Dosimetric Impact of Lesion Number, Size, and Volume on Mean Brain Dose with Stereotactic Radiosurgery for Multiple Brain Metastases

SIMPLE SUMMARY: There is an increasing incidence of patients diagnosed with multiple brain metastasis (MBM) in the modern era. Although stereotactic radiosurgery (SRS) has been shown to result in similar survival with less neurocognitive deterioration for patients with MBM, the intracranial disease extent, both in quantity and volume, able to be treated with modern dedicated SRS platforms has yet to be empirically demonstrated. In this study, we evaluate the effect of the number and volume of lesions treated on mean brain dose using two dedicated intracranial SRS delivery technologies. We found that mean brain dose linearly increased with the number of lesions and total gross tumor volume (GTV), while selected metrics associated with radiation necrosis risk (i.e., V8 Gy, V10 Gy, and V12 Gy) had quadratic correlations to the number of lesions and the total GTV. This study shows that it is dosimetrically feasible to treat MBMs with SRS. ABSTRACT: We evaluated the effect of lesion number and volume for brain metastasis treated with SRS using GammaKnife(®) ICON™ (GK) and CyberKnife(®) M6™ (CK). Four sets of lesion sizes (<5 mm, 5–10 mm, >10–15 mm, and >15 mm) were contoured and prescribed a dose of 20 Gy/1 fraction. The number of lesions was increased until a threshold mean brain dose of 8 Gy was reached; then individually optimized to achieve maximum conformity. Across GK plans, mean brain dose was linearly proportional to the number of lesions and total GTV for all sizes. The numbers of lesions needed to reach this threshold for GK were 177, 57, 29, and 10 for each size group, respectively; corresponding total GTVs were 3.62 cc, 20.37 cc, 30.25 cc, and 57.96 cc, respectively. For CK, the threshold numbers of lesions were 135, 35, 18, and 8, with corresponding total GTVs of 2.32 cc, 12.09 cc, 18.24 cc, and 41.52 cc respectively. Mean brain dose increased linearly with number of lesions and total GTV while V8 Gy, V10 Gy, and V12 Gy showed quadratic correlations to the number of lesions and total GTV. Modern dedicated intracranial SRS systems allow for treatment of numerous brain metastases especially for ≤10 mm; clinical evidence to support this practice is critical to expansion in the clinic.