Radiobiological effect of delivery time in treatments of multiple brain metastases with robotic radiosurgery

To analyze the dose effect of the delivery time in CyberKnife Stereotactic Radiosurgery treatments. A prediction tool and methods to minimize this effect are presented.

Eighty CyberKnife plans of multiple brain metastases (BMs) were analyzed. The Microdosimetric Kinetic Model was used to assess the biological effectiveness of each treatment at each BM. Results were obtained in terms of relative dose (RD) between delivered and planned treatments. A prediction algorithm of RD was developed by predicting beam delivery times of not yet delivered plans. Two strategies to optimize treatments in terms of biological effectiveness were proposed: beam reordering and treatment splitting.

An average RD of 92.3 % (SD = 2.6 %) was obtained for our patient population. Differences between treated and predicted RD were -0.12 % (SD = 0.6 %). Beam reordering improved RD by a mean value of 5.5 % (SD = 3.3 %) with 17.3 % mean increase in overall treatment time. Splitted treatments yielded a mean increase of 5.8 % in RD with a time increase of 35 %, while healthy brain was equally spared if treatments were delivered in different days. For those five treatments, an 8.0 % increase in RD and a 20 % increase in treatment time were obtained with reordered beams.

The effect of treatment time on the dose delivered to BMs in SRS with CyberKnife should be taken into consideration, especially when a large number of BMs are involved. This effect can be predicted and minimized by reordering beams at the cost of increasing overall treatment time.