Impact of 18 F-FDG PET/CT on target volume delineation in recurrent or residual gynaecologic carcinoma

Background To evaluate the impact of 18 F-FDG PET/CT on target volume delineation in gynaecological cancer. Methods F-FDG PET/CT based RT treatment planning was performed in 10 patients with locally recurrent (n = 5) or post-surgical residual gynaecological cancer (n = 5). The gross tumor volume (GTV) was defined by 4 experienced radiation oncologists first using contrast enhanced CT (GTV CT ) and secondly using the fused 18 F-FDG PET/CT datasets (GTV PET/CT ). In addition, the GTV was delineated using the signal-to-background (SBR) ratio-based adaptive thresholding technique (GTV SBR ). Overlap analysis were conducted to assess geographic mismatches between the GTVs delineated using the different techniques. Inter- and intra-observer variability were also assessed. Results The mean GTV CT (43.65 cm 3 ) was larger than the mean GTV PET/CT (33.06 cm 3 ), p = 0.02. In 6 patients, GTV PET/CT added substantial tumor extension outside the GTV CT even though 90.4% of the GTV PET/CT was included in the GTV CT and 30.2% of the GTV CT was found outside the GTV PET/CT . The inter- and intra-observer variability was not significantly reduced with the inclusion of 18 F-FDG PET imaging (p = 0.23 and p = 0.18, respectively). The GTV SBR was smaller than GTV CT p ≤ 0.005 and GTV PET/CT p ≤ 0.005. Conclusions The use of 18 F-FDG PET/CT images for target volume delineation of recurrent or post-surgical residual gynaecological cancer alters the GTV in the majority of patients compared to standard CT-definition. The use of SBR-based auto-delineation showed significantly smaller GTVs. The use of PET/CT based target volume delineation may improve the accuracy of RT treatment planning in gynaecologic cancer.