J Visak, D Parsons, T Chiu, A Godley, J Deng*, University of Texas Southwestern Medical Center, Dallas, TX

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  J Deng

SU-H330-IePD-F6-4 (Sunday, 7/10/2022) 3:30 PM - 4:00 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 6

Purpose: To fabricate a novel phantom insert for a commercial MR-compatible quality assurance motion phantom and dosimetrically assess the feasibility of manually gated treatments on Unity MR-linac (MRL) with optically stimulated luminescent detectors (OSLDs). Commercially available inserts solely support ionization chamber measurements that may become damaged by an MR-cine motion tracking sequence or degrade image quality via triax cable noise.

Methods: A custom phantom insert shell was 3D printed and filled with silicon. The shell has slots for two OSLDs suspended in a gadolinium-laced silicon plug. 4D-CT was acquired with novel insert using a superior-inferior cos4 simulated breathing cycle (amplitude 15mm, breathing period 6s). An internal target volume (ITV) was derived from maximum intensity projection and expanded 5mm to create planning target volume (PTV). A 12-beam plan designed to homogenously cover PTV at 600 cGy was created for MRL adaptive delivery with two set-ups: (1) range of motion of OSLDs was identical to simulation and plan delivered continuously; (2) motion amplitude was increased to 25mm, taking target out of PTV, and breathing period to 20s to allow for easier gating. When the OSLD was seen outside PTV in MR-cine, the beam was manually stopped. Both set-ups utilized adapt-to-position (ATP) delivery. PTV mean dose was recorded and compared to OSLD readings.

Results: The PTV mean doses from ATP plan for non-gated and gated set-ups were 610.3cGy and 609.5cGy, respectively. Average OSLD reading for non-gated treatment was 596cGy(-2.34%). Due to device limitations, dose from single OSLD for gated treatment was 590.9cGy(-3.14%) which preliminarily indicates MRL manual gating is feasible.

Conclusion: This study designed a custom insert for a commercial motion phantom for dose measurement during MR-cine imaging. The insert then demonstrated the accuracy of dose delivered in manually gated treatments on MRL. Additional study with realistic breathing traces is ongoing.

Keywords

Gating, MRI, Quality Assurance

Taxonomy

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

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