The right moves.
What happens if you don’t correct intrafraction motion during stereotactic radiosurgery (SRS) for brain metastases? In this retrospective study of patients treated with SRS and surface guidance, the authors identified patients with ≥1 mm shift after intrafraction cone beam CT (CBCT) and recalculated the delivered dose had those shifts not been made. Treatment planning was variable with prescriptions ranging from 15-21 Gy in 1 fraction to 27 Gy in 3 fractions. PTV margins were 0-3 mm, and both VMAT plans and cone-based delivery were used. Exceeding thresholds of 1 mm translation and 1 degree of rotation triggered an intra-fraction CBCT. Had these shifts not been made, the minimum GTV dose would have been 15.8% lower, the minimum PTV dose 10.2% lower, and the absolute decrease in minimum dose to the GTV would have been 3 Gy. Importantly, a larger PTV margin mitigated the effect of these shifts on minimum GTV dose. Last, the difference in PTV coverage was significantly greater for cone-based plans compared to MLC plans (12% v 5%).
- Foster, Adv Radiat Oncol 2022