Purpose: A novel PET-linac platform, RefleXion X1, was released recently. Utilizing real time PET signal to guide radiotherapy delivery demands the gantry, including both linac assembly and PET detector arrays, rotates at a high speed of 60 rpm. Therefore, a compact and stable linac assembly is required. In this work, we investigate the beam characteristics and reproducibility of multiple linacs designed for this purpose.
Methods: The novel compact linac is capable of providing a single 6MV FFF beam. Beam scanning and output factor measurements were performed with a 3D scanning system for two linac assemblies. A total of 72 profiles measured at 1.5 cm, 10 cm, and 20 cm depths and 6 PDDs were measured for each linac with 12 field sizes ranging from 1.25 × 1cm to 40 × 2cm. The output factors were measured at depth 5 cm for the 12 field sizes and two additional field sizes 0.625 cm x 1 cm and 0.625 cm x 2 cm (0.625 cm is the MLC width). The beam features and dosimetric differences between two linacs were analyzed and compared.
Results: PDD(10) at reference condition (10X2 cm) was 57.4% for both linacs. The profiles and PDDs agreed very well with maximum differences 0.47% for the crossline profile, 0.97% for the inline profiles, and 0.25% for the PDDs. The output factors agreed by less than 1% for all field sizes except 1.3% for 1.25 cm x 1 cm and 1.25 cm x 2 cm.
Conclusion: Beam characteristics, including PDD, profiles and output factors of two compact linacs designed for PET guided radiotherapy were investigated and described. The measurements showed the reproducibility of the beam characteristics between the two systems. This work provides preliminary evidence that a standard beam model may be possible for the current design of the PET-linac.
Not Applicable / None Entered.