CAS incidence is high after head and neck radiotherapy, gradually rising over time. No clear dose-response effect between carotid dose and CAS was identified for HNC patients. Carotid artery screening and preventative strategies should be employed in this high-risk patient population.

HighlightsAll MRI-linac plans met the coverage and predefined OAR constraints.The prone approach appeared to be more favorable with respect to the chest wall, and ipsilateral lung dose compared to the supine position.Overall, the MRI-linac and clinical plans were comparable, with minor absolute dosimetric differences.

Purpose: To evaluate the latent variance (LV) of Varian TrueBeam photon phase-space files (PSF) for open 10x10 cm 2 and small stereotactic fields and estimate the number of phase spaces required to be summed up in order to maintain sub-percent latent variance in Monte Carlo (MC) dose calculations. Method: BEAMnrc/DOSXYZnrc software was used to transport particles from Varian phase-space files (PSFA) through the secondary collimators. Transported particles were scored into another phase-space located under the jaws (PSFB), or transported further through the cone collimators and scored straight below, forming PSFC. Phase-space files (PFSB) were scored for 6MV-FFF, 6MV, 10MV-FFF, 10MV and 15MV beams with 10x10 cm 2 field size, and PSFC were scored for 6MV beam under circular cones of 0.13, 0.25, 0.35, 1.0, 1.2, 1.5 and 4 cm diameter. Both PSFB and PSFC were transported into a water phantom with particle recycling number ranging from 10 to 1000. For 10x10 cm 2 fields 0.5x0.5x0.5cm 3 voxels were used to score the dose, whereas the dose was scored in 0.1x0.1x0.5cm 3 voxels for beams collimated with small cones. In addition, for small 0.25 cm diameter cone-collimated 6MV beam, phantom voxel size varied as 0.02x0.02x0.5cm 3 , 0.05x0.05x0.5cm 3 and 0.1x0.1x0.5cm 3. Dose variances were scored in all cases and latent variance evaluated as per Sempau et al. Results: For the 10x10 cm 2 fields calculated LVs were greatest at the phantom surface and decreased with depth until reached a plateau at 5 cm depth. LVs were found to be 0.54%, 0.96%, 0.35%, 0.69% and 0.57% for the 6MV-FFF, 6MV, 10MV-FFF, 10MV and 15MV energies, respectively at the depth of 10cm. For the 6 MV phase-space collimated with cones of 0.13, 0.25, 0.35, 1.0 cm diameter, the LVs calculated at 1.5 cm depth were 75.6%, 25.4%, 17.6% and 8.0% respectively. Calculated latent variances for the 0.25 cm cone-collimated 6MV beam were 61.2%, 40.7%, 22.5% in 0.02x0.02x0.5cm 3 , 0.05x0.05x0.5cm 3 and 0.1x0.1x0.5cm 3 voxels respectively. Conclusions: In order to achieve sub-percent latent variance in open 10x10 cm 2 field MC simulations single PSF can be used, whereas for small SRS fields more PSFs would have to be summed.

Volumetric modulated arc therapy (VMAT) is a relatively new treatment modality for dynamic photon radiation therapy. Pre-treatment quality assurance (QA) is necessary and many efforts have been made to apply electronic portal imaging device (EPID)-based IMRT QA methods to VMAT. It is important to verify the gantry rotation speed during delivery as this is a new variable that is also modulated in VMAT. In this paper, we present a new technique to perform VMAT QA using an EPID. The method utilizes EPID cine mode and was tested on Varian TrueBeam in research mode. The cine images were acquired during delivery and converted to dose matrices after profile correction and dose calibration. A sub-arc corresponding to each cine image was extracted from the original plan and its portal image prediction was calculated. Several analyses were performed including 3D γ analysis (2D images + gantry angle axis), 2D γ analysis, and other statistical analyses. The method was applied to 21 VMAT photon plans of 3 photon energies. The accuracy of the cine image information was investigated. Furthermore, this method's sensitivity to machine delivery errors was studied. The pass rate (92.8 ± 1.4%) for 3D γ analysis was comparable to those from Delta(4) system (99.9 ± 0.1%) under similar criteria (3%, 3 mm, 5% threshold and 2° angle to agreement) at 6 MV. The recorded gantry angle and start/stop MUs were found to have sufficient accuracy for clinical QA. Machine delivery errors can be detected through combined analyses of 3D γ, gantry angle, and percentage dose difference. In summary, we have developed and validated a QA technique that can simultaneously verify the gantry angle and delivered MLC fluence for VMAT treatment.This technique is efficient and its accuracy is comparable to other QA methods.

Compared to other radiation therapy modalities, clinical electron beam therapy has remained practically unchanged for the past few decades even though electron beams with multiple energies are widely available on most linacs. In this paper, we present the concept of dynamic electron arc radiotherapy (DEAR), a new conformal electron therapy technique with synchronized couch motion. DEAR utilizes combination of gantry rotation, couch motion, and dose rate modulation to achieve desirable dose distributions in patient. The electron applicator is kept to minimize scatter and maintain narrow penumbra. The couch motion is synchronized with the gantry rotation to avoid collision between patient and the electron cone. In this study, we investigate the feasibility of DEAR delivery and demonstrate the potential of DEAR to improve dose distributions on simple cylindrical phantoms. DEAR was delivered on Varian's TrueBeam linac in Research Mode. In conjunction with the recorded trajectory log files, mechanical motion accuracies and dose rate modulation precision were analyzed. Experimental and calculated dose distributions were investigated for different energies (6 and 9 MeV) and cut-out sizes (1×10 cm(2) and 3×10 cm(2) for a 15×15 cm(2) applicator). Our findings show that DEAR delivery is feasible and has the potential to deliver radiation dose with high accuracy (root mean square error, or RMSE of <0.1 MU, <0.1° gantry, and <0.1 cm couch positions) and good dose rate precision (1.6 MU min(-1)). Dose homogeneity within ±2% in large and curved targets can be achieved while maintaining penumbra comparable to a standard electron beam on a flat surface. Further, DEAR does not require fabrication of patient-specific shields. These benefits make DEAR a promising technique for conformal radiotherapy of superficial tumors.