The primary tumour was concordantly staged with the tumour board consensus staging in 54% (12/22) of participants with 18F-FDG PET/MRI and 36% (8/22) with 18F-FDG PET/CT. Nodal staging was concordant with the tumour board consensus decision in 45% (10/22) with 18F-FDG PET/MRI and 50% (11/22) with 18F-FDG PET/CT. Our results show that within this small cohort, there is no statistically significant difference in T and N staging on 18F-FDG PET/CT and 18F-FDG PET/MRI, although 18F-FDG PET/MRI provides a non-significant higher concordance with the tumour board decision in terms of T staging, i.e. 54% (12/22) vs. 36% (8/22).
In terms of M staging, 18F-FDG PET/CT and 18F-FDG PET/MRI both exhibit 95% (21/22) concordance with tumour board decision, although, of note, 18F-FDG PET/MRI detected additional metastases in 3/10 participants with M1 disease. Comparing the staging to the tumour board consensus, the participants with discordant staging on 18F-FDG PET/CT tended to be the same participants with discordant staging on 18F-FDG PET/MRI. This is in keeping with other studies, also finding that 18F-FDG PET/MRI offers similar levels of diagnostic accuracy as 18F-FDG PET/CT; Linder et al. (2019) found that comparison of oesophageal tumour SUV measurements revealed strong correlations, without significant differences between 18F-FDG-PET/MRI or 18F-FDG-PET/CT.
Early studies showed disappointing results for the role of conventional MR imaging compared with CT in oesophageal cancer staging (Quint et al. 1985; Takashima et al. 1991), describing low accuracy in staging the tumours, mainly because of difficulty in detecting tumour invasion through the muscle layer into peri-oesophageal fat (Quint et al. 1985). However, MRI technology has advanced substantially since these initial studies. Furthermore, 18F-FDG PET/CT has emerged as a useful adjunct to conventional staging methods in oesophageal cancer and is of particular importance for the detection of unexpected distant metastases and recurrent disease (Rossum et al. 2015). Given the higher costs and lower availability of PET/MRI scanners, it is uncertain whether this technique could be performed routinely. However, as a combined modality, 18F-FDG PET/MRI may offer improvement in diagnostic accuracy, particularly as MRI methodology develops further, and also offers timesaving as a single scan, performed on a single attendance.
Wider studies in this sphere, such as that of Martin et al. (2019), which did not look specifically at oesophageal cancer, suggested that these benefits could be realised, finding 18F-FDG PET/MRI facilitates staging comparable to that of 18F-FDG PET/CT and improves lesion detectability in selected cancers, potentially helping to promote fast, efficient local and whole-body staging in one step, when additional MRI is recommended. Their results confirmed the potential for a reduction of radiation exposure by using 18F-FDG PET/MRI instead of 18F-FDG PET/CT, which may be of relevance in younger patients presenting with early disease.
There are limited studies looking specifically at 18F-FDG PET/MRI in oesophageal cancer, with results broadly in agreement. Lee et al. (2014), in a retrospective study of sequential 18F-FDG PET/MRI of 19 patients with non-metastatic oesophageal cancer, found that although 18F-FDG PET /MR is inferior to EUS, it was feasible for the identification of oesophageal wall layers, which was limited for CT and impossible for 18F-FDG PET/CT, suggesting 18F-FDG PET/MRI offers a diagnostic benefit for patients with oesophageal cancer in whom EUS is not suitable or appropriate. Our study did not include EUS results, as although EUS is conventionally included as part of the staging process for patients with localised tumours, as only 4/22 participants enrolled underwent an EUS, highlighting the number of patients for whom EUS may not be suitable.
Moving beyond diagnostics, 18F-FDG PET/MRI may also be useful in response evaluation. In a pilot study using 18F-FDG PET/MRI to evaluate the response of neoadjuvant therapy to predict resectability in patients with gastro-oesophageal junction adenocarcinoma, Belmouhand et al. (2019) found 18F-FDG PET/MRI response evaluation to be highly sensitive when predicting resectability, suggesting 18F-FDG PET/MRI may have a future role in this scenario.
Limitations
Endoscopic ultrasound and PET/CT are the standard staging investigations for oesophageal cancer. We were not able to include EUS as a comparative modality as many of the participants were ineligible for EUS due having an obstructing tumour which the EUS scope cannot traverse. The majority of the cancers included on this study were locally advanced (T3 or T4), and it is thought that EUS may not add additional information in some cases of locally advanced oesophageal cancers (DaVee et al. 2017).
Second, the small sample size in this exploratory study limits the generalisability of these findings. We also included a heterogeneous population of adenocarcinoma, squamous cell and small cell carcinoma. Although this is reflective of our clinical practice, our numbers are not large enough to allow subgroup analysis to compare the modalities across different oesophageal cancer subtypes.
Third, all participants underwent 18F-FDG PET/CT first followed by 18F-FDG PET/MRI. It is known that many malignant lesions will continue to increase target-to-background 18F-FDG uptake with delayed imaging (Chan et al. 2011; Kumar et al. 2005) and the absence of randomisation may bias the outcome of the comparison in favour of higher sensitivity for 18F-FDG PET/MRI.
Fourth, the majority of our participants underwent neoadjuvant therapy prior to surgery (if they went on to have surgery), limiting direct comparison with a contemporaneous histopathological reference standard for T and N status. This is a fundamental limitation of this study, as the reference standard (tumour board consensus staging) is based on standard investigations rather than histopathology and therefore is influenced by the results of contrast-enhanced CT and 18F-FDG PET/CT.