Epidermoid cyst is a rare benign lesion of the testis, comprising approximately 1-2% of all testicular masses. It is a cystic structure lined by keratinized squamous epithelium without dermal appendages (hair follicles, sebaceous glands) — this feature distinguishes it from dermoid cyst (mature cystic teratoma). On ultrasonography, it is characterized by the pathognomonic 'onion ring' or 'target' sign and is completely avascular on Doppler. Serum tumor markers (AFP, beta-HCG) are always normal. It is not a true neoplasm and is entirely benign; it does not contain GCNIS and has no metastasis risk. Treated with testis-sparing surgery.
Age Range
20-40
Peak Age
30
Gender
Male predominant
Prevalence
Uncommon
Epidermoid cyst is a benign cystic lesion lined by keratinized squamous epithelium. Concentric keratin lamellae accumulate in the cyst lumen, and this lamellar structure creates the pathognomonic 'onion ring' pattern on ultrasonography. The cyst wall consists solely of mature squamous epithelium without dermal appendages (hair follicles, sebaceous glands, sweat glands) — this feature provides definitive distinction from dermoid cyst (teratoma). Epidermoid cyst is monodermal (from a single germ layer — ectoderm) and does not develop on a background of GCNIS/IGCN; therefore, it is not classified as a germ cell tumor and has no malignant potential. The concentric ring pattern on ultrasonography results from alternating arrangement of keratin lamellae with different acoustic impedance values — compact keratin shows high impedance, loose keratin low impedance, and these alternating layers create the 'onion ring' appearance. The completely avascular structure reflects the cyst being a non-neoplastic, metabolically inactive lesion.
The combination of concentric alternating hyperechogenic-hypoechoic ring pattern ('onion ring' or 'target' sign) on ultrasonography with completely avascular structure on Doppler is pathognomonic for epidermoid cyst. This finding combination provides definitive distinction from all testicular neoplasms and allows avoidance of unnecessary radical orchiectomy.
Epidermoid cyst shows the pathognomonic 'onion ring' or 'target/bull's eye' sign on ultrasonography. Concentric alternating hyperechogenic and hypoechoic rings are seen within the lesion. These rings reflect the alternating arrangement of compact and loose keratin lamellae. The lesion is well-defined with round-oval shape, separated from surrounding testicular parenchyma by a sharp hyperechogenic capsule (fibrous cyst wall). No intralesional solid component or papillary projection.
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A well-defined cystic lesion with concentric ring ('onion ring') pattern is seen in the testicular parenchyma, consistent with epidermoid cyst.
Epidermoid cyst is completely avascular on color and power Doppler ultrasonography — no vascular flow is detected within the lesion. This finding is the most important finding providing definitive distinction from all testicular neoplasms showing intralesional vascularity (seminoma, embryonal carcinoma, Leydig tumor). Normal vascularity is preserved in surrounding testicular parenchyma.
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The lesion is completely avascular on color and power Doppler examination with no intralesional vascularity detected; neoplastic lesion is unlikely.
On T2-weighted MRI, epidermoid cyst shows concentric ring pattern. Hydrated keratin layers show high T2 signal, compact keratin layers low T2 signal. This alternating signal pattern reflects the 'onion ring' appearance on MRI as well. The cyst wall is seen as a thin linear structure with low signal on T2. MRI's high contrast resolution enables detailed visualization of keratin lamellae.
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The lesion demonstrates concentric alternating signal pattern on T2-weighted sequences, consistent with epidermoid cyst.
On T1-weighted MRI, epidermoid cyst shows low-to-intermediate signal. May show slightly higher signal than pure fluid due to proteinaceous keratin content. No enhancement on contrast-enhanced series — confirming avascular structure. No signal loss on fat-suppressed sequences (indicating no fat content, distinction from teratoma).
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The lesion shows low-to-intermediate signal on T1-weighted sequences with no enhancement on contrast-enhanced series and no signal loss on fat suppression.
Epidermoid cyst shows bright signal at high b-values on DWI. Variable ADC values are seen on the ADC map depending on density of keratin lamellae — compact keratin areas show low ADC, loose keratin areas higher ADC. This DWI hyperintensity has a similar mechanism to cholesteatoma (temporal bone) — keratin fibers restrict water molecule diffusion. DWI finding may be confused with malignant tumor; should be evaluated together with ADC and morphology.
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The lesion shows high signal on DWI with heterogeneous values on the ADC map, consistent with diffusion restriction related to keratin content.
On CT, epidermoid cyst appears as a well-defined, low-density (0-30 HU) intratesticular lesion. Does not contain calcification or fat density (distinction from teratoma). No enhancement on contrast-enhanced CT. CT is not the primary diagnostic modality for epidermoid cyst — ultrasonography and MRI are more informative.
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A well-defined, low-density, non-enhancing lesion is seen in the testicular parenchyma on CT.
Criteria
Most common type. Prominent concentric keratin lamellae. Typical onion ring pattern on US. Concentric rings with alternating T2 signal on MRI.
Distinct Features
Pathognomonic 'onion ring' pattern is most prominent in this type.
Criteria
Cyst lumen filled with homogeneous keratinous material, concentric lamellar structure not prominent. Homogeneous hypoechoic-isoechoic cystic lesion on US, no onion ring pattern.
Distinct Features
No onion ring pattern but completely avascular structure preserved. Differential diagnosis more challenging, MRI and frozen section recommended.
Criteria
Calcification in cyst wall. Rare. Thin peripheral calcified ring appears as hyperechogenic rim on US, high-density ring on CT.
Distinct Features
Peripheral calcification ring pattern is characteristic for epidermoid cyst. Keratin lamellae preserved in internal structure. Avascular structure continues.
Distinguishing Feature
Teratoma shows cystic + solid + calcification triad with vascularity in solid components. Epidermoid cyst is completely avascular without solid component. Teratoma shows signal loss on fat suppression MRI (dermoid component), no fat in epidermoid cyst.
Distinguishing Feature
Seminoma appears as homogeneous hypoechoic mass with marked hypervascularity. Epidermoid cyst shows concentric ring pattern and is completely avascular. Beta-HCG may be mildly elevated in seminoma.
Distinguishing Feature
Simple cyst is completely anechoic, thin-walled with posterior acoustic enhancement. Epidermoid cyst has concentric ring pattern and is not anechoic. Simple cyst shows no restriction on DWI, epidermoid cyst shows restriction due to keratin.
Distinguishing Feature
Leydig cell tumor appears as small, homogeneous hypoechoic mass with marked hypervascularity. Epidermoid cyst shows concentric ring pattern and is completely avascular. Hormonal symptoms accompany Leydig.
Urgency
lowManagement
Testis-sparing surgery (enucleation) is the standard treatment. Frozen section mandatory to confirm benign nature and exclude teratoma component. Radical orchiectomy is NOT indicated if diagnosis is confident. No GCNIS, no metastatic potential.Biopsy
Not NeededFollow-up
Minimal follow-up needed. Annual scrotal ultrasound reasonable for reassurance. No tumor marker monitoring needed. Prognosis is excellent — cure rate 100% with complete excision.Epidermoid cyst is an entirely benign lesion cured by testis-sparing surgery. Radical orchiectomy is not indicated — testis-sparing approach is standard if preoperative diagnosis is confident. Intraoperative frozen section is critically important in surgery — teratoma exclusion is required. As it does not contain GCNIS, there is no increased risk in the contralateral testis. No metastatic potential, and minimal follow-up is sufficient.
Epidermoid cyst is a completely benign lesion with no risk of malignant transformation. When onion-ring/target sign and avascularity are detected together, diagnosis can be made confidently and testis-sparing surgery (enucleation) can be performed. Frozen section pathology confirms benignity intraoperatively. Tumor markers are normal. No follow-up is required. Correct recognition of this lesion prevents unnecessary orchiectomy — therefore awareness is critical for radiologists.