IMRT
Meaning of IMRT: Radiation shaped to the tumor rather than aimed at a box around it, using beams whose strength varies across their own width.
Definition of IMRT
IMRT stands for intensity-modulated radiation therapy, a form of external beam treatment in which each beam is broken into many small segments of differing strength. Computer planning works backward from the dose the tumor needs and the limits set for nearby organs. The result is a radiation field that bends around healthy structures, which reduces the dose reaching them.
How does it differ from older radiation therapy?
Conventional treatment aimed a few uniform beams at a target from different angles. IMRT changes the intensity within each beam, so parts of the field deliver more dose than others.
| Conventional 3D treatment | IMRT | |
|---|---|---|
| Beam strength | Even across the field | Varies point to point |
| Dose shape | Roughly boxlike | Wraps around the target |
| Concave targets | Hard to spare the center | Can curve around an organ |
| Planning | Forward, by hand | Inverse, by computer |
How is the beam shaped?
Inside the treatment machine sits a multileaf collimator, a bank of narrow metal leaves that slide across the beam. During treatment the leaves move, so different parts of the field are exposed for different lengths of time. A part shielded for most of the delivery receives a low dose; a part left open receives the full dose. Combine several such fields from different angles and the high-dose region can be made concave, hugging the tumor while stepping around the spinal cord, a salivary gland or a section of bowel. Planning runs in reverse compared with older methods: the goals are entered first and software calculates the leaf movements that meet them.
What is it used for, and what are the trade-offs?
Head and neck, prostate and central nervous system tumors are common indications, because critical structures sit close to the target in all three. Sparing the salivary glands, for instance, reduces long-lasting dry mouth. The trade-offs are real. Planning and quality checks take longer, treatment sessions are longer, and a larger volume of surrounding tissue receives a low dose even though the high-dose region shrinks. Small daily shifts in position matter more when the dose falls off sharply, so image-guided radiation therapy is often used alongside it to confirm the target is where the plan assumed.
Also known as
Used in a sentence
The plan was delivered as IMRT to limit dose to the parotid glands.
Good to know
IMRT lowers the high dose delivered to nearby organs but spreads a low dose across a larger volume, and its accuracy depends on the patient being positioned as the plan assumed.