Intensity modulated radiation therapy (IMRT) is a relatively new way of delivering radiation that theoretically delivers more radiation to cancers, while delivering less radiation to normal tissues than conventional three-dimensional conformal radiation (3D-CRT). IMRT emerged through improvements and a decrease in the cost of server type computers, the development of multi-leaf collimators with multiple tungsten shields, which allowed the delivery of radiation through multiple ports (often referred to as “beamlets”) and the development of software that combined computerized tomography (CT) or other imaging of the cancer with control of the radiation delivered. The equipment allows for intensity modulation of the radiation beam during treatment. This is accomplished by the computer telling the machine to shield or not shield various ports with the tungsten shields. Theoretically, IMRT should allow the delivery of more radiation to the tumor while sparing normal tissues.

This technology to deliver IMRT has been available for at least 7 years and is in common use in most large radiation centers. However, the data to support an advantage of IMRT over 3D-CRT is sparse. In the August issue of the International Journal of Radiation Oncology-Biology-Physics, researchers from Memorial Sloan-Kettering Cancer Center report their early results of IMRT for the treatment of a large number of men with prostate cancer. They treated 772 patients with clinically localized prostate cancer with high-dose IMRT. They used a technique called inverse planning to determine the desired beam intensity. This method calculates the maximum allowable radiation that is to be delivered to normal tissue while delivering the desired dose to the tumor. The computer shapes the desired field and radiation is delivered by opening and closing of the tungsten shields or multileaf collumators. The researchers treated 698 men with 81.0 Gy and 74 with 86.4 Gy of radiation. These patients were followed for an average of 2 years after treatment.

Only 4.5% had moderate toxicity to the rectum, while none had severe toxicities. Twenty-eight had moderate urinary symptoms and one experienced urinary retention. There was mild rectal bleeding in 1.5% of patients and 4 patients (0.1%) experienced bleeding that required treatment. The researchers calculated that late moderate rectal toxicity occurred in 4%, late moderate urinary toxicity in 9% and severe urinary toxicity in 0.5% of cases. The evaluated the anti-tumor effect by measuring the prostate specific antigen (PSA). They classified their patients by extent of disease into favorable, intermediate and unfavorable categories. The 3-year actuarial PSA relapse-free survival rates for favorable, intermediate, and unfavorable risk group patients were 92%, 86%, and 81%, respectively. The authors provide information to substantiate that the acute and late rectal toxicities are probably reduced with the use of IMRT compared to 3D-CRT techniques. At the present time, this is the standard technique for treating localized prostate cancer at the Memorial Sloan-Kettering Memorial Cancer Center.

Comments: This is the largest series of patients treated with IMRT. The data appears to substantiate the theoretical advantages of IMRT over 3D-CRT. For practical purposes, most major radiation centers have replaced 3D-CRT with IMRT and there probably will be no randomized controlled trials to document the relative effectiveness of the two techniques.

Reference: Zelefsky MJ, Fuks Z, Hunt M, et al. High-dose intensity modulated radiation therapy for prostate cancer: early toxicity and biochemical outcome in 772 patients. International Journal of Radiation Oncology-Biology-Physics. 2002;53:1111-1116.