Response Monitoring in Metastatic Breast Cancer: A Prospective Study Comparing¹⁸F-FDG PET/CT with Conventional CT

This study aimed to compare contrast-enhanced CT (CE-CT) and ¹⁸F-FDG PET/CT for response monitoring in metastatic breast cancer using the standardized response evaluation criteria RECIST 1.1 and PERCIST. The objective was to examine whether progressive disease was detected systematically earlier by one of the modalities. <b>Methods:</b> Women with biopsy-verified metastatic breast cancer were enrolled prospectively and monitored using combined CE-CT and ¹⁸F-FDG PET/CT every 9-12 wk to evaluate response to first-line treatment. CE-CT scans and RECIST 1.1 were used for clinical decision-making without accessing the ¹⁸F-FDG PET/CT scans. At study completion, ¹⁸F-FDG PET/CT scans were unmasked and assessed according to PERCIST. Visual assessment was used if response criteria could not be applied. The modality-specific time to progression was defined as the time from the baseline scan until the first scan demonstrating progression. Paired comparative analyses for CE-CT versus ¹⁸F-FDG PET/CT were applied, and the primary endpoint was earlier detection of progression by one modality. Secondary endpoints were time to detection of progression, response categorization, visualization of changes in response over time, and measurable disease according to RECIST and PERCIST. <b>Results:</b> In total, 87 women were evaluable, with a median of 6 (1-11) follow-up scans. Progression was detected first by ¹⁸F-FDG PET/CT in 43 (49.4%) of 87 patients and first by CE-CT in 1 (1.15%) of 87 patients (<i>P</i> < 0.0001). Excluding patients without progression (<i>n</i> = 32), progression was seen first on ¹⁸F-FDG PET/CT in 78.2% (43/55) of patients. The median time from detection of progression by ¹⁸F-FDG PET/CT to that of CE-CT was 6 mo (95% CI, 4.3-6.4 mo). At baseline, 76 (87.4%) of 87 patients had measurable disease according to PERCIST and 51 (58.6%) of 87 patients had measurable disease according to RECIST 1.1. Moreover, ¹⁸F-FDG PET/CT provided improved visualization of changes in response over time, as seen in the graphical abstract. <b>Conclusion:</b> Disease progression was detected earlier by ¹⁸F-FDG PET/CT than by CE-CT in most patients, with a potentially clinically relevant median 6-mo delay for CE-CT. More patients had measurable disease according to PERCIST than according to RECIST 1.1. The magnitude of the final benefit for patients is a perspective for future research.