PET/MRI and PET/CT hybrid imaging of rectal cancer - description and initial observations from the RECTOPET (REctal Cancer trial on PET/MRI/CT) study

Abstract

Purpose: The role of hybrid imaging using ¹⁸F-fluoro-2-deoxy-D-glucose positron-emission tomography (FDG-PET), computed tomography (CT) and magnetic resonance imaging (MRI) to improve preoperative evaluation of rectal cancer is largely unknown. To investigate this, the RECTOPET (REctal Cancer Trial on PET/MRI/CT) study has been launched with the aim to assess staging and restaging of primary rectal cancer. This report presents the study workflow and the initial experiences of the impact of PET/CT on staging and management of the first patients included in the RECTOPET study.

Methods: This prospective cohort study, initiated in September 2016, is actively recruiting patients from Region Västerbotten in Sweden. This pilot study includes patients recruited and followed up until December 2017. All patients had a biopsy-verified rectal adenocarcinoma and underwent a minimum of one preoperative FDG-PET/CT and FDG-PET/MRI examination. These patients were referred to the colorectal cancer multidisciplinary team meeting at Umeå University Hospital. All available data were evaluated when making management recommendations. The clinical course was noted and changes consequent to PET imaging were described; surgical specimens underwent dedicated MRI for anatomical matching between imaging and histopathology.

Results: Twenty-four patients have so far been included in the study. Four patients were deemed unresectable, while 19 patients underwent or were scheduled for surgery; one patient was enrolled in a watch-and-wait programme after restaging. Consequent to taking part in the study, two patients were upstaged to M1 disease: one patient was diagnosed with a solitary hepatic metastasis detected using PET/CT and underwent metastasectomy prior to rectal cancer surgery, while one patient with a small, but metabolically active, lung nodulus experienced no change of management. PET/MRI did not contribute to any recorded change in patient management.

Conclusions: The RECTOPET study investigating the role of PET/CT and PET/MRI for preoperative staging of primary rectal cancer patients will provide novel data that clarify the value of adding hybrid to conventional imaging, and the role of PET/CT versus PET/MRI.

Trial registration: NCT03846882 .

Keywords: FDG-PET/CT; FDG-PET/MRI; Lymph nodes; PET/CT; PET/MRI; Rectal neoplasm; Rectal tumour; Staging; Tumour deposits.

Fig. 1

Flowchart for the included patients in the RECTOPET study

Fig. 2

An exemple of the matching process. Anatomically matched lymph node, measuring 2.5 mm in short axis, seen in a transaxial T2 weighted sequence perpendicular to the tumour, b transaxial T1-weighted sequence MRI of the surgical specimen, c in the finding-by-finding description using the photographed slices arrayed numerically, and d at microscopy, using hematoxylin & eosin stain at 1.5x magnification, where no malignant growth was seen

Fig. 3

Focally increased metabolic activity in the right liver lobe without corresponding morphological changes in the second FDG-PET/CT imaging after neoadjuvant treatment. a FDG-PET; b FDG-PET/CT; c CT

Fig. 4

Solid subpleural nodule measuring 7 mm with increased metabolic activity (SUV max 2.8) in the right lower pulmonary lobe in the staging FDG-PET/CT. a FDG-PET; b FDG-PET/CT; c CT

Fig. 5

A PET/MR image of the same patient as that in Fig. 3. a Transaxial T2 weighted sequence perpendicular to the tumour; b FDG-PET/MR image with a T2 weighted MR sequence; c Transaxial diffusion-weighted sequence (b = 800 s/mm2) d Static 3D MAC PET image