# Limited Additional Value of a Chest CT in Whole-Body Staging with PET-MRI: A Retrospective Cohort Study

## TL;DR

This study finds that PET-MRI is effective for detecting lung metastases but misses many small, indeterminate lung nodules that chest CT can detect.

## Contribution

The study demonstrates that PET-MRI has limited additional value over chest CT for detecting indeterminate lung nodules.

## Key findings

- PET-MRI detected 85% of lung metastases with 100% specificity.
- PET-MRI missed 77% of indeterminate lung nodules detected by chest CT.
- Missed indeterminate nodules were significantly smaller than those detected.

## Abstract

PET/MRI systems are being installed world-wide for the staging of cancer. As this combines MRI and PET in one system, the application of these systems offers a one-stop-shop for staging in Oncology. However, one of the pitfalls of these systems is that the MRI of the PET/MRI systems is not very adequate for detecting lung nodules. In this study, we show that the most relevant long-nodules suspected of metastasis can be detected with PET; it remains questionable if missing the indeterminate lung nodules are of clinical relevance.

Hybrid PET-MRI systems are being used more frequently. One of the drawbacks of PET-MRI imaging is its inferiority in detecting lung nodules, so it is often combined with a computed tomography (CT) of the chest. However, chest CT often detects additional, indeterminate lung nodules. The objective of this study was to assess the sensitivity of detecting metastatic versus indeterminate nodules with PET-MRI compared to chest CT. A total of 328 patients were included. All patients had a PET/MRI whole-body scan for (re)staging of cancer combined with an unenhanced chest CT performed at our center between 2014 and 2020. Patients had at least a two-year follow-up. Six percent of the patients had lung metastases at initial staging. The sensitivity and specificity of PET-MRI for detecting lung metastases were 85% and 100%, respectively. The incidence of indeterminate lung nodules on chest CT was 30%. The sensitivity of PET-MRI to detect indeterminate lung nodules was poor (23.0%). The average size of the indeterminate lung nodules detected on PET-MRI was 7 ± 4 mm, and the missed indeterminate nodules on PET-MRI were 4 ± 1 mm (p < 0.001). The detection of metastatic lung nodules is fairly good with PET-MRI, whereas the sensitivity of PET-MRI for detecting indeterminate lung nodules is size-dependent. This may be an advantage, limiting unnecessary follow-up of small, indeterminate lung nodules while adequately detecting metastases.