IMRT and IGRT: How Precision Radiotherapy Works

Intensity-modulated radiotherapy (IMRT) shapes the radiation dose to the tumour by dividing each beam into hundreds of small beamlets of different intensity, so the high-dose region wraps around the tumour and bends away from nearby organs. Image-guided radiotherapy (IGRT) adds imaging at the time of treatment to confirm the patient and tumour are exactly where the plan assumes. Together they are the foundation of modern precision radiotherapy.

Dr. Garima Singh, Principal Consultant - Radiation Oncology, BLK-Max Super Speciality Hospital, New Delhi, India, walked through each technique using plans from her own centre. This article sets out how IMRT, IGRT and helical tomotherapy work, what they allow clinicians to do that older techniques could not, and how motion, the heart and the salivary glands are protected in practice. The wider context is in the complete guide to advances in radiation oncology.

How IMRT works: multi-leaf collimators and inverse planning

The key hardware is the multi-leaf collimator (MLC), a set of thin metal leaves inside the head of the linear accelerator that move independently to shape the beam. Dr. Singh called it the backbone of IMRT. By moving the leaves during treatment, the machine divides the radiation beam into hundreds of small beamlets, each with its own intensity, and shapes the dose to the outline of the tumour while sparing the organs at risk next to it.

The second element is inverse planning. In older forward planning, the planner chose beam angles and weights and then checked the resulting dose. In inverse planning, the clinician specifies what the tumour must receive and what each organ at risk may receive, and the planning software works out the beam intensities that meet those goals. Dr. Singh described this capacity for dose painting, delivering different dose levels to different parts of the target, as one of the most valuable aspects of IMRT.

IMRT vs 3D conformal radiotherapy: what the plans show

The difference between IMRT and 3D conformal radiotherapy (3D-CRT) is easiest to see on a plan. Dr. Singh showed a teaching comparison prepared at her centre for a patient with oesophageal cancer, with the IMRT plan and the 3D-CRT plan side by side. In both the coronal and sagittal views, the IMRT plan covered the target closely with visibly less dose spilling into the surrounding tissue.

Her second case showed why the gain matters most for complex anatomy. It was a locally advanced nasopharyngeal carcinoma, staged T4 N2c M0. The nasopharynx has a notoriously complex geometry: the tumour sits close to the brain and to several critical structures, which made such tumours very difficult to treat in the era of two-dimensional planning. On the IMRT plan, the gross tumour volume (GTV, the visible tumour) was outlined in red, the nodal regions at different dose levels in other colours, and the lower-dose areas separately.

Each organ at risk is contoured individually. For this case Dr. Singh listed the brainstem, optic chiasm, temporal lobes, pharyngeal constrictor muscles, submandibular glands and parotid glands. IMRT then delivers the high dose to the tumour while keeping each of these within its limit. The precision of IMRT, in her words, made it possible to treat tumour shapes and locations that were previously very challenging.

The evidence: IMRT halves dry mouth in head and neck cancer

The landmark evidence came from a phase 3 trial published in Lancet Oncology in 2011, the PARSPORT trial, which compared parotid-sparing IMRT with conventional radiotherapy in head and neck cancer. Dr. Singh summarised its main finding: IMRT reduced xerostomia, the chronic dry mouth that is one of the most troublesome late side effects of head and neck radiotherapy, by about 50%. The mechanism is parotid sparing. Because IMRT can bend the dose around the parotid glands, they keep producing saliva.

That trial established IMRT as the standard for head and neck cancer. In daily planning at her centre, she said, the dose at the tumour site is optimised while the dose to every organ at risk is minimised.

Dose constraints for organs at risk in head and neck cancer

A dose constraint is the maximum dose an organ at risk should receive to keep the risk of a given side effect acceptable. Asked by Dr. Ivan Ipavu from Uganda about current constraints in head and neck cancer, Dr. Singh explained that the organs at risk depend on the area treated. For a cancer of the oral tongue, the relevant organs include the parotid and submandibular glands, the buccal mucosa, the dysphagia and aspiration-related structures (the muscles and tissues that control swallowing) and the oesophagus.

The figures she gave from her practice:

• Parotid gland: mean dose below 26 Gy.
• Oesophagus: mean dose below 45 Gy.
• Buccal mucosa: robust data on buccal mucosa constraints are lacking, so her institute's practice is to keep it within 32 to 35 Gy without compromising target coverage. Her full answer to Dr. Ipavu is recorded in the Live Discussion.

The constraints are drawn from published guidance. Dr. Singh listed the RTOG constraints, the Timmerman constraints for stereotactic treatment, QUANTEC (Quantitative Analyses of Normal Tissue Effects in the Clinic), and the more recent HyTEC (for hypofractionated treatment) and PENTEC (for children) reports.

IGRT: imaging at the moment of treatment

IGRT builds imaging into the treatment machine; Dr. Singh described its combination with IMRT as pivotal in enhancing treatment accuracy. Before or during each session, X-ray or CT imaging is taken on the treatment couch and compared with the planning scan, so the patient's set-up can be corrected before the beam is switched on.

The value is greatest where the margins are tightest. A highly conformal IMRT plan is only as good as the patient's position: if the tumour has shifted, a tight high-dose region can miss part of it or hit an organ at risk. IGRT is what makes it safe to shrink margins and escalate dose.

Helical tomotherapy: total body and craniospinal irradiation

Tomotherapy delivers IMRT in a different geometry. Instead of a conventional gantry, a 6 MV linear accelerator is mounted on a ring gantry and rotates continuously around the patient while the couch moves through the ring, so the dose is delivered in a helix. The ring also carries an image detector and a beam stopper, and a multi-leaf collimator shapes the beam. Dr. Singh's centre uses a new-generation tomotherapy system with adaptive radiotherapy and 3D imaging, capable of IMRT, IGRT, SRS, SBRT and total body irradiation.

Tomotherapy's particular strength is very long treatment fields, where a conventional linear accelerator would need several abutting fields.

• Total body irradiation (TBI) for haematological malignancies, given as part of conditioning before a bone marrow transplant. Dr. Singh showed a TBI plan covering the entire body properly while avoiding overlap between dose areas.
• Craniospinal irradiation (CSI), which treats the whole brain and spinal canal, for example in medulloblastoma.

For these long fields, tomotherapy provides fast treatment and better coverage of the target. Her centre uses it for paediatric patients needing long fields, discussed in stereotactic radiosurgery, SBRT and proton therapy.

Protecting the heart in left-sided breast cancer with deep inspiration breath hold

In left-sided breast cancer, the heart lies close to the chest wall that has to be treated. Dr. Singh's target when treating the left breast or chest wall is to keep the dose to the heart below 5 Gy. A helical tomotherapy plan she showed for a left chest wall abutting the heart achieved proper coverage of the treatment area while sparing the heart.

In some patients that is still not enough. In a lean, thin patient, the chest wall can sit so close to the heart that even tomotherapy cannot bring the heart dose below 5 Gy. For these patients Dr. Singh uses deep inspiration breath hold (DIBH).

DIBH works on simple anatomy. When the patient takes and holds a deep breath, the lungs expand, the heart moves towards the centre of the mediastinum and away from the chest wall, and the beam is delivered only during the breath hold. The result is a lower dose to the heart and lower cardiac toxicity. Surgical and systemic treatment of breast cancer is covered in the separate series on breast cancer treatment protocols.

Motion management: gating and tumour tracking in the lung

Some tumours move with every breath. Dr. Singh's example was the small, early lung tumour treated with SBRT, either in a patient who is not fit for surgery or for a lung metastasis. Because the lesion is small and moves with the breathing cycle, a tightly shaped beam can miss it.

There are two technical answers:

1. Gating: the beam is switched on only when the tumour is in a defined part of the breathing cycle.
2. Tracking: the beam follows the tumour as it moves.

Both optimise the dose to the tumour while keeping margins small. Four-dimensional radiotherapy, which plans on imaging that captures the tumour across the whole breathing cycle, belongs to the same family.

MRI-guided linear accelerators and adaptive radiotherapy

The newest platform combines an MRI scanner with a linear accelerator. Real-time MRI of the patient and tumour allows more precise alignment, and dynamic MRI in the treatment room shows both patient movement and tumour movement as they happen. Because MRI shows soft tissue far better than X-ray imaging, the plan can be adapted to the day's anatomy with the patient still on the couch. Dr. Singh highlighted that adaptive planning on this system is very fast.

Adaptive radiotherapy, more generally, means re-planning when the anatomy changes during a course, for example when a tumour shrinks or a patient loses weight.

Prostate cancer radiotherapy: dose escalation and hypofractionation

Prostate cancer shows what all these tools achieve together. The prostate sits directly against the bladder and the rectum, which made it very difficult to treat with early technology. In the early era of her illustration, which traced the field from 1935 to 2010, Dr. Singh explained, a dose of around 40 Gy was the practical limit. With IMRT and IGRT, the dose to the prostate can be escalated to about 80 Gy while the dose to the bladder and rectum is kept down. Newer techniques with protons and carbon ions can escalate further, above 80 Gy.

Higher dose means better tumour control and better survival. Prostate biology adds a second advantage. Dr. Singh put the alpha/beta ratio of prostate cancer at about 1.2 to 1.5, which means the tumour is especially sensitive to larger doses per session. Precision therefore allows not only higher doses but hypofractionated treatment, with minimal toxicity and better outcomes. The stereotactic option for early prostate cancer is covered in the SRS and SBRT article, and the cost of radiotherapy page gives indicative pricing.

What IMRT and IGRT change for the patient

The cumulative effect is the one Dr. Singh returned to throughout: more dose where the tumour is, less where it is not. For a patient with head and neck cancer, that means a better chance of keeping saliva. For a woman with left-sided breast cancer, a lower long-term risk to the heart. For a man with prostate cancer, a higher curative dose with fewer bladder and bowel effects, delivered in fewer sessions.