Patient setup variation on Elekta Unity and its impact on adaptive planning

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• The unique design of Magnetic Resonance Linear Accelerators (MR-linacs) can limit the use of immobilization devices.
• This limitation may lead to significant patient setup variations (PSVs), impacting treatment accuracy.
• Understanding and managing PSVs is critical for optimizing MR-linac treatments.

Purpose of the Study:

• To analyze patient setup variations (PSVs) on the Elekta Unity MR-linac system.
• To investigate the impact of these PSVs on adaptive radiotherapy planning workflows.
• To evaluate the effectiveness of adaptive planning in accommodating PSVs.

Main Methods:

• Patient setup variations (PSVs) were analyzed for brain, pancreas, prostate, and rectum treatments on the Elekta Unity.
• Prostate and pancreas treatment plans were used to simulate PSVs by shifting reference scans (1-3 cm LR, SI).
• Adaptive-to-position (ATP) and adaptive-to-shape (ATS) workflows were compared for planning time, monitor units (MUs), and dosimetric outcomes.

Main Results:

• Significant PSVs were observed across treatment sites, particularly in the superior-inferior and left-right directions for brain and pancreas/pelvis treatments.
• The adaptive-to-shape (ATS) workflow required two to three times longer than the adaptive-to-position (ATP) workflow.
• Both adaptive planning workflows successfully maintained target coverage and organ-at-risk sparing, irrespective of PSV magnitude, with only minor increases in MUs for ATP in prostate cases.

Conclusions:

• Suboptimal immobilization on the Elekta Unity MR-linac results in considerable patient setup variations (PSVs).
• Adaptive radiotherapy planning demonstrates efficacy in compensating for these PSVs in treatments like prostate and pancreas.
• Reducing PSVs, especially rotational errors, is recommended to mitigate intra-fraction motion and shorten overall treatment duration.