MR-Linac Successfully Guided the Treatment of a Free-Moving Mesenteric Mass.

A patient presenting with stage I follicular lymphoma was dispositioned for a definitive and curative radiation treatment. The tumor had significant daily movement, as much as 3.83, 3.26 and 12.42 cm in the anterior-posterior, superior-inferior and left-right direction respectively. The tumor volume increased from 16.72 to 44.23 cc in first 5 fractions, followed by a downward trend. To account for the inter-fraction tumor motion, online adaptive radiation therapy was delivered using Elekta Unity MR-Linac. Each fraction, the treating physician manually delineated the planning target volume (PTV) and Unity successfully generated an optimum adaptive plan with PTV coverage ≥ 95%. This case demonstrated that MR-Linac online adaptive workflow was superior in treating a free-moving and volume changing mesenteric mass, minimizing the margin needed in conventional static plan to cover the unpredictable large daily shift.