Intensity-Modulated Radiation Therapy (IMRT)
Intensity-Modulated Radiation Therapy (or radiotherapy), commonly referred to as IMRT is a newer and more sophisticated way of delivering conventional radiation therapy. Not all patients will benefit from this advance in technology but many will. The main advantage comes from being able to more closely ‘shape’ the radiation field so that the tumour or cancer is more precisely targeted and the other organs and tissues that do not require treatment are more easily avoided. This often allows higher doses to be delivered to the cancer whilst reducing the potential side effects that can come from treating other adjacent areas in the body. This leads to higher chance of cure for some cancers (e.g. prostate cancer) and significantly reduces the risk of damage from radiation. This effect (safely being able to give higher doses of radiation) is called dose escalation.
IMRT uses multiple beams of X-rays of varying intensity directed towards the cancer, angled from various directions around the patient. The standard treatment machine in Australia and New Zealand is called a linear accelerator and all newer machines have the ability to safely deliver IMRT. These machines are fitted with a device called a multi-leaf collimator (MLC) that allows the shape and intensity of the beam to vary as the treatment is delivered. IMRT is always combined with Image-Guided Radiation Therapy (IGRT).
IMRT allows treatment to be accurately delivered to unusually shaped cancers and can also create concave (hollow) areas within the high dose region radiation therapy. This allows the dose of radiation to a sensitive organ e.g. the rectum (lower bowel) or the spinal cord, to be kept to a minimum. With IMRT, cancers in some parts of the body that were not previously safe to treat with radiation therapy, now can be. IMRT is now widely used for most tumour types.
Specific examples include:
- Brain tumours – IMRT may allow the dose to the eyes and other critical parts of the brain to be reduced, hence avoiding damage. The dose to hormone-producing areas and even the memory areas of the brain can also be reduced.
- Head, neck and face cancers – IMRT can significantly reduce the dose to the salivary glands reducing one of the worst long-term side effects from treating some of these cancers in the past – a dry mouth (xerostomia).
- Breast and lung cancers – IMRT can be used to reduce doses to the heart and lungs. In breast cancer IMRT has been shown to reduce skin reactions from treatment. If the cancer is close to the spine, IMRT can allow a higher dose to be delivered without damaging the spinal cord. In the past, treatment close to the spinal cord often had to be completely avoided or the dose reduced as spinal cord damage is one of the most serious rare side effects of radiation (causing paralysis or weakness). With IMRT techniques, the risk of this becomes extremely low.
- Abdominal and pelvic cancers – IMRT can be very effective in reducing side effects from treatment of abdominal cancers for example in the stomach, pancreas and lower oesophagus. In the pelvis, radiation therapy for cancers of the bowel (colorectal cancers), often use IMRT to reduce the risk of damage to the bowel and bladder.
- Prostate cancer – IMRT is standard treatment now for treating localised prostate cancers and is often also used after an operation where the surgery (radical prostatectomy) has not removed all the cancer cells. IMRT allows higher doses to the prostate with a higher chance of controlling the cancer (cure). It also means that the lymph nodes can be more easily treated with reduced side effects than previously. The main organ close to the prostate that sometimes limited the dose that could safely be delivered is the rectum (lower bowel). IMRT allows the amount of rectum in the high dose area to be minimised. This means short and long-term side effects have become significantly less common.
- Cervix and uterine (endometrial or womb) cancer – IMRT is commonly used to treat the cancer with or without pelvic lymph nodes. Again, IMRT increases accuracy, and reduces short-term and late effects, also relating mainly to bladder and bowel irradiation.
Depending on the circumstances, the only minor disadvantage for patients having IMRT may be a slightly increased treatment time – though still only a few minutes per treatment. Other IMRT technologies, such as VMAT (volumetric modulated arc therapy), may reduce treatment time. IMRT may also give an overall larger body tissue dose (integral dose) due to low radiation doses delivered to tissues around the cancer. This may be relevant especially for young patients. Newer machines such as ‘Tomotherapy’ and ‘Cyber Knife’ are available in some treatment centres and can also deliver IMRT.