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  • Review Article
  • Published:

Innovations in image-guided radiotherapy

Nature Reviews Cancer volume 7, pages 949–960 (2007)Cite this article

An Erratum to this article was published on 06 December 2007

Key Points

  • In order to assure proper coverage of the clinical target volume (CTV) by radiation, a margin needs to be added to compensate for daily positioning errors and internal motion of organs, resulting in the planning target volume (PTV). The PTV therefore includes normal tissues near the tumour, to which radiation is intentionally delivered.

  • The dose of radiotherapy that is necessary to control a tumour is often not delivered because of a high probability of complications in nearby normal tissues. This problem can be tackled by the generation of conformal dose distributions that tightly match the volume of the PTV and/or by decreasing the amount of normal tissue in the PTV.

  • Image-guided radiotherapy (IGRT) is defined as frequent imaging in the treatment room that allows treatment decisions to be made on the basis of these images. IGRT aims at decreasing CTV-to-PTV margins from centimetres to millimetres.

  • The synergy between conformal radiotherapy (CRT) and IGRT has drastically improved the quality of radiotherapy and has broadened its possibilities and indications. Clinical implementations of CRT–IGRT have enabled dose escalation, conformal sparing and non-uniform dose distributions, and initiated a revision of fractionation schedules.

  • Research to improve image quality in radiotherapy is not new, but developments of software to quantify target localization errors, on the basis of in-room imaging and hardware allowing automated set-up, have stimulated mainstream clinical application of IGRT.

  • IGRT makes use of many different imaging techniques, using modalities ranging from planar imaging to fluoroscopy to cone-beam CT, and following procedures as simple as using a single set-up image or as complex as intra-fraction tumour tracking.

  • IGRT can be applied for managing of inter-fraction as well as intra-fraction geometric set-up uncertainties and for adapting treatments to tumour responses.

Abstract

The limited ability to control for the location of a tumour compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiotherapy (IGRT) these volumes can be optimized and tumoricidal doses can be delivered, achieving maximal tumour control with minimal complications. Moreover, with the ability of high-precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications for radiotherapy, some of which were previously considered infeasible.

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Figure 1: Megavolt computed tomography imaging.
Figure 2: Adaptive radiotherapy.
Figure 3: Combined computed tomography and daily MVCT image data.
Figure 4: Intensity-modulated radiotherapy and motion.

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Authors and Affiliations

  1. UZ Brussel, Oncologisch Centrum, Radiotherapie, Laarbeeklaan 101, Brussels, B-1090, Belgium

    Dirk Verellen, Mark De Ridder, Nadine Linthout, Koen Tournel, Guy Soete & Guy Storme

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  1. Dirk Verellen
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  2. Mark De Ridder
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  3. Nadine Linthout
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  4. Koen Tournel
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  5. Guy Soete
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  6. Guy Storme
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Correspondence to Dirk Verellen.

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DATABASES

National Cancer Institute

head-and-neck cancer

hepatic malignancies

Hodgkin disease

pancreatic cancer

prostate cancer

rectal cancer

Glossary

Gross tumour volume

The gross palpable or visible/demonstrable extent and location of malignant growth.

Clinical target volume

A tissue volume that contains the gross tumour volume and/or subclinical malignant disease, which is to be eliminated. This volume has to be treated adequately in order to achieve the aim of radiotherapy: cure or palliation.

Planning target volume

An additional margin added to the CTV that is a statistical construct to ensure that the desired dose can be anatomically realized in the CTV during treatment.

Image-guided radiotherapy

Frequent imaging in the treatment room during a course of radiotherapy to guide the treatment process.

Conformal radiotherapy

Describes the aim in radiotherapy of conforming or shaping the high-dose volume to the planning treatment volume. Alternatively, conformal avoidance refers to sparing of organs at risk.

Normal-tissue complication probability

With the introduction of 3D treatment planning systems, it has become possible to calculate and evaluate the dose distribution not only in tumours but also in nearby normal tissues. From these 3D dose distributions it is possible to model the outcome in biological terms of tumour control probability and normal-tissue complication probability. However, proper clinical validation of these models is still lacking.

Intensity-modulated radiotherapy

Radiotherapy technique in which the intensity of irradiation varies within a radiation field. This can be obtained by using differential dose absorbers or by varying the time of radiation at different points.

Dose painting by numbers

Experimental intensity-modulated radiotherapy strategy in which the intensity of radiation intentionally varies within a tumour, on the basis of estimated levels of radioresistance that were assessed by biological and physical imaging modalities.

Permanent seed implantation

Radiotherapy in which the radiation source is implanted into the tumour (also known as brachytherapy). Prostate brachytherapy can deliver high and concentrated doses of radiation to the prostate gland.

Tomotherapy

A specially designed collimator generates an intensity-modulated profile and at the same time the gantry rotates about the long axis of the patient and as such irradiates a slice of the patient. One approach is the slice-by-slice arc rotation approach, in which the patient is translated longitudinally between consecutive gantry rotations to treat sequential transaxial slices. In the other approach, helical tomotherapy, the patient is being translated longitudinally, slowly and continuously, during the gantry rotation.

Computed tomography

In radiotherapy, volumetric IGRT solutions can be based on conventional kV X-ray sources or high energy MV photon beams that are used for treatment.

Treatment simulator

The treatment simulator is a machine that emulates the geometry of the treatment unit, but uses diagnostic quality X-rays to carry out localization and verification of the patient in treatment position.

Linear accelerator

A device that uses high frequency electromagnetic waves to accelerate charged particles such as electrons to high energies through a linear tube. The high-energy electron beam itself can be used for treating superficial tumours or it can be made to strike a target to produce high-energy (MV) X-rays for treating deep-seated tumours.

Image registration

The process of registering different image sets of the same modality (for example, CT–CT) or different modalities (for example, CT–MRI or CT–PET). This registration can be affine (that is, one set is translated, rotated or rescaled to match with the primary set assuming the patient's anatomy was not deformed) or deformable (that is, the registration algorithm deforms the secondary set to cope with internal and external deformations of anatomy).

Electronic portal imaging device

A device that enables automated acquisition of images acquired with a treatment beam.

Offline patient set-up

The offline approach monitors the position of the patient during a limited number of fractions and adapts the safety margins accordingly. This approach does not allow for decreasing the treatment margins sufficiently for aggressive conformal radiotherapy.

Online patient set-up

The online approach offers the possibility of reducing most geometric errors (both systematic and random), but is considered to be time consuming and requires automated control of the treatment couch to make it efficient in clinical practice.

Peripheral solutions to IGRT

In-room imaging systems that are not mounted physically on the treatment-delivery system.

Isocentre

The point of intersection of the central axis of the radiation beam and the horizontal axis of rotation of the gantry. Traditionally the centre of the planning treatment volume coincides with the isocentre.

On-Board IGRT system

In-room imaging systems that are physically attached to the treatment-delivery system.

Cone beam volumetric CT

A CT scanning method in which the fan-beam and linear detector array is replaced with an open-beam and large-area flat-panel detector to generate volumetric images through a single rotation of the system.

Dose volume histogram

An alternative method for displaying the results of dose calculation. The histogram shows the percentage of the volume of any structure that is irradiated above a particular dose level. More correctly known as cumulative DVH.

Multi-leaf collimator

Computers have enabled the replacement of field-shaping beam blocks, which create irregularly shaped irradiation fields to spare vulnerable tissues, with an MLC. MLCs consist of 40–120 movable leaves, with a width varying between 0.2 and 1.0 cm, that are arranged in opposed pairs.

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Verellen, D., Ridder, M., Linthout, N. et al. Innovations in image-guided radiotherapy. Nat Rev Cancer 7, 949–960 (2007). https://doi.org/10.1038/nrc2288

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