[Frameless set-up with ExacTrac^® system for stereotactic radiotherapy of brain metastasis: Descriptive study from five French centres]

Purpose: This study aimed to analyse the positioning protocols with the ExacTrac^® system, associated with a dedicated linear accelerator such as Novalis^®, for stereotactic treatment of brain metastases in several French centres.

Materials and methods: A survey, including three questions about the prescription of irradiation and twenty-one questions about how the ExacTrac^® system is used, was sent to nine French centres owning a dedicated Novalis^® accelerator. Five centres have accepted to participate in the study.

Results: All centres checked the positioning before each treatment's bow, with residual mismatch tolerances of 0.5 to 0.7mm for the translations and 0.5 to 1° for the rotations. All centres except one also realised orthogonal planar images of classic incidences to help operators ensure proper isocentre positioning. Prescribed doses were 20Gy in one fraction, 30Gy and 33Gy in three fractions or 34Gy in four fractions, mainly depending on the size of the lesion. Finally, a physician validated the images at the treatment station before starting the irradiation.

Conclusions: The practices of the different centres concerning the positioning protocols were rather homogeneous, in agreement with the literature data on ExacTrac^® system's accuracy, as well as proposed fractionations. The systematic medical validation at the treatment station may, however, be questioned because of the waiting time between the doctor's call and validation itself and because of its usefulness; indeed, corrections by the radiation oncologist are very rare and in some centres, non-existent.