SU-F-P-60: Optimization of Cyberknife Treatment Planning Using a Dose-Limiting Auto-Shells Method for Brain Metastases

Purpose: In pursuit of high precision in target localization and steeper dose fall-off in stereotactic radiosurgery, we investigated the impact of optimizing dose-limiting auto-shell function on the quality of Cyberknife (CK) plans in treating brain metastases (BMs). Methods: Nineteen BMs previously treated using CK were selected for this study. The original CK plans (CK_original) had been produced using one to three dose-limiting auto-shells at prescription dose (PD) level and low dose levels of 10 to 30% of PD. In each case, a modified CK plan (CK_modified) was generated using five dose-limiting shells at PD level, intermediate dose level of 50% of PD, and low dose levels with an optimal shell-dilation size based on our experience. In addition, a Gamma Knife plan (GK) was also produced using the original contour set. Thus, a triplet data set of dosimetric parameters was generated and analyzed. Results: While no differences in conformity index (mean±SD 1.22 ± 0.1, 1.18 ± 0.1, and 1.24 ± 0.1 in CK_original, CK_modified, and GK, respectively; over all P > 0.05) and tumor coverage (mean±SD 99.5 ± 0.4%, 99.5 ± 0.3%, and 99.4 ± 0.2% in CK_original, CK_modified, and GK, respectively; over all P > 0.05) were observed among the plans, the normal tissue volume receiving 50% of PD was significantly decreased in CK_modified and GK compared with CKoriginal by 1.28 and 1.27-fold, respectively (P < 0.001, each). No significant differences in dose fall-off were observed between CK_modified and GK (P = 0.345). Conclusion: By optimizing auto-shell function, a significantly steeper dose fall-off can be achieved in CK system, while maintaining high precision in target localization.