In Silico Trial of MR-Guided Midtreatment Adaptive Planning for Hypofractionated Stereotactic Radiation Therapy in Centrally Located Thoracic Tumors

Purpose: Hypofractionated (>5 fraction) stereotactic radiation therapy (HSRT) may allow for ablative biologically equivalent dose to tumors with a lower risk of organ-at-risk (OAR) toxicity in central thoracic tumors. Adaptive planning may further improve OAR sparing while maintaining planning target volume (PTV) coverage. We hypothesized that midtreatment adaptive replanning would offer dosimetric advantages during HSRT for central thorax malignancies using magnetic resonance imaging (MRI)–guided radiation therapy. Methods and Materials: Twelve patients with central thorax tumors received HSRT using MRI-guided radiation therapy. Clinically delivered regimens were 60 Gy in 12 fractions or 62.5 Gy in 10 fractions, with low-field magnetic resonance (0.35 T) volumetric setup imaging acquired at each fraction. Daily gross tumor volume (GTV) and OARs were retrospectively redefined on fraction 1, 6, and 10 MRIs, and GTV response was recorded. Simulated initial plans prescribed a dose of 60 Gy in 12 fractions based on fraction 1 MRI. Midtreatment adaptive plans were created based on fraction 6 anatomy-of-the-day. All plans were created using an isotoxicity approach with a goal of 95% PTV coverage, subject to hard OAR constraints, to represent clinically ideal OAR sparing. Plans were then compared for projected OAR sparing and PTV coverage. Results: Patients demonstrated significant on-treatment MRI-defined GTV reduction (median 41.8%; range 16.7%-65.7%). At fraction 6, median reduction was 26.7%. All initial plans met OAR constraints. Initial plan application to fraction 6 and fraction 10 anatomy resulted in 8 OAR violations (5 of 13 patients) and 10 OAR violations (6 of 13 patients). All fraction 6 violations persisted at fraction 10. Midpoint adaptive planning reversed 100% of midpoint OAR violations and tended to reduce the magnitude of OAR violations incurred at fraction 10. In 40% of fractions (2 of 5) in which OAR violation resulted from initial plan application to fraction 6 anatomy, PTV coverage was increased concomitant with violation reversal. Conclusions: Midtreatment adaptive planning based on tumor response may be dosimetrically advantageous for sparing of surrounding critical structures in HSRT for central thorax malignancies and could be applied using either an online or offline paradigm.