Metastatic invasive lobular carcinoma (ILC) is a distinct breast cancer subtype characterized by loss of E-cadherin and a diffuse growth pattern that makes metastases difficult to detect with standard imaging such as computed tomography (CT) or 18F-Fluorodeoxyglucose Positron Emission Tomography (18F-FDG PET)/CT. As a result, disease burden in patients with ILC is frequently underestimated, progression is identified later than clinically optimal, and many patients are excluded from clinical trials due to insufficiently measurable disease.

Whole-body diffusion-weighted magnetic resonance imaging (WB-DWI/MRI) is a radiation-free imaging technique that has demonstrated improved sensitivity for detecting metastases-including peritoneal, bone, and nodal disease-in ILC. Retrospective studies suggest that WB-DWI/MRI can identify clinically relevant progression not visible on standard imaging. However, prospective evidence in ILC is lacking. Circulating tumor DNA (ctDNA) has also shown promise as a minimally invasive biomarker for monitoring treatment response, with early molecular changes often preceding radiologic progression, but data specific to ILC remain limited.

The DELILA study is a prospective, multicenter clinical trial conducted at University Hospitals Leuven and Institut Jules Bordet. The study aims to enroll 43 patients starting first-line systemic therapy for metastatic hormone receptor positive human epidermal growth factor receptor 2 negative (HR+/HER2-) ILC. Participants undergo serial dual imaging-WB-DWI/MRI and standard-of-care imaging-at baseline, at 1 month, and approximately every 3 months for up to 30 months or until disease progression. At each imaging time point, blood samples are collected for ctDNA analysis and Ca15.3 tumor marker assessment. Patient-reported psychological burden related to repeated imaging and blood sampling is evaluated using validated questionnaires.

The primary objective is to assess the added value of WB-DWI/MRI in detecting disease progression that informs clinical decision-making compared to standard imaging. Secondary objectives include evaluating whether ctDNA or Ca15.3 dynamics reflect disease evolution, assessing measurability of lesions with Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 and MRI-specific criteria, identifying early biomarkers of treatment response using Apparent Diffusion Coefficient (ADC) changes and ctDNA kinetics, and characterizing the psychological impact of trial procedures.

This study will provide the first adequately powered prospective evidence on the clinical utility of WB-DWI/MRI and liquid biopsy monitoring in metastatic ILC. Results may support implementation of WB-DWI/MRI as a routine imaging strategy, guide imaging frequency through biomarker-informed approaches, and improve patient experience and trial eligibility for individuals living with metastatic ILC.

Inclusion Criteria:

1. Voluntary written informed consent of the participant or their legally authorized representative has been obtained prior to any screening procedures
2. At least 18 years of age at the time of signing the Informed Consent Form (ICF)
3. ECOG from 0 to 2
4. Patient with stage IV ILC, either de novo or after prior (curative) treatment for early ILC. Histological subtype confirmed on primary tumor or on metastatic tissue. Patients that are diagnosed with stage IV breast cancer that have a mixed ILC/IBC-NST histology of the primary tumor (mixed histology within the same primary lesion) will also be able to participate and will be analyzed in an exploratory cohort.
5. Confirmation of hormone receptor positive (HR+)/HER2- disease either on new biopsy or archival tissue (e.g. primary tumor)
6. Multifocal or bilateral disease is allowed if all evaluated foci present with HR+/HER2- ILC
7. Starting first line of treatment for metastatic ILC

Exclusion Criteria:

1. Presence of a contraindication to perform WB-DWI/MRI
2. Presence of severe claustrophobia
3. Presence of a contraindication to use IV contrast for CT or PET/CT (e.g. allergy, severe renal failure) in case a non-contrast PET/CT cannot be performed
4. Patients considered to have oligometastatic disease who are expected to undergo locoregional treatment
5. Presence of other malignancies in recent medical history (<5 years)
6. Female who is pregnant, breast-feeding or intends to become pregnant or is of child-bearing potential and not using an adequate, highly effective contraceptive
7. Adults who are the subject of a legal protection measure or who are unable to express their consent.