Daily collimator size indicator tolerance for Non-IMRT, IMRT, and SRS-SBRT?

• A. Non-IMRT 2 mm; IMRT 2 mm; SRS-SBRT 1 mm
• B. Non-IMRT 2 mm; IMRT 2 mm; SRS-SBRT 3 mm
• C. Non-IMRT 1 mm; IMRT 2 mm; SRS-SBRT 1 mm
• D. Non-IMRT 2 mm; IMRT 1.5 mm; SRS-SBRT 1 mm

Answer: (A)

Daily collimator size indicator tolerance reflects how closely the machine’s indicated field size must match the true delivered size from day to day. For conventional and standard IMRT fields, the impact of a small reading error is less critical because the fields are larger and dose gradients are not as steep, so a 2 mm tolerance adequately accounts for normal mechanical and setup variations. In SRS/SBRT, the fields are very small and the dose gradients are extremely sharp, so even a 1 mm misread can cause meaningful dose differences to the target or nearby organs, requiring a tighter 1 mm tolerance. This combination—2 mm for Non-IMRT, 2 mm for IMRT, and 1 mm for SRS-SBRT—best aligns with how precision needs differ across these treatment modalities.