4D PET/CT integration: optimizing radiation and therapy

1706 Introduction: Respiratory motion presents significant challenges for accurate PET/CT imaging. The motion can artificially introduce apparent increase of lesion size, reduction of measured standardized uptake value (SUV), and mismatch in PET/CT fusion images. These artifacts lead to two major issues in radiotherapy treatment planning: 1. Estimation of the SUV of these lesions2. Estimation of tumor volumes This education exhibit will illuminate the role of motion correction, particularly 4D PET/CT, for radiation therapy planning. Current Solutions: Respiratory-gate (4D) PET/CT is one of the prime solutions for mitigating breathing motion artifact along with breathing instruction, motion-corrected PET reconstruction and post-processing methods. Introduction to Radiation Oncology planning concepts: GTV (Gross Tumor Volume) is the extent of tumor determined by clinical examination or imaging. As well as the primary site, gross tumor includes lymph nodes and/or other metastases in the treatment field. In general, the GTV corresponds to the part of the tumor where the tumor cell density is highest. CTV (Clinical Target Volume) contains the demonstrable GTV plus an estimated larger boundary designed to include presumed sub-clinical disease spread. The Planning Target Volume (PTV) is a geometric concept designed to ensure that the radiotherapy prescription dose is accurately delivered to the CTV. PTV is a volume related to the isocenter of the linear accelerator rather than to the anatomy of the patient. For this reason, the PTV may extend beyond anatomical barriers such as bony margins and may even extend outside the patient.Internal target volume (ITV) consists of amargin added to the CTV to compensate for internal physiologic movement and variations in size, shape, and position of the CTV. Advantages of 4D PET/CT? 1. FDG-PET provides higher sensitivity and specificity than CT for the detection of primary tumor and mediastinal nodes, and is now considered as a reference for the clinical staging of NSCLC. 2. In the radiotherapy field, FDG-PET has already been shown to significantly modify the size, location and shape of the primary Gross Target Volume and to improve the selection of neoplastic lymph nodes in the target volume. 3. Better identification of the target to be irradiated 4. Potential of identifying tumor subvolumes that are suspected of being radioresistant (high tumor burden hypoxia⋯), “dose-painting” strategy 5. Uniform dose escalation to the whole tumor would lead to too high doses to the normal tissues, with unacceptable subsequent toxicities. Restrictively boosting the parts of the tumor that show unfavourable responsiveness to radiation should thus reconcile tumor radiobiological imperatives with those related to treatment Where does 4D PET/CT work best? Current evidence suggests that gross tumor volume delineation based on 4D-PET/CT information may be the best approach currently available for its delineation in thoracic cancers (lung and non-lung lesions). Nehmeh et al demonstrated that 4D PET/CT can reduce tumor volume by 28% and increase maximum tumor SUV by 56.5%.Cohade et al. demonstrated that there is misregistration of free breathing PET and CT to be approximately 7.5 ±4.7 mm in lung fields. Liu et al demonstrated that diaphragmatic motion of 11 mm can cause maximum SUV underestimation of 28% and tumor volume overestimation of 130% on average for 1 cm lung lesions. Outside of the lung, Osman et al demonstrated that in size patients with liver metastasis the liver dome mis-localized to the right lower lobe. Other potential advantages 1. Patients undergoing radiation therapy employing 4D CT modality in therapy planning could undergo 4D PET/CT reducing radiation dose and patient visits. 2. The potential to reduce complications by reducing radiation dose to non-target tissue through the prevention of overestimation of tumor volumes has not been well defined in the literature