Physical Confirmation of Biology-Guided Radiotherapy Directed at Static Targets With Varying Shapes and Background Contrast Environments.

<bold>Purpose/objective(s): </bold>Biology-guided radiotherapy (BgRT), utilizes hardware that incorporates a PET detection system into a ring-gantry LINAC for real-time tracking delivery. For this study, we focus on the BgRT delivery performance in the case of static targets for dose accuracy measurement. During active beam delivery, the system operates by aiming beamlets of therapeutic radiation at malignant tumors in response to outgoing PET emissions with sub-second latency. Over a treatment fraction, these beamlets sum to the total intended dose prescribed by the physician. Physical demonstration of BgRT has not previously been reported. Here we report phantom experiments validating BgRT using static PET-avid targets of varying shapes and background PET environments.<bold>Materials/methods: </bold>An FDG-fillable insert with different shaped targets was developed to mimic different potential radiotherapy targets. Spherical and C-shaped targets were filled with 18F-flurodeoxyglucose (FDG) to represent tumors and/or organs-at-risk. The background material in the insert was either a homogenous water medium or a water filled heterogeneous medium with a Styrofoam mesh simulating lung tissue surrounding the target. Targets and OARs were filled with FDG to achieve a target/OAR: background ratio of 8:1, while the background concentration varied from 4.5-10 kBq/ml to simulate typical patient background activity concentrations. For our set-ups were investigated: Spherical target in homogenous background, spherical target in heterogeneous background, C-shaped target in homogenous background, and spherical target in homogenous background with nearby PET-avid, C-shaped OAR. For each combination, SBRT and BgRT treatment plans were created on the RefleXion treatment planning system (TPS). Next, SBRT and BgRT plans were delivered on a pre-commercial version of the RefleXion system and dosimetric accuracy was measured using the AC phantom. Gamma criteria (3%/3mm) were used to compare delivered and calculated dose. Paired Student's t-test assessed differences in SBRT and BgRT gamma pass results.<bold>Results: </bold>All BgRT deliveries met 3%/3mm gamma pass criteria greater than 95% (Range, 95.2% to 98.9%). Additionally, gamma passing rates were not significantly different than the companion SBRT plans performed on the same targets (P = 0.16).<bold>Conclusion: </bold>Dose accuracy results indicate that BgRT plans can be delivered accurately over a variety of test conditions (different target shapes, attenuating media and activity concentration levels) to FDG-avid targets, with performance similar to SBRT treatments. [ABSTRACT FROM AUTHOR]