Apparent Diffusion Coefficient of Subcutaneous Epidermal Cysts in the Head and Neck: Comparison With Intracranial Epidermoid Cysts 1

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1 Apparent Diffusion Coefficient of Subcutaneous Epidermal Cysts in the Head and Neck: Comparison With Intracranial Epidermoid Cysts 1 Chiori Suzuki, MD, Masayuki Maeda, MD, Akihiko Matsumine, MD, Toshio Matsubara, MD, Waro Taki, MD Stephan E. Maier, MD, PhD, Kan Takeda, MD Rationale and Objectives. Subcutaneous epidermal cysts and intracranial epidermoid cysts are pathologically identical. Although diffusion-weighted imaging (DWI) studies of intracranial epidermoid cysts have been numerously reported, those of subcutaneous epidermal cysts have not been sufficiently investigated. Our hypothesis for this study is that the apparent diffusion coefficient (ADC) values of subcutaneous epidermal cysts and intracranial epidermoid cysts are not different. This study was intended to evaluate the ADC of subcutaneous epidermal cysts of the head and neck in comparison with that of intracranial epidermoid cysts. Materials and Methods. The MR studies were performed in 14 patients with head and neck subcutaneous epidermal cysts and 10 patients with intracranial epidermoid cysts using line scan DWI (LSDWI). The ADC was measured and compared between the two types of cyst. Results. The ADC values (mean SD) were mm 2 /s in subcutaneous epidermal cysts and mm 2 /s in intracranial epidermoid cysts. A significant difference was found in ADC values between the two types (P.0019). Conclusion. Our preliminary study has shown that the ADC provides useful information regarding tissue characterization of subcutaneous epidermal cysts. However, the ADC of subcutaneous epidermal cysts was significantly lower than that of intracranial epidermoid cysts. Key Words. Cyst; epidermoid; epidermal; magnetic resonance imaging; diffusion-weighted imaging; apparent diffusion coefficient. AUR, 2007 Acad Radiol 2007; 14: From the Departments of Radiology (C.S., M.M., K.T.), Orthopedic Surgery (A.M.), and Neurosurgery (T.M., W.T.), Mie University School of Medicine, Edobashi, Tsu, Mie , Japan; and the Department of Radiology, Brigham and Women s Hospital, Boston, MA (S.E.M.). Received March 24, 2007; accepted May 15, Address correspondence to: M.M. mmaeda@clin.medic.mie-u.ac.jp AUR, 2007 doi: /j.acra Epidermal cysts are common, benign masses that occur in the skin. The lesions commonly involve the scalp, face, neck, trunk, and back (1). However, only a few reports regarding MR imaging findings have been issued on subcutaneous epidermal cysts (2 8). Typical MR imaging findings of subcutaneous epidermal cysts include a wellcircumscribed margin, iso-, or slightly high signal intensity relative to adjacent muscles on T1-weighted images, and very high signal intensity on T2-weighted images (7, 8). Usually, no apparent enhancement is visible inside the cyst if unruptured (8). Subcutaneous epidermal cysts and intracranial epidermoid cysts are pathologically identical: cysts filled with keratin debris and bounded by a wall of the stratified squamous epithelium (9). Unlike subcutaneous epidermal cysts, numerous MR imaging reports have described intracranial epidermoid cysts (10 17). Several studies of 1020

2 Academic Radiology, Vol 14, No 9, September 2007 ADC OF SUBCUTANEOUS EPIDERMAL HEAD AND NECK CYSTS diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) have had particular impact on the diagnosis of the intracranial epidermoid cysts (12 17). However, reports on DWI and ADC of subcutaneous epidermal cysts are insufficient. We hypothesize that ADC values of subcutaneous epidermal cysts and intracranial epidermoid cysts are not different because they are pathologically identical. This study was intended to evaluate the ADC of subcutaneous epidermal cysts of the head and neck in comparison with that of intracranial epidermoid cysts. MATERIALS AND METHODS Subjects We retrospectively studied 14 patients (five women and nine men; mean age, 52.1 years) with subcutaneous epidermal cysts in the head and neck and 10 patients (six women, four men; mean age, 50.6 years) with intracranial epidermoid cysts between July 2001 and April In all cases, the final diagnoses were made pathologically by surgery. Unruptured subcutaneous epidermal cysts were included in this study, but ruptured epidermal cysts were excluded because ruptured epidermal cysts contain exuberant foreign body reaction or hemorrhage inside the cyst. Lesion locations included the neck (n 5), scalp (n 5), and face (n 4) in patients with subcutaneous epidermal cysts, and cerebellopontine angle cistern (n 7), quadrigeminal cistern (n 2), and fourth ventricle (n 1) in patients with intracranial epidermoid cysts. The mean maximum diameter (mean SD) of lesions was mm in subcutaneous epidermal cysts and mm in intracranial epidermoid cysts. MR Imaging For this study, MR imaging was performed using a 1.5-T superconducting system (Signa CV/i; GE Medical Systems, Milwaukee, WI). In patients with intracranial epidermoid cysts, MR imaging was achieved using a head coil. Conventional MR imaging studies consisted of a sagittal T1-weighted sequence (repetition time [TR]/echo time [TE]/excitation, 400/14/2), axial T1-weighted (300/9/ 1), axial fast spin echo T2-weighted (TR/TE eff /excitation, 4000/100/2; echo-train length, 14) with or without fat suppression, and axial fast fluid-attenuated inversion recovery (FLAIR) (TR/TE eff /inversion time, 8000/133/2000; 1 excitation) sequences. Other parameters included a section thickness of 5 mm with a 1-mm intersection gap, a matrix, and a 20- to 22-cm field of view (FOV). Contrast-enhanced T1-weighted images were obtained for eight patients with intracranial epidermoid cysts. A three-dimensional (3D) heavily T2-weighted sequence was, in all patients, added to the conventional MR imaging for evaluation of the texture inside the cyst. The 3D heavily T2-weighted sequence was as follows: 6.3/ 1.8/2, a 24-cm FOV, a matrix, and a 1-mm section thickness without a gap. In patients with subcutaneous epidermal cysts, MR imaging was acquired using a head coil or a neurovascular array coil. An axial and/or coronal T1-weighted sequence ( /9/2) and an axial and/or coronal fast spin echo T2-weighted sequence ( /96 103/2 3, echo train length 12 16) with or without fat suppression was used with a matrix size of , an FOV of cm, and a section thickness of 3 5 mm with a 1-mm intersection gap. Contrastenhanced T1-weighted images with or without fat suppression were obtained in nine patients with subcutaneous epidermal cysts. The DWI was performed using line scan DWI (LSDWI). The LSDWI studies of patients were conducted within the guidelines of the research committees of our institution. Informed consent was obtained from patients or their authorized representatives. The LSDWI method has been described previously (18 21). Neither cardiac gating nor respiratory triggering was employed in LSDWI. No antisusceptibility devices on the neck were used to reduce susceptibility artifacts. The LSDWI images were acquired using the following scan parameters: TR ms, TE ms, one excitation, a FOV of cm, matrix size of columns, and bandwidth of khz. The effective section thickness was set to 3 5 mm with an inter-section gap of 1 mm. The LSDWI images were obtained with two different b values, with the maximum b value applied along three orthogonal directions: one with a low diffusion weighting (b factor) of 5 s/mm 2 and the other with a high (maximum) b factor of 1000 s/mm 2. The scan time per slice was s: in all, 3 5 slices were obtained according to the lesion size. Isotropic diffusion images with a b factor of 1000 s/mm 2 were generated from the three diffusion directions assessed. Trace ADC maps were generated using the equation described by Stejskal and Tanner (22), S S 0 e badc, where b is the diffusion weighting factor, S is the signal intensity of the diffusion trace for 1021

3 SUZUKI ET AL Academic Radiology, Vol 14, No 9, September 2007 Figure 1. A 52-year-old woman with intracranial epidermoid cyst in her left cerebellopontine angle cistern. T1-weighted (a) and T2-weighted (b) images show a mass that is equivalent to the signal intensity of cerebrospinal fluid. (c) 3D heavily T2-weighted image reveals that the lesion contains intervenient cerebrospinal fluid. (d) ADC map shows higher diffusion of the lesion than that of the adjacent normal brain. Note the heterogeneity of the ADC inside the lesion. The ADC of the lesion (circled area) is mm2/s. The ADCs of adjacent cerebellum and pons are 0.70 and mm2/s, respectively. (e) Photomicrograph shows keratin debris (arrow) and a wall of the stratified squamous epithelium. b maximum, and S0 is the signal intensity for b 5 s/mm2. The phantom study using water phantoms was performed to determine the variation between ADC values. The water phantoms were imaged at nine different positions within a coil using either a head coil or a neurovascular coil. The ADC values of those phantoms were compared between a head coil and a neurovascular coil. Imaging Data Analysis Signal intensities of masses were compared qualitatively with those of cerebrospinal fluid (CSF) on T1weighted and T2-weighted images. The assessment was achieved by consensus of two experienced neuroradiologists. The signal intensities of the cysts on T1-weighted images were rated as high (greater than those of CSF), low (equal to or lower than those of CSF), or mixed (high and low). The signal intensities of the cysts on T weighted images were rated as high (equal to or higher than those of CSF) or low (lower than those of CSF). The presence or absence of contrast enhancement was also evaluated. In addition, signal intensities of subcutaneous epidermal cyst were rated qualitatively relative to those of adjacent muscle on LSDWI images (b 1000 s/mm2) as high (greater than those of muscle) or low (equal to or lower than those of muscle). The ADC value measurements were obtained from the trace ADC maps using regions of interest (ROI) placed over the lesions by a neuroradiologist. The area of each ROI that was 80 mm2 or greater was included in the computation. When determining the ROI measurement of intracranial epidermoid cysts, special care was taken to avoid contamination of CSF as much as possible by referring to the corresponding 3D heavily T2-weighted images. The 3D heavily T2-weighted images showed that the signal intensity of CSF is very bright, whereas that of

4 Academic Radiology, Vol 14, No 9, September 2007 ADC OF SUBCUTANEOUS EPIDERMAL HEAD AND NECK CYSTS Figure 2. A 56-year-old man with intracranial epidermoid cyst in the fourth ventricle. T1-weighted (a) and T2-weighted (b) images show a mass that is equal to the signal intensity of cerebrospinal fluid, but the internal texture of the epidermoid cyst is slightly heterogeneous. (c) 3D heavily T2-weighted image reveals that the mass contains intervenient cerebrospinal fluid. (d) ADC map shows slightly increased diffusion of the epidermoid cyst compared with that of adjacent normal brain. Note the heterogeneity of the ADC inside the lesion. The ADC of the mass (circled area) is mm 2 /s. The ADC of the adjacent cerebellum is mm 2 /s. epidermoid cyst is low. Those findings facilitated appropriate selection of the areas with less intervening CSF for ROI measurements on ADC maps (Figs. 1 and 2). In addition, the ADC of the normal cerebellum and pons was measured in patients with intracranial epidermoid cysts. The obtained ADC values were expressed as mean SD. Statistical Analysis The Mann-Whitney U test was used to detect significant differences in ADC values between subcutaneous epidermal cysts and intracranial epidermoid cysts. Wilcoxon s signed rank test was used to detect significant differences in ADC values between intracranial epidermoid cysts and normal brain parenchyma (cerebellum and pons). A P value less than.05 was considered to indicate a statistically significant difference. RESULTS Signal Intensities and Contrast Enhancement of the Lesions The results of signal intensities of the cysts in comparison with CSF are summarized in Table 1. The signal intensities of intracranial epidermoid cysts were fundamentally similar to those of CSF both on T1-weighted and T2-weighted images (Figs. 1 and 2) although a slightly heterogeneous texture was occasionally found. In 1023

5 SUZUKI ET AL Academic Radiology, Vol 14, No 9, September 2007 Table 1 MR Imaging Findings of Subcutaneous Epidermal Cysts and Intracranial Epidermoid Cysts Imaging Findings Subcutaneous Epidermal Cyst (n 14) Intracranial Epidermoid Cyst (n 10) Signal intensity (T1-weighted) Low 0 9 High 14 0 Mixed 0 1 Signal intensity (T2-weighted) Low 3 0 High 9 10 Mixed 2 0 Contrast enhancement No enhancement 5 8 Thin and smooth rim 4 0 NA 5 2 NA, not applicable. one patient, T1-weighted images showed partly high signal areas inside the cyst. The signal intensities of subcutaneous epidermal cysts differed somewhat from those of intracranial epidermoid cysts (Table 1). On T1-weighted images, all cases showed slightly high signal intensity, usually similar to that of muscle (Figs. 3 and 4). On the other hand, signal intensities on T2-weighted images were varied, showing high signal intensity (n 9), low signal intensity (n 3), and mixed (n 2). Contrast enhancement was negative in all intracranial epidermoid cysts (n 8), whereas contrast enhancement was negative (n 5) and positive in the peripheral cyst wall (n 4) in subcutaneous epidermal cysts. The LSDWI images (b 1000 s/mm 2 ) showed that the signal intensities of the subcutaneous epidermal cyst were rated high in all cases (Fig. 4d). ADC Values The ADC (mean SD) of water phantoms was mm 2 /s using a head coil; it was mm 2 /s using a neurovascular coil. No significant difference was found between the two coils (P.698). For all patients, LSDWI provided excellent diagnostic images and permitted ADC values to be measured without significant motion artifacts or susceptibility artifacts (Figs. 1 4). The ADC was mm 2 /s in subcutaneous epidermal cysts, whereas the ADC was mm 2 /s in intracranial epidermoid cysts. A significant difference was found in ADC values between subcutaneous epidermal cysts and intracranial epidermoid cysts (P.0019). The ADC values of two patients with subcutaneous epidermal cysts were high and overlapped with those of patients with intracranial epidermoid cysts (Fig. 5). The respective ADCs of normal cerebellum and pons were mm 2 /s and mm 2 /s. A significant difference was found in the ADC between intracranial epidermoid cysts and normal brain (cerebellum and pons) (P.0051). DISCUSSION Subcutaneous epidermal cysts result from the proliferation of epidermal cells within circumscribed dermal spaces. They are cystic lesions lined with keratin-producing squamous epithelium filled with keratin debris. Several theories have been forwarded to explain their etiology, including remnant ectodermal tissues misplaced during embryogenesis (23), occlusion of the pilosebaceous unit, or traumatic or surgical implantation of epithelial elements (6, 24). Although epidermal cysts are common, particularly in subcutaneous masses of the head and neck, application of MR imaging is limited in a clinical setting. Therefore, only a few MR imaging reports have been issued on this type of lesion (2 8). Typical MR imaging findings of subcutaneous epidermal cysts include a well-circumscribed margin, iso- or slightly high signal intensity adjacent to the muscles on T1-weighted images and markedly high signal intensity on T2-weighted images, resembling a fluid-like signal (7, 8). However, variable signal intensities have also been reported (8). In our study, most cases showed slightly higher signal intensities than those of CSF on T1- weighted images, whereas variable signal intensity patterns were shown on T2-weighted images. These conventional MR imaging findings differ from those of intracranial epidermoid cysts, which usually exhibit similar signal intensity to CSF both on T1-weighted and T2-weighted images. We cannot sufficiently explain the reasons, but the CSF interstices in intracranial epidermoid cysts might be partially causative of the difference of the MR imaging findings between the two lesions. In intracranial epidermoid cysts, CSF interstices were mostly found on 3D heavily T2-weighted images in the current study. They might influence the gross signal intensity of the tumor on 1024

6 Academic Radiology, Vol 14, No 9, September 2007 ADC OF SUBCUTANEOUS EPIDERMAL HEAD AND NECK CYSTS Figure 3. A 58-year-old woman with subcutaneous epidermal cyst in her right cheek. (a) T1-weighted image shows that a subcutaneous epidermal cyst (arrow) in her right cheek is slightly higher in signal intensity than cerebrospinal fluid. Note a parotid tumor (subsequently confirmed as pleomorphic adenoma) shown on the right. (b) T2-weighted image shows that the signal intensity of the mass is equivalent to that of cerebrospinal fluid. (c) Contrast-enhanced T1-weighted image shows no enhancement of the subcutaneous epidermal cyst. However, the parotid tumor enhances strongly. (d) The ADC map shows lower diffusion of the subcutaneous epidermal cyst than that of the parotid mass. The ADC of subcutaneous epidermal cyst is mm2/s, whereas that of parotid tumor is mm2/s. (e) Photomicrograph shows keratin debris and a wall of the stratified squamous epithelium. T1-weighted and T2-weighted images. No enhancement or solely peripheral rim enhancement was observed in cases with unruptured subcutaneous epidermal cyst. Those MR imaging findings might be characteristic and useful for the suggestion of subcutaneous epidermal cysts, even though some other subcutaneous cystic masses such as cystic neurinoma might occasionally mimic epidermal cysts. The use of DWI allows water molecules to be observed moving within tissue structures. This technique has been regarded as useful for detection and differentiation of intracranial epidermoid cysts because conventional MR imaging sequences, such as T1-weighted and T2-weighted imaging, usually exhibit similar signal intensity of epidermoid cyst to that of CSF and arachnoid cyst (12, 13). In our study, the mean ADC of intracranial epidermoid cysts was mm2/s; this value was close to that reported previously using echo-planar DWI (EPDWI) (15, 17). The ADC of intracranial epidermoid cysts was significantly higher than that of the adjacent normal brain (cerebellum and pons), implying more active diffusion in the intracranial epidermoid cysts than in the adjacent normal brain parenchyma. This result was also in accord with those of previous studies (15, 17). We employed 3D heavily T2-weighted imaging to investigate the internal texture of the intracranial epidermoid cysts. This method provides high-resolution images with good contrast between CSF and solid structures (25). In our study, intervening CSF within the cyst was clearly depicted as a bright signal on the 3D heavily T2-weighted images. Therefore, we referred to the corresponding 3D heavily T2-weighted images in the measurement of the ADC of epidermoid cysts, avoiding CSF contamination as much as possible. Nevertheless, we assume that complete avoidance of CSF was difficult because CSF interstices extended deeply into the cysts. The ADC of subcutaneous 1025

7 SUZUKI ET AL Academic Radiology, Vol 14, No 9, September 2007 Figure 4. A 65-year-old man with subcutaneous epidermal cyst in his neck. (a) T1-weighted image shows that the signal intensity of the subcutaneous epidermal cyst (arrow) at the neck is almost equal to that of adjacent muscle. (b) T2-weighted image shows that the signal intensity of the mass is markedly high. (c) Contrast-enhanced T1-weighted image shows no enhancement of the epidermal cyst. (d) LSDWI image (b 1000 s/mm 2 ) shows a bright signal of the epidermal cyst compared with that of adjacent muscle. (e) On ADC map, the ADC of the epidermal cyst is mm 2 /s. Figure 5. Box plot of the ADC in subcutaneous epidermal cysts and intracranial epidermoid cysts. The ADC of subcutaneous epidermal cysts is significantly lower than that of intracranial epidermoid cysts (P.0019). epidermal cyst was significantly lower than that of intracranial epidermoid cyst in the current study. Higher ADC of intracranial epidermoid cysts might be explainable for intervening CSF of the lesions. Other explanations might exist for the observed difference in ADC values between subcutaneous epidermal cysts and intracranial epidermoid cysts. The two cysts are histologically identical if unruptured. On the other hand, trauma, at least minor, can be expected with the subcutaneous epidermal cyst but not with the intracranial epidermoid cyst. The trauma might engender hemorrhage and/or inflammatory reactions within the cyst, resulting in diffusion changes. Those histological findings are observed occasionally in ruptured subcutaneous epidermal cysts (6, 8). However, in the current study, we excluded ruptured epidermal cyst cases based on the histological analysis. We used the LSDWI technique because the technique is inherently insensitive to susceptibility artifacts (18 21). The major limitation of the EPDWI outside the brain is that EPDWI images occasionally suffer from considerable susceptibility artifacts in the head and neck because of the presence of dental work as well as adjacent air and bone. Therefore, it might be 1026

8 Academic Radiology, Vol 14, No 9, September 2007 ADC OF SUBCUTANEOUS EPIDERMAL HEAD AND NECK CYSTS difficult to obtain excellent DWI images and precise ADC measurements of lesions. For that reason, it appears necessary to use DWI techniques such as LSDWI, which is insensitive to susceptibility artifacts, for the evaluation of head and neck lesions. Moreover, we consider that the LSDWI is suitable for application to intracranial epidermoid cyst because this lesion is frequently located at the cerebellopontine angle cistern, where susceptibility artifacts might be considerable because of the adjacent air and bones. Subcutaneous epidermal cysts are usually very small and are usually diagnosed clinically without imaging. Rarely, they grow sufficiently large to necessitate additional workup (7). In addition, large or medium epidermal cysts might be located at unusual sites close to specific organs, such as the parotid gland, mimicking tumors originating from that tissue. In that situation, dermatologists or head and neck surgeons might infer the possibility of some other tumor rather than an epidermal cyst. Occasionally, MR imaging might be used to investigate such lesions. There are several reports of giant epidermal cysts or those at unusual sites confirmed at surgery that were not clinically suspected as epidermal cysts, but which were retrospectively implicated as such by MR imaging findings (4, 6 8, 26). Thus, it should be of clinical value to characterize the MR imaging findings of subcutaneous epidermal cysts including the DWI features as well as conventional MR imaging findings. A major limitation of our study is that we used different coils such as a head coil for intracranial epidermal cyst and a neurovascular coil for neck epidermal cyst. The water phantom study demonstrated that the variation of the ADC values was very small and that the ADC was not significantly different between the two coils. Therefore, we consider that the difference of coils did not significantly contribute to the ADC results between the two cysts. Another limitation is the small number of patients studied. Investigations with more numerous cases with intracranial epidermoid cyst and subcutaneous epidermal cyst should be continued in future studies. CONCLUSION Our preliminary study has shown that the ADC provides useful information regarding tissue characterization of subcutaneous epidermal cysts. However, the ADC of subcutaneous epidermal cysts was significantly lower than that of intracranial epidermoid cysts. REFERENCES 1. Vincent LM, Parker LA, Mittelstaedt CA. Sonographic appearance of an epidermal inclusion cyst. J Ultrasound Med 1985; 4: Sundaram M, McGuire MH, Herbold DR, Beshany SE, Fletcher JW. High signal intensity soft tissue masses on T1-weighted pulsing sequences. Skeletal Radiol 1987; 16: Krandorf MJ, Jelinek JS, Moser RP, et al. Soft tissue masses: diagnosis using MR imaging. 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