Target volume delineation of anal cancer based on magnetic resonance imaging or positron emission tomography.

Purpose: To compare target volume delineation of anal cancer using positron emission tomography (PET) and magnetic resonance imaging (MRI) with respect to inter-observer and inter-modality variability.
Methods: Nineteen patients with anal cancer undergoing chemoradiotherapy were prospectively included. Planning computed tomography (CT) images were co-registered with 18F-fluorodexocyglucose (FDG) PET/CT images and T2 and diffusion weighted (DW) MR images. Three oncologists delineated the Gross Tumor Volume (GTV) according to national guidelines and the visible tumor tissue (GTV _T ). MRI and PET based delineations were evaluated by absolute volumes and Dice similarity coefficients.
Results: The median volume of the GTVs was 27 and 31 cm ³ for PET and MRI, respectively, while it was 6 and 11 cm ³ for GTV _T . Both GTV and GTV _T volumes were highly correlated between delineators (r = 0.90 and r = 0.96, respectively). The median Dice similarity coefficient was 0.75 when comparing the GTVs based on PET/CT (GTV _PET ) with the GTVs based on MRI and CT (GTV _MRI ). The median Dice coefficient was 0.56 when comparing the visible tumor volume evaluated by PET (GTV _T_PET ) with the same volume evaluated by MRI (GTV _T_MRI ). Margins of 1-2 mm in the axial plane and 7-8 mm in superoinferior direction were required for coverage of the individual observer's GTVs.
Conclusions: The rather good agreement between PET- and MRI-based GTVs indicates that either modality may be used for standard target delineation of anal cancer. However, larger deviations were found for GTV _T , which may impact future tumor boost strategies.