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Online Adaptive Planning Strategy for 1.5T MR-Linac

J Yang*, A Sobremonte, S Vedam, K Brock, A Ohrt, C Fuller, S Choi, A Jhingran, P Castillo, B Lee, J Wang, N Hughes, M Mohammadsaid, P Balter, MD Anderson Cancer Center, Houston, TX


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Elekta Unity (1.5T MR-Linac) provides two online plan adaptation approaches: (1) adapt-to-position (ATP) with plan isocenter shift only; and (2) adapt-to-shape (ATS) including contour deformation for shape changes. We develop an adaptation strategy to stratify patients for ATP or ATS in order to optimize the MR-Linac treatment workflow.

Ten patients treated with the Elekta Unity in our institution were included for this study: 5 head-and-neck (30-33 fractions) and 5 prostate and pelvis SBRT (5 fractions). All patients were treated with ATP for online adaptation. ATP plans were created on the reference CT. Fraction dose of each ATP plan were added together to create a summed ATP plan, which represented the planned delivered dose. In parallel, we shifted each ATP plan to their corresponding MR image, and deformed the dose back to the reference CT image for accumulation using an in-house deformable registration tool. The accumulated deformed dose approximated the actual delivered dose. The accumulated deformed dose was compared with the summed ATP dose to evaluate the accuracy of the ATP approach.

All head-and-neck patients had the primary target dose difference within 1% except one patient that has about 3% less in actual delivered dose, possibly due to the anatomical change during the treatment. Most organs-at-risk (OARs) receiving substantial dose have a difference within 10%; Small structures (e.g. cochlea, and carotids) tended to have large difference. For prostate and pelvis SBRT, all target dose is within 1% difference. Most OAR dose was low and the absolute dose difference is small. One prostate patient has a significant higher rectum dose (15.1Gy versus 11.1Gy).

ATP could provide accurate dose delivery in most cases. ATS is recommended if the OARs that may move day to day receive substantial dose, or the patient anatomy changes during a long treatment course.

Funding Support, Disclosures, and Conflict of Interest: MD Anderson is a founding member of the Elekta Unity MRL consortium. Some of the coauthors received corporate funding support from Elekta, but it is not related to this work.


MR, Image-guided Therapy, Deformation


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking

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