According to the National Comprehensive Cancer Network (NCCN) 2018 guidelines 18F-2-fluoro-2-deoxy-D-glucose (FDG)-PET/CT may be performed as an alternative to a contrast-enhanced CT of the chest, abdomen and pelvis and Tc-99m methylene diphosphonate (MDP) bone scan for evaluation of distant metastatic disease in newly diagnosed stage III breast cancer patients. FDG-PET/CT is usually not obtained for stage I or stage II breast cancer patients as change in patient management is rare. Prior studies have demonstrated FDG-PET/CT can identify sites of unsuspected metastatic disease in newly diagnosed breast cancer patients thereby altering treatment decisions given that palliative management is typical for stage IV disease, whereas neoadjuvant therapy followed by surgery and postoperative radiation may be considered for stage II and operable stage III disease. These guidelines consider invasive breast cancer as a single entity and do not consider whether tailoring imaging techniques for subtypes of breast cancer may be beneficial. However, prior research suggests that FDG-PET/CT may be more appropriate as an alternative to CT and bone scan for patients with invasive ductal carcinoma (IDC) rather than invasive lobular carcinoma (ILC) as FDG demonstrates comparatively reduced sensitivity for ILC metastases. Compared to IDC, ILC is more often occult on mammography, ultrasound, and FDG-PET/CT; which is of importance for clinical management as ILC is more often multifocal and bilateral compared to IDC. Clinical breast examination also has lower sensitivity for detection of ILC compared to IDC, even for large tumors, as ILC may be indistinguishable from normal breast tissue on palpation.

A prior study evaluating systemic staging of newly diagnosed patients with stage I-III invasive breast cancer found that FDG-PET/CT is 1.98 times less likely to reveal unsuspected distant metastatic disease for women with ILC compared to IDC. In this study, all IDC metastases demonstrated FDG avidity whereas 25% of ILC metastases (3 of 12) were not FDG avid. Detection of local axillary metastatic disease on FDG-PET/CT was also lower for ILC (0 of 146 patients) compared to IDC (7 of 89 patients) despite data from the Surveillance, Epidemiology and End Results (SEER) database demonstrating similar rates for lymph node metastases between IDC and ILC. Another study evaluating FDG-PET/CT for the diagnosis of primary breast cancer found that the false negative rate for detection of ILC by FDG was 65% (15 of 23 cases) compared to 23% for IDC (23 of 97 cases) when matching for tumors of the same size. A final study reported a false negative rate of FDG for ILC detection of 13% (2 of 15 patients). Mechanistically, ILC may not take up FDG as avidly as IDC due to lower tumor microvascularity, cellular density, proliferation rate, and number of glucose transporters (GLUT). ILC osseous metastatic disease is also more frequently occult on FDG-PET/CT compared to IDC as ILC osseous metastases are more frequently sclerotic, whereas FDG-PET/CT is more sensitive for lytic osseous metastases. Sclerotic ILC osseous metastases also may be indistinguishable from benign bone islands on CT at initial staging, thereby necessitating biopsy or imaging follow-up for confirmation of osseous metastatic disease. Improved imaging strategies for primary and metastatic ILC are therefore warranted.

Multiple studies have proven the efficacy of FES-PET/CT for imaging evaluation of ER+ invasive breast malignancy (evaluating both IDC and ILC together, with the large majority of cases comprising IDC) but, to our knowledge, no prior study has focused FES-PET/CT evaluation only to cases of ILC, nor have prior studies compared FES-PET/CT directly with FDG-PET/CT for evaluation of newly diagnosed ILC. Given that all prior studies on FES-PET/CT have grouped a small number of ILC cases with a larger number of IDC cases, the imaging performance of FES-PET/CT specifically for ILC is unknown. ILC demonstrates higher rates of ER positivity than IDC with prior studies showing greater than 90% positivity for cases of ILC. Data from the SEER database also shows ILC demonstrates higher overall expression of ER than IDC (ILC 95% positive for ER, n=17,503 vs IDC 74% positive for ER, n=172,379). FES-PET/CT may therefore be suitable for imaging evaluation of a high proportion of patients with ILC.