Introduction

Cutaneous leiomyosarcoma (CLMS) is a rare malignant tumour that forms from smooth muscle cells in the skin. CLMS makes up a small percentage of both soft tissue sarcomas and skin cancers.^1,2 As CLMS is a rare sarcoma subtype and this raises several challenges in diagnosis, research and treatment guidelines as a result.³

CLMS primarily arises from the smooth muscle in blood vessel walls, dartos muscles, or arrector pili muscles in hair follicles. The cutaneous form can be divided into subcutaneous, which involves tissue just beneath the dermis, or dermal, which is strictly within the dermis.^4,5 Skin-limited tumours usually have better prognostic outcomes than those that penetrate deeper into the dermal layers.^1,2

Differences between the cutaneous, subcutaneous and deep soft tissue leiomyosarcomas

Cutaneous Leiomyosarcoma, also known as dermal leiomyosarcoma, is usually a tumour that forms in the dermis. It develops from thin smooth muscles under the genital skin (dartos) or beneath the skin attached to the hair follicle (arrector pili) and appears as firm nodules. There is a significant risk of it coming back locally, but a lower chance of spreading to other parts of the body.^5,6

Subcutaneous leiomyosarcoma starts in the smooth muscle of blood vessel walls in the subcutaneous tissue. It has larger and more mobile masses, unlike the cutaneous leiomyosarcoma. It has a higher risk of local recurrence and a noticeably higher chance of cancer spreading.^5,6

Soft Tissue Leiomyosarcoma: This type is generally more aggressive and tends to have worse outcomes than the cutaneous forms. Leiomyosarcoma occurs in deeper soft tissues, such as the muscles of the limbs.⁷

Epidemiology

• CLMS accounts for about 2 to 3% of all cutaneous soft tissue sarcomas and only 0.04% of all neoplasms¹ 
• Age: It is most often diagnosed between the ages of 50 and 80^1,8
• Sex Distribution: Studies show a slight male predominance, with a male-to-female ratio of up to 3 to 1 for cutaneous cases⁴
• Distribution: It usually affects the extremities, particularly the lower limbs. However, it can also occur on the trunk, scalp, and, less frequently, the face or other areas⁹
• Recurrence and Metastasis: Subcutaneous forms of CLMS have higher rates of both distant spread and recurrence³

Origin of cutaneous leiomyosarcoma

The Cutaneous leiomyosarcoma’s clinical and biological features depend on its specific development within the skin.  

1. Superficial Dermal CLMS: This subtype mainly arises from the arrector pili muscles, which are small smooth muscles connected to hair follicles in the dermis. Occasionally, other smooth muscles in the skin, such as the genital dartos or mammary areola, can also serve as origins^2,9
2. Subcutaneous CLMS comes from vascular smooth muscle found in the walls of small arteries and veins in the fatty layer beneath the dermis^2,9

Clinical features of CLMS

CLMS often appears as a single, hard nodule that is skin-colored or varies from red to brown. These nodules usually measure 1 to 3 cm in diameter and are most commonly found on the extremities, especially the legs. However, they can also develop on the torso, scalp, face, and, less frequently, on fingers or other unusual skin areas.^3,5

Typical clinical characteristics

• Solitary and firm nodules: They are generally firm to the touch and have a smooth surface. The colour can range from skin-coloured to red^3,9
• Location: Most often, these lesions affect the lower limbs, particularly the legs. Occasionally, they may appear on the scalp, trunk, and rarely on the face and fingers^5,9
• Symptomatology: Patients often report pain, soreness, itching, or tingling at the site of the lesions. Up to 95% of patients may experience discomfort, while some remain asymptomatic^3,9
• Size and growth: Most lesions grow slowly over months or years, though the clinical course can vary quite a bit^3,10

Atypical clinical characteristics

• Atypical presentations may include pain, ulceration, or bleeding sores. While most CLMS appear as non-ulcerated nodules, trauma or tumour development can lead to ulceration and bleeding³
• Fast growth or multifocal involvement: Although most instances are solitary and slow-growing, some show rapid enlargement or multiple nodules, which complicates the diagnosis^3,10
• CLMS can occasionally affect uncommon areas like the scalp, face, or fingers. These locations are less commonly reported but are clinically significant due to the diagnostic challenges they present^5,9
• Presentations in immunocompromised or pediatric patients: Although CLMS mainly affects middle-aged to older adults, unusual cases in immunocompromised individuals or children have been recorded. These patients may show more aggressive behaviour or unusual clinical features, but data on these groups is limited³

How to diagnose CLMS

Clinical evaluation

• Presenting features: A single, hard, skin-colored or reddish nodule, usually found on the limbs
• Assessment: A thorough skin examination and a complete patient history are necessary
• Clinical Note: CLMS can look like benign lesions, which makes early diagnosis challenging^3,11

Histopathology

Microscopic features:

• There are elongated spindle cells in fascicles
• Eosinophilic cytoplasm with blunt-ended nuclei, like the cigar-shaped form of cigar-shape
• Nuclear pleomorphism, atypia, and mitotic figures (more than 1-2 per 10 HPF)
• Possible necrosis and expansion into the dermis and subcutaneous tissue

These features help in distinguishing between well-differentiated and poorly differentiated cancers.^3,9,12,13

Immunohistochemistry

• Positive markers confirm smooth muscle origin. These include SMA, desmin, vimentin, muscle-specific actin, h-caldesmon, and calponin^9,12,13,14
• Negative markers help exclude other conditions that mimic CLMS. For example, S100 excludes melanoma and nerve tumours, CD34 excludes Dermatofibrosarcoma Protuberans (DFSP), and Cytokeratin excludes sarcomatoid cancer^4,12

Differential diagnosis

Key Conditions to Differentiate:^4,13

• Dermatofibrosarcoma Protuberans: CD34 positive, SMA/Desmin negative
• Atypical Fibroxanthoma: lacks SMA/Desmin and shows significant pleomorphism
• Pilar Leiomyoma: Benign; no atypia, SMA/Desmin positive, but not malignant
• Spindle Cell Melanoma (S100 positive)
• Other spindle cell cancers are classified by histology and marker profile

Imaging studies

Indications for imaging studies:

• Suspected deep extension, large tumours, or metastases.

Preferred modalities:

• Magnetic Resonance Imaging (MRI) is best for determining the extent of a local tumour and planning surgeries¹⁵
• Computed Tomography (CT)/ Positron Emission Tomography-Computed Tomography (PET-CT): Used to stage and find metastases¹⁶

It is necessary to note that Imaging is often not needed for superficial skin cancers unless deeper involvement is expected.

Related cases of the rare presentation of CLMS

A rare smooth muscle cancer, cutaneous leiomyosarcoma (CLMS) can show various clinical symptoms, some of which are very uncommon and hard to identify. Below are selected case examples from published literature that highlight clinical and diagnostic issues related to unusual or atypical presentations.

CLMS on the face: basal cell carcinoma mimicry  

Although facial CLMS is very rare, it can appear like more common skin cancers, including basal cell carcinoma (BCC). In one case, a patient had a hard, red lump on their face that initially seemed to be BCC based on its appearance and location.

Immunohistochemistry and histopathology played a key role in diagnosing CLMS. The lesion showed spindle-shaped cells with varied nuclei grouped in fascicles. These cells were positive for desmin and smooth muscle actin (SMA), but negative for cytokeratin or S100, which indicates melanoma or BCC. This case emphasises the importance of histologic confirmation, especially for facial nodules that do not respond to standard treatments for common cancers.^3,17

Young CLMS with unusual histology  

Most people affected by CLMS are middle-aged to older adults; cases in children are very rare. A pediatric CLMS case exhibited atypical histological features, including a high mitotic index and significant nuclear atypia. It is important to separate these tumours from other juvenile spindle cell neoplasms, such as atypical fibroxanthoma or rhabdomyosarcoma.

Immunohistochemical profiling confirms smooth muscle origin, especially when smooth muscle markers (SMA, desmin) are positive. Due to their rarity and potential for more aggressive behaviour, pediatric cases can present diagnostic challenges, requiring close monitoring and often more intensive treatment.¹⁷

Dermal CLMS recurrence following incomplete excision  

Incomplete initial excision can lead to recurring cutaneous CLMS. In one case, a patient with cutaneous CLMS on the trunk underwent a limited excision initially. The tumour came back locally several times over the years, despite having histologically negative margins. Eventually, the disease was managed through further excisions with wider margins and careful histological margin evaluation. Histology linked recurrence to larger tumours and deeper skin invasion. These cases show that, even in dermal subtypes of CLMS that typically have lower metastatic potential, inadequate margins significantly increase the chance of recurrence.^17,18

Summary

Cutaneous leiomyosarcoma (CLMS) forms from smooth muscle cells in the skin, specifically from either vascular smooth muscle (subcutaneous) or arrector pili muscles (dermal). Because of its rarity and unclear symptoms, CLMS is often misdiagnosed, leading to delays in getting the proper treatment. It is essential to distinguish between the subcutaneous and dermal types, as they behave very differently. Subcutaneous CLMS is usually more aggressive and has a higher chance of recurrence and spreading, while dermal CLMS generally has a better outlook and a lower risk of spreading. An accurate diagnosis relies on early detection, thorough histopathologic investigation, and immunohistochemical confirmation.