Incidental Finding of Anterior Abdominal Wall Endometriosis Masquerading as Metastasis on FDG PET/CT in a Breast Cancer Patient

Accurate disease staging is critical in the management of breast cancer to guide optimal therapeutic strategies. While 18F-fluorodeoxyglucose positron emission tomography/ computed tomography ( 18F FDG PET/CT) is an invaluable modality for detecting locoregional and distant spread, [1] it is not without limitations. Because 18F-FDG naturally accumulates in tissues with high glucose metabolism, benign inflammatory, infectious, or fibrotic conditions can yield false-positive results.[2] When these incidental hypermetabolic findings occur during oncologic staging, they create a significant clinical dilemma, often masquerading as metastatic deposits and threatening to inappropriately upstage the patient. One such confounding benign condition is endometriosis, a chronic fibro-inflammatory disorder defined by the presence of functional endometrial tissue outside the uterine cavity. [3] Although it is predominantly a pelvic disease, extra-pelvic implants can occur. Abdominal wall endometriosis (AWE), for instance, is an uncommon presentation that typically manifests as a localised subcutaneous nodule.[4] Driven by active inflammation and localised tissue remodelling, these endometriotic lesions can exhibit varying degrees of 18F-FDG avidity that frequently overlap with the standardised uptake values of malignant lesions. [5] Consequently, an unexpected finding of hypermetabolic AWE can closely mimic metastatic disease in female cancer patients. CASE REPORT A 44-year-old woman patient had recently been diagnosed with biopsy-proven carcinoma of the left breast. Initial breast biopsy revealed an invasive ductal carcinoma [not otherwise specified (NOS)], Grade 2, which was Luminal B-like (estrogen and progesterone receptor positive, Human epidermal growth factor receptor 2 (Her2Neu) negative, Ki- 67: 20-25%).  e patient was subsequently referred for an 18F FDG PET/CT scan for disease staging. Her menstrual cycles

Accurate breast cancer staging using 18F FDG PET/CT is crucial, though benign inflammatory conditions can yield false-positive results mimicking metastasis. We present a 44-year-old woman with newly diagnosed invasive ductal carcinoma who underwent a baseline PET/CT. Imaging revealed the primary tumour, axillary node involvement, and an unexpected hypermetabolic nodule in the anterior abdominal wall. Highly suspicious for a metastatic deposit, this finding threatened to inappropriately upstage the patient to Stage IV , which would shift her treatment from curative to palliative intent. An ultrasound-guided core biopsy was performed, and histopathology definitively diagnosed abdominal wall endometriosis, ruling out distant metastasis. is confirmation allowed the patient to safely proceed with a curative-intent mastectomy. is case underscores the necessity of tissue biopsy for isolated hypermetabolic findings to prevent erroneous cancer upstaging.

Abdominal wall endometriosis, Breast cancer, FDG PET/CT *Corresponding author: Meghana Prabhu, Department of Nuclear Medicine, Amrita School of Medicine (Amrita Vishwa Vidyapeetham – Deemed University), Faridabad, 121002, Haryana, India. [email protected] Received: 02 March 2026 Accepted: 12 June 2026 EPub Ahead of Print: 10 August 2026 Published: ** *** **** DOI: 10.25259/IJNM_57_2026 Article in Press Jha, et al.: Endometriosis Masquerading as Metastasis Indian Journal of Nuclear Medicine • Article in Press | 2 were regular, and the last menstrual period was documented as 6days prior to the 18F FDG PET/CT study.  e baseline scan demonstrated the primary hypermetabolic left breast tumour (measuring 2.2 x 2.0 x 2.3 cm, SUVmax 8.0) and multiple FDG-avid left axillary and internal mammary lymph nodes, consistent with locoregional spread [Fig 1A-E]. However, the scan also incidentally identified an FDG-avid, discrete enhancing nodular lesion in the right lower anterior abdominal wall [Fig 1F-G, blue arrows].  e lesion closely abutted the underlying rectus muscle, measured 1.5 x 1.7 cm, and demonstrated mild hypermetabolism with a SUVmax of 2.7. Given the context of a primary breast malignancy, this abdominal wall lesion was flagged as a potential mitotic or metastatic deposit, and a biopsy correlation was strongly recommended. Other incidental findings included bilateral adnexal cysts with mild right adnexal uptake (SUVmax 4.1), cholelithiasis, and hepatomegaly. To rule out distant metastasis, an ultrasound-guided core needle biopsy of the abdominal wall nodule was performed, and multiple linear tissue cores were retrieved. Microscopic examination of these cores revealed fibrocollagenous and adipose tissue with scattered, intervening benign endometrial glands and stroma.  ere was no evidence of neoplastic or metastatic pathology.  e impression conclusively diagnosed the suspicious nodule as abdominal wall endometriosis. With distant metastasis ruled out, the patient proceeded with a left mastectomy with axillary clearance as the curative-intent surgery, along with the excision of the right-sided anterior rectus sheath tumour. Gross examination of the excised abdominal wall soft tissue [Fig 2A-C] revealed a 4 x 3 x 2 cm specimen. Final histopathologic evaluation of this anterior rectus sheath mass confirmed fibroadipose tissue containing benign endometrial glands and stroma, with no cytoarchitectural atypia, definitively consistent with endometriosis. Meanwhile, the final breast pathology confirmed Grade 2 invasive ductal carcinoma (Luminal B-like) staged as pT2 N3b Mx (with 22 out of 23 lymph nodes positive for tumour). Fig 1: FDG PET/CT images. (A) Maximum intensity projection (MIP); (B-E) Transaxial fused PET/CT and corresponding CT images reveal a primary hypermetabolic left breast tumour (measuring 2.2 x 2.0 x 2.3cm, SUVmax 8.0) and metastases to multiple FDG-avid left axillary lymph nodes. (F and G) Incidentally identified FDG-avid discrete enhancing nodular lesion (measuring 1.5 x 1.7cm, SUVmax of 2.7) in the right lower anterior abdominal wall (blue arrows). FDG: Fluorodeoxyglucose; PET/CT: Positron emission tomography/computed tomography; SUV: Standardised uptake value Jha, et al.: Endometriosis Masquerading as Metastasis Indian Journal of Nuclear Medicine • Article in Press | 3

Accurate staging of invasive breast carcinoma using 18F FDG PET/CT is essential for identifying locoregional spread and distant metastasis, a process that fundamentally dictates the treatment paradigm. [1] In this patient, while the primary tumour and significant axillary nodal burden (pT2 N3b) appeared as expected on imaging, an incidental hypermetabolic nodule in the anterior abdominal wall presented a critical diagnostic challenge. Assuming this lesion was a metastasis would have categorised the patient as Stage IV , shifting the clinical strategy from curative-intent surgery to palliative systemic therapy. is scenario highlights a well-documented limitation of functional imaging: FDG, a glucose analogue, accumulates not only in malignant cells but also in active inflammatory, infectious, and certain benign neoplastic tissues. [2,5] One such benign entity is endometriosis: a chronic, oestrogen- dependent fibro-inflammatory disorder defined by the presence of ectopic endometrial glands and stroma outside the uterine cavity.[3] It is broadly classified into three phenotypes: (i) superficial peritoneal endometriosis (SPE), comprising flat, shallow implants on the peritoneal surface that seldom exceed 5 mm in depth; (ii) ovarian endometrioma (OMA), also termed “chocolate cysts, ” characterised by haemorrhagic cystic lesions within the ovary; and (iii) deep infiltrating endometriosis (DIE), the most aggressive subtype, defined by lesions penetrating more than 5 mm beneath the peritoneal surface and frequently invading the rectovaginal septum, bowel, bladder, or ureters.[6,7] A fourth category, extrapelvic endometriosis, includes abdominal wall, diaphragmatic, pleural, and umbilical disease, and is typified by isolated nodules within or adjacent to surgical scars or, less commonly, arising spontaneously. [4] With respect to FDG avidity, these subtypes behave distinctly. SPE lesions, being predominantly fibrotic and metabolically quiescent, are largely FDG-negative and thus rarely identified on PET/CT. Ovarian endometriomas are characteristically low-avidity lesions (reported SUVmax range 1–4) due to their predominantly cystic, haemorrhagic architecture rather than active cellularity, although pericystic inflammatory tissue can occasionally elevate uptake. [8,9] DIE lesions, owing to their intense stromal fibrosis and active inflammatory microenvironment, demonstrate the widest and most unpredictable range of uptake (SUVmax 4.88- 5.66 in one series), and can closely mimic malignancy. [8,10] Abdominal wall and other extrapelvic implants, as present in this case, similarly show variable but potentially significant avidity (SUVmax 1.7–4.8 in case series), driven by the degree of local inflammation, tissue remodelling, and macrophage infiltration within the lesion.[8] A critical and clinically underappreciated determinant of FDG avidity in endometriotic implants is the phase of the menstrual cycle at which the PET/CT scan is performed. Because ectopic endometrial tissue retains functional responsiveness to ovarian hormones, its metabolic activity mirrors the cyclical fluctuations of the eutopic endometrium.[7] During the perimenstrual and early menstrual phase, oestrogen-driven cellular proliferation, prostaglandin- mediated inflammation, and retrograde shedding of ectopic tissue create a microenvironment of heightened glucose utilisation, resulting in peak FDG uptake within endometriotic implants. Studies of normal endometrial FDG uptake demonstrate that SUVmax is highest during menstruation (mean SUVmax ~5.0) and at mid-cycle ovulation (mean SUVmax ~3.7), while significantly lower values are observed during the proliferative and secretory phases (mean SUVmax ~2.6 and ~2.5, respectively). [7] By extension, endometriotic implants are most likely to be FDG- avid, and therefore most likely to be detected or to mimic malignancy when scanning coincides with the perimenstrual or ovulatory window. Conversely, they may appear entirely Fig 2: Gross examination of the excised abdominal wall soft tissue reveals (A) Haematoxylin and eosin (H and E) at 50X showing core composed of scattered endometrial glands and stroma in fibrotic tissue; (B) H and E at 200X showing endometrial gland surrounded by endometrial stroma, having a hemosiderin-laden macrophage (red arrows); (C) Immunohistochemistry for CD10 highlights the endometrial stroma (200X). H and E: Haematoxylin and eosin Jha, et al.: Endometriosis Masquerading as Metastasis Indian Journal of Nuclear Medicine • Article in Press | 4 quiescent if imaged during the mid-proliferative or secretory phase.[5,10] is cyclical variability of FDG uptake is the fundamental explanation for the inconsistent sensitivity of [ 18F]F-FDG PET/CT in detecting endometriosis across published studies, in which scans were often performed without standardisation of cycle timing. [5,10] In the present case, the patient’s last menstrual period was documented as only 6 days prior to the PET/CT scan, placing the study within the early post-menstrual/proliferative phase.  e relatively modest SUVmax of 2.7 observed in the AWE nodule is therefore entirely consistent with the expected tapering of FDG avidity as menstrual-phase inflammation subsides but remained sufficient to flag the lesion as metabolically suspicious and indistinguishable from a low-grade malignant deposit on imaging alone. is underscores the importance of documenting the day of the menstrual cycle in the clinical request for any 18F FDG PET/CT performed in a premenopausal woman, as this information is indispensable for accurate scan interpretation.[7] Because there is a significant overlap in the SUVmax of AWE and low-to-moderate grade malignancies, functional imaging alone cannot reliably differentiate an endometriotic implant from a metastatic deposit. [5,8,10] Consequently, the decision to perform an ultrasound-guided core needle biopsy of the isolated distant lesion was pivotal. Given the patient’s Luminal B-like invasive ductal carcinoma and aggressive lymphatic spread (22 out of 23 positive lymph nodes), the pre-test probability for distant metastasis was substantial. Fortunately, histological examination revealed benign fibroadipose tissue with endometrial glands, effectively ruling out disseminated disease. is confirmation allowed the patient to safely proceed with a curative-intent left mastectomy and axillary clearance.  e concurrent excision of the rectus sheath mass definitively treated the AWE and provided final pathological correlation.

Ultimately, this case reinforces a core oncological principle: isolated, unexpected findings on advanced imaging must always be corroborated by tissue biopsy before upstaging a patient and irrevocably altering their treatment trajectory. Author contributions: AJ: Literature review, data collection, manuscript drafting, reference management, and manuscript revision; MP: Conceptualisation, study design, PET/CT image interpretation, supervision, critical manuscript review, correspondence, and final approval; AB: Nuclear medicine image interpretation, validation of imaging findings, and critical manuscript review; GK: Histopathological examination, pathological interpretation, and clinicopathological correlation; SR: Patient management, surgical data collection, clinical documentation, and manuscript review. Ethical approval: Institutional Review Board approval is not required. Declaration of patient consent:  e authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given consent for their images and other clinical information to be reported in the journal.  e patient understand that the patient’s names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship: Nil. Conflicts of interest:  ere are no conflicts of interest. Use of artificial intelligence (AI)-assisted technology for manuscript preparation:  e authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript, and no images were manipulated using AI.

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