Epidermal inclusion cyst of the breast: A case report

Abstract

Epidermal inclusion cysts are nonmalignant proliferations of squamous epithelium situated within the dermis. Their presence in the breast is uncommon and presents considerable diagnostic difficulty. This case report aims to elucidate the multimodal imaging findings and differential diagnosis of a rare breast epidermal inclusion cyst, highlighting its capacity to mimic malignant neoplasms. We present a case of a 38-year-old female with a palpable right breast nodule that had progressively increased over a period of 15 years. A clinical examination identified a firm mass in the upper inner quadrant of the right breast. Ultrasound revealed a heterogeneous solid nodule, whilst mammography detected a sizable, well-defined hyperdense mass (BI-RADS 4). Magnetic resonance imaging demonstrated a well-defined nodule with T2/STIR hyperintensity, characterized by a ring-like internal enhancement pattern and restricted diffusion, classified as BI-RADS 4. An ultrasound-guided core biopsy was performed, and histological analysis confirmed the diagnosis of an epidermal inclusion cyst. This case underscores the importance of considering epidermal inclusion cysts in the differential diagnosis of breast cancers with imaging features to avoid unnecessary invasive interventions.

Introduction

Epidermal inclusion cysts are benign lesions formed by the proliferation of squamous epithelium within the dermis, commonly located on the scalp, trunk, or extremities [1]. Their occurrence in the breast gland is exceedingly rare and poses a unique diagnostic challenge, particularly when involving deeper breast tissue, where imaging features may closely mimic those of malignant tumors [2].

The pathogenesis of epidermal inclusion cysts in the breast has not been fully elucidated. Proposed mechanisms include post-traumatic or iatrogenic implantation of epidermal tissue into the breast parenchyma following surgery or minor trauma [3], obstruction of pilosebaceous units with progressive keratin accumulation, and less commonly, congenital inclusion of squamous epithelium during embryologic development. These mechanisms may explain the rarity of this lesion in breast tissue and its variable clinical and imaging presentation [[4], [5], [6]].

Epidermal inclusion cysts generally appear as well-defined nodules on mammography, although ultrasonography may display diverse patterns—solid, cystic, or mixed—based on keratin composition [7]. Magnetic resonance imaging often reveals uneven signal intensity, and contrast enhancement can further complicate the separation from malignancy, particularly when diffusion restriction is present. Despite typically exhibiting a gradual and benign clinical trajectory, instances of increasing growth, inflammation, infection, or, in rare cases, malignant transformation have been documented [4].

We report the case of a young female patient with a chronic palpable breast nodule. Initial imaging studies suggested malignancy; however, histological analysis confirmed the diagnosis of an atypical breast epidermal inclusion cyst.

Case report

A 38-year-old woman was admitted for evaluation of a 3-month history of upper gastrointestinal symptoms, leading to hospitalization for investigation of progressive dysphagia. She had no prior history of breast disease, breast surgery, chest wall trauma, cosmetic breast procedures, chronic inflammatory skin conditions, recurrent cutaneous infections, or thoracic radiotherapy. Her surgical history was unremarkable, with no previous breast or thoracic interventions.

On general physical examination, a solid mass was detected in the right breast (Fig. 1). The patient reported initially noticing a small palpable nodule approximately 15 years before presentation, which had shown very slow, indolent growth and remained asymptomatic. In the months preceding admission, she perceived a more evident increase in size, prompting further evaluation. There were no associated breast pain, nipple discharge, skin changes, or systemic symptoms attributable to the breast lesion. Focused breast examination revealed a firm, mobile, nontender mass measuring approximately 6 × 5 cm located in the upper inner quadrant of the right breast. The overlying skin was normal, without erythema, ulceration, dimpling, or peau d’orange, and there was no nipple retraction or pathological discharge. No axillary or supraclavicular lymphadenopathy was identified. Gynecological and hormonal history revealed regular menstrual cycles, with no use of hormone replacement therapy or prolonged oral contraceptives. Obstetric history was unremarkable, and there was no personal or family history of breast, ovarian, or other hereditary malignancies.

Fig. 1.

Postbiopsy changes reveal the large volume previously occupied by the right breast lesion, resulting in contralateral nipple displacement.

Breast ultrasonography revealed a heterogeneous, well-defined nodule involving almost all internal quadrants of the right breast, with internal echogenic linear tracts arranged in a laminated “onion ring” pattern and absence of internal vascularity on Power Doppler imaging (Fig. 2). Mammography demonstrated a large, hyperdense, well-circumscribed nodule in the lower inner quadrant of the right breast, better delineated on tomosynthesis (Fig. 3). Postbiopsy imaging highlighted the large volume previously occupied by the lesion, with contralateral displacement of the nipple.

Fig. 2.

Breast ultrasound (Right breast): Heterogeneous echogenic nodule associated with linear echogenic tracts arranged in an “onion ring” pattern, and showing no vascularity on Power Doppler imaging.

Fig. 3.

(A) Bilateral digital mammography (Craniocaudal projection): In the lower inner quadrant of the right breast, a hyperdense nodule with well-circumscribed margins is identified (yellow arrow) and (B) Unilateral right breast tomosynthesis provides greater detail of the well-circumscribed margins (red arrow).

Breast MRI was performed on a 3.0-T system using standard breast protocols, including T1-weighted, T2-weighted, STIR, diffusion-weighted imaging (b values of 0 and 800 s/mm²), and dynamic contrast-enhanced sequences. Contrast-enhanced breast magnetic resonance imaging demonstrated an oval, well-defined mass with intermediate signal intensity on T1-weighted images and a predominantly hyperintense, heterogeneous signal on T2-weighted and STIR sequences. Diffusion-weighted imaging showed marked diffusion restriction with corresponding signal drop on the apparent diffusion coefficient map (ADC value: 0.4 × 10⁻³ mm²/s). Dynamic contrast-enhanced sequences revealed nodular enhancement with an internal ring enhancement pattern and a progressive type 1 kinetic enhancement curve (Fig. 4). Apparent diffusion coefficient values were obtained by placing a region of interest within the solid-appearing portion of the lesion on the ADC map, avoiding peripheral enhancement areas, using diffusion-weighted images acquired with b-values of 0 and 800 s/mm². Based on these findings, the lesion was classified as BI-RADS 4, and tissue sampling was recommended. Routine laboratory tests performed during hospitalization, including complete blood count and inflammatory markers, showed no abnormalities related to the breast lesion. No laboratory findings suggestive of infection or systemic inflammatory disease were identified.

Fig. 4.

Contrast-enhanced breast MRI: The nodule demonstrates intermediate signal intensity on T1-weighted images and is predominantly hyperintense on T2-weighted images, with marked restriction on diffusion-weighted imaging and corresponding signal drop on the ADC map. The dynamic study reveals nodular enhancement with an internal ring enhancement pattern, displaying a type 1 kinetic enhancement curve.

Given the lesion size and suspicious imaging features, an ultrasound-guided core needle biopsy using a 14-gauge needle was performed, obtaining multiple tissue samples. Histopathological examination demonstrated a cystic lesion lined by stratified squamous epithelium filled with laminated keratin, consistent with an epidermal inclusion cyst (Fig. 5, Fig. 6).

Fig. 5.

(A) Microscopy with H&E staining at 10x magnification shows abundant keratin lamellae as well as scattered erythrocytes, consistent with the material typically found in epidermal inclusion cysts and (B) Microscopy with H&E staining at 4x magnification reveals numerous tracts of keratin. Courtesy of Dr. Cerrillo, Anatomical Pathologist.

Fig. 6.

Biopsy specimens demonstrate the liquid content corresponding to the cystic area of the lesion and the dense reddish-brown material. Courtesy of Dr. Cerrillo, Anatomical Pathologist.

Surgical excision of the lesion was subsequently performed, confirming the diagnosis. The postoperative course was uneventful, and no recurrence was observed during the available follow-up period.

Discussion

Epidermal inclusion cyst (EIC) is a benign cystic lesion frequently encountered in the skin; however, its occurrence within the breast parenchyma is exceedingly rare. As of 2023, approximately 100 cases have been reported in the literature, yielding an estimated incidence of nearly 3 cases per year [8]. This rarity contributes to diagnostic uncertainty, particularly when the lesion is deeply located within the breast rather than presenting as a superficial cutaneous abnormality. The present case emphasizes the importance of including EIC in the differential diagnosis of well-circumscribed breast masses larger than 3 centimeters, as misinterpretation may lead to unnecessary invasive diagnostic or therapeutic procedures.

The pathogenesis of breast EIC remains incompletely understood. Proposed mechanisms include traumatic or iatrogenic implantation of epidermal elements into deeper breast tissue, obstruction of pilosebaceous units with progressive keratin accumulation, and less commonly, congenital inclusion of squamous epithelium during embryologic development. Once ectopic squamous epithelium becomes sequestered within the breast parenchyma, continuous keratin desquamation results in gradual intralesional accumulation and slow enlargement. This mechanism provides a plausible explanation for the long-standing, indolent clinical course frequently reported in breast EICs, including the fifteen-year history observed in our patient. Importantly, most published cases lack a clear history of prior breast surgery or trauma, and causal associations remain poorly defined [9].

The imaging findings in this case closely mirror those described in previous reports by Giess et al. [4] and Chung et al. [5], breast EICs typically appear as oval or spherical, well-circumscribed masses with heterogeneous internal architecture. On ultrasonography, alternating hypoechoic and hyperechoic layers may produce the characteristic “onion-ring” appearance. Doppler evaluation usually demonstrates absent or minimal internal vascularity, supporting a benign cystic process [10]. On mammography, these lesions are commonly described as well-defined, hyperdense nodules, with occasional internal calcifications attributed to keratin debris or chronic inflammation [11].

Magnetic resonance imaging plays a pivotal role in the evaluation of complex cystic breast lesions with suspicious features. Typical findings include low to intermediate signal intensity on T1-weighted images and heterogeneous, predominantly hyperintense signal on T2-weighted and short tau inversion recovery sequences, correlating with keratin-filled cystic spaces and areas of low cellularity [6,12]. In the present case, peripheral ring enhancement further supported the cystic nature of the lesion. Diffusion-weighted imaging demonstrated marked diffusion restriction with a very low apparent diffusion coefficient value. Although diffusion restriction is commonly associated with malignancy, in keratin-rich lesions such as EIC it more accurately reflects high viscosity and restricted proton mobility rather than increased cellularity, a distinction that is essential to avoid diagnostic overestimation [12]. The presence of a progressive type 1 kinetic enhancement curve further favored a benign etiology [13].

The differential diagnosis of breast EIC includes a range of well-circumscribed cystic and solid breast lesions. Fibroadenomas and phyllodes tumors may present with similar size and circumscribed margins; however, they more commonly demonstrate internal vascularity on power Doppler imaging and different internal architecture. Myxoid fibroadenomas may show T2 hyperintensity on magnetic resonance imaging, potentially overlapping with EIC, although lesion size and internal composition often aid differentiation. Of greater concern are malignant entities such as mucinous or medullary carcinoma, which may appear well defined but more typically exhibit microlobulated margins, internal vascularity, and a predominantly hypoechoic solid appearance on ultrasonography, in contrast to the heterogeneous, often predominantly hyperechoic echogenicity seen in epidermal inclusion cysts [6].

From a diagnostic standpoint, evaluation followed a stepwise clinical and radiological reasoning pathway. The long-standing history of slow growth, absence of skin changes, nipple retraction, or palpable lymphadenopathy initially suggested a nonaggressive process. Nevertheless, the lesion’s large size, heterogeneous internal architecture, peripheral enhancement, and marked diffusion restriction raised concern for malignancy, justifying BI-RADS 4 categorization and tissue sampling. Despite these suspicious features, additional imaging findings, including circumscribed margins, lack of internal vascularity, and a benign-type kinetic enhancement curve, favored a nonaggressive lesion. Definitive exclusion of malignancy required histopathological confirmation, which demonstrated a cyst lined by stratified squamous epithelium filled with laminated keratin, establishing radiology–pathology concordance.

Compared with previously reported cases, the present lesion shares typical imaging characteristics but is distinguished by its exceptionally long-standing clinical course, large size, deep intraparenchymal location, and marked diffusion restriction, all of which contributed to heightened diagnostic concern [8,9]. These features highlight a key diagnostic pitfall, whereby marked diffusion restriction and specific enhancement patterns in keratin-rich lesions may mimic malignancy and must be interpreted with caution.

Management of breast epidermal inclusion cysts depends on lesion size, symptoms, diagnostic certainty, and patient preference. While small, asymptomatic lesions with typical imaging features may be managed conservatively, surgical excision is generally recommended for large lesions, progressive enlargement, recurrent inflammation, rupture risk, cosmetic concerns, or persistent diagnostic uncertainty. Complete excision of the cyst wall is essential, as incomplete removal may predispose to recurrence, although recurrence rates after complete excision remain low [5].

In endemic regions, rare parasitic cystic lesions, such as hydatid disease, may also be present as complex cystic breast masses and should be considered in a broader differential diagnosis [14]. Emerging explainable artificial intelligence models in breast imaging may further assist in differentiating benign cystic lesions from malignancy by quantitatively analyzing texture, enhancement kinetics, and diffusion characteristics, potentially reducing diagnostic uncertainty in rare benign breast entities [15].

This case contributes to the existing literature by demonstrating how a long-standing epidermal inclusion cyst of the breast can closely mimic malignancy on multimodal imaging, particularly on magnetic resonance imaging. Awareness of this entity and careful integration of clinical history, imaging features, and pathological correlation are essential to avoid diagnostic overestimation and unnecessary aggressive interventions.

Conclusion

An epidermal inclusion cyst of the breast is an uncommon and benign condition that can closely resemble malignant results on imaging examinations. Clinical and radiological connection is essential to prevent misdiagnosis and unnecessary procedures. Consequently, this lesion must be included in the differential diagnosis concerning breast masses.

Limitations and future scope

The main limitation of this report is the inherent rarity of breast epidermal inclusion cysts, which restricts generalization and precludes standardized diagnostic or management guidelines.

CRediT authorship contribution statement

All authors contributed equally to the work. Their contributions included conceptualization, methodology design, validation, investigation, writing the original draft, review and editing. All authors have read and approved the final version of the manuscript.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the author(s) used Grammarly in order to improve readability. After using this tool, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

Patient consent

The authors declare that informed consent was obtained from the patient for the publication of this case report and any accompanying images.