Reducing the dosimetric impact of positional errors in field junctions   for craniospinal irradiation using VMAT
  Aim: To improve treatment plan robustness with respect to small shifts in patient position during the VMAT treatment by ensuring a linear ramp-like dose profile in treatment field overlap regions.   Background: Craniospinal irradiation (CSI) is considered technically challenging because the target size exceeds the maximal field size, which necessitates using abutted or overlapping treatment fields. Volumetric modulated arc therapy (VMAT) is increasingly being examined for CSI, as it offers both better dose homogeneity and better dose conformance while also offering a possibility to create field junctions which are more robust towards small shifts in patient position during the treatment.   Materials and Methods: A VMAT treatment plan with three isocenters was made for a test case patient. Three groups of overlapping arc field pairs were used; one for the cranial and two for the spinal part. In order to assure a ramp-like dose profile in the field overlap region, the upper spinal part was optimised first, with dose prescription explicitely enforcing a ramp-like dose profile. The cranial and lower spinal part were done afterwards, taking into account the dose contribution of the upper spinal fields.   Results: Using simple geometrical reasoning, we demonstrated that hot- and cold spots which arise from small displacement of one treatment field relative to the other treatment field can be reduced by taking two precautions: (a) widening the field overlap region, and (b) reducing the field gradient across the overlap region. The function with the smallest maximal gradient is a linear ramp. We present a treatment planning technique which yields the desired dose profile of the two contributing fields, and minimises dosimetric dependence on minor positional errors in patient set-up. 
Andrej Strojnik, Ignasi Mendez, Primoz Peterlin
physics.med-ph