Evaluation of internal mammary lymphadenopathy in patients with breast cancer by correlating CT/MRI visibility with FDG-PET avidity.

Background and purpose: Breast cancer staging involves assessment of tumour and regional lymph nodes. Imaging can non-invasively assess internal mammary nodes (IMNs), which are difficult to biopsy. While fluorodeoxyglucose-positron emission tomography (FDG-PET) is the gold-standard for detection of pathological nodes, CT/MRI is limited by a lack of evidence-based size criteria. This retrospective study aims to interrogate the relationship between IMN CT/MRI visibility and FDG-PET avidity, to determine whether CT/MRI can identify IMN metastases. Methods: An audit of all breast FDG-PET/CT/MRIs performed at SVHM from 1-July-2017 to 30-June-2022 was conducted. Patient records (demographics, history, pathology, radiology) were analysed for IMN involvement and other risk factors. Results: A total of 205 patients had 216 FDG-PETs, 154 CTs, and 55 MRIs in the study period. Patients were predominantly female (99.4%), middle-aged (median 54, IQR 45-67), and presented symptomatically (73.5%). IMN positivity was 10.2% (22), 11.7% (18), and 9.1% (5) for FDG-PET, CT, and MRI, respectively. 64 patients (31%) did not have SVHM medical records and were excluded from analysis. Only scans taken within 60 days were compared to reduce interval treatment effect, leaving 15 unique patients for descriptive analysis. When positive IMN were detected, primary tumours were more commonly left sided (58%), in outer quadrants (60%), high grade (79% BRE-G3), moderately sized (47% 2-5cm; 27% >5cm), with ipsilateral nodes (94%), and commonly involved distant disease (40% stage IV). Hormone receptors were common (67% ER+; 53% PR+; 44% HER2+). Median abnormal IMN size was 8mm on CT (IQR 4.86-10.75mm) and 5mm on MRI (IQR 4.25-6mm). CT performed better than MRI (sensitivity 100% vs 80%; specificity 98.45% vs 98%). Conclusion: CT is a useful alternative to FDG-PET for detection of IMNs.