Impact of High Maximum Dose Constraints Within the Gross Tumor Volume on the Quality of Stereotactic Radiosurgery...

Purpose This planning study aimed to clarify the significance of maximum dose constraints within a gross tumor volume (GTV), exceeding 1.5 times the prescription dose, in stereotactic radiosurgery (SRS) using volumetric-modulated arcs (VMA) with the Monaco® planning system (Elekta AB, Stockholm, Sweden) for single brain metastases (BMs). Materials and methods Thirty five lesions were included with the GTV ranging from 0.33 cc to 48.09 cc (median 7.34 cc). Four VMA plans were developed for each GTV using a multileaf collimator (MLC) Agility^® (Elekta AB): one without any maximum dose constraint (None group) and three with increasing constraints (55%, 60%, and 65% groups). In the 55%, 60%, and 65% groups, maximum dose constraints of ≤181.82%, ≤166.67%, and ≤153.85% of the prescription dose were assigned to achieve the GTV coverage by ≥55%, ≥60%, and ≥65% isodose surfaces (IDSs), respectively. The arc and MLC angle configurations and optimization method are unified, except for the maximum dose constraint. The GTV coverage by the prescription dose was rescaled to the D _{V-0.01 cc}: the minimum dose to a GTV minus 0.01 cc (D _>95%) for GTV >0.20 cc or D _95% for GTV ≤0.20 cc. Results The total planning time was significantly shorter in the None group. This group showed the most statistically significant superiority in GTV dose conformity, the appropriateness of the dose attenuation margin, and the steepness and concentric lamellarity of dose gradients 2 mm outside and 2-4 mm inside the GTV boundary. The None group also showed favorable trends in the steepness of dose fall-off from the prescription dose to 50%. The 55% and 60% groups showed a favorable trend in smaller prescription isodose volume spillage; however, no significance was observed in other evaluation metrics. Conclusions Maximum dose constraints within the GTV, at 1.5-1.8 times the prescription dose, likely impaired GTV dose conformity, the appropriateness of the dose attenuation margin, and the steepness and concentric lamellarity of dose gradients outside and inside the GTV boundary. Furthermore, they deteriorated the efficiency of VMA optimization in maximizing dose conformity and gradients. Therefore, dose constraints to ensure ≥55-65% IDS coverage are not recommended, and alternative methods should be considered to adjust excessively steep dose attenuation margins in small GTVs.