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Analysis of Target Volume Definition Using CT, MRI and FDG-PET in Radiotherapy Treatment Planning of Anal Cancer

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Abstract

Purpose/Objective: The main objectives were to explore the intermodality and intra-observer variations of anal cancer GTV delineations on CT, MRI and PET images. Materials and Methods: 22 patients with biopsy-proven anal carcinoma scheduled for curative RT underwent FDG-PET/contrast enhanced CT and MRI. The delineations of GTV were done twice for each imaging modality with a minimum of 3 months in between. Delineations on the CT and MRI were done by routine methods. For the PET part three different cut-off values were used: SUV 2.5, 40% and 50% of maximum SUV, respectively. The GTVs were compared for: size, volume changes, center-of-mass (COM) and non-overlapping regions, regarding inter-modality and intra-observer variations. Wilcoxon rank sum test and ANOVA analysis were used, where p < 0.05 was considered significant. Results: 1. Volume size: The mean GTV-CT of the 1st and 2nd delineation was significantly larger compared to the mean GTV-MRI and the median GTVs from the different PET cut-offs (33.9 cm3, 18.0 cm3, 15.6 cm3, 14.8 cm3 and 10.3 cm3, respectively). The intraobserver differences in voluma size were small and non-significant. It was additionally found that the mean GTV-CT increased when overlaying either mean GTV-MRI or one of the three mean GTV-PETs (17%, 24%, 56% and 36 %, respectively). 2. COM: The inter-modality differences in COM were found to be > 1cm between CT-MRI, CT-PET2.5 and MRI-PET2.5. However, the intraobserver differences in COM were found to be minor. 3. Non-overlap: There were significant differences in mean of the non-overlapping regions using the CT or MRI (20.5% and 22.5%) compared to the different PET cut-offs (5.5%, 6.4% and 5.8%), respectively. Conclusions: Delineations of anal tumors are difficult. GTV-CT was the largest compared to GTV-MRI and the different GTV-PETs. Although the GTV-CT were larger, it did not always cover the GTVs of MRI and PET. The GTV-CT increased if overlaying GTV-MRI or one of the GTV-PETs. Based on our study, our recommendations are not to use CT solely for delineating anal tumors; MRI or PET should be considered as a second imaging modality.