4D PET/CT images, showing a tumor after treatment. Image source: University of Pittsburgh Medical Center

A whole-body PET/CT protocol that includes respiratory gated 4D PET/CT is likely to be recommended for future oncological studies and radiation therapy applications and could reduce internal margins and deliver more effective radiation therapy, according to a study published in the September issue of Radiotherapy and Oncology.

Cardiac and respiratory induced organ movement impact the utility of PET/CT in radiation oncology in two ways, according to Valentino Bettinardi, MD, of San Rafael Scientific Institute in Milan, Italy, and colleagues. Movement reduces image quality and quantitative accuracy for diagnostic purposes and compromises the ability to define accurate target volumes.

“Respiratory gated (RG) 4D PET/CT … represents an innovative methodology for accurate imaging of tumors, particularly those located in the thorax and upper abdomen,” wrote Bettinardi and colleagues, who suggested the method could improve detection and offer more accurate metabolic characterization of tumors affected by respiratory movements and more accurately assess tumor volume and motion for radiation therapy applications.

The authors noted that the protocol may be helpful in the characterization of tumors, therapeutic monitoring and target definition in radiation therapy planning. The protocol may assist evaluation, including staging and re-staging, of small lesions located in regions affected by respiration. RG 4D-PET/CT techniques also may facilitate evaluation of response to treatment by providing comparative quantification of metabolic activity. Finally, the technique can provide true assessment of target motion, potentially allowing increased radiation dose to the target and better sparing of normal tissues.

Two acquisition protocols suffice for RG 4D PET/CT: prospective and retrospective. Although the retrospective protocol entails a higher radiation dose than the prospective mode, “it is preferable as it allows detection of the complete organ/lesion motion and the attenuation correction of PET images with CT phase-matched images,” according to Bettinardi and colleagues.

The retrospective technique relies on data acquisition at each table position for at least one complete breathing cycle. Then images are sorted into phases that represent anatomical volume during a specific part of the respiratory cycle. “Raw data are processed and reconstructed into a number of new datasets corresponding to the phases of the 4D CT study,” explained the authors.

After the 4D CT phases are generated they can be used for target volume definition with the contour of the target in each 4D-CT phase corresponding to the gross tumor volume during specific parts of the respiratory cycle.

Bettinardi and colleagues urge appropriate patient selection for RG 4D PET/CT; both a regular breathing pattern and good patient collaboration are essential as “the rationale for the clinical use of RG 4D-PET/CT relies on its capability to improve image quality in terms of lesion/background contrast; improve quantification accuracy in the estimation of the radiotracer uptake, and assess and measure organ lesion/motion.”

The authors acknowledged the real benefit of RG 4D PET/CT “in routine clinical setting and its possible impact on patient management have not been established yet.” Although routine clinical use is possible, simple procedures for scanner setting, fast acquisition protocols and powerful reconstruction-processing algorithms are needed to introduce the technique into normal department workflow, summed Bettinardi and colleagues.