Incidental pulmonary leiomyoma in a patient with invasive mucinous breast carcinoma: A diagnostic challenge

Abstract

Pulmonary leiomyomas are rare benign smooth muscle tumors that are often discovered incidentally, particularly during oncologic staging, and can closely mimic metastatic lesions. We present the case of a 73-year-old postmenopausal woman newly diagnosed with low-grade ductal carcinoma in situ of the right breast with a focal invasive mucinous carcinoma component. As part of staging, contrast-enhanced chest CT revealed 2 well-circumscribed solid nodules in the left lower lobe, initially raising suspicion for metastatic spread. ^18F-FDG PET/CT demonstrated low metabolic activity, and subsequent CT-guided biopsy confirmed a well-differentiated smooth muscle neoplasm consistent with pulmonary leiomyoma. In view of the benign histology, low FDG avidity, and radiologic stability, surgical excision was deferred in favor of surveillance. The patient underwent breast-conserving surgery with radiotherapy and adjuvant endocrine therapy, later switched from letrozole to exemestane due to side effects. During multi-year follow-up, the pulmonary nodules remained stable without evidence of recurrence or metastasis. This case underscores the diagnostic dilemma of incidental pulmonary nodules in oncology patients and highlights the importance of integrating multimodality imaging with histopathology to avoid unnecessary interventions.

Introduction

Pulmonary leiomyomas are rare benign smooth muscle tumors of the lung. The majority of reported cases represent benign metastasizing leiomyoma (BML), in which uterine fibroids spread to the lung. Fewer than 100 cases of pulmonary BML have been documented worldwide as of 2020, occurring almost exclusively in women with a history of uterine leiomyomas [1,2]. Histologically, BML nodules consist of bland spindle cells and share the same immunophenotypic profile as uterine fibroids [3].

By contrast, primary pulmonary leiomyoma is even more uncommon, accounting for only ∼2% of all benign pulmonary tumors [4]. These may originate from smooth muscle elements of the tracheobronchial tree, pulmonary vessels, or heterotopic foci within the lung parenchyma.

In patients with an established malignancy such as breast carcinoma, the incidental discovery of pulmonary nodules typically raises strong suspicion for metastatic disease. However, studies have demonstrated that most small pulmonary nodules in breast cancer patients are actually benign [5]. The imaging characteristics of pulmonary leiomyomas, often presenting as well-circumscribed round nodules, can closely mimic metastatic lesions. BML typically manifests as multiple, well-defined pulmonary nodules on CT [6].

Here, we present the case of a postmenopausal woman with invasive mucinous breast carcinoma in whom staging CT revealed pulmonary nodules initially concerning for metastasis, but ultimately diagnosed as pulmonary leiomyoma.

Case presentation

A 73-year-old postmenopausal woman (G4P4) presented to the surgical outpatient clinic with a 1-month history of right-sided nipple pain and intermittent bloody discharge. She denied any history of breast lumps, nipple retraction, skin changes, axillary symptoms, or systemic features such as weight loss or fever. The left breast was clinically unremarkable. On physical examination, a 1 cm palpable nodule was detected in the right breast at the 10 o’clock position, approximately 3 cm from the nipple. Gentle pressure on the nipple elicited a small amount of bloody discharge, and no axillary lymphadenopathy was noted.

Breast ultrasound demonstrated a hypoechoic lesion corresponding to the palpable abnormality in the right breast at the 10 o’clock position, approximately 3 cm from the nipple. Cytology of the nipple discharge revealed clusters of epithelial cells with mild atypia, consistent with an intraductal process, and core needle biopsy confirmed low-grade ductal carcinoma in situ (DCIS) with strong ER and PR positivity. Histopathological analysis confirmed low-grade DCIS, with strong estrogen receptor (ER) and progesterone receptor (PR) positivity.

Following multidisciplinary team (MDT) review, the patient underwent wide local excision (WLE) and was scheduled for whole-breast radiotherapy (WBRT). Subsequent pathology revealed extensive DCIS with a focal invasive mucinous carcinoma component. For staging, contrast-enhanced CT chest, abdomen and pelvis was performed, which incidentally revealed 2 well-defined, solid, non-calcified lesions in the left lower lobe of the lung, the largest measuring up to 4 × 3 cm. These findings raised concern for possible metastatic disease versus benign pulmonary pathology such as hamartomas or smooth muscle tumors.

To further evaluate the lesions, whole-body ¹⁸F-FDG PET/CT was performed. No hypermetabolic activity was detected in the right breast surgical bed, right axilla, or the pulmonary lesions, reducing the likelihood of metastatic disease (Fig. 1). The remainder of the examined organs were unremarkable. However, due to the solid nature of the nodules, a CT-guided percutaneous biopsy of the dominant left lower lobe mass was performed (Fig. 2). Histopathological analysis confirmed a well-differentiated smooth muscle neoplasm, consistent with pulmonary leiomyoma. Although surgical excision was initially considered to definitively exclude a low-grade malignancy, the patient remained clinically and radiologically stable, and the decision was made to pursue imaging surveillance.

Fig. 1.

Whole-body F-18 FDG PET/CT. Selected axial (A–C) and coronal (D, E) images show a well-defined solid lesion in the anteromedial segment of the left lower lobe, measuring 4 × 3 cm, with smooth margins and no internal calcification, fat, or cavitation (A, B; solid arrow). A second, smaller lesion with similar morphology is also noted (A; dashed arrow). PET and fused PET/CT images (C–E; solid arrow) demonstrate low-grade FDG uptake with SUVmax 3.2. FDG = Fluorodeoxyglucose; PET = Positron Emission Tomography.

Fig. 2.

Limited plain CT chest (axial, prone position). CT-guided tru-cut biopsy was performed using a coaxial biopsy needle with a single pass targeting the anteromedial segment lesion of the left lower lobe. No immediate post-procedural complications were observed.

The patient underwent WLE of the breast lesion and received WBRT. Sentinel lymph node biopsy revealed no nodal involvement (0/4 nodes), confirming a final staging of pT1cN0M0. Immunohistochemistry demonstrated ER positivity (100%), PR positivity (10%), HER2 negativity, and a low proliferative index (Ki-67 at 6%). Based on the hormone receptor–positive profile, adjuvant endocrine therapy with letrozole 2.5 mg daily was initiated, along with calcium supplementation (Caltrate).

She tolerated therapy well initially but later developed joint pain, fatigue, and nausea, prompting discussion regarding a switch to exemestane, which she accepted. At her most recent follow-up, she remained clinically stable, with no radiologic evidence of locoregional recurrence or distant metastasis, and the pulmonary lesions remained stable (Fig. 3). Serial imaging over several years demonstrated no progression of the pulmonary lesion (Fig. 4). Ongoing surveillance with periodic breast ultrasound and chest imaging was planned.

Fig. 3.

Multisequence axial contrast-enhanced MRI of the breast. A well-defined left lower lobe lesion demonstrates heterogeneous signal intensity on T2-weighted (A) and T1-weighted (B) sequences, with post-contrast enhancement (C, D). Diffusion-weighted imaging was degraded by artifact. The larger lesion is indicated by a solid arrow, and the smaller lesion by a dashed arrow.

Fig. 4.

Frontal chest radiograph. A dense, partially well-defined mass is seen in the left lower lung zone, with smooth lateral margins but medially blending into the mediastinum. No calcification, cavitation, or overlying rib destruction is evident. The left cardiac border is preserved, consistent with an intraparenchymal pulmonary mass. Right chest wall surgical clips are noted, consistent with prior breast surgery.

Discussion

DCIS is a non-invasive breast cancer in which malignant epithelial cells remain confined within the ductal-lobular system without breaching the basement membrane. It is frequently detected incidentally on screening mammography, often presenting as microcalcifications or a mass [7]. In this case, the patient presented with nipple pain and bloody discharge, less common but clinically significant symptoms of DCIS. Unilateral bloody nipple discharge warrants prompt investigation due to its known association with underlying malignancy [8]. Low-grade DCIS typically follows an indolent course and is associated with favorable outcomes following appropriate treatment [9]. Hormone receptor status plays a critical role in guiding adjuvant therapy; ER and PR positivity, as seen in this case, predicts a good response to endocrine therapy, particularly aromatase inhibitors such as letrozole [10]. This finding prompted staging imaging to exclude metastatic disease, confirmed the need for adjuvant WBRT, and supported the initiation of endocrine therapy with an aromatase inhibitor.

Mucinous carcinoma is a rare histologic subtype, accounting for approximately 2% of all invasive breast cancers [11]. It is commonly ER/PR-positive, HER2-negative, and tends to follow a relatively indolent course with a favorable prognosis. Radiologically, mucinous carcinomas can show distinctive features, as highlighted in recent case reports, which may complicate their distinction from metastatic or benign lesions [12].

Incidental findings of pulmonary nodules have become increasingly common due to the widespread use of cross-sectional imaging in oncology. In cancer patients, such findings typically raise concern for metastatic disease; however, benign causes must also be considered. In this patient, the nodules’ smooth, well-circumscribed, and non-calcified morphology warranted further investigation [13]. While 18F-FDG PET/CT is a sensitive modality for detecting malignancy, it may yield false negatives in tumors with low metabolic activity, such as mucinous carcinoma [14]. In addition, FDG PET/CT has several well-recognized limitations, as benign inflammatory or reactive conditions may mimic malignancy [15]. Both benign leiomyomas and malignant smooth-muscle tumors may show a wide spectrum of FDG uptake, depending on cellularity and mitotic activity [16,17]. Uptake depends on tumor cellularity, mitotic activity, lesion size, and the presence of abundant acellular mucin or fibrosis, factors that commonly produce low metabolic activity, whereas highly cellular or mitotically active lesions may be relatively hypermetabolic. Consequently, a lack of FDG avidity reduces but does not eliminate the possibility of malignancy; PET findings need to be interpreted alongside morphology, IHC, and, when doubt persists, tissue sampling.

On CT, pulmonary leiomyomas most often appear as well-defined, homogeneous soft-tissue nodules without calcification [18]. MRI findings are variable but may show intermediate T1 signal, heterogeneous T2 signal, and contrast enhancement reflecting cellularity or fibrous stroma [19]. Histopathological evaluation of the pulmonary nodules confirmed a diagnosis of pulmonary leiomyoma, especially uncommon in postmenopausal women, where hormonal influence is diminished [20]. This accurate diagnosis prevented unnecessary aggressive treatment and underscored the importance of histological confirmation before initiating systemic therapies. Histologically, pulmonary leiomyomas demonstrate fascicles of bland spindle cells with minimal atypia, low mitotic activity, and the absence of necrosis [21]. Immunohistochemical staining typically shows strong positivity for smooth muscle markers such as smooth muscle actin (SMA), desmin, and h-caldesmon while epithelial markers (cytokeratins) are negative [22,23]. Management of early-stage breast cancer in older adults should be individualized. Breast-conserving surgery combined with radiotherapy has been shown to offer survival outcomes equivalent to mastectomy, with superior cosmetic and quality-of-life results [24,25]. Hormonal therapy with aromatase inhibitors like letrozole significantly reduces recurrence and improves survival in postmenopausal women [26]; however, side effects such as arthralgia and fatigue can impact adherence [27].

Many benign smooth-muscle pulmonary tumors, including BML express estrogen and PRs, suggesting hormone-driven growth [2]. Persistence or progression of ER/PR-positive lesions in postmenopausal women when circulating estrogen levels are low is less common and therefore renders this case more unusual. In this case, switching to exemestane was considered to improve tolerability. The role of the MDT is essential in optimizing outcomes. MDT-driven care improves survival rates and patient satisfaction by ensuring coordinated diagnostic, surgical, and adjuvant decision-making [28]. In this case, MDT involvement guided every stage of the patient's care. Importantly, this case emphasizes the diagnostic dilemma posed by incidental pulmonary nodules in oncology patients. Misinterpreting a benign lesion as metastatic disease could lead to overtreatment and patient distress. The Fleischner Society guidelines support the use of biopsy and serial imaging for the evaluation of indeterminate pulmonary nodules [29]. Long-term follow-up remains vital due to the risk of recurrence or the development of new malignancies [30].

In summary, this case exemplifies the complexity of breast cancer management and the nuances involved in interpreting incidental findings. It highlights the importance of individualized, multidisciplinary, and evidence-based care, while reinforcing the need for patient-centered strategies particularly when managing treatment-related side effects to support adherence and achieve optimal outcomes. This case is a useful teaching example for radiology and multidisciplinary tumor boards as it highlights that even in patients with a known primary malignancy, well-circumscribed pulmonary nodules may be benign and require a measured diagnostic approach.

Learning points

• •Pulmonary leiomyomas are rare benign tumors that can mimic metastatic lesions on imaging, particularly in oncology patients. Careful evaluation and histopathologic confirmation are essential to avoid misclassification and overtreatment.
• •Mucinous carcinoma of the breast, although rare and often indolent, may show low FDG uptake on PET/CT, which can complicate the differentiation of metastatic versus benign pulmonary lesions.
• •Multidisciplinary team (MDT) involvement is critical in cases with synchronous or incidental findings, ensuring accurate diagnosis, appropriate staging, and optimal treatment planning.
• •Incidental pulmonary nodules discovered during cancer staging should be approached with a structured diagnostic algorithm, integrating imaging characteristics, metabolic activity, and tissue biopsy where appropriate to avoid unnecessary surgical intervention.

Patient consent

Written informed consent was obtained from the patient for publication of this case report and accompanying images.