Planning Target Volume Implications of Residual Setup Uncertainty and Intrafraction Motion During MRI Guided Brain Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Magnetic resonance image guided brain radiotherapy (MRIgRT) is a promising advance to maximize the therapeutic ratio. An unexplored uncertainty in this approach is the residual setup uncertainty after translation-only MR based patient setup. This work quantified this uncertainty, including intrafraction motion, and determined the impact on planning target volume (PTV) margins. <h3>Materials/Methods</h3> Sixty-six patients treated on a 1.5T-based MR-Linac were included. Of these, 49 and 17 patients received conventional (15-30 fractions for glioblastoma) and hypofractionated (5 fractions for post-operative brain metastases) treatments, respectively; 1,329 total patient fractions were analyzed. At each fraction, patients were setup using online MR guidance by translation-only (T) fusion of the T1 volumetric MR to the planning image set to determine the treatment isocenter shift. These fusions were independently validated offline with translational and rotational (T+R) fusion of bony anatomy. The difference between the T and T+R co-registrations was applied to the original clinical target volume (CTV) to yield the CTV at each fraction (CTV_treat). The set of all CTV and CTV_treat volumes was incorporated in a population-based model parameterized by the relative thresholds (<b>CTV_min, Fx_min, Pt_min</b>). This model determined the minimum planning target volume (PTV) margin such that the PTV encompassed <b>CTV_min</b> of the CTV_treat volume in ≥ <b>Fx_min</b> of fractions in ≥ <b>Pt_min</b> of patients. In this work, we determined the PTV margin that for two sets of thresholds: A) (<b>CTV_min, Fx_min, Pt_min</b>) = (98%, 95%, 90%) and B) (<b>CTV_min, Fx_min, Pt_min</b>) = (95%, 95%, 90%). In a subset of 412 fractions, intrafraction motion was determined as the spatial registration difference between a post-treatment acquired MRI and the pre-treatment MRI. <h3>Results</h3> Residual setup uncertainty (difference between T+R and T registrations) and intrafraction motion results are summarized in Table 1. Across all patient fractions, the vector magnitude of the setup uncertainty and intrafraction motion was 1.3 ± 0.8 mm (mean ± SD) and 0.6 ± 0.5 mm, respectively. To accommodate the setup uncertainty, PTV margins of 2.5 and 1.8 mm were required to meet criteria sets A and B, respectively. <h3>Conclusion</h3> We have quantified residual setup and motion uncertainties for a large series of treated MR-Linac brain tumor patients, generated a model for a population-based PTV, and recommend a minimum PTV of 2-3 mm for this indication. This work is a crucial step towards developing an adaptive brain treatment based on dynamic tumor changes and tumor response.