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.