Feasibility of Ablative SBRT Treatment of Pancreas Patients on an MR-Linac.

PURPOSE/OBJECTIVE(S) To report on our early experience of treating patients with pancreas ductal adenocarcinoma (PDAC) with ablative SBRT (50 Gy in 5 fractions) on the Elekta Unity MR-Linac. MATERIALS/METHODS Ten PDAC patients were treated with abdominal compression (AC) and daily online plan adaptation using Adapt-to-Shape (ATS) workflow. AC is the method of choice to reduce breathing motion since automatic breath-hold/gating or other motion management options are not yet available on Unity MR-Linac. Three tumors were located in the head of the pancreas and seven were in the body. Seven patients were considered inoperable per surgeon and six were node positive. Average GTV volume during simulation was 42.5 ± 24.8 cc. Three orthogonal plane cine MRI were acquired to assess AC belt pressure during MR simulation as well as during each treatment fraction to assess stability of AC in minimizing tumor motion. Three sets of 3D T2w MR scans, pre-treatment (MRIpre), verification (MRIver) and post-treatment (MRIpost) MRI, were acquired for online planning. To assess the dosimetric impact of intrafraction organ motion, a post-treatment quality assurance (QA) was performed before the next fraction by propagating pre-treatment plan and structures to both MRIver and MRIpost, editing the contours and recalculating dose. GTV coverage and selected organs-at-risk (OAR) dose constraints (e.g., < 33 Gy to 0.035 cc, < 25 Gy to 2 cc and < 25 Gy to 5cc volume) were evaluated on MRIver and MRIpost. RESULTS Median total treatment time was 75.5(49-132) minutes. Average contouring, planning, physics QA and beam-on time was 25.1 ± 11.9, 12.2 ± 6.0, 1.9 ± 1.5 and 14.1 ± 3.7 mins respectively. Average AP, LR and SI motion in FB and with compression was 0.3 ± 0.1, 0.6 ± 0.2 and 0.7 ± 0.2 and 0.2 ± 0.1, 0.2 ± 0.1, 0.4 ± 0.1 cm respectively, indicating that compression belt was effective in minimizing patient breathing motion. Average tumor motion in AC belt for all fractions was 0.2 ± 0.1, 0.2 ± 0.1 and 0.4 ± 0.2 cm in AP, LR and SI direction. Median GTV coverage was 78.7% of Rx dose for all fractions. Average GTV Dmax and GTV mean dose for all the fractions was 57.4 ± 0.7 and 51.2 ± 1.6 Gy respectively. Average 0.035 cc, D2cc and D5cc stomach dose was 34.4 ± 5.4, 26.7 ± 2.9 and 24.3 ± 2.1 Gy on MRIver and 35.3 ± 6.2, 27.0 ± 3.3 and 24.6 ± 2.5 Gy on MRIpost. Average 0.035 cc, D2cc and D5cc small bowel dose was 35.1 ± 6.3, 27.1 ± 3.6 and 24.4 ± 2.6 Gy on MRIver and 35.5 ± 6.5, 27.3 ± 3.9 and 24.6 ± 2.5 Gy on MRIpost. CONCLUSION MR-guided adaptive RT enables delivery of curative dose to pancreatic tumors by taking into account interfraction motion of gastrointestinal OARs in daily adaptive planning. To manage intrafraction motion in our current clinical workflow, the organ motion is assessed for first two fractions and conservative strategies (e.g., bigger PRV margins or change the directive to keep stomach 0.035 cc dose to < 30 Gy) are employed for later fractions. ATS workflow with AC and the post-treatment QA allow safe delivery of ablative radiation doses for selected cases of PDAC on Unity MR-Linac. AUTHOR DISCLOSURE N. Tyagi: Honoraria; Elekta. Travel Expenses; Elekta; ISMRM.J. Liang: None. S. Burleson: None. E. Subashi: None. P. Godoy Scripes: None. K.R. Tringale: None. P.B. Romesser: Travel Expenses; Elekta. M. Reyngold: None. C.H. Crane: Honoraria; Elekta. Travel Expenses; Elekta.