(S009) In Silico Trial of MR-Guided Mid-Treatment Adaptive Planning for Hypofractionated Stereotactic Radiotherapy in Centrally Located Thoracic Tumors

Abstract

Hypofractionated (>5fx) stereotactic radiotherapy (HSRT) may allow for biologically equivalent dose to tumor with a lower risk of organ at risk (OAR) toxicity in centrally located thoracic tumors (CTT). Adaptive planning may further improve OAR sparing while maintaining planning target volume (PTV) coverage. We evaluate potential dosimetric advantages of mid-treatment adaptive re-planning during HSRT for CTT using magnetic resonance image-guided radiotherapy (MR-IGRT). Thirteen patients with CTT received HSRT using MR-IGRT. Clinically delivered regimens were 60Gy/12fractions (n=8) or 62.5Gy/10fractions (n=3), with low-field-MR (0.35T) volumetric setup imaging acquired at each fraction. Daily GTV/OAR were retrospectively re-defined on fx1, 6, and 10 MR-image sets, and tumor volume response was recorded. Simulated initial plans (PI) were created with prescribed dose of 60Gy/12fx based on fx1 MRI. Fx6 and fx10 adaptive plans (PA) were created based on fx6 and fx10 anatomy-of-the-day. All PI/PA were created using an isotoxicity approach with goal 95%PTV coverage, subject to hard OAR constraints, to represent clinically ideal OAR sparing. PI/PA were then compared for projected OAR sparing/PTV coverage. All patients demonstrated on-treatment MRI-defined GTV reduction (median 52.1%; range 30.5-70.8%). At fx6, median reduction was 34.8%. All PI met initial hard OAR constraints. PI application to fx6 and fx10 anatomy resulted in 8 OAR violations (5/13 patients) and 12 OAR violations (6/13 patients), respectively. All violations observed in fx6 were persistent in fx10; average magnitude of OAR violation was higher in fx10 than fx6. Adaptive planning reversed 100% of OAR violations. In 55%(6/11) of fractions where OAR violation resulted from PI application to fx6/fx10 anatomy, PTV coverage was increased concomitantly with violation reversal. Midpoint adaptive planning based on tumor response may be dosimetrically advantageous for sparing of surrounding critical structures in HSRT for central thorax malignancies.