An empirical method for splitting arcs in VMAT


      • The core findings from our study are:
      • Presents new prior knowledge based method to determine the optimal split arc in VMAT.
      • Fast and effective approach to improve the Plan quality for VMAT.
      • The proposed split arc VMAT strategy is shown to reduce the dose to the OARs.
      • Low dose volume, beam on time and monitor units are reduced by proposed split arc strategy.



      We present a new approach to determine the optimal arc split for VMAT beams which is an extension of our recently published algorithm for selecting optimal beam angles in Intensity Modulated Radiation Therapy (IMRT)

      Material and Methods

      The proposed approach uses an objective function based scoring method called “ ψ – score” to determine optimal arc splitting strategy. To validate our approach, we applied it in different clinical cases: Abdomen-Para aortic node, Lung, Pancreas and Prostate. Basically, for all clinical cases, two set of plans were created, namely VMAT plan and VMAT_S plan using Pinnacle3 (V16.2, Philips Medical Systems (Cleveland), Inc.). In the VMAT plans, full arc (360°) with 4-degree gantry spacing was used during optimization to compute the “ ψ – score”. Subsequently the avoidable arc portions were identified and removed using the ψ – score plot followed by the final optimization (VMAT_S).


      Equivalent or better OAR sparing, and similar target coverage were achieved in VMAT_S plans compared to VMAT plans. VMAT_S reduced the number of control points and monitor units by 24.2% and 12.9% respectively. On the average, beam on time was reduced by 21.9% and low dose volume (5 Gy isodose volume) to healthy tissues was reduced by 4.9% in VMAT_S compared to VMAT plans.


      The results demonstrated that the proposed method is useful for reducing the monitor units, beam on time and low dose volume without significantly compromising plan quality and most useful for non-centrically located targets.


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