Intensity modulated radiation therapy (IMRT) is an advanced mode of high-precision radiotherapy that utilizes computer-controlled x-ray accelerators to deliver precise radiation doses to malignant tumour or specific areas within the tumour. The radiation dose is designed to conform to the three dimensional (3-D) shape of the tumour by modulating or controlling the intensity of the radiation beam to focus a higher radiation dose to the tumour while minimizing radiation exposure to surrounding normal tissue.
Treatment is planned by using 3-D computed tomography (CT) images of the tumour in conjunction with computerized dose calculation to determine the dose intensity pattern that will best conform to the tumour shape. Combinations of several intensity modulated fields coming from different beam direction produce a custom tailored radiation dose that maximizes the dose to the tumour while protecting adjacent normal tissues.
Currently, IMRT is being used to treat cancers of the prostate , head and neck, breast, thyroid, lung, liver, brain tumours, lymphomas and sarcomas: as well as in gynecological cancers. IMRT is also beneficial for treating paediatric malignancies.
The precise 3-dimensional shaping of the radiation dose with IMRT is accomplished by using multiple shaped beams focused on the tumour, as in 3-D conformal radiation. With IMRT, each beam has the intensity of the radiation varied, or modulated, to deliver varying amounts of dose. These hundreds of “beamlets” are made possible by using computer-controlled multileaf collimators which are mounted inside the linear accelerator. These move continuously during the treatment to deliver varying doses of radiation within the field. (2 pics of pelvis CT, modulated beams)
In preparation to any IMRT treatment, an extensive planning process needs to be performed which includes a CT scan. This is necessary even if a recent CT scan had been done elsewhere. This “planning” CT scan will be performed by placing you in the exact same position that you will be in for each of your radiation treatment.
It is not unusual to prepare custom-fabricated moulds or casts, which will be used each day for your treatment. The purpose of these is to ensure that you are in the exact same position each day, to achieve the greatest precision. On completion of the scan, you will have three marks placed on the skin, which will be used for alignment purpose your upcoming treatments.
IMRT has fewer side effects than conventional radiation therapy. It varies from patient to patient, depending on the location of your cancer. Some patients experience no side effects at all. General side effects of radiation therapy may include: fatigue, hair loss, skin changes and loss of appetite. Specific side effects associated with the area of the body being treated.
Although uncomfortable, the side effects associated with radiation therapy are usually not serious, and is easily controlled with medication and diet. Your radiation oncologist can explain the side effects you are likely to experience and help determine the best strategies of managing them.
IMRT is not appropriate for all cancers. The types of cancer where IMRT offers the most benefit are the ones which are relatively small and localized, which can be targeted with pinpoint precision. Speak to your physician to see if it may be right for you.
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