What is the monthly gantry/collimator angle indicator tolerance?

• A. 0.5°
• B. 2°
• C. 1.5°
• D. 1°

Answer: (D)

The monthly tolerance for the gantry and collimator angle indicators is 1 degree. This means the angle readout on the machine’s display should match the actual physical angle within one degree during monthly QA checks. The purpose is to ensure the beam is delivered with the intended orientation, since even small angular errors can affect dose distribution, especially in complex setups or arc treatments.

TG-142 sets this tolerance at about 1° because it reflects a practical balance: it’s tight enough to catch drift or miscalibration that could impact treatment accuracy, but not so strict that it makes routine QA overly burdensome or unrealistic given the mechanical precision of standard equipment. If the measured difference is greater than 1°, the system should be recalibrated or serviced.

Other tighter or looser numbers would either impose unnecessary QA burden (0.5°) or risk larger misalignments in delivery (1.5° or 2°).