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 cm(2) 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.