R. Chuter

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‐Linacs 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.

Assessing MR-linac radiotherapy robustness for anatomical changes in head and neck cancer

The MR-Linac will provide excellent soft tissue contrast for on-treatment imaging. It is well known that the electron return effect (ERE) results in areas of increased and decreased dose at air/tissue boundaries, which can be compensated for in plan optimisation. However, anatomical changes may affect the quality of this compensation. In this paper we aim to quantify the interaction of anatomical changes with ERE in head and neck (H&N) cancer patients. Twenty patients treated with either 66 Gy or 60 Gy in 30 fractions were selected. Ten had significant weight-loss during treatment requiring repeat CT (rCT) and ten had PTVs close to the sinus cavity. Plans were optimised using Monaco to meet the departmental dose constraints and copied to the rCT and re-calculated. For the sinus patients, we optimised plans with full and empty sinus at both 0 T and 1.5 T. The effect of the opposite filling state was next evaluated. No clinically relevant difference between the doses in the PTV and OARs were observed related to weight-loss in 0 T or 1.5 T fields. Variable sinus filling caused greater dosimetric differences near the walls of the sinus for plans optimised with a full cavity in 1.5 T, indicating that optimising with an empty sinus makes the plan more robust to changes in filling. These findings indicate that current off-line strategies for adaptive planning for H&N patients are also valid on an MR-linac, if care is taken with sinus filling.