Technical Note: Investigating the impact of field size on patient selection for the 1.5T MR-Linac

Purpose: The 1.5 T Elekta MR-Linac, due to the construction of the system will have a maximum radiation field size in the superior-inferior patient direction of 22 cm at isocentre. The field size may impact on the patient groups which can be treated on the system. This technical note aims to address the question of which treatment sites will be affected by field size limitations on the MR-Linac.

Methods: Using historical data for 11 595 cases over 2 yr treated at the authors' institution, the proportion of plans that would fit the MR-Linac's field size was determined for eleven patient groups. In addition, cervix plans were analyzed to determine the length of the two Clinical Target Volumes (CTVs) and any overlap between them.

Results: With a 1 cm margin to allow for online plan adaption, 80% of all plans would be suitable for the MR-Linac due to the field size. This percentage increases to 100% for smaller tumor volumes such as prostate and brain. However, for cervix and three dose-level head and neck plans the percentage becomes 61% and 66%, respectively.

Conclusion: The maximum radiation field size of the MR-Linac in the superior-inferior patient direction is 22 cm. With a 1 cm margin approximately 80% of all plans would be suitable for the MR-Linac with the available field size, decreasing to 61% for larger tumor volumes. For cervix patients this may motivate investigations into treating each CTV with a separate isocentre, allowing for careful control of matching fields.