Image-Guided Radiation Therapy (IGRT) is a broad concept which includes the use of modern imaging to improve target delineation and the use of in-room imaging to improve the delivery of radiation therapy.

In-room imaging adjusting for changes in tumor and patient position (between and during treatments) is a particularly exciting aspect of IGRT. In lay terms, it is the use of imaging in the room to ensure that patients are accurately treated on a daily basis.

What Makes IGRT Special?                                                                       

IGRT is a major change in the practice of Radiation Oncology.  Before IGRT, patients were imaged at most once at the beginning of treatment and only periodically during treatment. Now, patients can be imaged everyday immediately prior to treatment.

A major clinical and research focus for UCSD physicians and physicists, many patients are currently receiving IGRT at our center, notably patients with prostate, head and neck, lung and gynecologic tumors.

How is IGRT Performed?                                                                           

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

As shown in the photo, 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.

The Varian Trilogy is arguably the most sophisticated IGRT device in clinical use today.  It provides the ability to rapidly acquire planar (kV) images of the patient on the treatment table with significantly less radiation exposure than traditional electronic portal imaging devices (EPID).  And significantly higher quality images are obtained.

How are Planar IGRT Images Used in the Clinic?                                   

Planar images of patients are used to ensure that they 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.

Reference Image (left) obtained at Simulation is compared with the kV OBI image (right) in a cervical Cancer patient undergoing extended field IMRT

In addition to using planar images to align patients based on bones, such images provide the ability to track internal markers.  In prostate cancer patients, small seeds are implanted prior to treatment and used daily localize the prostate.  The treatment couch is then adjusted to return the prostate to the same position as at simulation. 

Why is Daily Prostate Localization Important?                                        

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.

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.

What is a Cone Beam CT (CBCT) Scan?                                                 

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.

CBCT 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.

What Other Uses Exist for IGRT?                                                             

Perhaps more importantly, CBCT imaging also opens the door to the future by providing the possibility to adapt treatment to tumor response.

It has long been known that many tumors shrink (often significantly) during Radiation Therapy.  However, it has not been common practice to re-plan patients during treatment to account for such changes. 

There is increasing evidence, however, that adapting treatment to changes may significantly improve treatment delivery.  Hopefully, adaptation will reduce side effects and perhaps allow the delivery of higher more effective doses, improving cure rates.

Another exciting area of IGRT research is in management of respiratory-induced tumor motion (Respiratory Gating).

IGRT Publications by UCSD Radiation Oncology Faculty                                 

Cui Y, Dy JG, Sharp, Alexander B, Jiang SB. Robust fluoroscopic respiratory fating for lung cancer radiotherapy without implanted fiducial markers Phys Med Biol 52:741-55, 2007

Pawlicki T, et al. Prostate cancer therapy with stereotactic body radiation therapy. Front Radiat Ther Oncol 2007;40:395-406

Pawlicki T, et al. Investigation of linac-based image-guided hypofractionated prostate radiotherapy. Med Dosim 2007;32:21-29

Hsu A, Pawlicki T et al. A study of image-guided intensity-modulated radiotherapy with fiducials for localized prostate cancer including pelvic lymph nodes. Int J Radiat Oncol Biol Phys 2007;68:898-902

Yoo S, Kim GY, Hammoud R, Elder E, Pawlicki T, et al. A quality assurance program for the on-board imagers. Med Phys 33:4431-47, 2006

Berbeco RI, Nishioka S, Shirato H, Jiang SB. Residual motion of lung tumors in end-of-inhale respiratory gated radiotherapy based on external surrogates. Med Phys 33:4149-56, 2006

Keall PJ, Mageras GS, Balter JM, Emery RS, Forster KM, Jiang SB et al. Management of respiratory motion in radiation oncology report of AAPM Task Group 76. Med Phys 33:3874-900, 2006

Jiang SB. Radiotherapy of mobile tumors. Semin Radiat Oncol 16:239-48, 2006

Flampouri S, Jiang SB, et al. Estimation of the delivered patient dose in lung IMRT treatment based on deformable regristration of 4D CT data and Monte Carlo simulations. Phys Med Biol 51:2763-79, 2006

Jiang SB. Technical aspects of image-guided respiration-gated radiation therapy. Med Dosim 31:141-52, 2006
Link to IGRT Paper 3

Neicu T, Berbeco R, Wolfgang J, Jiang SB. Synchronized moving aperture radiation therapy (SMART): improvement of breathing pattern reproducibility using respiratory coaching. Phys Med Biol 51:617-36, 2006

Berbeco RI, Mostafavi H, Sharp GC, Jiang SB. Towards fluoroscopic respiratory fating for lung tumors without radiopaque markers. Phys Med Biol 50:4481-90, 2005

Berbeco RI, Neicu T, Rietzel E, Chen GT, Jiang SB. Technique for respiratory-gated RT treatment verification with an EPID in cine mode. Phys Med Biol 50:3669-79, 2005

Berbeco RI, Nishioka S, Shirato H, Chen GT, Jiang SB. Residual motion of lung tumors in gated radiotherapy with external respiratory surrogates. Phys Med Biol 50:3655-67, 2005

Wu H, Sharp GC, Salzberg B, Kaeli D, Shirato H, Jiang SB. A finite state model for respiratory motion analysis in image-guided radiation therapy. Phys Med Biol 49:5357-72, 2004

Sharp GC, Jiang SB et al. Tracking errors in a prototype real-time tumour tracking system. Phys Med Biol 49:5347-56, 2004

Berbeco RI, Jiang SB et al. Integrated radiotherapy imaging system (IRIS): design considerations of tumor tracking with linac gantry-mounted diagnostic x-ray systems with flat-panel detectors. Phys Med Biol 49:243-55, 2004

Sharp GC, Jiang SB et al. Prediction of respiratory tumor motion for real-time image-guided radiotherapy. Phys Med Biol 49:425-40, 2004

Neicu T, Shirato H, Seppenwoolde Y, Jiang SB. Synchronized moving aperture radiation therapy (SMART): average tumor trajectory for lung patients. Phys Med Biol 48:587-98, 200