This work investigates the incorporation of visibility parameters and constraints into the optimization of volumetric modulated arc therapy (VMAT) plans using fiducial markers. We propose that by incorporating fiducial marker constraints into the optimization, one may produce treatment plans that ensure a higher visibility of fiducials throughout the irradiation (a requirement for real-time tumour tracking techniques), in addition to simultaneously satisfying dosimetric requirements. We investigated this approach on a dynamic thorax phantom and multiple patient disease sites (prostate, liver and lung) using a radiotherapy optimization development software (MonArc).

For all the investigated datasets, three fiducial markers were implanted inside or around the planning target volume (PTV) and a VMAT plan was created for each patient. We modified MonArc to analyze beam’s-eye-views (BEV) of the gantry arc control points to include marker-based visibility constraints of type ‘hard’ (i.e. requiring 100% visibility of all markers, HC) and ‘soft’ (i.e. penalizes visibility for one marker [SC¬I] or two markers [SCII] only) in the optimization process. Dose distributions from the constrained plans (i.e. HC, SCI, and SCII) were compared to the non-constrained plan (NC) using several metrics including the conformity index, homogeneity index, PTV average index and doses to organs-at-risk (OAR).

Across all the disease sites, one marker is always fully visible at all BEV apertures (i.e. 100% of the gantry arc control points) for the constrained plans. All three markers were fully visible in at least 33% of BEV apertures for the constrained plans, while also satisfying the required dosimetric objectives. Although dose metrics showed some deterioration for constrained plans (-6% for SCI up to -15% for HC, when compared to NC using the PTV average index), the required dosimetric objectives were still satisfied in at least 90% of patients. In conclusion, we demonstrated that marker-based constraints can be incorporated into VMAT, to produce treatment plans satisfying both the visibility and dosimetric objectives. This approach should ensure greater clinical success when applying real-time tracking algorithms for VMAT delivery.