Abstract
PurposeTo 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.MethodsNineteen 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 (GTVT). MRI and PET based delineations were evaluated by absolute volumes and Dice similarity coefficients.ResultsThe median volume of the GTVs was 27 and 31 cm3 for PET and MRI, respectively, while it was 6 and 11 cm3 for GTVT. Both GTV and GTVT 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 (GTVPET) with the GTVs based on MRI and CT (GTVMRI). The median Dice coefficient was 0.56 when comparing the visible tumor volume evaluated by PET (GTVT_PET) with the same volume evaluated by MRI (GTVT_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.ConclusionsThe 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 GTVT, which may impact future tumor boost strategies.
Highlights
Squamous cell carcinoma of the anal canal is a rare cancer where the primary treatment is combined radiotherapy (RT) and chemotherapy, with surgery being reserved for salvage treatment [1]
The rather good agreement between Positron Emission Tomography (PET)- and Magnetic Resonance Imaging (MRI)-based gross tumor volume (GTV) indicates that either modality may be used for standard target delineation of anal cancer
Larger deviations were found for guidelines and the visible tumor tissue (GTVT), which may impact future tumor boost strategies
Summary
Squamous cell carcinoma of the anal canal is a rare cancer where the primary treatment is combined radiotherapy (RT) and chemotherapy, with surgery being reserved for salvage treatment [1]. The main challenge is local control, though a general escalation of the RT treatment dose is associated with acute toxicity and late effects which potentially inhibit treatment and impact quality of life [2]. The RT target volume originates from the gross tumor volume (GTV) and includes regions with possible subclinical disease together with margins accounting for patient motion and setup. RT volumes such as GTV are commonly defined on Computed Tomography (CT) images suitable for dose calculations. Tumor stage is evaluated by tumor diameter and degree of infiltration into normal tissue, often visualized by Magnetic Resonance Imaging (MRI) with its high resolution and softtissue contrast in pelvic tumors [7]. Positron Emission Tomography (PET) provides high sensitivity in detecting presence of tumor, regional nodal status and distant metastatic spread [8], though in general has somewhat lower resolution compared to CT and MRI.
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