Gated 4D PET/CT: Improving Radiation Treatment Planning for Patients With Lung Cancer

TEANECK, New Jersey-Respiratory gating during positron emission tomography (PET) and computed tomography (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 46th Annual Meeting of the American Society for Therapeutic Radiology and Oncology (abstract 1056). "We have been using PET/CT at our facility for about 3 years now, so we are very comfortable using functional imaging with PET in treatment planning for radiation oncology," said lead author Allan J. Caggiano, MS, a medical physicist at Holy Name Hospital, Teaneck, New Jersey. "We decided to try to extend this technology to include imaging the patients while they are breathing, trying to eliminate some of the errors associated with moving tumors under respiration," he told ONI in an interview. Two-Day Protocol Eighteen patients with lung cancer have participated in the 2-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 (see Figure 1), and was classified as low (< 1.0 cm), medium (1.0 to 2.0 cm), or high (> 2.0 cm). Overall, 15 (83%) of the 18 patients completed the rigorous imaging protocol. The gated PET and CT images were successfully fused for each of the 10 respiratory phases, and the phases were combined into a movie loop. Analyzing Tumor Motion Analysis showed that tumor centroids moved 0 to 3.0 cm during breathing, and one third of patients each 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 any kind of gated treatment, Mr. Caggiano noted. Not surprisingly, most of the tumor motion occurred in the superior-inferior direction. Clinicians reported that having the gated PET/CT movie loops helped in planning radiation therapy and identifying patients who were good candidates for respiratory-gated radiation therapy. "We found that at least 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," Mr. 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 for people 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, according to Mr. Caggiano. Acknowledging the small study size, he noted that "patients who have gone through this protocol seem to have much less respiratory compromise going out from radiation therapy onward-a good indicator that this is probably a good technique to use for them." A comparison of the treatment plans generated from the ungated and gated PET/CTs revealed that gating also helped to ensure that all of the tumor was targeted, especially edges lying in the main direction of motion, Mr. Caggiano commented. "Even with good standard margins on the ungated field, there are cases when the tumor moves a lot and traverses in and out of the path of the ray...So you would be missing superior and inferior segments of the tumor and the patient would get less dose," he said. According to Mr. Caggiano, the investigators plan to continue developing their 4D PET/CT process. One area that they will be looking at in particular is the consistency of tumor motion from day to day during treatment for lung cancer. "PET/CT is a field ripe for research right now. There are many adjustments that could be made to make it better," he concluded.