Characteristic MRI Findings of Epidermal Cysts Categorized by Size

Materials and Methods: Unruptured epidermal cyst cases were analyzed according to 3 cyst size categories: large (≥4 cm), medium (≥2 to <4 cm), and small (<2 cm). MRI assessments focused on the keratinized material using low signal intensity on T2-weighted images because of intracystic diffuse foci (a sign of intracyst debris) and serpentine structures at the periphery (a sign of a peripheral lining) of cysts.


INTRODUCTION
Epidermal cysts, or epidermoid cysts, are common, benign, intradermal, or subcutaneous lesions [1,2].They are lined with stratified squamous epithelium and filled with keratinous debris originating from the squamous epithelium [1,2].Cyst pathogenesis has been proposed to be the result of a migration of epidermal cells into the dermis, aberrant embryogenesis during cellular differentiation, transplantation of epidermal cells into the dermis following trauma, or inflammation of pilosebaceous structures causing a cystic reaction [2].
Epidermal cysts are frequently asymptomatic [3] and grow via the accumulation of epithelial and keratinous debris [1,4].Surgical excision of the lesion is a frequent curative treatment [5].The size of most epidermal cysts ranges from 1 to 4 cm [3].Rarely do these cysts exceed 5 cm in diameter, but when they do they are referred to as giant epidermal cysts [1, 6 -8].Malignant transformation of epidermal cysts has been observed, although the reported frequency of a squamous cell carcinoma arising from epidermal cysts is low, ranging from 0.011 to 0.045% [9,10].

Fig. (1).
A 40-year-old male with a large-size epidermal cyst in his buttock.The MRI demonstrates a well-circumscribed, ovalshaped lesion with hyperintense signal relative to the muscle on a T1-weighted image (A) and a hypointense signal relative to adipose tissue on a T2-weighted image (B).Keratinized material with a low signal intensity on T1-and T2-weighted images is observed as intracystic diffuse foci (a sign of intracyst debris: yellow arrow) and at the periphery close to the wall (sign of a peripheral lining: white arrow) (A and B).

Fig. (2).
A 54-year-old male with a medium size epidermal cyst at the knee.The MRI displays a well-defined oval-shaped lesion with a rather monotonous signal that was hyperintense relative to muscle on a T1-weighted image (A) and high signal intensity on a T2-weighted image without (B) and with (C) fat suppression.Keratinized material, comprised of intracyst debris or a peripheral lining, is not apparent on the T1-and T2-weighted MRIs.

Fig. (3).
A 12-year-old male with a small size epidermal cyst in the finger.The MRI depicts a well-defined oval-shaped lesion with slightly high signal intensity on a T1-weighted image (A) and low signal intensity on a T2-weighted image (B).No intracyst debris is apparent, although a peripheral lining (yellow arrow) can be observed at the wall (B).
Magnetic Resonance Imaging (MRI) can reveal an epidermal cyst as a well-demarcated oval-shaped mass with a cystic wall [11].On T1-weighted images, epidermal cysts are reported to demonstrate isointense to slightly hyperintense signals, and on T2-weighted images the signals are of intermediate to high signal intensity [11].The variable amount of keratinous debris, that is, lower signal foci or serpentine structures, on T2-weighted images has been reported to be 43% [11] and 58% [5]  The purpose of this study was to analyze the characteristic MRI findings of epidermal cysts categorized by size in the current cases.

MATERIALS AND METHODS
Surgically resected epidermal cysts were evaluated in our analysis.Diagnosis of epidermal cysts was histopathologically confirmed in all resected cases.Epidermal cysts with a history of rupture were excluded from this study.Using MRIs, cysts were evaluated by size and assessed using low signal intensity on T2-weighted images for keratinous materials which included intracystic diffuse foci and serpentine structures at the periphery, or signs of "intracyst debris" and "peripheral lining" respectively.Lesions were divided into 3 size categories as follows: large (≥4 cm), medium (≥2 to <4 cm), and small (<2 cm).
There were 8 cases of small-sized cysts in patients that included 7 males and 1 female of mean age 48.0 ± 22.7 (range 12 to 87) years.Mean cyst size was 1.1 ± 0.42 (range 0.6 to 1.5) cm.Cyst locations were in the finger (5 cases), as well as back, buttock, and thigh (1 case each).Among these 8 cases, on T1-weighted images the signal for cysts was hyperintense relative to muscle (100%).On T2-weighted images, the signal was hyperintense relative to muscle, but lower than adipose tissue (4 cases, 50%), isointense relative to adipose tissue and hyperintense relative to adipose tissue (each had 2 cases, 25%).Intracyst debris was not observed in any of these 8 cases, but a peripheral lining was detected (4 cases, 50%).

DISCUSSION
In the current cases, characteristic MRI findings of epidermal cysts by size category were analyzed.MRIs showed the epidermal cysts as well-demarcated and oval-shaped lesions in all cases.The signal intensity of the cystic wall shows low signal intensity on T1-and T2-weighted images [8], which is consistent with what was observed among all our cases.The signal intensity for the content of the cyst was intermediate to slightly increased on T1-weighted images and of high intensity on T2 -weighted images [5,12].On T1-weighted images, a hyperintense signal relative to muscle was a characteristic in 85% of the cases in the current series.For the same 9 cases reported previously, T2-weighted images of the lesions were of high and intermediate signal intensity 67% and 22% of the time, respectively [11].In our series, on T2-weighted images, 61% of the cases displayed hyperintense signals relative to muscle, but hypointense signals relative to adipose tissue, such that our results appear consistent with the previous report.Overall, the variation in signal intensity was the same among the 3 different size categories for cysts even though they had different anatomical distributions; small-sized lesions were located peripherally in the fingers whereas larger-sized cysts were located more centrally in the torso, in the back or buttock for example Fig. (3).
Epidermal cysts are filled with keratinous material.The foci of keratinous debris with low signal intensity on T2weighted images have been reported in 43% [11] and 58% of cases [5].Heterogeneity due to the foci of keratinous debris is reported to be useful in differentiating epidermal inclusion cysts from other fluid-filled masses such ganglion cysts with homogeneous signal intensities [5,12].In our case series overall, intracyst debris was observed in 45% cases.However, in cases involving cysts categorized as large, signs of intracyst debris were present in all cases.Intracyst debris reflects the pathology of epidermal cysts and represents a useful sign to observe, not only for the diagnosis of an epidermal cyst, but it would also provide information that assists in the differentiation of this type of cyst from one that is mimicking a malignancy.Furthermore, when cyst size is large enough for sampling with a needle, we propose that cytological confirmation of keratinous material would be helpful not only in making a diagnosis, but also in excluding malignancy.
The keratinous content in epidermal cyst material originates from the cyst wall and is often in a laminated arrangement.In our case series, the peripheral lining structure of the cyst has been emphasized.In a previous study, the peripheral lining structure with its low signal intensity on T2-weighted images was observed in 4 out of 9 cases (44%) [11].In our cases in this study, a peripheral lining was observed 76% of the time.A peripheral lining was clearly evident at a high positive rate in patients with cysts in the large size category, while only half of the cases with small Fig. (1 and 2).

Table 3 . MRI signs on epidermal cysts according to size category.
Intracyst foci sign of intracyst debris, Peripheral lining sign of a peripheral lining