Image-Guided Radiation Therapy (IGRT) Treatment
Image-Guided Radiation Therapy (IGRT) is a broad concept applying both to the use of modern imaging to improve the target delineation and to improve the delivery of treatment. The key behind the second part of IGRT is the ability to image patients in the treatment room with the patient on the treatment table.
IGRT represents a major change in the practice of Radiation Oncology. Before IGRT, patients were imaged once at the beginning of treatment and only periodically during treatment. Now, patients are imaged everyday immediately prior to treatment.
At UCSD, IGRT is delivered on a Varian Trilogy, a sophisticated, state-of-the-art linear accelerator equipped with an on-board imager (OBI).
Varian Trilogy Machine
The OBI on the Trilogy consists of a kilovoltage (kV) source and a detector mounted at right angles to the treatment beam on the machine gantry. The kV source and detector are operated via robotic arms and can be extended for imaging and retracted when not in use.
Planar images are used to ensure that patients are accurately setup every time every day. Varian software rapidly compares the images with references images obtained at simulation and adjustments in table position are made automatically.
In prostate cancer patients, small seeds are routinely implanted prior to treatment and used to daily localize the prostate. The treatment couch is then adjusted to return the prostate to the same position as at simulation.
Such adjustments are needed for the position of the prostate may change from day to day, due to differences in the amount of air in the rectum and urine in the bladder. Daily prostate localization is essential to ensure that treatment is delivered consistently and accurately.
Reference Image (left) obtained at simulation is compared with the kV OBI image (right) in a prostate cancer patient undergoing IMRT. Note the 5 seeds outline in green on the reference image. These markers are clearly seen on the OBI film.
An additional feature of the Varian Trilogy is the ability to generate volumetric images of the patient, known as cone-beam CT (CBCT) imaging. CBCT images are obtained by rotating the OBI imager around the patient and reconstructing the 2D images into 3D ones.
High-quality CBCT images are produced by the Varian Trilogy quickly and with only a modest dose to the patient. In contrast, other IGRT approaches (such as Tomotherapy) use megavoltage X-rays. The result is inferior quality images and higher doses to the patient.
Image of a prostate cancer patient undergoing IMRT
The ability to generate high-quality kV CBCT images of the patient on the treatment table immediately prior to treatment is nothing short of revolutionary. Such images can be used to more accurately setup patients based on internal anatomy without the need for implanted fiducial markers.
In the future, it is hoped that volumetric imaging will allow treatment to be adapted to changes in the patient and/or tumor. Adaptive radiotherapy is a focus of intensive research at our center.
Other Genitourinary Tumors
A variety of other genitourinary tumors are also treated with radiotherapy. Patients with testicular seminomas are commonly treated following surgery to reduce the risk of relapse in the abdominal lymph nodes (view paper).
While commonly used in the past, chemoradiotherapy is less frequently used today in patients with bladder cancers due to significant advancements in surgery. However, in medically inoperable patients, radiotherapy with or without chemotherapy is associated with favorable outcomes (view paper).