Significance of Inverse Planning With Variable Dose Rate and the Segment Shape Optimization in Dynamic Conformal Arcs Using the High-Definition Dynamic Radiosurgery Platform for Single Brain Metastases.

Purpose This planning study aimed to clarify the significance of inverse planning with variable dose rate (VDR) and the segment shape optimization (SSO) in the quality and efficiency of dynamic conformal arcs (DCA) using the high-definition dynamic radiosurgery (HDRS) platform for stereotactic radiosurgery (SRS) of single brain metastases (BMs). Materials and methods Twenty clinical BMs were included, with the gross tumor volume (GTV) ranging from 0.33 cc to 48.09 cc (median: 7.05 cc). The HDRS platform included the 5-mm leaf-width, 160-leaf collimator Agility® (Elekta AB, Stockholm, Sweden) and the Monaco® planning system (Elekta AB). Prior to the main comparison, the high-precision leaf positions (HPLP) values of between five and 20 in the SSO were compared to determine which was optimal in six lesions. Using the constant dose rate (CDR) optimization as a baseline (the CDR group), the effects of changing to VDR (the VDR group), and further adding the SSO with the suitable HPLP value (the SSO group) on the DCA planning were investigated. The same prescription dose was assigned to the GTV  (minimum dose of GTV minus 0.01 cc). Results The HPLP value of 20 in the SSO (SSO_20) was suitable in terms of the total calculation time (tCT), the total monitor units (MU) per fraction, and the GTV dose conformity and gradients. The tCT was significantly longer in the order of the SSO_20, the VDR, and the CDR. The total MU was the highest in the SSO_20, and the MU assignments to the three arcs were automatically optimized in each group. The change from CDR to VDR significantly improved the GTV dose conformity, the appropriateness of dose attenuation margin outside the GTV, the steepness of dose gradient outside the GTV, and the concentric lamellarity of dose increase 2-4 mm inside the GTV boundary. The addition of the SSO_20 further significantly improved the GTV dose conformity, the dose attenuation margin, the steepness, and the concentric lamellarity of dose gradients outside and inside the GTV boundary. In the SSO_20, the beam segments were shaped by anisotropic leaf adaptations to the GTV boundary with extensions of some of the leaf edges beyond the GTV boundary (minus leaf margins) and the practically <5 mm variable widths of the outermost leaves in the leaf movement direction through the dynamic shielding by the diaphragms (jaws), along with the position controls of both the leaves and the diaphragms in 0.1 mm increments. Conclusions The inverse planning with VDR and the SSO_20 significantly improved the quality of DCA plans in terms of the dose conformity and gradients outside and inside the GTV boundary. However, the SSO_20 with VDR required longer tCT and higher total MU per fraction. The SSO_20 with VDR was recommended for DCA-based SRS planning using the HDRS for BMs.