Purpose: In this work, we evaluated the accuracy of biology-guided radiotherapy (BgRT) delivery to a PET-avid target using the QUASAR™ MRI4D motion phantom on an investigational radiotherapy system having an integrated PET system.
Methods: The motion phantom is a water-fillable oval phantom that consists of two cylindrical inserts. A target was placed in the central insert and moved within the oval in the superior-inferior direction. We placed an EBT-XD Gafchromic film at the center of the target for dose measurements. The target (22Na point source) motion was simulated with a sinusoidal waveform with a frequency of 15 cycles per minute and an amplitude of 20 mm. The prescription dose of 10 Gy was delivered to the target under two conditions: (1) static and (2) for 20 mm amplitude sinusoidal motion. A planning target volume (PTV) of 22 mm was used while a biology tracking zone of 30 mm around the PTV was used to encapsulate the motion of the point source. Dosimetric analysis was performed by analyzing the margin loss and clinical target volume (CTV) coverage dose between the delivery dose on the film plane with respect to the planned dose. The margin loss is defined as the amount of margin between the PTV and CTV lost from motion of the target.
Results: The margin loss for the 20 mm motion case was found to be around 1.78 mm. The minimum dose on the CTV was 1053 cGy for the plan and 1083 cGy for the film. BgRT was able to ensure adequate dose coverage to the target.
Conclusion: This study evaluated the tracking accuracy of BgRT delivery to a moving PET-avid target. In addition, this study showed the potential of the QUASAR phantom to model different motion patterns for continuous tracking and dose delivery based on the PET signal.
Funding Support, Disclosures, and Conflict of Interest: Research supported by RefleXion Medical, Inc
Image-guided Therapy, PET, Radiation Therapy
TH- External Beam- Photons: onboard imaging (development and applications)