NEWPORT NEWS, VA – Riverside and University of Virginia Radiosurgery Center clinicians have shown that a new image guidance technology from Elekta can help target cancer in patients whose tumors move with their breathing, enabling a distinct picture, without blurring.
Elekta's Symmetry 4D technology helps show a clearer picture of the tumor's position and its motion. Physicians have used it to improve their targeting of tumors, thereby avoiding having to treat a larger margin of healthy tissue around the lesion, officials say.
A Symmetry scan is acquired just before treatment to ensure the patient is correctly positioned, and to visualize tumor movement.
[See also: Imaging tech narrows research for brain cancer.]
"Symmetry scans give us clearer information about the movement of the tumor, allowing us to increase the safety of treating our patients by decreasing the dose to healthy tissues, while hopefully providing a better clinical outcome," according to Riverside medical physicist Kelly Spencer.
Before they began using the Symmetry feature in Elekta's XVI package of IGRT tools, Riverside clinicians were using sophisticated XVI 3D cone beam CT (VolumeView CBCT) imaging technology integrated with their Elekta Synergy S to image tumors.
These pre-treatment scans provide physicians with added confidence in the margins that they have planned. Although seeing a target with IGRT technology such as VolumeView has been a key clinical improvement, the motion still created a blur that encompassed the tumor's range of motion.
"For our current protocol, we would create an ITV [internal target volume] to cover the blur we see on the VolumeView," Spencer says. "We began using Symmetry on a couple of patients with lower lung lesions near the diaphragm where we would expect tumor motion to be an issue. We were actually quite mesmerized by the images Symmetry provided. We could clearly see the actual motion of the delineated tumor with respect to the patient's breathing."
[See also: Cancer research initiative will make heavy use of IT.]
The same Symmetry benefit applied to a recent patient with a liver tumor situated near the diaphragm. In this case, clinicians inferred tumor motion by observing the movement of the liver with the patient's breathing.
"We wanted to use Symmetry to see how the superior border of the liver moved, and what we observed was that it did not move simply superior to inferior – there was almost a rolling pattern to the liver motion," Spencer recalls. "We knew that the liver didn't necessarily move symmetrically on CBCT scans, but it was harder to appreciate due to motion artifact. This movement probably varies between patients, so that is an excellent reason to use Symmetry to evaluate this motion on a case-by-case basis."
The extent of tumor motion as depicted in a Symmetry scan helps clinicians confirm that the margins around the tumor that they applied during the planning phase are as small as possible, thereby protecting healthy uninvolved tissues.
"For example, The Radiation Therapy Oncology Group SBRT Lung protocol criterion is 1.0 cm margins superior and inferior on the gross tumor volume," he notes. "With Symmetry we can potentially reduce the standard margins and thereby decrease the dose to the uninvolved lung tissues."
Accounting for tumor motion is especially important for patients with lung tumors, as Riverside's protocol calls for beam delivery while the patient is breathing freely.
"Many of our lung cancer patients are older, they often have chronic obstructive pulmonary disease or emphysema, so their shortness of breath and erratic breathing pattern makes them unsuited for breath hold treatments or any sort of gating," Spencer says, adding that eight out of 10 patients with lung tumors could benefit from a Symmetry scan.
