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

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. 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. 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. 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.