Image-Guided Radiotherapy—A Case Study

US Oncology Review, 2005;1(1):62-4

Image-guided External Beam Radiotherapy
External beam radiotherapy is the most common form of radiation treatment offered to cancer patients. It consists of irradiating the tumor using high energy X-rays and/or electrons using a machine called a medical linear accelerator. Most treatments consist of multiple X-ray beams pointed at the tumor from different directions. This approach allows more energy to be absorbed in the tumor where the X-ray beams overlap, improving the potential for simultaneously eradicating the tumor and sparing healthy tissue.

The medical linear accelerators used at the St Agnes Cancer Center are the most advanced treatment machines available in the field today. These machines each have two X-ray beam energies and four to five electron beam energies. This variety of radiation beams is clinically required to provide the most flexibility in optimizing a patient’s treatment. The machines also have multileaf collimators (computer controlled radiation beam shaping devices that minimize exposures to healthy tissues) and integrated record-and-verification systems (computer software that monitors the treatment set-up and delivery to improve treatment accuracy). One of the machines is outfitted with an electronic portal imager that allows filmless imaging of the patient set-up prior to treatment.

Integrated Software
A sophisticated treatment planning system is necessary to best utilize the capabilities of these advanced machines. St Agnes Cancer Center’s threedimensional (3-D) treatment planning system is widely recognized as the most advanced treatment planning software tool in the field of radiotherapy today. The system is networked to a variety of imaging systems on the campus, including CT, MRI, and PET. This allows the information from different types of images to be used to better define the tumor volume using a method called image fusion. It is then possible to combine information about tissue function using PET with anatomical information from CT or MRI, a method currently under development in the Cancer Center by Dr Richard Hudes, Director of Radiation Oncology, in collaboration with Dr Ethan Speigler, Director of Nuclear Medicine.

Enhanced Imaging Capability
Most 3-D treatment planning utilizes CT images obtained in another area. Aside from the minor inconvenience to the patient, there are limitations to how a patient can be positioned for a treatment due to the opening, or bore, of a diagnostic CT scanner. To overcome these issues, in 2002, the St Agnes Cancer Center acquired a state-of-the-art, large bore (85cm) helical CT simulator.This was the first device of its kind in the greater Baltimore area, illustrating St Agnes’s commitment to advanced cancer care in this community.