Intensity-Modulated Radiation Therapy (IMRT)

Modulated-intensity external beam radiotherapy. The linac's multi-leaf collimator moves during delivery to create spatially varying fluence across each treatment field, producing highly conformal dose distributions that shape radiation dose around tumor targets while sparing adjacent organs at risk.

Clinical standard for most treatable cancers since the mid-2000s. Predecessor of VMAT (which delivers IMRT faster via continuous gantry rotation).

Workflow

Simulation — CT simulation (sometimes with MRI / PET fusion). Patient positioned in treatment-intent immobilization.
Contouring — physician defines GTV (gross tumor volume), CTV (clinical target volume accounting for microscopic extension), PTV (planning target volume with setup margin). Physicist contours OARs (organs at risk).
Planning — inverse planning in the TPS (Eclipse, Monaco, Pinnacle, RayStation). Optimizer finds MLC leaf sequences that deliver prescribed dose to target while meeting OAR constraints.
Plan QA — physicist-signed. Patient-specific QA (portal dosimetry, ion chamber or array measurement) verifies delivered dose matches plan.
Delivery — daily fraction with image-guided setup (CBCT or kV-kV or portal imaging). 15–30 min per fraction. 20–40 fractions typical course.

Common clinical sites

Head and neck cancer — sparing parotids reduces xerostomia; IMRT is standard
Prostate cancer — dose escalation with rectum/bladder sparing
Gynecologic — cervix, endometrium with bowel sparing
Lung — when SBRT is not appropriate (larger volumes, nodal coverage)
Brain — conformal CNS radiation with hippocampal sparing
Pediatric — when protons aren't available

Systems

Every modern linac supports IMRT. Common delivery platforms:

Dose + motion considerations

TG-142 QA program maintains delivery accuracy
Daily IGRT (image-guided setup) verifies patient position
Motion management for thoracic + abdominal targets (ITV, breath-hold, gating)
Adaptive replanning for significant anatomic changes during treatment course

Step-and-shoot vs sliding-window

Two IMRT delivery modes:

Step-and-shoot — MLC moves to configuration, beam turns on, delivers dose, beam off, MLC moves, repeat. Simpler QA; slower.
Sliding-window (dynamic MLC) — MLC moves continuously with beam on. Faster; requires tighter QA.

Most modern systems default to sliding-window.