External Beam Radiation Therapy:

External beam radiation started in the 1970s with the use of box-like fields, which often did not give adequate doses to the prostate but gave a significant dose to nearby structures. This caused acute and long-term side effects. However, with the advent of 3D conformal radiation in the early 1990s, radiation oncologists were ab1e to shape the radiation field much more precisely to the region of the prostate, allowing more radiation to be delivered to the prostate and less to the normal tissues nearby, improving not only the cure rate, but also reducing the side effects of the treatment. In the 1ate 1990s, the advent of intensity-modulated radiation therapy (IMRT) increased this advantage, allowing clinicians to increase the radiation dose to the prostate to a level where cure rates approached or equaled those of surgery while reducing side effects to levels equal to or less than those of surgery. It allows for better control, lower side effects, increased flexibility of radiation delivery, and improved results.

IMRT is a three-dimensional conformal radiation treatment that combines the power of today’s computers with advanced delivery devices to produce a highly focused radiation dose that conforms to the area of disease identified by the radiation oncologist and reduces the radiation received by nearby normal tissues.

IMRT radiation beamlets are combined by the computer into a precise treatment delivery that results in a high dosage to the cancerous tumors and a lowered dose to the surrounding healthy tissues. IMRT is, without a doubt, the most updated technologically advanced external beam radiation method available.

The medical linear accelerator was first used in the 1950s, which revolutionized the way external beam radiation was delivered to treat prostate cancer. Linear accelerators accelerated electrons to produce radiation that could be aimed with greater precision. By using electromagnetic waves, the device accelerated electrons through a specially designed tube to generate X-rays. However, until the advent of digital diagnostic imaging capability, powerful computers, and specialized software, it was almost impossible for radiation oncologists to conform the external beam radiation to the shape of the tumor.

Prior to the advent of CT-based treatment planning, it was actually possible to miss the target. Heavy lead blocks were used and constantly repositioned for each patient. In addition, the therapist had to leave the radiation room to change the direction of the machine and the field size and to insert a new block and any other field changes.

The introduction of the multi-leaf collimator (MLC) and other devices has changed the dynamic role of external beam radiation in treating prostate cancer.

The advantages of IMRT include:

Greater reduction of dose to surrounding healthy tissues, thus reducing side effects.

Increased flexibility in treating difficult lesions surrounding critical organs.

Improved conformal dose distributions around the cancerous area.

Advanced approach to 3-dimensional radiation therapy.

Improved potential for curing patients.

Higher tumor controls and lower normal tissue toxicity.

With the more complex delivery of IMRT, there are now too many treatment parameters to be transferred by nonelectronic means; information must be sent from the treatment-planning computer to the delivery device using media such as a floppy disk or by direct network connection. Once delivered to the treatment machine, key parameters are verified by the clinic’s medical physics staff to insure a correct delivery.

In the typical planning process, the radiation physicists and physicians design a treatment plan and make use of a computer to display the dose that would be received if the plan were delivered. This plan, which consists of a number of beams from several directions, as well as their relative weights, is changed until which time an adequate dose is achieved. IMRT takes a different approach.

Certain dose prescriptions and dose constraints are given to the computer, and the computer generates a plan that meets these dose goals. There are a number of optimization of types. Each one allows the computer to determine the “best” solution among millions of possible combinations of beam directions and weights.

This particular process is called “inverse planning.” The physician and physicist determine what doses to deliver to the tumor and what limits on dose should be applied to nearby organs, and the computer determines how to deliver that dose.

The process is similar to the one used in image reconstruction in computed tomography (CT). The Corvus treatment planning system of NOMOS Corporation is the first inverse planning system to be used in radiation therapy.

Currently, it is used by more than a hundred hospitals and clinics around the world to create complex treatment plans used with IMRT. However, other inverse planning systems are now in clinical use, and it is anticipated that soon all treatment planning for IMRT will be of the “inverse mode.”