Margin derivation from intrafraction patient motion of multi-target, single isocentre, brain stereotactic radiosurgery treatments

Background

Brain metastases are the most common intracranial malignancy and remain a substantial source of morbidity and mortality in cancer patients. Linear accelerator based stereotactic radiosurgery (SRS) is widely used and is frequently delivered by hypo-fractionnated volumetric modulated arc therapy using non-coplanar beams, where geometric accuracy and planning margins are a major concern.

Purpose

To give a practical analysis of intrafraction patient motion for multi-target, single isocentre, brain SRS treatments and to derive adapted GTV-to-PTV margins.

Methods

Data of 154 lesions, spread over 85 fractions from 56 patients treated in our institution with the Varian HyperArc  SRS solution was processed. Intrafraction patient motion were recorded using an Optical Surface Monitoring System during irradiation. The present study focuses on small tumor volumes, roughly equal or inferior to 1.5 ${\mathrm{cm}}^3$, and frameless mask-based immobilization. For each treatment session, a tumor displacement vector matrix was calculated from the patient drifts as a function of time. Data were combined together into a representative treatment scenario and the dosimetric impact of GTV displacement was calculated.

Results

Recommended margins due to patient motion range between 0.3 and 1 mm, depending on the distance tumor-isocentre, and the desired GTV edge dose coverage. Those values should be added quadratically with other sources of uncertainty, such as mechanical isocentre and kV-MV misalignment.

Conclusion

Thorough analysis of intrafraction patient motion was performed, the dosimetric impact was calculated for different scenarios, and adequate GTV-to-PTV margins were derived. These values vary according to the distance isocentre-to-GTV, as well as the desired dose coverage, and should be chosen adequately.