResultsThe magnitude of the mean linear translations from the MR registrations was 0.6 ± 0.3 mm. The mean differences in distance from target to skull on a per target basis were 0.3 ± 0.2 mm. The MI was 0.582 ± 0.042. Registration between a comparison group of 30 CT scans that had the same data resampled and 30 scans that were intercompared with different patients gave MI = 0.721 ± 0.055 and MI = 0.359 ± 0.031, respectively. The mean gamma passing rates were 0.997 ± 0.007 for 1 mm/1% criteria.ConclusionsThe rMR can be aligned to the iCT to accurately define targets. The skull shows minimal change between scans so the iCT can be used for set‐up at repeat treatments. The dosimetry provided by the iCT dose calculation is adequate for repeat SRS. Treatment based on iCT is feasible.