Dosimetric Benefits and Practical Pitfalls of Daily Online Adaptive MRI-Guided Stereotactic Radiation Therapy for Pancreatic Cancer.

PURPOSE: Magnetic resonance imaging guided (MRI-g) radiation therapy provides visualization of the target and organs at risk (OARs), allowing for daily online adaptive radiation therapy (OART). We hypothesized that MRI-g OART would improve OAR sparing and target coverage in patients with pancreatic cancer treated with stereotactic body radiation therapy (SBRT).

METHODS AND MATERIALS: Ten patients received pancreas SBRT to a dose of 33 to 40 Gy in 5 fractions. The dose was prescribed to 90% coverage of the planning target volume at 100% isodose (PTV100). After each fraction's setup magnetic resonance imaging scan, the target position was aligned by 3-dimensional shifts, the normal anatomy was recontoured, and the original radiation therapy plan was recalculated to create a nonadaptive plan. A reoptimized (adaptive) plan was then generated for each fraction and renormalized to 90% coverage of PTV100. Target and OAR doses between nonadaptive and adaptive plans were compared to assess the dosimetric impact of daily adaptation.

RESULTS: The PTV100 mean for adaptive and nonadaptive techniques was 90% and 80.4% (range, 46%-97%), respectively (P = .0008). Point maximum (Dmax) 38 Gy duodenum objectives were met in 43 adaptive fractions compared with 32 nonadaptive fractions (P = .022). Both PTV100 ≥90% and all OAR objectives were achieved in 28 adaptive fractions compared with only 3 nonadaptive fractions. For nonadaptive plans, interfraction increases in stomach volume correlated with higher stomach V33 (P = .004), stomach Dmax (P = .009), duodenum V33 (P = .021), and duodenum Dmax (P = .105). No correlation was observed between stomach volume and OAR doses for adaptive plans. OART plans with Dmax violations of the spinal cord (20 Gy) in 4 fractions and large bowel (38 Gy) in 5 fractions were identified (although not delivered).

CONCLUSIONS: MRI-g OART improves target coverage and OAR sparing for pancreas SBRT. This benefit partially results from mitigation of interfraction variability in stomach volume. Caution must be exercised to evaluate all OARs near the treatment area.