J Med Ultrasonics (2009) 36:91–97 DOI 10.1007/s10396-009-0215-5

© The Japan Society of Ultrasonics in Medicine 2009

ORIGINAL ARTICLE In Yong WHANG · Hang Joo CHO · Su Lim LEE Na Young JUNG · Kyung Ah CHUN · Ki Tae KIM

Epidermoid cyst appearing as a malignancy-mimicking subcutaneous lesion on ultrasonography

Received: July 16, 2008 / Accepted: February 3, 2009

Abstract Purpose. It is challenging to diagnose epidermoid cysts on ultrasonography; except in typical, benign-appearing cases. The purpose of this study was to include epidermoid cysts in the differential diagnosis of diverse subcutaneous lesions, especially malignancy-mimicking lesions, as seen on ultrasonography. Methods. We reviewed 19 cases of pathologically confirmed epidermoid cysts in 19 patients (male, 8; female, 11). Three radiologists, who were blinded to the pathology data, classified (by consensus) these epidermoid cysts as benign or malignancy-mimicking lesions, according to generally accepted ultrasonographic criteria, including the margin, shape, echotexture, and transitional zone with surrounding tissue, and also including the growth pattern and adjacent tissue change. The ultrasonographic data were then correlated with the pathology results regarding the ruptured or unruptured status of the cysts. Results. Epidermoid cysts have been noted as showing a wide-spectrum of findings on ultrasonography. Twelve of our cases showed benign ultrasonographic features: six cases had typical, benign ultrasonographic features with unruptured status; two cases with ruptured status did not have clear ultrasonographic features, although we decided by consensus that there were benign ultrasonographic features; and four cases with unruptured status had peculiar internal echogenicities, described as “internal rod-like contents”, that could be considered to be a variation of the typical ultrasonographic finding of epidermoid cysts. Seven

I.Y. Whang (*) · S.L. Lee · K.A. Chun · K.T. Kim Department of Radiology, Catholic University College of Medicine, Uijongbu St. Mary’s Hospital, CPO Box, Kyunggido 480-717, Korea Tel +82-31-820-3138; Fax +82-31-846-3080 e-mail: [email protected] H.J. Cho Department of Surgery, Catholic University of Korea, College of Medicine, Uijongbu St. Mary’s Hospital, Kyunggido, Korea N.Y. Jung Department of Radiology, Catholic University of Korea, College of Medicine, Holy Family Hospital, Kyunggido, Korea

cases showed malignancy-mimicking ultrasonographic features; all seven of these had ruptured status. Conclusion. The diagnosis of ruptured epidermoid cysts should be included in the differential diagnosis of malignancy-mimicking subcutaneous lesions. The internal rod-like contents can be regarded as another typical ultrasonographic finding of epidermoid cysts. Keywords epidermoid cysts · subcutaneous tumor · ultrasonography

Introduction There are several types of cysts of epidermal origin, including epidermoid cysts, trichilemmal cysts, mila, steatocytoma multiplex, dermoid cysts, branchial cysts, and preauricular cysts. Of these types, the most common cysts detected on physical examination are epidermoid cysts, and clinicians often use terms such as keratin cysts, sebaceous cysts, epidermal (inclusion) cysts, and epithelial cysts to refer to them.1 Vincent et al.2 and Denison et al.3 have indicated that an epidermoid cyst may be interpreted as a solid mass and even as a malignant tumor because its internal echogenicity varies according to its composition. Beginning with this proposition, we reviewed our ultrasonography results in 19 patients with a pathologically confirmed epidermoid cyst.

Patients, materials, and methods Our institution’s Ethics Committee did not require patients’ informed consent for this retrospective study. Between March 2006 and September 2007, we scanned 19 patients (8 male; 11 female) who had been referred to the Ultrasound Division of the Department of Radiology for the characterization and differential diagnosis of subcutaneous nodules in the whole body and nodules in the breast. We used a broad-bandwidth 14-MHz linear scan

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head (L12-5, HDI 5000 [Philips, Bothell, WA, USA] and 15L8w, Acouson Sequoia 512 [Siemens, Mountain View, CA, USA]). All lesions were first scanned using B-mode imaging and they were then scanned with color Doppler imaging. The pathology results were obtained by surgical excision of the skin lesions. Surgical excision of palpable lesions was performed if patients complained or were concerned about their skin nodules for cosmetic reasons (10 cases) or were anxious about the possible malignant transformation of their lesions (2 cases). Surgical excision was performed in the other patients because the radiologist suspected a malignancy (7 cases). Following surgical excision, we retrospectively compared the pathology results with the initial ultrasonographic images. We classified the initial ultrasonographic images as benign or malignant, because the purpose of this study was to include the ultrasonographic diagnosis of benign epidermoid cyst in the differential diagnosis of diverse subcutaneous lesions, especially malignancy-mimicking lesions. Before evaluating the ultrasonography of epidermoid cysts, we selected the characteristic ultrasonographic findings that general radiologists agree on as indicating benign status, i.e., circumscribed margin, ovoid or elongated shape, homogeneous internal echotexture, and a clear or narrow transitional zone between the lesion and the surrounding tissue. Findings opposite to the above-mentioned findings, i.e., uncircumscribed margin; irregular shape; inhomogeneous internal echotexture; and a wide transitional zone, such as perilesional increased echogenicity; were considered to be malignant findings. In evaluating breast lesions, the ultrasonographic criteria we used differed from those used for all other body parts. For the breast lesions, we used the American College of Radiology Breast imaging reporting and data system (ACR BI-RADS)4 categorization, as almost all breast radiologists worldwide use this system for evaluating breast images, including ultrasonographic evaluation. We considered category 4 as a point of reference; category 4 or 5 lesions are considered as possible malignancies requiring biopsy. Therefore, we used category 4 findings as malignant criteria for breast evaluation. Two experienced radiologists (I.Y.W. and S.L.L) reviewed all the available ultrasonography images. In cases where a consensus could not be reached on the interpretations by these two radiologists, the lesions were classified as not correctly matched, and three radiologists, including another experienced radiologist (N.Y.J.), made the final decision by consensus, based on their ultrasonographic experience with the musculoskeletal and breast regions. All of the ultrasonography images were stored in our hospital’s Picture Archiving and Communication Systems, and we reviewed the images on dedicated monitors. The pathognomonic histopathology finding of epidermoid cysts indicates that the lesion is lined by epidermaltype epithelium and is filled with keratin material. The only difference among the numerous types of epidermoid cysts is the presence of inflammatory infiltrate cells or a possible foreign-body reaction in the adjacent tissue, which could suggest the diagnosis of ruptured cyst. The pathology reports of the excised epidermoid cyst specimens were

retrospectively reviewed from this point of view. We then compared the ultrasonographic features indicating the possibility of malignancy and the pathology report regarding the presence of rupture.

Results Our study population consisted of 8 male (mean age, 47.5 years) and 11 female (mean age, 37 years) patients. Each patient had one lesion, and the scanned region encompassed the subcutaneous layers of the back (n = 2), flank (n = 1), buttock (n = 6), suprapubic region (n = 1), neck (n = 2), occipital region (n = 1), axilla (n = 1), and breast (n = 5). The ultrasonography and pathology results are shown in Table 1. As seen on ultrasonography, in 12 of the 19 patients the cases had benign features. In these 12 cases, the two radiologists reached a decision regarding benign status (without interpretation differences) in only 6 cases with the typical ultrasonographic appearance of epidermoid cyst (see example in Fig. 1). However, in the remaining 6 cases, the radiologists could not agree on which cases matched their criteria and therefore classified the lesions as not correctly matched. Findings of loculated fluid with perilesional skin thickening (Fig. 2), cord-like structure (Fig. 3), and internal rod-like contents (Fig. 4) were shown in these 6 cases. By consensus of the three radiologists, these types of lesions were also considered to be benign. The other 7 cases exhibited malignant ultrasonographic features (see examples in Fig. 5). In the pathology report, nine cases were identified as having ruptured epidermoid cysts. Of these nine cases, seven showed malignancy-mimicking ultrasonographic features and two showed benign features. In one of these two cases with benign features, we first considered the possibility of abscess formation in the subcutaneous layer, because

Fig. 1. Ultrasonographic appearance of an uncomplicated epidermoid cyst detected in the breast of a 25-year-old woman. This well-defined, ovoid lesion lies partially within the skin. We could easily have made the diagnosis of a cyst of epidermal origin; however, when this cyst was optimally scanned using an acoustic standoff, it was more easily seen that the lesion was located in the skin and that a dilated gland neck would have been found if the lesion were an epidermoid cyst

M/63 F/30 F/38 M/57 F/49 F/35 M/45 M/38 M/55 M/68 F/34 M/33 F/31 M/44 F/25 F/22 F/50 F/38 F/44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Rt. flank Suprapubic Rt. buttock Back Post. neck Rt. occipital Back Rt. buttock Rt. buttock Post. neck Rt. axilla Rt. buttock Lt. buttock Lt. buttock Breast Breast Breast Breast Breast

Location

Unruptured Unruptured Ruptured Ruptured Unruptured Ruptured Unruptured Unruptured Unruptured Ruptured Unruptured Ruptured Unruptured Unruptured Unruptured Ruptured Ruptured Ruptured Ruptured

Pathology status

Benign (Fig. 4B) Benign (Fig. 4A) Malignant Benign (Fig. 2) Benign Malignant Benign Benign (Fig. 4C) Benign (Fig. 4D) Malignant (Fig. 5B) Benign Benign (Fig. 3) Benign Benign Benign (Fig. 1) Malignant Malignant Malignant Malignant (Fig. 5A)

Overall feature Ovoid Ovoid Irregular Irregular Ovoid Irregular Ovoid Ovoid Ovoid Irregular Ovoid . Ovoid Ovoid Ovoid Irregular Round Ovoid Irregular

Shape

a

NC, not circumscribed; PST, perilesional skin thickening; CLS, cord-like structure ACR BI-RADS4 category

Sex/Age (years)

Case no.

Table 1. Ultrasonography and pathology results

Circumscribed Circumscribed NC NC Circumscribed NC Circumscribed Circumscribed Circumscribed NC Circumscribed NC Circumscribed Circumscribed Circumscribed NC Circumscribed Circumscribed NC

Margin Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Homogeneous Inhomogeneous Homogeneous Inhomogeneous Inhomogeneous Inhomogeneous Homogeneous Inhomogeneous Homogeneous Homogeneous Homogeneous Homogeneous Inhomogeneous Inhomogeneous Inhomogeneous

Internal echotexture

US finding

Clear Clear Wide . Clear Wide Clear Clear Clear Wide Clear . Clear Clear Clear Wide Clear Clear .

Transitional zone

+ + − − − − − + + − − − − − − − − − −

Rod-like contents

3a 4a 4a 4a 4a

CLS

PST

Others

5.0 × 2.5 4.8 × 3.0 4.0 × 4.2 3.0 × 0.8 0.6 × 0.4 3.8 × 3.0 0.5 × 0.5 5.0 × 3.5 4.6 × 2.3 3.0 × 1.8 0.6 × 0.4 5.0 × 0.8 0.5 × 0.4 0.6 × 0.5 0.6 × 0.3 2.2 × 1.5 1.5 × 1.5 2.2 × 1.7 2.0 × 1.6

Size (cm)

93

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of the thickening of the overlying skin and the pure, cystlike internal echotexture of the lesion with its ill-defined margin (Fig. 2). The other case had cord-like or serpentineshaped, low echogenicity in the subcutaneous layer (Fig. 3).

Fig. 2. A right paramedian-back skin lesion in a 57-year-old man. Ultrasonography revealed fluid echogenicity with perilesional skin thickening. We first considered the possibility of abscess formation in the subcutaneous layer. Pathology slides showed a completely ruptured epidermoid cyst with abscess formation

Fig. 4A–D. Peculiar findings of “internal rod-like contents”. A Epidermoid cysts in the suprapubic skin of a 30-year-old woman. This case illustrates that epidermoid cysts can have multiple internal rod-like contents that can be detected as a peculiar ultrasonographic finding. This peculiar echo undoubtedly indicates fragments of detached keratin layers produced in the cyst after layering of the keratin. The keratin layering or onion-like echogenicity of epidermoid cysts has been verified on pathology slides in several studies; however, microscopic verification of this rod-like echogenicity was impossible due to the degradation of the structure during the slideprocessing steps. We selected the finding of multiple rod-like internal contents as a benign criterion, as this pattern is not seen in malignancies. B, C, D Findings similar to those in A are shown in epidermoid cysts in the flank region of a 63-yearold man (B), the perianal buttock of a 38-year-old man (C), and the buttock of a 55year-old man (D)

Although both of these cases were first included among the not-correctly-matched lesions, they were ultimately included in the benign ultrasonography cases, as mentioned above. All of the ten cases whose pathology reports revealed unruptured epidermoid cysts showed benign features on

Fig. 3. The right buttock of a 33-year-old man. Ultrasonography revealed a cord-like, low-echogenicity structure in the subcutaneous layer. Pathology slides showed a completely ruptured epidermoid cyst

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A

B Fig. 5A, B. Malignant-mimicking ruptured epidermoid cysts. A Breast of a 44-year-old woman. As this lesion had a heterogeneous internal echotexture with perilesional increased echogenicity, we assigned it a ACR BI-RADS category of 4. B Posterior neck of a 68-year-old man. This lesion had uncircumscribed margins with perilesional increased echogenicity, irregular shape, and heterogeneous internal echotexture

ultrasonography; six cases had typical epidermoid cyst ultrasonography and four cases had internal rod-like contents.

Discussion Epidermoid cysts are keratin-filled, epithelial-lined cysts. They can occur anywhere in the body, but most commonly occur in areas with hair, such as the scalp, face, neck, and trunk. Fewer than 10% of epidermoid cysts occur in the lower extremities, while even lower numbers occur in the palms of the hand, soles of the feet, and breasts. On physical examination, an epidermoid cyst typically appears as a smooth-surfaced, small, round mass adhering to the skin; these cysts have mobility compared to the deeper tissue and may have a small, black hole in the dome

of the lesion. In general, one or several epidermoid cysts may be seen; in Gardner syndrome, multiple cysts may be seen, especially on the scalp and back. Epidermoid cysts are most commonly the result of plugged pilosebaceous units. Epidermoid cysts may be congenital or they may occur following trauma or surgery.3,5,6 Residual epidermal components in sutured skin sites or in infolded dermoglandular structures can also cause epidermoid cysts. Diverse complications can result from the presence of epidermoid cysts, including spontaneous rupture and their development into squamous cell cancer.7,8 In the case of spontaneous rupture, keratin not absorbed from the cyst functions as an irritant and causes secondary foreign-bodytype reactions, granulomatous reactions, or abscess formation.9 Squamous cell cancer originating from epidermoid cysts has been reported by several authors, with the incidence varying widely, from 0.5% to 19%.10 However, the number of case reports is too small to allow accurate determination of the incidence. Squamous cell cancers arising from epidermoid cysts grow rapidly and are accompanied by pain. Asymptomatic lesions do not require treatment, and if there are typical ultrasonographic and physical examination findings, biopsy seems unnecessary.1 However, in order to prevent inflammatory and malignant changes, surgical intervention may be appropriate, and some authors recommend surgical excision in order to avoid possible malignant change.11 Several reports of the ultrasonographic findings of epidermoid cyst have depicted this lesion as a solid mass, a well-defined mass, or a combined-form mass.3,5,12,13 However, when we performed ultrasonography to assess patients’ skin nodules, only a few cases of typical lesions confined to the skin layer were suspected of being epidermoid cysts on initial ultrasonography. Many other cysts, including lesions that should be ruled out for malignancy on ultrasonography, were ultimately determined to be benign epidermoid cysts only after surgical excision. We were initially unsure why this simple subcutaneous disease entity shows varying ultrasonographic findings, thereby making us hesitant to suggest the possible diagnosis of subcutaneous epidermoid cyst. Based on the assumption that ultrasonography findings reflect the anatomy or histopathology of a scanned region, we first analyzed the histopathology of the excised specimens of epidermoid cysts. The histopathologic differences in the excised epidermoid cysts included only the presence of inflammatory cells around the cyst, thus indicating whether or not the cyst had been ruptured. Therefore, we proposed that the ultrasonographic differences were caused by the status of the epidermoid cysts. We focused on this point and planned to compare the ultrasonographic features with the presence of rupture of the cyst. Though we had an insufficient number of cases to be able to arrive at any definite conclusions, we found that when ultrasonography showed benign features (12 of the 19 cases), most of these lesions (10/12) had an intact cyst wall without any surrounding tissue abnormality (Fig. 6). In all

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Fig. 6. Photomicrography of an epidermoid cyst with benign ultrasonographic features shows a large cyst filled with keratin (arrows) and surrounded by stratified squamous epithelium; these features are pathognomonic for an epidermoid cyst. There is no inflammatory infiltrate in the subepithelial stroma. H & E, ×40

A

B

Fig. 7A, B. Photomicrography of a ruptured epidermoid cyst with malignant ultrasonographic features shows the epithelial lining filled with keratin (A); the box in (A) reveals inflammatory cells or a foreignbody reaction (B). A H & E, ×40; B H & E, ×100

7 of the cases of malignant-mimicking ultrasonographic features, a ruptured cyst wall was shown, with a surrounding inflammatory reaction (Fig. 7). Of our 12 cases with benign ultrasonographic features, 6 were classified as not-correctly-matched lesions. However, these lesions were also considered to be benign according to consensus by the three radiologists. The reasons they were considered benign, according to the ultrasonographic findings of loculated fluid with perilesional skin thickening and cord-like structure, were that the adjacent tissue showed inflammatory changes, and a malignancy usually does not tend to exhibit these growth patterns. The reason that the four cases with ultrasonographic findings of internal rod-like contents were considered benign was because a well-circumscribed lesion or multiple rod-like echogenicity

distinctly differs from the generally accepted characteristics of malignant features, e.g., non-circumscribed margin or irregular internal echotexture without specific patterns. During our retrospective study and our literature review, we found two specific ultrasonographic findings that may be used with confidence for diagnosing epidermoid cysts. In addition to the classical finding of epidermoid cysts, i.e., a well-circumscribed, skin-layer nodule, a mass with multiple keratin layers has been identified by several authors as a specific ultrasonographic finding.14–16 We also identified another peculiar finding; namely, multiple “internal rodlike contents”, in four of our cases of cysts (Fig. 4). Epidermoid cysts continuously produce keratin layers that, when they detach from the mass, can have an onion-skin-like echogenic multilayered appearance; when these multilayers are composed of less organized keratin, they are connected to the inner cyst wall and appear as keratin floating in the cyst. The internal rod-like contents could consist of these floating keratin materials, which seem to be another typical finding of epidermoid cysts. In a study by Lee et al.,17 we found their 3 cases of linear or branching ultrasonographic structures in their total of 24 epidermoid cysts to be very similar to our “rod-like” echogenicities. They also interpreted these echogenicities as fragmentation of keratin layers. Therefore, we conclude that the internal rod-like contents are a common ultrasonographic finding of epidermoid cysts, as a variant of multiple keratin layers. In our study, only 6 of 19 cases showed the classical findings of typical epidermoid cysts. Therefore, the number of correct diagnoses based on ultrasonography would be too small to be significant. However, if the finding of “internal rod-like contents” was also included, it would distinctly raise the possibility of reaching the correct diagnosis of epidermoid cyst on the initial ultrasonography. We did not take into consideration the clinical data at the image interpretation step. The usually associated clinical findings are frequent recurrence of a lesion even after completely subsiding as a result of medical treatment, pus discharge over the skin, or induration of the overlying skin with heating sensation. If we had been aware of the clinical data plus the diverse ultrasonography findings, the suggestion of benign disease based on ultrasonography would have been more obvious. The differential ultrasonographic diagnosis of epidermoid cysts includes synovial lesions, fibromas, xanthomas, lipomas, sacrococcygeal teratomas, and other soft-tissue masses or complicated cystic masses.2,9,18 In summary, the most trivial and most common types of epidermal-origin cysts or epidermoid cysts can be noted as showing a wide spectrum of findings on ultrasonography, i.e., ranging from a well-defined, benign nodule to a lesion mimicking a malignancy. Ultrasonographic findings such as a mass with internal rod-like contents, suggestive of detached keratin material within a cyst, could be another specific finding other than the multilayered appearance. In the ultrasonographic findings of a subcutaneous malignancy-mimicking lesion, clinicians should include ruptured epidermoid cysts in the differential diagnosis.

97 Acknowledgments We would like to thank Bonnie Hami, MA (USA), for her editorial assistance in the preparation of the manuscript.

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