Time-resolved biological effective dose distributions in stereotactic radiosurgery for vestibular schwannomas: a comparative analysis between two dedicated stereotactic radiosurgery systems

Stereotactic radiosurgery (SRS) delivered using a step-and-shoot approach, using either isocentric or non-isocentric radiation beams, typically results in distinct treatment delivery patterns across different SRS systems, reflected in treatment time. In this study, we performed a comparative analysis of dosimetric and radiobiological properties of the CyberKnife^TM(CK) and the Gamma Knife^TM(GK) systems in the management of vestibular schwannomas (VS). 
Approach: The treatment plan and delivery data of 21 patients underwent GK SRS for VS tumors were retrospectively evaluated and compared to corresponding treatment plan calculations performed for the CK. A a marginal dose of 12Gy was prescribed in each case. Corresponding Biological Effective Dose (BED) distributions were calculated and compared accounting for sublethal DNA damage repair during treatment delivery.
Main Results: Statistical analyses showed comparable plan quality indices, except for coverage, gradient index and R50, where GK plans exhibited higher coverage and lower gradient and R50 values. CK plans had higher minimum target doses, while D98 values were higher for GK. GK plans exhibited higher mean and maximum target doses due to lower prescription isodose levels. Integral doses (ID) were similar between systems. Treatment times were comparable, although Beam-On-Times were shorter for CK. CK plans presented higher minimum but lower maximum target BED values. Increased BED values were observed in target regions within the auditory canal and near the cochlea for GK plans. No significant differences were found in BED98, mean BED, and integral BED. The marginal BED decreased linearly with treatment time, following: BED(Gy_2.47)= -0.29*T(min)+67.47 for both systems. A novel finding comprised the mutual dependency shown for target integral BED (IBED) increasing linearly with ID according to: IBED(mJ_2.47)=5.63*ID(mJ). 
Significance: Based on the same VS patient cohort, our results suggest that the same dose prescription results in comparable time-resolved BED distributions across the two SRS systems.&#xD.