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.