Figure 2. Principles of dose prescription and distribution design for 10-fraction stereotactic radiosurgery of brain metastases.

The most prioritized foundation for dose distribution design is the gross tumor volume (GTV) boundary, not the margin-added planning target volume (PTV). The dose prescription is based on the biologically effective dose (BED) derived from the linear-quadratic formula with an alpha/beta ratio of 10 (BED₁₀) and three-tiered dose gradient optimization both just outside and inside the GTV boundary. The GTV margin was covered by 53 Gy, with a BED₁₀ of ≥80 Gy, which was highly prioritized. To ensure a moderate, not too steep or gradual, dose spillage (attenuation) margin outside the GTV boundary, 2-3 mm outside the GTV margin was covered by 37 Gy, with a BED₁₀ of ≈50 Gy. To attain a concentrically laminated steep dose increase inside the GTV boundary, 2 mm inside the GTV margin was covered by ≥62 Gy, with a BED₁₀ of ≥100 Gy, by intentionally increasing the centrum dose by ≥71 Gy, a BED₁₀ of ≥120 Gy. The BED₁₀ values of 37, 53, 62, and 71 Gy correspond to 50.7, 81.1, 100.4, and 121.4 Gy, respectively. The constancy of the maximum dose or % isodose of 53 Gy, that is, the degree of GTV dose inhomogeneity, is considered insignificant, with the variations being allowed up to approximately 106 Gy at the maximum dose.