Susan London

A new 5D mathematical model seems to accurately predict the motion of lung tumors on CT scans obtained while patients breathe, according to a preliminary report presented at the American Society for Therapeutic Radiology and Oncology meeting in October.

The ability to quantitate the motion of internal organs and tumors during respiration would make it possible to compensate for or counteract the effects of breathing, thereby improving the accuracy of radiation treatment planning, said Daniel A. Low, Ph.D., an associate professor of radiation oncology at Washington University in St. Louis.

"A method that modeled the motion as a function of a noninvasive external measurement would be a valuable aid for reducing the effective motion during irradiation. For example, the linear accelerator can be programmed to activate only during a predetermined phase of breathing, based on advanced treatment planning optimization techniques," Low said.

Most groups studying the motion of lung tumors view breathing as a function of time (the fourth dimension in 4D CT), which assumes that breathing is predictably cyclic and that tidal volume remains the same from one breath to the next, he said.

"The reality is absolutely not that. These patients do not breathe, for the most part, in little sine waves. They have severe lung disease, independent of cancer. They have emphysema, and they have serious underlying physiologic damage caused by smoking-they are not normal breathers," Low said.

In addition, most radiation oncology groups use coached breathing, in which patients are instructed when and how to breathe during scanning.

"We specifically avoided coached breathing because we want to understand how well these models work when a patient is not concentrating on breathing," he said.

The investigators first assessed the performance of a 4D CT process in which multislice CT scans were sorted (gated) by tidal volume. Each of 12 patients with lung cancer underwent 15 consecutive CT scans while spirometry measurements were obtained. The scans were sorted and reconstructed to create a 4D data set. Analyses showed near-perfect correlation between tidal volume and the motion of internal objects, which was assessed from internal air content on the CT scan.

They then developed a mathematical breathing motion model based on five dimensions: the spatial location of the object (x, y, and z) during a reference breathing phase, tidal volume, and airflow. To test the model, they compared the actual positions of 40 moving objects in the lung on CT scans obtained during breathing with the predicted positions from the model. This comparison revealed incredibly good fits, according to Low. The objects tracked moved 4 to 11 mm during breathing, and the mean discrepancy between actual and predicted locations was about 0.5 mm.

Although the model had been well tested in only one patient at the time of the ASTRO meeting, its excellent performance was especially noteworthy because that patient had widely ranging tidal volumes, Low said.

Nonetheless, the findings using the new model must be viewed as preliminary, Low said. His team is now rigorously testing the model to see how it performs in a variety of patients and sessions.

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New 5D model may predict motion of lung tumors

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.

Susan London

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New 5D model may predict motion of lung tumors

Gated 4D PET/CT improves lung treatment planning