MU
Monitor Unit — the unit of linac dose-delivery measurement, defined by the dose registered on the in-head ionization chamber under a calibrated reference condition (typically 1 MU = 1 cGy at d_max in water at 100 cm SSD with a 10×10 cm field, energy-specific). Each treatment plan specifies MU per field; the linac delivers the prescribed MU and stops. Core unit of radiotherapy dose delivery — every clinical decision and QA check ultimately resolves to MU.
Why it matters to buyers: Treatment-planning systems (Eclipse, Monaco, RayStation, Pinnacle) compute MU per field per fraction. Across an entire treatment course, total MU delivered determines wear on the linac (klystron / magnetron / target / MLC) and is the operational unit for both planning and billing context.
Why it matters to engineers: Daily output constancy (TG-142) verifies MU calibration hasn't drifted from the absolute reference, with tolerances typically ~3% daily, tighter monthly / annual. Calibration is traceable to NIST via an annual physicist absolute-dose calibration. Output drift is one of the most-watched QA parameters; persistent drift is investigated immediately.