Hybrid RT device joins MR and linear accelerator

A combined open-bore MR scanner and linear accelerator, in development by researchers at the Alberta Cancer Board, promises to overcome the problems of motion and poor tumor definition encountered in the multistep process that typically defines conventional radiation therapy.

The prototype, scheduled for completion in December 2007, was described July 24 by B. Gino Fallone, Ph.D., director of the medical physics division in the University of Alberta department of oncology, at the American Association of Physicists in Medicine meeting in Minneapolis.

Fallone and Alberta Cancer Board researchers Marco Carlone and Brad Murray contend that the hybrid will improve the accuracy of radiation treatments for solid tumors and make radiation therapy a viable option for lung, liver, stomach, and pancreatic cancers. Because these organs move, cancers in them are often difficult to treat. The hybrid also could improve treatment for prostate cancer, they said.

Conventional 2D x-ray or 3D CT imaging systems are good at depicting bones but poor at imaging organs and tumors. Also, the images are typically produced before treatment rather than during. Often, by the time a patient receives radiation, the position and shape of the tumor have changed. Compensatory techniques, particularly the common practice of adding an extra "treatment margin" to the tumor to ensure complete coverage, can result in collateral damage to healthy tissue and increase adverse side effects.

Combining a radiation treatment machine with real-time MR guidance should allow more precise positioning of the radiation beam on the patient's tumor by accounting for patient changes and movements. Misalignments related to patient breathing that can occur on conventional systems will not pose a problem, according to the researchers, as the hybrid will track the moving tumor in real-time with MR imaging and adjust the radiation beam accordingly.

The hybrid will lock the linear accelerator and open-bore MRI system into fixed positions in relation to each other, allowing them to rotate in unison around the patient, delivering treatments over all angles. This fixed system will reduce interference between the linear accelerator and MR scanner, providing an unobstructed treatment beam and real-time imaging, according to the researchers. The open-bore MR design provides more room than a cylindrical scanner and, for most procedures, allows the patient's head to remain outside the machine.