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The Basis of Image Guided Radiotherapy (IGRT)

31 October, 2011 | imaging equipment, Radiology

What is radiotherapy and what does it do?

Radiotherapy, together with conventional surgery, is one of the most common cancer treatment options available. Radiation can shrink a tumor by killing tumor cells or interfering with the tumor’s ability to grow. With conventional radiotherapy the radiation dose needed to destroy the tumor is applied in low doses during many sessions.

An effective radiation delivery method used in radiotherapy is Intensity Modulated Radiation Therapy (IMRT). During IMRT the radiation dose is matched to the three-dimensional shape of a patient’s lesion, focusing higher radiation doses on the tumor while minimizing exposure to healthy tissue surrounding the treatment area. IMRT utilizes multiple radiation beams from more than one direction that constantly adjust to achieve the three-dimensional shape of the tumor.

Radiation therapy stops tumor cells from growing and dividing. In many cases radiation therapy can effectively kill cancer cells by shrinking or eliminating the tumor all together.

One of the most important steps on the way to improve radiotherapy was the introduction of computed tomography with direct applications within treatment planning. This new imaging technique, coupled with improvements in computer processing capabilities and speed, meant computer planning systems rapidly developed to allow individualized patient planning in 3 dimensions. This was followed by the introduction of multi-leaf collimators, which resulted in an increase in the conformality of the dose distribution achievable around the treatment target.

How is image-guided radiotherapy (IGRT) superior to other methods?

More sophisticated methods of planning and beam delivery are now available in the form of intensity modulated radiotherapy, IMRT in which the intensity of the radiation is varied during radiation beam delivery. This enables better sparing of organs at risk and the possibility of escalating the dose to the target without compromising surrounding healthy tissue. This benefit can only be fully realized if the radiation distribution is assured to be delivered where it is planned in relation to patient structures.

Image-guided radiotherapy (IGRT) uses imaging techniques to improve the accuracy of radiotherapy delivery to the target tumor, allowing more accurate and precise targeting of the treatment volume and avoidance of organs at risk. This may lead to a reduction in the radiation-induced complications and side effects that are caused by irradiation of normal tissues. It may also allow an increased dose to be delivered to the target tissues, thereby maximizing the chances of successful control or eradication of the tumor.

Image-guided radiotherapy (IGRT) improves the radiotherapy treatment in the following ways:

  • To visualize the anatomical target and organs at risk in 3D
  • To identify changes in position, shape and size of target anatomy relative to that seen when the treatment was planned
  • To quantify the variation in position of the anatomical target between the planned and initial setup treatment images
  • To correct any patient misalignment by changing the relative geometry of the treatment machine before the treatment is delivered.

What is the Image-Guided Radiotherapy (IGRT) Unit Comprised of?

The typical image-guided radiotherapy (IGRT) system is a kV-cone beam CT system integrated onto a precise linear accelerator. The system consists of an X-ray tube and amorphous silicon flat panel detector both of which are mounted with a view direction that is perpendicular to the treatment beam axis. The tube is deployed for imaging while the detector unfolds from its stored position against the face of the gantry under motorized control. The configuration of the system has the X-ray source at 1000 mm from the machine’s isocenter, which is the same standard distance of the therapeutic source to the isocenter.

There is also another, technically different, approach to image-guided radiotherapy (IGRT): Tomotherapy is a new way of delivering radiation treatment for cancer and literally means “slice therapy”. The tomotherapy system can deliver small beamlets of radiation from every point on a spiral, providing exceptional accuracy.

The more angles that a radiation treatment beam can be delivered from, the better the focus on the tumor and the less effect on surrounding tissue.

What makes tomotherapy truly revolutionary, however, is the ability to create a computed tomography (CT) image just prior to radiation treatment. This means that we can now view a full three-dimensional image of a patient’s anatomy and adjust the size, shape and intensity of the radiation beam to the precise location of the patient’s tumor.