Beyond the lung: recognizing incidental breast lesions on chest computed tomography.

Introduction
Breast cancer is one of the leading causes of cancer-related mortality among women, and early detection remains the most important factor for improving survival (1). Mammography, ultrasound, and breast magnetic resonance imaging (MRI) remain the standard imaging modalities for diagnosing breast cancer (1,2). In contrast, chest computed tomography (CT)—although it routinely includes the entire breast—is not considered a primary diagnostic tool for breast lesions, and radiologists may consequently devote limited attention to the breast region during interpretation.
Recent large cohort analyses report that incidental breast nodules are identified in 7–8% of routine chest CTs, emphasizing the non-negligible prevalence of these findings in clinical practice. In patients with such nodules undergoing further workup, the proportion ultimately proven to be malignant has been reported to exceed 20%, with irregular margins and spiculation on CT strongly predictive of malignancy (3-5). Notably, in lung cancer screening CT, breast cancer accounts for approximately one in eight incidental breast abnormalities (6). With the rapidly increasing use of chest CT in oncologic imaging and screening settings, radiologists are more frequently confronted with unexpected breast findings outside the context of dedicated breast imaging.
Despite this growing clinical relevance practical CT-oriented guidance on how to recognize, categorize, and appropriately triage incidental breast lesions remains limited in the thoracic imaging literature. Key studies addressing incidental breast findings on chest CT are summarized in Table 1. A structured understanding of typical CT appearances—including benign entities, suspicious malignant features, and expected post-treatment changes—is therefore essential for thoracic radiologists and general clinicians.

Anatomy and imaging considerations on chest CT
The breast is composed of the nipple-areolar complex, branching ductal system, fibroglandular tissue, Cooper’s ligaments, retromammary fat, and the underlying pectoralis major muscle, all of which may be visualized on routine chest CT examination (7). Although chest CT performed for thoracic evaluation is not optimized for dedicated breast imaging—unlike emerging technologies such as cone-beam breast CT designed specifically for breast assessment—it routinely includes the entire breast and may reveal incidental breast abnormalities (8) (Figure 1). On chest CT, the axilla is conventionally divided into three nodular levels according to the relationship to the pectoralis minor muscle: level I, II, III to the muscle (9) (Figure 2). Standards for axillary lymph node assessment differ between CT and ultrasound: ultrasound primarily relies on cortical thickness (≥3 mm) and loss of fatty hilum, whereas CT generally applies size-based criteria such as a short-axis diameter >10 mm (10,11). However, CT is less sensitive and specific than ultrasound for the detection of axillary lymph node metastasis, particularly in early-stage disease (10). Understanding normal anatomic structures is essential to avoid misinterpreting normal variants as pathology. A unilateral axillary muscular arch or the sternalis muscle, for example, may mimic a breast mass or axillary lymphadenopathy on axial images (Figure 3).
Breast density—defined by the proportion of fibroglandular tissue relative to fat—is a key concept well established in mammography, where Breast Imaging Reporting and Data System (BI-RADS) categorizes density into four groups ranging from almost entirely fatty to extremely dense (9). Dense breasts may obscure lesion detection and independently increase breast cancer risk. Although density assessment is not formally included in chest CT reporting, CT-based density estimation correlates well with mammographic categories. On CT, 0–24% fibroglandular tissue is considered almost entirely fatty, 25–50% scattered, 51–75% heterogeneously dense, and >75% as extremely dense (Figure 4) (12). Recognizing these patterns on CT can help radiologists anticipate limitations in lesion detection and contextualize the likelihood of subtle incidental abnormalities, particularly in patients with dense parenchyma.
Background parenchymal enhancement (BPE) represents contrast uptake by normal fibroglandular tissue and, although physiologic in origin, is influenced by hormonal status and may affect diagnostic performance and risk stratification on breast MRI (13-15). On contrast-enhanced chest CT, a similar pattern may appear as multiple small enhancing foci within fibroglandular tissue, and when bilateral and symmetric, it favors a benign physiologic process rather than true focal lesions (14) (Figure 5). When bilateral and symmetric, this pattern strongly favors a benign background process rather than true enhancing lesions. Previous studies have demonstrated good correlation of BPE severity between contrast-enhanced breast CT and breast MRI (16).

Benign breast lesions detected on chest CT
Benign breast lesions constitute a substantial proportion of incidental abnormalities detected on chest CT, and recognizing their characteristic imaging features prevents unnecessary additional workup while maintaining diagnostic confidence.

Gynecomastia
Gynecomastia represents benign proliferation of male breast glandular tissue caused by hormonal imbalance and is associated with systemic disease, endocrine disorders, and medications. Although gynecomastia is most commonly bilateral, it is often asymmetric and may occur unilaterally (17). On CT, retroareolar soft-tissue thickness of ≥2 cm on axial imaging is a practical diagnostic threshold (18). A discrete mass separate from the retroareolar glandular tissue or an eccentrically located lesion should prompt further evaluation to exclude malignancy (17).

Duct ectasia
Duct ectasia manifests as dilated ducts beneath the nipple–areolar complex and may occasionally appear as a tubular retroareolar structure on cross-sectional imaging (9,19). Although usually benign and age-related, duct ectasia may mimic a mass on axial CT images (Figure 6); its continuity with the ductal system and absence of suspicious enhancement patterns helps in differentiation.

Hamartoma
Breast hamartomas are benign mixed-tissue tumors composed of fat, fibrous tissue, and glandular elements (9). On CT, a well-circumscribed lesion with internal fat and soft tissue produces the characteristic “breast-within-a-breast” appearance (Figure 6).

Fibroadenoma
Fibroadenoma is the most common benign breast tumor and presents as a well-defined calcified or noncalcified mass on CT (20). Densely calcified fibroadenomas demonstrate the characteristic “popcorn” appearance that allows confident diagnosis on both mammography and CT (9,20,21). Small enhancing nodules identified on CT often correspond to smooth, ovoid, hypoechoic masses on ultrasound (20). However, non-calcified fibroadenomas may be difficult to distinguish from other benign or malignant breast masses on chest CT alone (Figure 6). Therefore, correlation with prior chest CT examinations and dedicated breast imaging studies is essential to confirm lesion stability or prior characterization.

Malignant breast lesions detected on chest CT

CT features suggestive of malignancy
Although most incidental breast findings on chest CT are benign, identifying features suspicious for malignancy is critical, as breast cancer may first be detected on CT performed for unrelated clinical indications (21). Several CT characteristics are consistently associated with breast cancer, including irregular or spiculated margins, abnormal mass-like enhancement, unilateral axillary lymph node enlargement, and enhancement of ≥33 Hounsfield unit (HU) on contrast-enhanced CT, a threshold reported to demonstrate a sensitivity of 83% and specificity of 95% for malignancy (21-23). Interval growth of a breast lesion should likewise raise suspicion. When breast cancer is suspected on CT, radiologists should evaluate not only secondary features such as skin thickening, nipple retraction, chest wall invasion, and regional nodal metastasis, but also primary lesion characteristics including irregular or spiculated margins, rim or eccentric enhancement, and deceptively well-defined masses that enhance homogenously despite malignant morphology on MRI (Figure 7).

Limitations of CT compared with breast MRI
Interpreting malignant masses on CT is challenging because features that are clearly malignant on MRI—such as irregular shape, spiculated margins, heterogeneous or rim enhancement, and dark internal septations—may appear subtle or even inconspicuous on CT (Figure 7). Rim enhancement, in particular, is a strong MRI predictor of malignancy but may manifest only as faint peripheral enhancement on CT. Non-mass enhancement, another MRI descriptor associated with malignancy, can be especially difficult to appreciate on CT, where enhancement patterns are less conspicuous and architectural distortion is less reliably depicted (Figure 8). Small cancers may appear deceptively circumscribed or homogeneous on CT despite demonstrating overtly malignant morphology on MRI, underscoring CT’s inherent limitations in characterizing lesion margins and internal architecture (Figures 7,8).

Detection performance and missed lesions on chest CT
A recent study by Park et al. demonstrated the difficulty of CT-based detection, with 64.3% of incidental breast cancers initially missed and detection more likely in lesions ≥9 mm with stronger enhancement (24). Small, weakly enhancing cancers—particularly those with isoattenuating margins—are commonly overlooked.

CT detection in mammographic blind regions
In contrast to its limitations, chest CT may detect breast cancers located in mammographic blind regions, including far posterior lateral, or inferior breast tissue. These lesions may be incompletely visualized on screening mammography but subsequently confirmed with targeted diagnostic imaging. Recent studies have emphasized that incidental breast cancers detected on chest CT often arise in these mammographically challenging locations, underscoring the complementary role of chest CT in breast cancer detection (25).
These findings highlight the need for radiologists to adopt a systematic approach when evaluating the breast on chest CT, maintaining vigilance for even subtle enhancing lesions, especially when accompanied by asymmetric density, unilateral trabecular thickening, or ipsilateral axillary adenopathy. Incorporating these principles into routine CT interpretation may help reduce the proportion of missed breast cancers and facilitate earlier diagnosis.

Post-treatment findings

Expected early postoperative and postradiation changes
Postoperative and postradiation changes show a wide spectrum of CT appearances, reflecting the diversity of surgical techniques, reconstructive methods, and radiation protocols (7,26). Expected early findings include fluid collections, parenchymal and skin thickening, edema and fat stranding. Recognition of these temporal patterns helps avoid overcalling recurrence.

Breast augmentation and implant types
Breast augmentation and reconstruction may use saline, silicone, hybrid implants, or autologous fat or tissue flaps (7,26). On CT, saline implants appear as homogeneous low-attenuation sacs containing a visible fill valve, whereas silicone implants demonstrate slightly higher attenuation and may be single- or dual-lumen. Implant position—prepectoral or retropectoral—alters breast contour and the displacement of native parenchyma (Figure 9). Early postoperative fluid collections small air foci, or diffuse edema should not be misinterpreted as complication (7,26).

Implant-related complications
Complications are categorized as early or late. Early complications include seroma, hematoma, and infection, which may appear as fluid collections, soft-tissue thickening, or inflammatory fat stranding. Late complications include fat necrosis, implant rupture, capsular contracture, and breast implant-associated anaplastic large-cell lymphoma (27,28). Intracapsular rupture may be suggested by the classic linguine sign, representing collapsed elastomer shell fragments within the silicone gel, whereas extracapsular rupture is characterized by high-attenuation silicone material outside the capsule (Figure 10). Although CT can demonstrate these findings, MRI remains more sensitive when rupture is equivocal (7,26). Capsular contracture, a common late complication, presents as spherical deformation of the implant with thickened or calcified capsule and may be associated with discomfort (7,26).

Breast-conserving surgery (BCS) and radiation therapy
BCS with radiation results in characteristic temporal CT findings (7,26,29,30). Early postoperative changes include seroma, parenchymal and skin thickening, and edema, while fat necrosis may develop later and can calcify over time (7,26) (Figure 11). Radiation-related changes range from acute skin thickening and edema to late subpleural reticulation or fibrosis, and in rare cases, chest wall osteoradionecrosis (7,26) (Figure 12). New or enlarging enhancing masses or suspicious nodal enlargement should prompt concern for local recurrence and warrant further dedicated evaluation (7,26).

Mastectomy techniques and reconstruction
Mastectomy range from skin-sparing and nipple-sparing to modified radical mastectomy, the latter involving removal of the nipple-areolar complex and typically performed for advanced breast cancer or for prophylactic purposes in high-risk patients (Figure 13). Reconstruction may be achieved using tissue expanders—identified on CT by their chest-wall–oriented metallic port—followed by placement of permanent implants (26). Nipple reconstruction and contour-modifying procedures, including autologous fat grafting, further contribute to the diversity of postoperative morphology (Figure 14).

Autologous flap and adjunctive reconstruction methods
Autologous flap reconstruction, such as the transverse rectus myocutaneous abdominis (TRAM) flap, deep inferior epigastric perforator (DIEP) flap, and latissimus dorsi flap, creates predictable patterns of redistributed fat and muscle on CT, with occasional visualization of vascular pedicles (Figure 14). These expected findings should not be mistaken for tumor recurrence. Adjunctive reconstructive materials, including acellular dermal matrix (ADM), are frequently incorporated into implant-based reconstruction; ADM appears on CT as a thin curvilinear soft-tissue density partially enveloping the implant, representing a normal postoperative feature (31). On CT, ADM appears as a curvilinear soft tissue density partially encasing the implant, a normal finding that should not be mistaken for neoplasm (7,14). Reduction mammoplasty produces characteristic imaging patterns—including dermal or skin calcifications, architectural distortion, and focal fat necrosis—that are typically stable on serial studies, reflecting expected postsurgical changes rather than recurrent disease (7,32).

Imaging findings encountered across benign, malignant, and post-treatment conditions

Skin and trabecular changes
Skin thickening greater than 2-mm thickness and trabecular prominence are nonspecific CT findings that may arise from a wide spectrum of benign, malignant, and post-treatment processes. Benign etiologies include postoperative change, post-radiation effect, mastitis, congestive heart failure, and systemic causes of edema, all of which produce diffuse dermal thickening with subcutaneous stranding. Trabeculation, reflecting prominence of Cooper’s ligaments or stromal edema, becomes more visible when the breast is edematous. In contrast, marked or asymmetric skin thickening raises concern for inflammatory breast cancer, which manifests as diffuse dermal lymphatic invasion, or for rare etiologies such as radiation-induced sarcoma. Physiologic states such as lactation can also produce prominent skin and trabecular thickening with interval regression postpartum (Figure 15). Clinical correlation is essential to differentiate among these entities.

Calcifications
Most calcifications identified on chest CT are benign, as microcalcifications—generally less than 1 mm in size—lie below the spatial resolution of CT and therefore may not be reliably visualized. Common benign causes include fat necrosis, oil cysts, involuting fibroadenomas, and vascular or sutural calcifications. Coarse “popcorn-like” calcifications are characteristic of fibroadenoma. Although malignant microcalcifications are typically poorly visualized on CT, an associated irregular or enhancing mass may still be detectable. Therefore, calcifications accompanied by suspicious enhancement or mass-like morphology should prompt dedicated breast imaging for further evaluation (Figure 16).

Conclusions
This review provides a practical, structured overview of breast lesions encountered on routine chest CT, focusing on key imaging features, diagnostic pitfalls, and post-treatment appearances. A systematic review of the breast on chest CT helps reduce missed clinically significant disease.
Understanding typical benign patterns, suspicious malignant features, and expected postoperative changes allows radiologists to appropriately triage incidental findings. If a breast mass has no prior characterization or long-term imaging stability, additional diagnostic mammography and targeted ultrasound are generally recommended based on prior literature.
With the growing use of chest CT, consistent attention to the breast region during routine interpretation is a simple but important step for earlier detection and appropriate patient management.

Supplementary
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