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Purpose/objective(s): To evaluate Biology-guided Radiotherapy (BgRT) plans generated using PET data acquired on the first installation of the Reflexion™ X1 system. BgRT treatment planning optimizes for fluence indirectly by calculating firing filters that best translate the X1 acquired PET image to the fluences that deliver the prescribed radiation dose. During a treatment session, the firing filters are applied to limited time sampled (LTS) PET images to calculate delivery fluences.Materials/methods: An FDG fillable phantom insert that consists of a 22 mm diameter spherical target (PTV1, ball) and a C-shape object (PTV2 with 26mm axial length, 255° partial annulus with major and minor radii of 27 mm and 12 mm) was imaged using a CT Simulator to create planning contours. The water filled insert surrounding these objects was filled with FDG to simulate a warm background. Both targets were filled with FDG, simulating a target to background ratio of 8:1. Two different BgRT treatment plans were generated for PTV1 simulating homogenous uptake, and PTV2 with an inhomogeneous FDG uptake due to necrosis (a cylinder encompassing the C shape PET avid object). A structure that is 12.5 mm away from these targets was used to simulate OAR. Two PET/CT imaging acquisitions were performed on the RefleXion X1 masking the signal outside of each target plus 10 mm margin. Two different BgRT plans were generated prescribing 50 Gy delivered in 5 fractions to each target, separately. The quality of each treatment plan was evaluated based on PTV coverage, Conformity Index at 50% and 80% isodose lines (CI50, CI80) and OAR sparing. Both BgRT plans then delivered to the independent quality assurance tool for patient specific QA verification where the 3%/3 mm gamma criteria was used.Results: BgRT plans for both homogenous and heterogenous FDG uptake targets were created successfully. Prescription dose coverages were 94.2% and 96.3% for PTV1 and PTV2, respectively. The maximum OAR doses were 30.9 and 33.0 Gy and average OAR doses were 14.4 and 20.1 Gy for PTV1 and PTV2 plans, respectively. CI50 were 7.7 and 6.8 where CI80 were 2.5 and 2.6 for PTV1 and PTV2 plans. The patient specific QA passed with 100 % for both plans.Conclusion: This is the first report of the BgRT plan creation and QA delivery using a clinical installation of the Reflexion X1 system, opening the potential towards BgRT dose delivery using the real time FDG uptake measurements to biologically guide active radiation beam delivery. The X1 system was able to optimize and deliver the BgRT plans for both homogenous and heterogenous FDG uptake scenarios. BgRT is a new paradigm of dose optimization, where a new set of optimization and evaluation approaches will be needed. [ABSTRACT FROM AUTHOR]

Purpose/objective(s): A biologically guided radiotherapy (BgRT) system was developed and equipped with a novel O-ring gantry Linac, positron emission tomography (PET) and computed tomography technologies. This architectural design is to support real-time PET-guided treatment, termed biology-guided radiotherapy. To reduce latency between PET data acquisition and radiation delivery, the BgRT system incorporates a fast-transitioning binary multi-leaf collimator (MLC) leaves sandwiched between a split-jaw. This study reports the dosimetric commissioning of the first clinical BgRT machines at our institution, and compared with the reference data provided by the manufacturer.Materials/methods: The dosimetric commissioning of the 6 MV flattening filter-free (FFF) photon beam was characterized using a 3D water phantom and diode detectors at a source to surface distance of 85 cm. In-air profiles of 40cm x 2cm field, 40cm x 1cm field, and various leaf combinations plus in water PDD data of 12 field sizes (1.25, 2.5, 5, 10, 20 and 40 cm MLC opened leaves with 1 and 2cm jaw settings) were acquired first for beam modeling. Additional longitudinal and transverse profile scans were performed for these 12 field sizes at the depth of 1.5, 5, 10, 15 and 20 cm to verify the beam model accuracy. The relative outputs factors were measured with the W2 1mmx1mm scintillator at the depth of 10cm for the field sizes from the smallest 0.6cm x 1cm to the maximum 40cm x 2cm.Results: For the PDDs of reference field size of 10cm x 2cm measured at Stanford, the maximum doses (dmax) were 12.5mm within the manufacturer's reference range of 13 ± 0.75 mm. The percentage doses at 10cm depth (d10) at reference field size were 56.9% and 57.6% for Stanford and RefleXion measurements, respectively. The longitudinal field sizes for all tested fields were agreed within 0.5mm and the transverse off-axis ratio is within 1% in the beam core (80% of nominal field size). The output factors measured at our institute were ranging from 0.706 to 1.012 relative to the reference 10cm x 2cm field.Conclusion: This evaluation represents the first commissioning evaluation of the dosimetric characteristics of the BgRT system with architecture designed to accommodate both PET detector and LINAC subsystems. This data will serve as a clinical reference for the BgRT treatment planning system's beam model and can act as a standard for comparison and beam model improvement for units installed at future clinical sites. [ABSTRACT FROM AUTHOR]