Gated 4D PET/CT improves lung treatment planning

Respiratory gating in PET and CT can reduce uncertainties about tumor location, thereby improving radiation treatment planning for patients with lung cancer, according to a pilot study presented at the American Society for Therapeutic Radiology and Oncology meeting in October.

"We have been using PET/CT at our facility for about three years, so we are very comfortable using functional imaging with PET in treatment planning for radiation oncology," said lead author Allan J. Caggiano, a medical physicist at Holy Name Hospital in Teaneck, NJ. "We decided to extend this technology to include imaging patients while they are breathing, trying to eliminate some of the errors associated with moving tumors under respiration."

Eighteen patients with lung cancer have participated in the two-day protocol. On the first day, an immobilization device was made, patients were trained in regular breathing, and baseline measures were obtained.

On the second day, ungated and gated (4D) PET/CT images were obtained, the latter during coached breathing.

Treatment plans were generated from both ungated and gated PET/CTs. Tumor motion on gated PET/CTs was assessed during 10 phases of respiration, ranging from one full inhalation to the next and classified as low ( 2 cm).

Overall, 15 (83%) of the 18 patients completed the rigorous imaging protocol. The gated PET/CT images for each of the 10 respiratory phases were combined into a movie loop.

Analysis showed that tumor centroids moved 0 to 3 cm during breathing, and one-third of patients had tumors with high, medium, and low motion. This information is useful because patients who fall into the low-motion group probably do not need gated treatment, Caggiano said. Not surprisingly, most of the tumor motion occurred in the superior-inferior direction.

Clinicians reported that the gated PET/CT movie loops helped in planning radiation therapy and identifying patients who would be good candidates for respiratory-gated radiation therapy.

"We found that about 50% of the patients benefited from our knowing ahead of time what the movement was, so we could establish that in our treatment margins," Caggiano said. "This is a very different approach from what we have done in the past, which was to look at how we thought the tumor was moving in general and then just (expand) the margins uniformly in all directions, probably overirradiating normal lung tissue."

The tailored margins may in turn reduce the adverse effects of radiation therapy. Acknowledging the small study size, Caggiano noted that patients who have experienced this protocol seem to have much less respiratory compromise after radiation therapy.

A comparison of treatment plans generated from the ungated and gated PET/CTs revealed that gating also helped ensure that all of the tumor was targeted, especially edges lying in the main direction of motion, he said.

"Even with good, standard margins on the ungated field, there are cases when the tumor moves a lot, traverses in and out of the ray. So you would be missing superior and inferior segments, and the patient would get less dose," Caggiano said.

The investigators plan to continue developing their 4D PET/CT process. One area they will look at in particular is the consistency of tumor motion from day to day during treatment for lung cancer, he said.