4.2. Treatment Planning for Comparative Analysis

Comparative treatment planning, volumetric and dosimetric analysis were performed on RaySearch’s RayStation v6.1.1.2 (RaySearch Laboratories, Stockholm, Sweden), utilizing the comprehensive scripting capabilities for semi-automated plan generation and analysis. Treatment plans were created for HFSRT using volumetric modulated arc therapy (VMAT) with four non-coplanar arcs at a VersaHD™ linear accelerator (Elekta, Stockholm, Sweden) with an Agility™ (Elekta Stockholm, Sweden) multi-leaf collimator (MLC) featuring 5 mm leafs at the isocenter. Prescribed dose for all comparative plans was 35 Gy in 7 fractions to the PTV, aiming for a coverage of at least 98%, The prescribed dose was scaled to the conformally surrounding 70% isodose (maximum = 100%).

To evaluate the volumetric and dosimetric impact of different treatment settings (pre vs. post-operative) and margin concepts, three hypothetical clinical scenarios were constructed and compared for each patient. Target volume definition and treatment planning for those hypothetical scenarios were done independently from the de facto performed treatment in order to achieve a homogeneous sample with regard to delineation and treatment planning strategy. Comparative plans were not actually used for treatment. Target delineation with strict adherence to the specifications outlined below was performed by a radiation oncologist with extensive experience in intracranial radiosurgery. All target volumes were then reviewed and approved by a second experienced radiation oncologist and differences were resolved by individual discussion. The evaluated treatment scenarios are illustrated in Figure 1 and were defined as follows:

The pre-operative MRI was registered with the post-operative treatment-planning CT to delineate the location and extent of the resected lesion. For a hypothetical pre-operative HFSRT, a planning target volume (PTV) margin of 1 mm by isotropic expansion was added to the macroscopic tumor, as is the institutional standard for the SRS/HFSRT of unresected brain metastases.

On the post-operative MRI, the resection cavity and—if applicable—residual tumor without an additional margin and without the surgical tract were defined as GTV. The clinical target volume (CTV) was created by adding an isotropic margin of 3 mm to the GTV. As above, for PTV creation, an additional margin of 1 mm was added by isotropic expansion.

GTV delineation was performed as in post-op-I. The CTV was created by adding an isotropic margin of 3 mm to the GTV and—if necessary—enlarging the resulting structure to encompass the complete surgical tract and all contrast-enhancing parenchymal and meningeal tissue associated with it. As above, for PTV creation, an additional margin of 1 mm was added to the CTV.