Memory circuit dosimetry in patients with brain metastases treated with Sparing Memory with Advanced Radiosurgery...

Radiation dose reduction to major memory circuit substructures (e.g., hippocampus, amygdala, fornix, and corpus callosum) may reduce patients' neurocognitive toxicities. We evaluated memory circuit (MC) dosimetry and determined the feasibility of the "Sparing Memory with Advanced Radiosurgery Targeting" (SMART) technique in patients with brain metastases.

Retrospective contouring of MC substructures was performed for 101 patients that received single-fraction, linear accelerator (LINAC)-based, monoisocentric SRS plans for increasing numbers of brain metastases: 1, 2-4, 5-9 and ≥ 10. Patients with ≥10 lesions (n = 13) were reoptimized using SMART by maximizing dose reduction to MC while maintaining >95% PTV coverage (V100% > 95%). Dosimetric parameters such as D_mean, D_max, and D_median to MC, cumulative PTV and MC volume were evaluated.

Patients with ≥10 brain metastases received significantly higher D_mean (2.9 vs. 0.41 Gy, p < 0.0001), D_median (2.9 vs. 0.34 Gy, p < 0.0001) and D_max (9.0 vs. 1.9 Gy, p < 0.0001) to the MC compared to those with a single metastasis. SMART replanning yielded a significant decrease in D_mean, D_median, D_max, and D₁₀₀ doses to the MC when compared to clinical dosimetry (2.9 vs. 1.8 Gy, p = 0.0001; 2.9 vs. 1.8 Gy, p = 0.0002; 9.0 vs. 4.7 Gy, p = 0.0007; 1.5 vs. 0.98 Gy, p = 0.0010, respectively).

Increased number of brain metastases results in higher MC doses. Patients with ≥10 lesions received increased D_mean/D_median/D_max doses to the MC compared to patients with <10 brain metastases. SMART technique is clinically feasible. Studies evaluating the neurocognitive benefits of SMART are warranted.