Intensity modulated radiation therapy (IMRT)

Intensity Modulated Radiation Therapy (IMRT) represents the radiotherapeutic modality where the intensity of the radiation delivered, could be modulated during the treatment to focus on the tumour tissue and spare the adjacent anatomical structures/tissue(s). Therefore the increased dose of radiation is delivered to the tumour. IMRT is the type of Conformal Radiation that represents the modality where the radiation beams are shaped according to the approximate shape of the tumour.

IMRT is based on the use of the linear accelerator (LINAC) based technology, being the promising cancer treatment that brings the radiation to the tumor and produces less damage to the adjacent healthy anatomical structures.

The goal of the IMRT is to deliver the radiation more precisely to the target area, i.e., to enable the more precise conformal radiation dose distribution by allowing the operator to control the intensity of the radiation beam. The idea is to deliver a much higher dose of radiation to the tumour without the increase in radiation being delivered to the surrounfing healthy anatomical structures/tissue(s).

IMRT system starts with the target volume and places a uniform, conformal dose around the tumor. The computer “backprojects” through the patient’s tissue to the LINAC source and “finds” the radiation exposure that must be delivered by the LINAC to give this conformal dose pattern. The system, like the CT scan, uses a slice-by-slice rotation approach. The planning includes the 3D planning for conformal radiation. IMRT is an individualized radiotherapy according to the location of the tumor and anatomical structures.

Anatomical position of the tumor and surrounding anatomical structures/healthy tissue(s) need to be accurately defined. Computed tomography (CT), positron emission tomography (PET) and magnetic resonance (MR) imaging provide the necessary three-dimensional anatomical information prior to the treatment.

During the treatment, the radiation intensity of each beam is controlled, and the beam shape changes hundreds of times during each treatment. As a result, the radiation dose “bends” around important anatomical structures/healthy tissues in a specific way, characteristic only for this treatment modality. However, special high-speed computers, treatment-planning software, diagnostic imaging and patient-positioning devices to plan treatments and control the radiation dose during the treatmen are being used.

Treatment delivery is considered very efficient as there is no need for different energies of photons or mixed photon / electron beams. A special multileaf collimating system (multileaf intensity modulating collimator – MIMiC) is used to deliver spatially nonuniform radiation exposure to the patient in order to create a relatively uniform dose distribution at the target.

The patients are required to use the localization devices like masks or body frames in order to assist the targeting and provide better accuracy. The devices are molded according to the contours of each patient.

It is important to mention that even small movements result in significant deviations from the calculated doses. Therefore, patient and organ movements are of extreme concern when delivering conformal radiation therapy. In general, the movement can be minimised by either the invasive fixation device or a specially reinforced mask, being the noninvasive immobilisation device.

For example, the prostate motion during radiotherapy leads to under dosing the target tumor tissue within the prostate and/or overdosing of the critical anatomical structures like rectum and bladder. Therefore, a rectal catheter with an inflated balloon was developed in order to minimize the prostate motion.

IMRT is based on the use of LINAC-based technology with the application of the multileaf collimators, that could be turned on or off during the treatment, in order to vary the radiation beam intensity across the field. The radiation beams could be moved several times, each time with the different intensity that results in the radiation being delivered virtually in "three dimensions ". The dosage could vary within the tumorous tissue aiming at better tumour control and less damage being produced to the adjacent structures, including the better quality of life (QoL) for the patients.

Applications of IMRT

Creation of multiple targets

Multiple treatment targets can be created and treated simultaneously. For example, multiple brain metastases can be treated simultaneously to high-dose radiation while other parts of the brain are spared. This modality decreases the treatment-related CNS toxicity. The IMRT also enables planning and treatment of multiple targets of different origins, i.e., different tumors, or even two completely unrelated tumors.

Creation of multiple avoidance normal structures

Numerous critical anatomical structures are surrounding the targets, particularly if the targets belong to the head and neck or brain cancers, like optic nerves, chiasm, lenses, brainstem, temporal lobe etc. having lower radiation tolerance, praticularly lower than the dose being delivered to the tumour. IMRT allows the creation of the avoidance structures, leading to the concept of conformal avoidance.

The IMRT enables the definition of the PRIMARY TARGET (palpable/imaging documented tumours and lymph nodes) and the SECONDARY TARGET(S) (microscopic disease sites at risk, like the draining lymphatic vessels). Those targets are treated simultaneously at the dose of 60Gy and 50Gy respectively in the period of four weeks, the daily fractions being 2.4Gy and 2Gy respectively. The dose is the most important variable in curing a given stage of cancer. The delivery of the once-daily treatments is five days per week. The radiation than reaches the tumour cells at different stages of cell cycle, causing more damage to the cancerous cells.

Clinically relevant data for the use of this method are available for several types / primary cancer localizations like prostate, lung, breast, kidney, liver, pancreas, larynx, tongue and.sinuses.

Conclusion

The goal of radiation therapy is to deliver a high-dose of radiation to the tumour/target to improve the local control of disease and a low-dose radiation to the adjacent anatomical structures/healthy tissue(s) to limit the side effects. Intensity modulated radiotherapy (IMRT) was developed to achieve the desired dose distribution. The precise imaging of internal anatomy is required so that the radation it is well adapted to the tumour/target and organs at risk. Morphological imaging such as computed tomography (CT) is already recommended for radiotherapy planning.
Intensity Modulated Radiation Therapy (IMRT) represents the radiotherapeutic modality where the intensity of the radiation delivered, could be modulated during the treatment in order to focus on the tumour tissue and spare the adjacent anatomical structures/tissue(s). Therefore, the increased dose of radiation is delivered to the tumour. IMRT is the type of Conformal Radiation that represents the modality where the radiation beams are shaped according to the approximate shape of the tumour. IMRT is based on the use of the linear accelerator (LINAC) based technology, being the promising cancer treatment that brings the radiation to the tumour and produces less damage to the adjacent healthy anatomical structures.

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Article information

rID: 8139
System: Oncology
Section: Physics
Synonyms or Alternate Spellings:
  • Intensity Modulated Radiation Therapy (IMRT)

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