A simple plan strategy to optimize the biological effective dose delivered in robotic radiosurgery of vestibular schwannomas.

Using the concept of biologically effective dose (BED), the effect of sublethal DNA damage repair (SLR) on the bio-efficacy of prolonged radiotherapy treatments can be quantified (BED_SLR). Such treatments, lasting more than 20 min, are typically encountered in stereotactic radiosurgery (SRS) applications using the CyberKnife (CK) and Gamma knife systems. Evaluating the plan data from 45 Vestibular Schwannoma (VS) cases treated with single fraction CK-SRS, this work demonstrates a statistically significant correlation between the marginal BED_SLRdelivered to the target (m-BED_SLR) and the ratio of the mean collimator size weighted by the fraction of total beams delivered with each collimator (wmCs), to the tumor volume (Tv). The correlation betweenm-BED_SLRandwmCsTvdatasets was mathematically expressed by the power functionm-BEDSLR=85.21 (±1.7%)⋅(wmCsTv)(0.05±7%) enabling continuousm-BED_SLRpredictions. Using this formula, a specific range ofm-BED_SLRlevels cana prioribe targeted during treatment planning through proper selection of collimator size(s) for a given tumor volume. Inversely, for a selected set of collimators, the optimization range ofm-BED_SLRcan be determined assuming that all beams are delivered with the smallest and largest collimator size. For single collimator cases or when the relative usage of each collimator size is known or estimated, a specificm-BED_SLRlevel can be predicted within 3% uncertainty. The proposed equation is valid for the fixed CK collimators and a physical dose prescription (D_pr) of 13 Gy. For alternateD_prin the range of 11-14 Gy, a linear relationship was found between relative changes ofm-BED_SLR(D_pr) andD_prwith respect tom-BED_SLR(13 Gy) and 13 Gy, respectively. The proposed methodology is simple and easy to implement in the clinical setting allowing for optimization of the treatment's bio-effectiveness, in terms of the delivered BED, during treatment planning.