Dosimetric Analysis of Stereotactic Body Radiotherapy for Pancreatic Cancer using MR-Guided Tri-60Co Unit, MR-Guided LINAC, and Conventional LINAC Based Plans

Publication date: Available online 1 March 2018
Source:Practical Radiation Oncology
Author(s): S.J. Ramey, K.R. Padgett, N. Lamichhane, H.J. Neboori, D. Kwon, E.A. Mellon, K. Brown, M. Duffy, J. Victoria, N. Dogan, L. Portelance
PurposeTo perform a dosimetric comparison of two MR-guided radiotherapy systems capable of performing on-line adaptive radiotherapy (ART) versus a conventional radiotherapy system for pancreas stereotactic body radiation therapy.Methods and MaterialsTen cases of patients with pancreatic adenocarcinoma previously treated in our institution were used for this analysis. MR-Cobalt and MR-LINAC plans were generated and compared to conventional LINAC (VMAT) plans. The prescription dose was 40Gy in five fractions covering 95% of the planning tumor volume (PTV40) for the 30 plans. The same organs at risk (OARs) dose constraints were used in all plans. Dose volume-based indices were used to compare PTV coverage and OAR sparing.ResultsThe PTV40 conformity index (CI) demonstrated higher conformity in both LINAC-based plans compared to MR-Cobalt plans. While there was no difference in mean CI betweenLINAC and MR-LINAC plans (1.08 in both), there was a large difference between LINAC and MR-Cobalt plans (1.08 vs. 1.52). Overall, 79%, 72%, and 78% of critical structure dosimetric constraints were met with LINAC, MR-Cobalt, and MR-LINAC plans. The MR-Cobalt plans delivered more dose to all OARs when compared to the LINAC plans. In contrast, the doses to the OARs of the MR-LINAC plans were similar to LINAC plans except in two cases—liver mean dose (MR-LINAC—2 .8Gy versus LINAC—2.1Gy) and volume of duodenum receiving at least 15Gy (MR-LINAC—13.2 cubic centimeters [cc] versus LINAC—15.4cc). Both differences are likely not clinically significant.ConclusionThis study demonstrates that dosimetrically similar plans were achieved with conventional LINAC and MR-LINAC while doses to OARs were statistically higher for MR-Cobalt compared to conventional LINAC plans due to low dose spillage. Given the improved tumor-tracking capabilities of MR-LINAC, further studies should evaluate potential benefits of ART-capable MR-guided LINAC treatment.



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