Independent monitor unit verification for dynamic flattened beam plans on the Halcyon linac

Independent monitor unit verification (MUV) methods for the dynamic beam-flattening (DBF) technique have not been established. The purpose of this study was to clarify whether MU values for the DBF technique can be calculated using in-air and in-water output ratios (S_c and S_cp ). S_c and S_cp were measured in the DBF mode, and the phantom scatter factor (S_p ) was calculated. The difference between calculated and planned MUs with square and rectangle fields and clinical plans for different treatment sites was also evaluated. S_c values for the 4 × 4 to 24 × 24 cm² fields of the distal multi-leaf collimator (MLC) layer at 2-cm intervals were 0.887, 0.815, 0.715, 0.716, 0.611, 0.612, 0.511, 0.373, 0.374, 0.375, and 0.374, respectively. No collimator exchange effect was observed. S_c also depends slightly on the field size of the distal MLC layer. If the distal-MLC-layered field size was less than 20% of the corresponding MLC sequence size in the proximal MLC layer, S_c was affected by >1%, which was compensated using a correction factor (CF). S_p increased as the field sizes of the MLC sequence and distal MLC leaves increased. MUs calculated using measured S_c , S_p , and CF for square and rectangle fields agreed with planned MUs within ±1.2%. A larger difference (-1.5%) between calculated and planned MUs was observed for clinical plans, whereas differences in MUs were within 2 MU for most fields (56 out of 64 fields). MU calculation for the DBF technique can be performed with S_c , S_p , and CF for independent MUV.