One of the greatest challenges to delivering radiation therapy treatment is to provide the maximum coverage to the tumor while minimizing normal tissue exposure. One of the tools with which radiation oncologists are meeting this challenge is the utilization of daily on-board imaging with 3D cone beam CT (CBCT) prior to treatment.

CBCT, unlike a conventional CT scanner, does not image slices. Instead, its cone-shaped beam illuminates a complete volume. A beam is rotated around a patient and an image is captured in pre-defined degree segments. This allows the area of study to be observed from multiple angles. Utilizing image reconstruction algorithms, the study area can be reconstructed in 3D at high resolution. Physicians are then able to use the 3D images to identify and localize the tumor, allowing them to deliver radiation to the neoplasm while conserving the surrounding healthy tissues.

David Yoo, MD, PhD, from the department of radiation oncology at Duke University Medical Center in Durham, N.C., and colleagues, conducted a study measuring the effectiveness of CBCT in target localization compared with 2D orthogonal imaging for stereotactic body radiation therapy (SBRT). Their results were presented at the recent American Society for Therapeutic Radiation Oncology (ASTRO) meeting in Los Angeles.

“Effective and safe application of stereotactic body radiotherapy requires extremely accurate and precise localization,” Yoo said. “In addition, we must be sure that the target lesion is immobilized and/or we compensate for motion of this target lesion during treatment.”

The researchers treated 32 patients with 40 lesions who received 123 fractions of SBRT. Customized alpha cradles were used for immobilization, and all patients had CT-based simulation planning studies. Patients with lung, liver, or adrenal masses underwent 4D CT, a series of CT scans that measure how much a tumor moves when a patient breathes and allows radiation oncologists to personalize radiation treatment for this motion.

Based on tumor- and patient-specific factors, either gated or free breathing techniques were used, Yoo noted.

The patients were initially aligned with lasers to external marks drawn on their skin and the cradles. They then had 2D orthogonal images taken and aligned to the digitally reconstructed planning radiographs using bony landmarks. The treatment table was shifted based on 2D matching, and changes to the isocenter in the three standard orthogonal planes -- anterior-posterior (AP), cranial-caudal (CC), and medial-lateral (ML) -- were recorded, Yoo said.

Pre-treatment CBCT images were acquired and matched to the planning CT based on soft tissue anatomy, clips, fiducial markers and bone; in addition, changes in isocenter position as a result of CBCT imaging were recorded, according to the researchers.

Post-treatment 2D or 3D CBCT images were taken and compared to pre-CBCT images to reflect any intra-fractional changes in the liver, lung, spine, and overall. The researchers then calculated absolute averages, standard deviations, and root mean squares (RMS) for 2D imaging compared with laser alignment; pre-treatment CBCT compared with 2D imaging; and post-treatment on-board imaging (OBI) compared with pre-treatment CBCT.

The scientists found that after initial set up to external marks with laser guidance, 2D images revealed set up deviations of 0.60 cm (RMS). The utilization of CBCT resulted in additional isocenter shifts of 0.39 cm (RMS). Post-treatment OBI demonstrated intra-fractional variations of 0.14 cm (RMS), the researchers reported.

They noted that individual shifts seen within the AP, CC, and ML orthogonal planes showed no obvious directional error bias. With regard to site-specific shifts, the deviations for liver lesions appeared to exceed those for lung and spine masses, according to the research team.

“For patients undergoing SBRT, CBCT allows further refinement of treatment set up and target localization beyond that provided by 2D bony registration,” Yoo said. “These advances in target tracking/immobilization during treatment allow for improved confidence in the accuracy and precision of treatment delivery in SBRT.”