Image Guided Radiation Therapy

In the past, large target margins and treatment volume were used to accommodate the positioning error and organ motion associated with radiation therapy treatment. Because highly conformai radiation therapy can now be delivered, accurate target determinations (i.e., CTV, GTV, PTV) have become a very important issue. Various image modalities can now guide highly conformal therapy such as IMRT by locating the target before daily fractionated radiation therapy. Ultrasound imaging is often used for internal target positioning, mostly for localized prostate cancers.45 However, because the ultrasound cannot penetrate air or bone, its use is limited to certain anatomic sites. Additionally, ultrasound, similar to CT, does not provide any functional information on tumor biology.

Traditionally, the determination of tumor volumes for radiation treatment planning, as well as determining responses following radiation therapy, has been limited to imaging techniques such as CT and magnetic resonance imaging (MRI) scanning. Based on these scanning techniques, we now can deliver higher radiation doses using 3-D CRT and IMRT techniques in certain situations such as localized prostate cancer46 and head and neck cancers,47 which translate into improved local tumor control and survival. With the

figure 3.11. The orthogonal images for a head and neck cancer show the dose distribution from an intensity-modulated radiation treatment (IMRT) plan with nine beams at equally spaced gantry angles. Markedly reduced doses to the spinal cord (4,300cGy) and the right parotid (2,200cGy) are noted, while delivering 7,200cGy to the primary tumor and 5,400cGy to the at-risk lymph nodes.

figure 3.11. The orthogonal images for a head and neck cancer show the dose distribution from an intensity-modulated radiation treatment (IMRT) plan with nine beams at equally spaced gantry angles. Markedly reduced doses to the spinal cord (4,300cGy) and the right parotid (2,200cGy) are noted, while delivering 7,200cGy to the primary tumor and 5,400cGy to the at-risk lymph nodes.

clinical development of positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance spectroscopy (MRS), radiation oncologists are attempting to integrate these function imaging techniques into radiation treatment planning. This evolving field of image-guided radiation therapy (IGRT) holds the potential to radically alter the way radiation therapy is used to treat cancer over the next decade. A comprehensive overview was recently published.48

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