Initial clinical diagnosis of a pilar cyst revised to sebaceous carcinoma of the occipital scalp in a 36-year-old male: a case report

Introduction

Sebaceous carcinoma (SC) is a rare malignant tumor of the skin appendages, constituting approximately 0.7% to 1.3% of primary cutaneous malignancies. Its incidence is substantially higher in men than in women. SC predominantly occurs in older individuals, with peak incidence between 61 and 70 years of age and a median age of 65 years (1). Generally, SC is classified by tumor location as either periocular or extraocular. Periorbital tumors mainly arise from the meibomian glands, particularly in the upper eyelid; extraocular tumors typically occur in the head and neck, although cases involving the trunk, limbs, and genitals have also been reported. The most common sites are the face (37.7%), eyelids (26.2%), head and neck (19.7%), and trunk and limbs (16.4%) (2).

Notably, SC is an aggressive malignancy, with a reported 5-year mortality rate of 4% to 20% (3). Prognosis is closely dependent on early detection and prompt treatment. Delayed or missed diagnoses can lead to serious outcomes, including local invasion into adjacent structures such as the skull, regional lymph node metastasis, and distant spread to visceral organs, all of which markedly reduce survival. Despite its malignant potential, SC often presents with nonspecific clinical features, making it easily mistaken for benign lesions. This diagnostic challenge contributes to a high rate of misdiagnosis and subsequent treatment delays.

Scalp SCs are particularly challenging diagnostically due to their rarity at this site and clinical similarity to common benign scalp lesions such as pilar cysts. They typically present as firm, small nodules and often lack overt signs of malignancy, including rapid growth, ulceration, or pain. This subtle and deceptive presentation frequently leads to misdiagnosis, which may result in inappropriate initial management and unfavorable oncologic outcomes. In some cases, metastasis is already present at the time of diagnosis (4).

The present case highlights several noteworthy features: a 36-year-old male with extraocular SC located in the occipital region, initially misdiagnosed clinically and radiologically as a benign pilar cyst. This case emphasizes the diagnostic challenges posed by rare SC mimicking common benign scalp lesions, alerting clinicians that even seemingly typical scalp masses in young adults may harbor malignancy. Given the limited literature on scalp SC in younger patients, this report underscores the importance of greater vigilance, early biopsy, and accurate pathological evaluation to prevent misdiagnosis and treatment delays. We present this article in accordance with the CARE reporting checklist (available at https://acr.amegroups.com/article/view/10.21037/acr-2025-354/rc).

Case presentation

Chief complaint

A 36-year-old male presented with a slowly enlarging subcutaneous mass in the left occipital region, present for more than two months.

Medical history

• Month 0: The patient initially noticed a small, asymptomatic subcutaneous nodule, approximately 0.5 cm in diameter, in the left occipital region, with no identifiable predisposing factors. Because the lesion was painless and caused no discomfort, he did not seek medical attention at that time.
• Month 2: The mass gradually enlarged in a slow, painless manner, prompting the patient to consult a dermatology clinic. He reported no functional impairment, pain, itching, or tenderness. He also denied systemic symptoms such as weight loss, fatigue, fever, or night sweats. The patient had no significant medical history, history of local trauma or radiation exposure, or family history of malignant tumors or hereditary dermatological disorders.

Physical examination

Systemic physical examination was unremarkable. No palpable superficial lymphadenopathy was detected in the head, neck, axillary, or inguinal regions. Cardiopulmonary and abdominal examinations were also normal.

Dermatological examination revealed a solitary, raised subcutaneous mass in the left occipital region, approximately 2 cm in diameter. The lesion was round, firm, and mobile, with no obvious fixation to the overlying skin or underlying structures. The overlying skin showed mild erythema but remained intact, without ulceration, scaling, or crusting. No central punctum was observed, and the mass was non-tender on palpation (Figure 1).

Figure 1 Clinical presentation showing mild erythema of the overlying skin. The mass was painless and exhibited no signs of ulceration. The initial diagnosis was a pilar cyst.

The clinical presentation—a small, firm, mobile, well-circumscribed, asymptomatic, slowly enlarging subcutaneous nodule without a central punctum, tenderness, or ulceration—was highly suggestive of a benign pilar cyst and showed no overt signs of malignancy. Typical features of malignancy, such as rapid growth, induration, fixation to adjacent structures, ulceration, bleeding, or regional lymphadenopathy, were absent, contributing to the initial misdiagnosis.

Color Doppler ultrasonography

Color Doppler ultrasound was performed by a qualified ultrasonographer using a Siemens ACUSON Sequoia 10L4 linear array probe (Siemens Healthineers, Erlangen, Germany) at 4–10 MHz. The scan revealed a well-defined, hypoechoic scalp nodule measuring 2.3 × 2.0 × 1.0 cm, with homogeneous internal echotexture, no significant calcification or cystic degeneration, and minimal internal vascularity. These sonographic findings supported the preoperative impression of an epidermoid or pilar cyst.

Surgical treatment and histopathological examination

Under local anesthesia, the lesion was completely excised based on the presumptive diagnosis of a benign pilar cyst. The excised specimen was fixed in 10% neutral buffered formalin and processed for hematoxylin and eosin staining.

Histopathological examination revealed an epithelial neoplasm with ill-defined borders and architectural asymmetry. The tumor cells were arranged in variably sized lobular clusters, with no connection to the overlying epidermis. Cytologically, the cells demonstrated sebaceous differentiation, mild to moderate atypia, and identifiable mitotic figures. The lesion was relatively well-circumscribed, with no evidence of capsular invasion or epidermal involvement (Figure 2A,2B).

Figure 2 Histopathological and immunohistochemical examination of the excised skin tissue. (A,B) H&E staining demonstrated a large epithelial-derived tumor with poorly defined margins and asymmetry (A, 3.5×; B, 200×). Tumor cells formed lobular aggregates of varying sizes and shapes without connection to the epidermis. Features of sebaceous differentiation and mitotic figures were evident. The tumor was clearly demarcated from the surrounding tissues and showed no invasion of the capsule or epidermis. (C-F) Immunohistochemical staining results were as follows: AR(+) (C, 3.7×), Ki-67(30%) (D, 3.7×), P63(+) (E, 3.7×), and GCDFP-15(−) (F, 3.7×).

Immunohistochemical analysis demonstrated positive expression of androgen receptor (AR) and P63, while gross cystic disease fluid protein-15 (GCDFP-15) was negative. The Ki-67 proliferation index was approximately 30% (Figure 2C-2F). Based on these histomorphological and immunohistochemical findings, a definitive diagnosis of SC in the left occipital region was established.

Subsequent recommendation and follow-up

Adjuvant Mohs micrographic surgery (MMS) was recommended to further minimize the risk of recurrence; however, the patient was lost to long-term follow-up.

All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s), and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Discussion

SC is a rare but aggressive cutaneous malignancy with a significant risk of local invasion, regional metastasis, and mortality. Hamid et al. (5) reported a case involving a 64-year-old woman who presented to the clinic for evaluation of a scalp mass. Initially, she was scheduled for excision of a suspected pilar cyst; however, histopathological examination ultimately revealed a diagnosis of SC. The present case describes a 36-year-old male with extraocular SC in the occipital region, initially misdiagnosed clinically and radiologically as a benign pilar cyst. This report underscores the diagnostic challenges of SC in younger patients and highlights the importance of clinical vigilance, early biopsy, and appropriate genetic and oncologic evaluation.

The molecular mechanisms underlying SC are increasingly being elucidated. The Wnt/β-catenin signaling pathway plays a central role in the development, homeostasis, and malignant transformation of sebaceous glands. Aberrant activation of this pathway promotes uncontrolled cell proliferation, disrupts sebaceous differentiation, and contributes to the development of invasive carcinoma (6). In addition, inactivation of the p53 tumor suppressor gene is commonly observed in SC, resulting in impaired DNA repair, evasion of apoptosis, and accumulation of genetic alterations that drive tumor progression (7). Together, these molecular abnormalities underpin the aggressive biological behavior of SC, including its capacity for local invasion and metastasis.

SC of the scalp often mimics benign lesions, such as pilar cysts, epidermal cysts, and lipomas. In this patient, the lesion exhibited characteristic benign features, including slow growth, firm consistency, mobility, well-circumscribed borders, absence of tenderness or ulceration, and no lymphadenopathy, which contributed to the preoperative misdiagnosis. Although different types of skin masses exhibit variations in echo uniformity, edge clarity, and the layers affected, high-frequency ultrasound still faces challenges in providing a definitive diagnosis for skin tumors. High-frequency ultrasound has limited ability to distinguish SC from benign cystic lesions, as both may appear as well-circumscribed, hypoechoic nodules. These observations highlight that imaging alone cannot reliably rule out malignancy, and histopathological examination remains the definitive diagnostic standard. In our case, the clinical diagnosis was an epidermal cyst; however, the ultrasound did not yield a more accurate diagnosis in a timely manner, also indicating an epidermal cyst. This illustrates that ultrasound has a notable misdiagnosis rate in the assessment of soft tissue masses.

The gold standard for diagnosing SC remains histopathological examination. Tumor cells typically display a diffuse infiltrative growth pattern in the dermis and subcutaneous tissue, with frequent extension into the epidermis. Tumors with well-demarcated borders and simple displacement of surrounding tissues are uncommon. Poorly differentiated tumors are mainly composed of polygonal basal-like cells with large, irregular nuclei and multiple nucleoli. More mature sebaceous cells exhibit vacuolated, pale cytoplasm and atypical fan-shaped nuclei, showing clear sebaceous differentiation. The degree of cellular atypia varies, and mitotic activity is evident, including atypical mitotic figures. Well-differentiated tumors exhibit clear sebaceous gland differentiation and are primarily composed of mature tumor cells with minimal atypia. In contrast, poorly differentiated tumors are characterized by sheet-like proliferation of basal-like cells, squamous metaplasia, and necrosis. These tumors typically demonstrate lobular infiltrative growth of basal-like cells into adjacent tissues, with tumor cells forming irregular nest-like structures. Keratin pearls may occasionally be observed, frequently accompanied by central comedo-type necrosis.

Further immunohistochemical analysis of SC reveals several characteristic features. Tumor cells commonly express lipophilin, AR, epithelial membrane antigen, cytokeratin 7, and epithelial cell adhesion molecule (specifically detected by antibody clone BerEP4). Decreased AR expression may indicate poor prognosis and has been proposed as both a prognostic factor and a potential therapeutic target, particularly in SC of the eyelid (8). In this case, the immunohistochemical analysis showed positive AR expression. Patients should be closely monitored following surgery.

Complete surgical excision with adequate margin control remains the cornerstone of SC treatment. MMS is widely considered the gold standard, particularly for lesions in the head and neck region. MMS allows for comprehensive, real-time histological assessment of all surgical margins, reducing recurrence rates while preserving surrounding healthy tissue (9). For high-risk cases—defined by large size, high grade, deep invasion, or prior recurrence—MMS is strongly preferred over conventional wide local excision. In the present case, MMS was recommended after the diagnosis was confirmed; however, the patient was lost to follow-up.

Of significant clinical relevance, young patients with extraocular SC should be promptly evaluated for Muir-Torre syndrome (MTS), an autosomal dominant cancer predisposition syndrome caused by germline mutations in mismatch repair (MMR) genes, including MLH1, MSH2, MSH6, and PMS2 (10,11). MTS is characterized by the occurrence of sebaceous neoplasms, such as adenomas and carcinomas, alongside visceral malignancies, most frequently involving the colorectal, endometrial, urinary tract, and gastrointestinal systems (10,11). The Mayo MTS Risk Score is a validated tool for risk stratification. Clinical features suggestive of MTS include: age at onset of the first sebaceous neoplasm (1 point if under 60 years), total number of sebaceous neoplasms or keratoacanthomas (2 points if two or more), and personal history of Lynch syndrome-associated internal malignancies (1 point) (12). Given the patient’s young age (36 years) and the presence of extraocular SC, immunohistochemical analysis of MMR proteins and genetic testing are strongly recommended to rule out underlying Lynch syndrome/MTS and to guide screening for associated visceral malignancies. This evaluation is essential for the early detection and prevention of secondary cancers and for improving long-term survival. The lack of such screening in this case represents a notable limitation and highlights an important clinical lesson for practitioners.

SC carries an estimated 5-year mortality rate of 4% to 20%, with local recurrence and distant metastasis representing major adverse outcomes. Delayed diagnosis increases the risk of invasion into the skull, regional lymph node metastasis, and distant spread to organs such as the lungs and liver. Early biopsy, accurate pathological diagnosis, and timely definitive surgical management are therefore critical for improving patient outcomes.

This case demonstrates that SC can mimic benign scalp cysts in young adults, often lacking classic signs of malignancy. Both clinical and imaging findings may be misleading. Consequently, a preoperative biopsy should be strongly considered for scalp nodules that persist or enlarge, even when they appear clinically benign. Greater awareness of SC, along with routine evaluation for metastatic disease and hereditary cancer syndromes in young patients, is essential to reduce misdiagnosis, guide appropriate treatment, and optimize long-term outcomes.

As a single case report, the present work cannot establish broader patterns or change diagnostic guidelines. In addition, loss to follow-up prevents assessment of long-term outcomes, limiting the prognostic insights that can be drawn from this case. Larger case series of scalp SC are necessary to establish optimal diagnostic approaches. Furthermore, long-term follow-up is essential to determine effective treatment strategies and evaluate patient outcomes.

Conclusions

In conclusion, we report a rare case of primary extraocular SC of the scalp that was initially misdiagnosed as a pilar cyst in a young adult. Clinicians should recognize that such uncommon neoplasms can exhibit atypical clinical behavior and mimic benign lesions. Imaging studies, including ultrasound, carry a substantial risk of misdiagnosis; therefore, prompt skin biopsy is essential for early and accurate diagnosis. Young patients with extraocular SC should undergo evaluation for MTS, including mismatch repair protein testing and appropriate cancer screening. MMS remains the preferred surgical approach to minimize recurrence. This case highlights the diagnostic challenges of SC and emphasizes the importance of clinical vigilance in the management of atypical scalp lesions.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the CARE reporting checklist. Available at https://acr.amegroups.com/article/view/10.21037/acr-2025-354/rc

Peer Review File: Available at https://acr.amegroups.com/article/view/10.21037/acr-2025-354/prf

Funding: This study was supported by the Open Research Fund of the Ningxia Clinical Research Institute, People’s Hospital of Ningxia Hui Autonomous Region (2024KF09), and the Key R&D Program Talent Introduction Special Project (2024BEH04005).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://acr.amegroups.com/article/view/10.21037/acr-2025-354/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s), and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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