New “contouring” radiation therapy making an impact at Cancer Centre

By Paul Michna

It takes a lot of planning to perform the new intensity modulated radiation therapy (IMRT) treatment at the Cancer Centre of Southeastern Ontario at Kingston General Hospital.

Careful computer programming, physician consultation, the involvement of treatment planners and some fastidious calibration and review by physicists is required before a patient can undergo this form of radiation treatment.

But when these steps are complete, and a patient is ready to undergo IMRT to treat their cancer, there are tremendous benefits to this new form of radiation therapy.

“Intensity modulated radiation therapy allows us to give radiation to the actual tumor area much more precisely, while sparing the normal tissues,” explains Cynthia Stulp, manager of radiation therapy at the cancer centre’s oncology radiation therapy department.

Conventional radiation therapy exposes more normal tissue to radiation doses. While healthy tissue can repair itself, IMRT avoids exposing that tissue by projecting a more concentrated dose of radiation on the affected organ or tumor mass.

Dr. John Schreiner, chief of medical physics at the cancer centre’s oncology medical physics department, says that conformal therapy shapes the radiation to the tumor, nodes and other targets.

“What makes IMRT a little different is we don’t irradiate with a uniform field of X-rays from any one direction. We actually change the intensity of the field in different parts,” says John. “We modulate the field. So we’re now using a much more complicated beam from every direction.”

This means that radiation is more concentrated on cancerous cells, and can be calibrated to avoid normal tissue and organs.

Physicians will “contour” their targets and other critical structures, such as organs at risk and organs that don’t require radiation. Then the treatment planner will use the treatment planning system to calculate the best way to bring the radiation in to achieve all of the dose objectives, says John. This is done by using three dimensional CT images of the patient, along with computerized dose calculations that determine the dose intensity pattern that best matches a tumor’s shape. A medical linear accelerator generates the X–rays used in IMRT. The patient lies on a treatment table, while the linear accelerator, about the size of a small car, delivers multiple beams of radiation to the tumor from various directions.

Because of this, IMRT also results in fewer side effects – welcome news for anyone who has undergone conventional radiation therapy. In the case of a typical head and neck cancer patient, for example, radiation hits the parotid gland, which controls salivation and affects taste.

“This means that patients who are typically on head and neck treatment will have difficulty swallowing, and have difficulty producing saliva,” says Cynthia. “Patients may not enjoy eating because nothing tastes the same anymore. When they use a fork, all they taste is metal. With IMRT, we can save some of that parotid and then also save the amount of saliva that’s being produced, which helps maintain good dental status.”

A plastic mesh system, known as an S-frame, is employed to help ensure that patients with head and neck cancers do not move while receiving treatment. While some claustrophobic patients might not enjoy it, there’s a reason for the device.