Allows modulation of the intensity of each radiation beam, so that each field may have one or more areas of high intensity radiation and any number of lower intensity areas within the same field, allowing for greater control of the dose distribution at the target site.
Intensity-modulated radiation therapy (IMRT) is an advanced form of external beam radiation therapy that shapes and modulates radiation beams to conform tightly to a tumor while limiting dose to nearby normal tissues.
IMRT is a computer-controlled radiation technique that uses a linear accelerator to deliver many thin beams or “beamlets” of varying intensity from multiple angles.
Allows for conformed therapy when treating different volumes by altering beam intensities using multilevel beam collimators.
Non-uniform delivery results in sharper dose falloff, allowing higher doses to the tumor and lower doses to surrounding tissues and permitting targeting of irregular contoured volumes.
Computer-driven enhancement of 3-D technique that allows improvement in rates of cancer control and reduces the risk of adverse effects.
IMRT Planning uses 3‑D imaging-CT, often fused with MRI or PET, and inverse planning algorithms that prescribe target and organ-at-risk constraints with software optimizing the dose distribution.
A multileaf collimator (MLC) with many individually movable leaves sculpts the beam shape and intensity, creating highly conformal dose distributions around complex or concave targets.
Delivers highly conformal doses while sparing surrounding normal structures.
Contours target volumes and normal tissue that needs to be spared with establishment of a set of dose constraints for each target and normal tissue volume calculated by a computer algorithm.
Two-tangential fields by IMRT significantly reduces contralateral breast radiation compared with conventional tangential techniques.
Associated with decreased morbidity compared to 3D radiotherapy.
A large population based cohort study, found that IMRT for prostate cancer was not associated with an increased risk of second primary cancers, either solid or hematologic (Pithadia K).
Compared with 3D conformal RT, IMRT allows nonuniform beam intensity simultaneous treatment of multiple targets at different doses, and use of techniques such as SMART (simultaneous modulated accelerated RT) to escalate dose while shortening overall treatment time.
IMRT is widely used where targets abut critical organs, including prostate, head and neck, CNS, lung, gastrointestinal, breast, gynecologic, and selected pediatric tumors.
Typical fractionation is once daily, 5 days per week over several weeks, but hypofractionated or accelerated schedules are used depending on site and protocol.
Advantages include improved target conformity, better sparing of organs at risk (e.g., spinal cord, parotids, optic apparatus), reduced acute and late toxicity, and feasibility of safe dose escalation to improve local control.
Limitations include increased planning workload, longer treatment times, greater need for image guidance, and a modest increase in integral dose and low-dose bath to normal tissues.
Delivery requires detailed contouring of targets and organs at risk, sophisticated treatment planning by medical physicists/dosimetrists, and rigorous pre-treatment measurements.
Daily setup and positioning are performed by radiation therapists, often with on-board kV imaging or cone-beam CT to verify alignment before each fraction.