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

Abstract

Background: To evaluate the impact of 18F-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 (GTVCT) and secondly using the fused 18F-FDG PET/CT datasets (GTVPET/CT). In addition, the GTV was delineated using the signal-to-background (SBR) ratio-based adaptive thresholding technique (GTVSBR). 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 GTVCT (43.65 cm3) was larger than the mean GTVPET/CT (33.06 cm3), p = 0.02. In 6 patients, GTVPET/CT added substantial tumor extension outside the GTVCT even though 90.4% of the GTVPET/CT was included in the GTVCT and 30.2% of the GTVCT was found outside the GTVPET/CT. The inter- and intra-observer variability was not significantly reduced with the inclusion of 18F-FDG PET imaging (p = 0.23 and p = 0.18, respectively). The GTVSBR was smaller than GTVCT p ≤ 0.005 and GTVPET/CT p ≤ 0.005.

Conclusions: The use of 18F-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.

Figure 1

Comparison of mean tumor volumes using the different manual and SBR delineation techniques. Error bars indicate standard deviation (SD) on the mean. Results are shown for the gross tumor volume (GTV) delineated on CT (GTV_CT1 and GTV_CT2) and PET/CT-based GTVs obtained by manual delineation of contours (GTV_PET/CT1 and GTV_PET/CT2), and signal-to-background ratio (SBR)-based adaptive thresholding (GTV_SBR).

Figure 2

¹⁸F-FDG PET with coregistered contrast enhanced CT showed a local recurrence with a SUV_max of 16.16 in a 60 year old patient 6 months after total hysterectomy, adnexectomy and pelvic lymphadenectomy for an endometrial cancer FIGO IIIA. (A) GTV_CTs defined by four observers on axial, sagittal and coronal contrast enhanced CT. (B) GTV_PET/CTs defined by four observers on axial, sagittal and coronal ¹⁸F-FDG PET/CT. In panel B we observed a greater interobserver agreement and the GTV_PET/CTs were smaller than the GTV_CTs.