Memory circuit dosimetry in patients with brain metastases treated with sparing memory with advanced Radiosurgical targeting (SMART) technique

Purpose 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 Radiosurgical Targeting" (SMART) technique in patients with brain metastases. Materials and methods 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. Results 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 D 100 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). Conclusion 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.