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Introduction: We present a Trajectory-based Volumetric Modulated Arc Therapy (TVMAT) technique for Stereotactic Radiosurgery (SRS) that takes advantage of a modern linacs ability to modulate dose rate and move the couch dynamically. In addition, we investigate the quality of the developed TVMAT method and the dosimetric accuracy of the technique. Methods: The main feature of the TVMAT technique is a standard beam trajectory formed by dynamic motion of the treatment couch and the linac gantry. The couch rotates slowly through 180 degrees while the gantry delivers radiation through continuous sweeps of the gantry. The number of partial arcs that constitute the trajectory can be varied between two and eight and as the number of partial arcs increases, the trajectory more finely samples 4p geometry. Along these trajectories, the multi-leaf collimator (MLC) and dose rate are optimized through an inverse planning framework. The TVMAT method was tested on ten cranial SRS patients who were previously treated with the Dynamic Conformal Arc (DCA) technique. The plans were compared with the DCA and a four-arc VMAT technique with regards to dose to the OAR, dose falloff, V12Gy, and V4Gy. Validation measurements were performed using ion-chamber and Gafchromic film. In addition, the trajectory-log files were analyzed and compared with the treatment plan beam data. Results: The TVMAT treatment plans were successfully delivered with a treatment time between 3-8 min which mostly depended on total cumulated dose. Ion chamber measurements had an average measured error of 1.1 AE 0.6% and a maximum value of 2.2% of the delivered dose. The 2%, 2 mm gamma pass rates for the film measurements were 96% or greater. In a preliminary comparison of ten patients who underwent SRS treatments with the DCA technique, the TVMAT and VMAT techniques were able to produce plans with comparable dose falloff and OAR doses, while achieving better dose conformality, V4Gy and V12Gy when compared to the original DCA plans. The improvement of the TVMAT plans were as follows (mean % improvement AE standard err): Conformity (10 AE 2%), V4 (20 AE 20%), V12 (27 AE 10%), volume weighted mean dose to organs at risk (13 AE 13%), homogeneity index (2 AE 2%) and falloff (4 AE 2%). Conclusion: We have developed and validated a trajectory-based dose delivery method which has dose distribution improvements while having a treatment time of 3-8 min. In addition, it has the potential for a simpler planning experience while maintaining an accurate delivery on the Varian Truebeam Linac.

cute aortic syndrome (AAS) is a life-threatening emergency, accounting for 1/2000 presentations of acute chest or back pain to the emergency department. 1 It is a clinical spectrum of diagnoses including aortic dissection, intramural hematoma and penetrating atherosclerotic ulcer at any location along the aorta. 1 The incidence of AAS is about 3 per 100 000 persons. 2,3 Many physicians do not consider AAS in their initial differential diagnosis, which is in part why 25% of patients with AAS are not diagnosed with the condition until 24 hours after presenting to the emergency department. 4 Prognosis is most favourable when patients are treated early, while they are clinically stable. Mortality follows a linear increase with diagnostic delay and can be as high as 2% per hour of delay. 5 The misdiagnosis rate during the initial emergency department visit for AAS (i.e., patient admitted for an alternative diagnosis and later diagnosed as AAS; discharged from the emergency department and presenting again with a diagnosis of AAS; or diagnosed on postmortem examination) is estimated to be as high as 38%. 4,6-16 Patients with suspected AAS are typically investigated with electrocardiogram (ECG)-gated contrast-enhanced computed tomography (CT). 2 Current use of this investigation in patients with a clinical suspicion for AAS is inefficient. 17,18 The unnecessary use of CT leads to a direct increase in health care costs but also an increase in contrast-associated complications (e.g., allergic reactions), increased length of emergency department stay or incidental findings requiring further follow-up, additional im aging and increased stress or anxiety for the patient. 17 Use of CT in a low-prevalence population can result in an increase in false-positives, which can lead to further testing, unnecessary transfer and even surgical intervention. 19 There are 2 high-quality guidelines related to the diagnosis of AAS, from the American Heart Association (2010) and the European Society of Cardiology (2014). 20,21 However, there is still considerable variation in how clinicians investigate for AAS in Canada. 17 This variation is likely multifactorial but may be a result of lack of key stakeholder involvement in the development of the guidelines or the difference in threshold for investigation within the Canadian health care system. 22,23 The aims of this guideline are to update the available guideline recommendations with current evidence; include key stakeholders to allow interpretation of the evidence in context of values and preferences; and make practice recommendations that are applicable to the Canadian health care system. The full guideline, including supplemental documents, is available at Appendix 1