Principal Consultant, Radiation Oncology, BLK-Max Super Speciality Hospital, New Delhi
Part 2 of 10 in Radiation Oncology - Advances & Latest Trends
IMRT and IGRT: The Foundations of Precision Radiotherapy
August 28, 2026
Intensity-modulated radiotherapy (IMRT) is built around a deceptively simple piece of hardware: the multi-leaf collimator inside a linear accelerator, which divides a single radiation beam into hundreds of small beamlets. Each beamlet can be shaped to the tumor's geometry, which lets a radiation oncologist paint a high dose onto a complex, irregular volume while sparing the organs sitting right next to it. Dr. Garima Singh, Principal Consultant in Radiation Oncology at BLK-Max Super Speciality Hospital, describes this capability, dose painting through inverse planning, as one of IMRT's defining features, and walked through two of her own cases to show what it looks like in practice.
Case One: Carcinoma of the Esophagus
The first was a teaching case built from a patient treated at her center: carcinoma of the esophagus, prescribed a dose of 40.4 Gy. Her team generated both an IMRT plan and a 3D-CRT plan for the same patient for academic comparison. In both the coronal and sagittal views, the IMRT plan showed markedly less dose spillage into surrounding tissue while still achieving full coverage of the target area. The comparison is a clean illustration of what conformality buys a patient: the same prescribed dose, delivered with far less collateral exposure.
Case Two: Locally Advanced Nasopharyngeal Cancer
The second case, a T4N2cM0 carcinoma of the nasopharynx, illustrates why IMRT matters most in anatomically difficult tumors. The nasopharynx sits close to the brainstem, the optic apparatus and the temporal lobes, and this kind of complex, irregular geometry was extremely difficult to treat safely in the era of two-dimensional planning. With IMRT, the gross tumor volume and nodal regions can be delineated separately and assigned different dose levels using color-coded planning, so gross disease receives a higher dose while elective nodal areas receive less. In head and neck planning more broadly, Dr. Singh's team routinely delineates the brainstem, optic chiasm, temporal lobes, constrictor muscles, submandibular glands and parotid glands as organs at risk, so that long-term toxicity to each can be tracked and limited individually.
The Trial That Made IMRT Standard of Care
A phase III trial published in The Lancet Oncology in 2011 compared IMRT against conformal radiotherapy and found a 50 percent reduction in xerostomia, the dry-mouth side effect that is among the most common late complications of head and neck radiotherapy, in patients treated with IMRT. The improvement traced directly to parotid gland sparing: because IMRT can shape dose around the parotid glands rather than through them, daily planning can optimize dose at the tumor while minimizing it at the organs at risk. That trial is the reason IMRT is now standard of care in many head and neck malignancies rather than an optional refinement.
Adding Imaging: What IGRT Changes
Image-guided radiotherapy (IGRT) is the next layer on top of IMRT: the integration of imaging, typically X-ray or CT, during the treatment session itself rather than only at planning. This allows real-time verification of patient positioning and target alignment immediately before and during delivery, reducing set-up error and improving the accuracy of the highly conformal doses IMRT makes possible. The two technologies are usually discussed together because IMRT's precision is only as good as the positioning accuracy IGRT provides.
Dose Constraints in Head and Neck Cancer
Organ-at-risk planning is site-specific and follows established constraint guidelines, including RTOG, QUANTEC and, more recently, HyTEC and PENTEC. In oral cavity cancers such as carcinoma of the tongue, the parotid and submandibular glands, buccal mucosa and dysphagia-related structures are the relevant organs at risk. Dr. Singh's working constraints, drawn from these guidelines and her own institute's practice, keep the parotid gland's mean dose below 26 Gy and the esophagus and trachea's mean dose below 45 Gy. Buccal mucosa constraints are less robustly established in the literature, but her center aims to keep dose to that structure within roughly 32 to 35 Gy without compromising coverage of the target, and to keep overall oral cavity dose below 45 Gy. These numbers are not arbitrary: each represents a threshold below which the relevant late toxicity, whether xerostomia, esophagitis or mucosal injury, becomes substantially less likely.
This guide is based on a live Jivo Masterclass — Dr. Garima Singh taught doctors across Africa on February 22, 2026.
FROM THE LIVE Q&A
Dr. Abraha Gebreegziabher, Ethiopia
What has been your experience treating pediatric cancer patients?
Dr. Garima Singh
I have treated around ten pediatric patients from Ethiopia specifically, as part of a broader pediatric caseload. Pediatric malignancy needs to be treated very carefully. In medulloblastoma, for example, craniospinal irradiation planning has to be extremely precise to prevent radiation-related toxicity. Because survival in cancers like ependymoma and medulloblastoma is often good, the most important concern becomes preventing secondary malignancy later in life, which means paying close attention to low-dose spillage, or integral dose, across the whole treatment field. For pediatric cases needing long treatment fields, we generally use tomotherapy, and we get very good outcomes.
Frequently Asked Questions
When is radiotherapy a viable treatment option for a patient, and when is it not?▼
It depends on the site and the stage. In early-stage head and neck cancer, surgery alone can be sufficient, but high-risk features on post-surgical pathology can still require adjuvant radiotherapy. In locally advanced disease, radiotherapy is generally needed as part of definitive treatment. In stage IV disease, radiotherapy is mostly palliative: relieving pain, bleeding, cord compression or hemoptysis. More than 50 to 60 percent of patients need radiotherapy at some point, whether as radical treatment, adjuvant therapy or palliation, and we rely on a multidisciplinary tumour board to decide the exact timing for each patient.
What is the guiding principle for when to integrate chemotherapy with radiotherapy, versus using either alone?▼
We follow international guidelines. Using cervical cancer as an example: stage IA, IB and IIA disease is treated with surgery first. Adjuvant treatment then depends on the Sedlis and Peters criteria: positive margins or positive nodes call for concurrent chemoradiation; deep stromal invasion beyond one-third, a tumour over 4 centimetres, or lymphovascular space invasion, without positive margins or nodes, call for radiation alone; if none of those features are present, no adjuvant treatment is needed and the patient goes to follow-up. Locally advanced disease, stage IB3 to IVA, needs concurrent chemoradiation. Stage IVB generally starts with chemotherapy, though a bulky tumour or high nodal burden may call for six weeks of neoadjuvant chemotherapy before concurrent chemoradiation. Every site, endometrium, lung, breast, has its own guideline, and we make these calls through tumour board discussion.
Does imaging or histopathological cell type weigh more heavily in this decision?▼
They are complementary, not competing. Staging starts with clinical examination: if the tumour looks confined to the cervix, with no fornix or parametrial involvement and a size under 4 centimetres, surgery looks feasible on clinical grounds. MRI is then used specifically to confirm there is no parametrial invasion, which gives us a clinico-radiological diagnosis. Only after surgery does histopathology decide whether adjuvant treatment is needed. This stepwise approach, clinical exam, then imaging, then surgery, then histopathology, is important because operating on an advanced or bulky tumour without confirming operability first raises the risk of a positive margin, which then commits the patient to more aggressive treatment than necessary.
Is there a role for radioactive bead implants (brachytherapy) in this therapy?▼
Yes, that is brachytherapy. Historically, cervical cancer brachytherapy used preloaded sources, meaning the source had to be implanted directly. Technology has evolved to after-loading systems: the applicator, such as the Fletcher-Suit system, is placed first, and the radioactive source is then transferred into the tandem remotely through the treatment machine. That shift from preloading to after-loading has improved safety for patients and staff.
What are the current dose constraints for organs at risk in head and neck cancer?▼
It depends on the treatment area. For oral cavity cancers such as carcinoma of the tongue, the parotid and submandibular glands, buccal mucosa and dysphagia-related structures are the relevant organs at risk. We keep parotid gland mean dose below 26 Gy, and esophagus and trachea mean dose below 45 Gy. Buccal mucosa constraints are not as robustly established in the literature, but our institute's practice is to keep dose there within about 32 to 35 Gy without compromising target coverage, and to keep overall oral cavity dose below 45 Gy. We follow RTOG, QUANTEC, Timmerman, and more recently HyTEC and PENTEC guidelines for these constraints.
How did an IMRT plan compare with a 3D-CRT plan in a teaching case of esophageal cancer?▼
For the same prescribed dose of 40.4 Gy, the IMRT plan showed markedly less dose spillage into surrounding tissue while still achieving full coverage of the target area, visible in both coronal and sagittal views.
What does image-guided radiotherapy add on top of IMRT?▼
IGRT integrates real-time imaging, typically X-ray or CT, during the treatment session itself to verify patient positioning and target alignment immediately before and during delivery, reducing set-up error.
Why is IMRT especially valuable for treating nasopharyngeal cancer?▼
The nasopharynx sits close to the brainstem, optic apparatus and temporal lobes. IMRT allows the gross tumor volume and nodal regions to be delineated separately and assigned different dose levels through color-coded planning, so gross disease receives a higher dose while elective nodal areas receive less.
Which organs at risk are typically tracked in head and neck radiotherapy planning?▼
Planning routinely delineates the brainstem, optic chiasm, temporal lobes, constrictor muscles, submandibular glands and parotid glands as organs at risk, so long-term toxicity to each can be tracked and limited individually.
In This Series: Radiation Oncology - Advances & Latest Trends
- 1.Radiation Oncology Advances
- 2.IMRT and IGRT: The Foundations of Precision Radiotherapy
- 3.Tomotherapy and Dose Escalation in Prostate Cancer
- 4.Stereotactic Radiosurgery and SBRT: High-Precision Treatment for Brain and Body
- 5.Proton Therapy, Pediatric Radiotherapy and Emerging Modalities
- 6.Cardiac Sparing and Motion Management in Radiotherapy
- 7.Hypofractionation and the Global Access Gap in Radiotherapy
- 8.Brachytherapy in Gynecological Cancer: From Cervix to Endometrium
- 9.Cervical Cancer: A Staging-Based Treatment Framework
- 10.When Is Radiotherapy the Right Choice? A Site-by-Site Decision Guide