IntensityModulated

Intensity-modulated refers to the deliberate variation of the strength of a beam or signal across its cross-section or over time. In medical physics, the term is most commonly encountered as intensity-modulated radiation therapy (IMRT), a cancer treatment technique that shapes the delivered dose to the tumor while sparing surrounding healthy tissue.

IMRT achieves modulation by adjusting beam fluence with multileaf collimators, dynamic shaping, and sometimes changing the gantry angle. Modern systems use inverse planning: a computer algorithm determines the optimal spatial fluence map (beamlet intensities) that satisfies dose constraints for the target and organs at risk. Delivery modalities include step-and-shoot and sliding-window dynamic MLC, and in recent years, volumetric modulated arc therapy (VMAT) or rapid arc that modulates intensity while the gantry rotates.

Clinical advantages include improved target coverage with sharper dose gradients and reduced exposure of normal tissues, enabling dose escalation and potentially improved tumor control with fewer side effects. It is used for various sites, especially where critical structures are nearby, such as the head and neck, prostate, and brain.

Limitations include increased planning complexity, longer or more involved quality assurance, and sensitivity to patient setup and motion. The technique requires precise imaging, immobilization, and verification of MLC positions to ensure treatment accuracy.

Outside radiotherapy, the concept of intensity modulation appears in optics and imaging to adjust light distribution, illumination, or signal strength, illustrating the broader utility of modulating intensity in precise, controlled ways.