Reducing the dosimetric impact of positional errors in field junctions for craniospinal irradiation using VMAT
Strojnik, Andrej ; Méndez Carot, Ignasi ; Peterlin, Primož
To improve treatment plan robustness with respect to small shifts in patient positionduring the VMAT treatment by ensuring a linear ramp-like dose profile in treatment fieldoverlap ...regions.Background: Craniospinal irradiation (CSI) is considered technically challenging because thetarget size exceeds the maximal field size, which necessitates using abutted or overlappingtreatment fields. Volumetric modulated arc therapy (VMAT) is increasingly being examinedfor CSI, as it offers both better dose homogeneity and better dose conformance while alsooffering a possibility to create field junctions which are more robust towards small shifts inpatient position during the treatment.Materials and methods: A VMAT treatment plan with three isocenters was made for a test casepatient. Three groups of overlapping arc field pairs were used; one for the cranial and twofor the spinal part. In order to assure a ramp-like dose profile in the field overlap region, theupper spinal part was optimised first, with dose prescription explicitly enforcing a ramp-likedose profile. The cranial and lower spinal part were done afterwards, taking into accountthe dose contribution of the upper spinal fields.Results: Using simple geometrical reasoning, we demonstrated that hot- and cold spotswhich arise from small displacement of one treatment field relative to the other treatmentfield 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 maxi-mal gradient is a linear ramp. We present a treatment planning technique which yields thedesired dose profile of the two contributing fields, and minimises dosimetric dependenceon minor positional errors in patient set-up.
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