Intensity Modulated Radiation Therapy (IMRT)
What is intensity modulated Radiation Therapy?
Intensity Modulated Radiation therapy, abbreviated as IMRT, is the next generation of advanced radiation therapy treatment.
How is it delivered?
IMRT is delivered by a linear accelerator using multiple tiny beams or "beamlets" are used to deliver the radiation. Compared to standard conformal radiation therapy which uses up to 6-10 beams, IMRT incorporates 40-60 beamlets. The result is a more conformal radiation therapy plan, for any shape or size cancer. These smaller beamlets are achieved through the use of computer-controlled multi-leaf collimators (MLCs). MLCs are tiny leaves of lead, that are located inside the head of the linear accelerator and can be configured to any shape beam. They also can move continuously during the radiation therapy to deliver varying doses of radiation.
What is the benefit of IMRT?
With a more conformal radiation therapy plan, this may result in less dose to the adjacent surrounding normal tissues and higher doses to the cancer. This may translate to less early and permanent side effects from radiation, and with higher doses, a higher chance of curing cancer.
Which cancers is IMRT most effective for?
IMRT has mainly been used in prostate cancer and head and neck cancers. In prostate cancer, it allows for higher doses to be delivered to the prostate cancer, whilst sparing dose to the rectum (bowel). For head and neck cancer ( which includes tonsil, pharynx and nasopharynx), it allows the reduction of dose delivered to normal tissues such as the spinal cord, salivary glands and nerves. Less common uses include anal cancer, brain tumours, and when needing to retreat to high doses. Further research with IMRT continues in breast, cervix and uterine cancers.
Is there evidence that IMRT is effective?
Since the year 2000, IMRT has become increasingly used. There are several studies in prostate cancer demonstrating higher rates of cancer control1 (biochemical PSA control) and less side effects2,3. In head and neck cancer, recent publications shows rates of controlling cancer are just as high as standard conformal radiation therapy4-6, with less side effects and improved quality of life.7-9
What equipment is used to deliver IMRT?
IMRT is delivered by a capable linear accelerator (treatment machine), but also requires sophisticated planning software to optimize the radiation beamlets. It also requires more specially trained staff including radiation therapists and physcists to ensure the treatment planned is the treatment delivered. IMRT requires more time, man-power and equipment.
What steps are involved in IMRT?
The steps taken before starting treatment are very similar to standard conformal radiation therapy. Most of the extra work is required during the planning process, and does not require the patient to be present during this time. Planning IMRT can take a number of hours to days.
Am I candidate?
IMRT is not appropriate for every cancer, and best used if higher doses are required or the need to spare dose to adjacent surrounding tissues. Please consult your radiation oncologist to see if IMRT is right for you.
1. Wong WW, Vora SA, Schild SE et al, Radiation dose escalation for localized prostate cancer. Cancer 2009 Aug 10.
2. Zelefsky MJ, Levin EJ, Hunt M et al. Incidence of late rectal and urinary toxicities after three-dimensional conformal radiotherapy and intensity-modulated radiotherapy for localized prostate cancer. Int J Radiat Oncol Biol Phys 2008 March 15: 70(4): 1124-9.
3. De Meerleer G, Vakaet L, Meersschuot S et al, Intensity-modulated radiotherapy as primary treatment for prostate cancer. Int J Radiat Oncol Biol Phys. 2004 Nov 1; 60(3):777-87
4. Lee N, Xia P, Fischbein NJ et al, Intensity-modulated radiation therapy for head-and-neck cancer. Int J Radiat Oncol Biol Phys 2003. Sep 1; 57(1): 49-60
5. Chao KS, Ozyigit G, Tran BN et al, Patterns of failure in patients receiving definitive and postoperative IMRT for head and neck cancer. Int J Radiat Oncol Biol Phys, 2003 Feb 1; 55(2):312-21.
6. Kam MK, Teo PM, Chau RM et al, Treatment of nasopharyngeal carcinoma with intensity-modulated radiotherapy. Int J Radiat Oncol Biol Phys. 2004 Dec 1;60(5):1440-50.
7. Vergeer MR, Doornaert PA, Rietveld DH et al, Intensity-modulated radiotherapy reduces radiation-induced morbidity and improves health-related quality of life. Int J Radiat Oncol Bio Phys, 2009 May 1:74(1):1-8. Epub 2008 Dec 26.
8. van Rij CM, Oughlane-Heemsbergen WD, Ackerstaff AH et al, Parotid gland sparing IMRT for head and neck cancer improves xerostomia related quality of life. Radiat Oncol, 2008 Dec 9;3:41.
9. ow EH, Kwong DL, McMillan AS e al, Xerostomia and quality of life after intensity modulated radiotherapy vs conventional radiotherapy for early stage nasopharyngeal carcinoma. Int J Radiat Oncol Biol Phys. 2006 Nov 15; 66(4):981-91.