PurposeDosimetric accuracy is critical when switching a patient treated with stereotactic body radiation therapy (SBRT) or stereotactic fractionated radiotherapy (SRT) among beam‐matched linacs. In this study, the dose delivery accuracy of volumetric modulated arc therapy (VMAT) plans for SBRT/SRT patients were evaluated on three beam‐matched linacs.MethodBeam data measurements such as percentage depth dose (PDD 10), beam profiles, output factors, and multi‐leaf collimator (MLC) leaf transmission factor for 6 MV photon beam were performed on three beam‐matched linacs. The Edge™ diode detector was used for measurements of beams of field size less than 5 × 5 cm2. Ten lung and 15 brain plans were generated using VMAT with the same beam model. Modulation complexity score of the VMAT plan (MCSv) was used as a plan complexity indicator. Doses were measured using ArcCHECK™ and GafChromic™ EBT3 films. The measurements were compared with calculated doses through absolute dose gamma comparison using 3%/2 mm and 2%/2 mm criteria. Correlation between difference in passing rates among beam‐matched linacs and MCSv was evaluated using the Pearson coefficient. Point doses were measured with the A1SL micro ion chamber.ResultsDifference in beam outputs, beam profiles, and MLC leaf transmission factors of beam‐matched linacs were all within ±1%, except the difference in output factor for 1 × 1 cm2 field between linac 1 and 3 (1.3%). For all 25 cases, passing rates of measured doses on three linacs were all higher than 90% when using 2%/2 mm gamma criteria. The average difference in point dose measurements among three beam‐matched linacs was 0.1 ± 0.2% (P > 0.05, one‐way ANOVA).ConclusionMinimal differences in beam parameters, point doses, and passing rates among three linacs proved the viability of swapping SBRT/SRT using VMAT among beam‐matched linacs. The effect of plan complexity on passing rate difference among beam‐matched linacs is not statistically significant.

Four‐dimensional, cone‐beam CT (4D CBCT) substantially reduces respiration‐induced motion blurring artifacts in three‐dimension (3D) CBCT. However, the image quality of 4D CBCT is significantly degraded which may affect its accuracy in localizing a mobile tumor for high‐precision, image‐guided radiation therapy (IGRT). The purpose of this study was to investigate the impact of scanning parameters (hereinafter collectively referred to as scanning sequence) and breathing patterns on the image quality and the accuracy of computed tumor trajectory for a commercial 4D CBCT system, in preparation for its clinical implementation. We simulated a series of periodic and aperiodic sinusoidal breathing patterns with a respiratory motion phantom. The aperiodic pattern was created by varying the period or amplitude of individual sinusoidal breathing cycles. 4D CBCT scans of the phantom were acquired with a manufacturer‐supplied scanning sequence (4D‐S‐slow) and two in‐house modified scanning sequences (4D‐M‐slow and 4D‐M‐fast). While 4D‐S‐slow used small field of view (FOV), partial rotation (200°), and no imaging filter, 4D‐M‐slow and 4D‐M‐fast used medium FOV, full rotation, and the F1 filter. The scanning speed was doubled in 4D‐M‐fast (100°/min gantry rotation). The image quality of the 4D CBCT scans was evaluated using contrast‐to‐noise ratio (CNR), signal‐to‐noise ratio (SNR), and motion blurring ratio (MBR). The trajectory of the moving target was reconstructed by registering each phase of the 4D CBCT with a reference CT. The root‐mean‐squared‐error (RMSE) analysis was used to quantify its accuracy. Significant decrease in CNR and SNR from 3D CBCT to 4D CBCT was observed. The 4D‐S‐slow and 4D‐M‐fast scans had comparable image quality, while the 4D‐M‐slow scans had better performance due to doubled projections. Both CNR and SNR decreased slightly as the breathing period increased, while no dependence on the amplitude was observed. The difference of both CNR and SNR between periodic and aperiodic breathing patterns was insignificant (p>0.48). At end‐exhale phases, the motion blurring was negligible for both periodic and aperiodic breathing patterns; at mid‐inhale phase, the motion blurring increased as the period, the amplitude or the amount of cycle‐to‐cycle variation on amplitude increased. Overall, the accuracy of localizing the moving target in 4D CBCT was within 2 mm under all studied cases. No difference in the RMSEs was noticed among the three scanning sequences. The 4D‐M‐fast scans, free of volume truncation artifacts, exhibited comparable image quality and accuracy in tumor motion reconstruction as the 4D‐S‐slow scans with reduced imaging dose (0.60 cGy vs. 0.99 cGy) due to the use of faster gantry rotation and the F1 filter, suggesting its suitability for clinical use.PACS number: 87.55.Qr

ObjectiveTo evaluate the inter-rater and intra-rater reliability of ultrasonographic measurements of axillary recess (AR) thickness in healthy individuals, and to analyze the factors affecting the thickness of the AR capsule.MethodsWe recruited 20 healthy individuals (10 male, 10 female) with a mean age of 37 years (standard deviation ±10). Two physiatrists (an experienced and a novice rater) independently investigated the AR thickness in three rounds. The AR thickness was measured for each individual at three shoulder abduction angles (50°, 70°, and 90°). Intra-class correlation (ICC) coefficients were used to assess the reproducibility of each measurement.ResultsExcellent intra-rater reliability coefficients were observed at the three shoulder abduction angles, in the analysis of both raters. The inter-rater reliability coefficient was also was excellent in both studies. There were significant differences in the AR thickness, according to the angle of shoulder abduction. The AR was thicker at 50° than at 70° and 90° (all p<0.001), and the AR was thicker at 70° than at 90° (p<0.001). Height (r=0.62, p=0.003) and body mass index (r=0.52, p=0.019) were positively correlated with AR thickness. Males had a thicker AR capsule than females at all three angles (all p<0.001).ConclusionUltrasonographic measurements of AR thickness in healthy individuals demonstrate excellent intra-rater and inter-rater reliability. AR thickness may depend on anthropometric variables and position of the shoulder.

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