P46. Use of 18-FDG PET/CT in the radiotherapy planning of lung cancer—our updated experiences

Purpose: To compare the definition of gross tumor volume using computed tomography (CT) alone versus 18-FDG positron emission tomography/computed tomography (PET/CT), show the role of integration of PET in radiotherapy planning and its impact on definition of the planning target volume, for its coverage by prescribed dose and the dose to the lung in patients with non-small lung cancer (NSCLC).

Material and methods: Total of 40 patients with non-small cell lung cancer underwent three-dimensional conformal radiotherapy (3D CRT). Before radiotherapy planning, there were performed 18-FDG PET/CT or CT alone in all of patients. Three groups of gross tumor volumes were defined: Group 1: CT volumes alone (CT-tumor and CT-lymph nodes); Group 2: PET volumes alone (PET-tumor and PET-lymph nodes); Group 3: fused PET with CT volumes (fused PET/CT-tumor and fused PET/CT-lymph nodes). The median total dose was 66 Gy, 1.8-2 Gy per day, 5 times a week (min =30 Gy, max=70 Gy). There were compared two 3D-CRT plans (AP-PA or oblique fields, three fields or four fields) with respect to the gross tumor volume (GTV), planning target volume (PTV), the mean lung dose (MLD) and lung volume which received ≥20 Gy (V20).

Results: Several studies have addressed with possibility of using 18-FDG PET in radiotherapy contouring of individual volumes to improve the overall accuracy and reduce interobserver variability. The biggest changes in the definition of gross tumor volume were recorded in cases of atelectasis and in the definition of affected mediastinal lymph nodes which were not clearly differentiable by CT alone. Improvement of the target volume coverage and reduction of the target volume using 18-FDG PET/CT in the planning of radiotherapy leads to the possibility of dose escalation to the tumor and reduction to the surrounding healthy tissues. Information obtained by 18 FDG-PET enabled to significantly reduce the endpoints: GTV at 40%, PTV at 68%, MLD at 64%, and V20 at 68% patients. Use of PET/CT in the radiotherapy planning causes a reduction in dose to lung (median =17 Gy MLD-CT versus MLD-PET/CT =12 Gy; median =29% V20-CT versus V20-PET/CT =21%).

Conclusions: Although the target volumes defined by CT are similar to the target volumes defined by PET, but more often were the cases that the use of CT alone without fusion with PET occurred under-dosage, and not covered the entire target volume. Radiotherapy treatment planning by PET/CT is more accurate in terms of definition of tumor and lymph nodes and more accurate differentiation atelectasis from the tumor than by CT alone.

Keywords: Non-small lung cancer (NSCLC); positron emission tomography/computed tomography (PET-CT); radiotherapy