Objective To study the specific splitting mode of fluence in Monaco planning system with single-beam multi-arc.

Methods (1) Simulation plan: two, three, and four cylindrical structures were drawn along the x direction on the CT image of the cylindrical Delta4 phantom with uniform density to simulate three cases that need to irradiate two, three, and four isolated target volumes at the same time, respectively, to observe the relationship between the shape of the irradiation fields and the target volumes and analyze their regularity. (2) Patient irradiation: the following past cases in Jilin Cancer Hospital were completely random selected: one case of whole brain prophylactic irradiation, one case of nasopharyngeal carcinoma, and one case of cervical cancer. Three volume-modulated arc therapy plans were designed for each patient. A 360° beam was used in all plans, and the difference is that the numbers of arcs were two, three, and four, respectively. The gantry angles of all real case plans were set to 0°, the real patient plans were delivered on the accelerator, and the Matrixx dose verification system was used to measure the total dose of the plans and the partial dose per arc. Compare the positional relationship between the total dose for each plan and its dose per arc.

Results The results of simulation plan and patient irradiation study showed that on the beam's eye view (BEV), the fluence was divided along the x-axis, and the number of fluence divisions was equal to the number of arcs in the plan. Each arc was illuminated from left to right in the BEV direction in sequence.

Conclusion The fluence splitting mode in Monaco planning system was elucidated, providing guidance for its practical application in clinic.