Correlation of V12 to the Number of Lesions, Total Tumor Volume and Dose Prescription in Multiple Brain Metastases Stereotactic Radiosurgery Treatment

Abstract

Patients with brain metastases (BM) present with variable parameters in terms of number, size and location of lesions. In single-fraction stereotactic radiosurgery (SRS) for patients with multiple BM, smaller total tumor volume, higher prescription dose, and lower volume of normal brain receiving >12 Gy (V12) were associated with increased survival rate. V12 larger than 10 cc has been reported to relate to radiation brain necrosis. In this study, we investigated the correlation of V12 to the number of lesions, total tumor volume and dose prescription in multiple BM SRS treatment using Gamma Knife ICON. The dose fall off near the target periphery follows the power-law relationship of D(r, n) ∼ 1/rp(n) (1), where D is the peripheral dose near the target, r is the distance from center of target to a spherical surface surrounding the target, n is the number of targets being treated, and p(n) is the power of the dose fall off. By assuming all the targets to be spheres, the dose of x Gy encompassed volume (Vx) is derived as (Vx/(VT*CI))=(Dp/Dx)γ(n) (2). Here γ(n) = 3/p(n) (3), VT: total target volume, CI: conformity index and Dp: prescription dose. Furthermore, by derivative equation (2), the change of V12 with number of targets and dose prescription are derived respectively as δV12 = VT*CI*(Ln(Dp/12))*(Dp/12)γ(n) δγ(n) (4) and δV12 = VT*CI*γ(n)*(Dp/12)(γ(n)-1) δDp (5). Therefore, we hypothesize that γ(n) increases with number of targets and total target volumes; V12 increases with increasing number of targets and dose prescriptions. In order to prove the above, five single fraction mask-based Gamma Knife treatment plans with numbers of lesions of 3, 13, 8, 3+13=16 and 3+13+8=24 (3 consecutive courses treated for one patient in 2 years) were analyzed and power fitted based on the derived equation (2) and γ(n) was determined for different target number. A significant positive correlation between γ(n) and number of targets was found (R2 = 0.7455), but no strong correlation between γ(n) and total target volume (R2 = 0.4356). A significant positive correlation between increased V12 (δV12) and number of targets were determined (R2 = 0.9998) and the relationship between V12-VT and γ(n) were established (R2 = 0.5174). The results can help clinicians in making decisions about which prescription to choose and what maximum number of targets can be treated to avoid potential complication in brain. The results are listed in the Table below. In this study, we developed a model to correlate V12 in terms of a higher number of targets and dose prescription. The model was derived from clinical multiple BM cases treated with the Gamma Knife ICON in our institution. Our findings can be used as additional clinical guidelines in treatment planning strategy and plan evaluation.Abstract 3851; Table 1n38131624γ(n)1.6321.8941.8441.8082.162VT (cc)7.9438.3261.3579.31817.644Average Dp (Gy)18.615.416.61716.5V12 -VT (cc)7.2313.2862.91211.64125.824V12 (cc)15.17411.6124.26920.95943.468 Open table in a new tab