Overview
Sebaceous cell carcinomas account for 1–5% of all eyelid malignancies and primarily affect older adults with a slight female gender bias. Despite representing a small fraction of all eyelid tumors, proper identification and treatment of these tumors are critical because the rate of misdiagnosis has been estimated to be as high as 50%, with a mortality rate of at least 20% ( Box 52.1 ). Sebaceous cell carcinomas typically arise from the meibomian glands, but can also develop within the pilosebaceous glands of the eyelid cilia (glands of Zeis) and the caruncle. Although the clinical science of diagnosing and treating sebaceous cell carcinomas of the eyelid has advanced significantly in the past two decades, treatment is still surgical and hampered by poor understanding of the biology of these tumors. Given that cell biology and molecular signaling are intimately related to the function and development of these glands, this chapter will: (1) review the functional role of the sebaceous glands on the eyelid and normal sebaceous gland biology; (2) discuss the clinical and pathologic features of sebaceous cell carcinoma; and (3) summarize the important genetic, molecular, and cellular regulators of sebaceous cell carcinoma and demonstrate how these might shed light on the clinical behavior of these tumors.
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Sebaceous cell carcinomas account for 1–5% of all eyelid malignancies
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The eyelids contain at least two major anatomic structures that can degenerate into sebaceous cell carcinomas: the cilia-associated glands of Zeis and the meibomian glands
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Proper identification and treatment of these tumors are critical because the rate of misdiagnosis has been estimated to be as high as 50%, with a mortality rate of at least 20%
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The diagnosis of sebaceous cell carcinoma is difficult as it often masquerades as more common processes
Sebaceous gland physiology
Sebaceous glands can be grouped based on their location, association with hair follicles, and function. Although sebaceous glands are found throughout the body, certain areas are particularly rich in sebaceous gland content. While most sebaceous glands are associated with hair follicles (i.e., pilosebaceous glands), free glands can be found throughout the body, and some of them have evolved to perform specialized functions such as hormone signaling and odor production. The meibomian glands of the eyelids are one type of specialized sebaceous gland. While this chapter will focus on the biology of sebaceous cancer of the eyelids, much of the science of sebaceous glands and sebaceous cell carcinoma is based on studies of the common pilosebaceous glands.
The evolutionary origins of pilosebaceous glands are not clear, and their function in humans is controversial. Sebaceous glands are holocrine glands which release sebum via the disintegration of mature, lipid-laden cells and, as a result, require continual cellular proliferation, differentiation, and maturation. These processes and, hence, the kinetics of sebum secretion, are regulated by a complex set of signals under both endocrine and neuroregulatory control.
The function of human sebum is controversial. One interesting hypothesis suggests that sebum assumes unique functions at different temperatures. At temperatures below 30°C, the sebum of pilosebaceous glands serves to create a water-repellent skin cover. At temperatures above 30°C, human sebum assumes the characteristics of a surfactant. Sweat with high surface tension that drips off skin will cause dehydration without contributing to evaporative cooling. By acting as a surfactant for eccrine secretions, sebum lowers the surface tension of sweat and allows the sweat to be retained on the skin to achieve its thermoregulatory function. The meibomian glands of the eyelid have a similar function, namely to stabilize the tear film. The normal evaporation rate of the tear film is 25 µg/cm 2 /min. This increases 4–20-fold in the absence of the lipid layer. However, the makeup of sebum is quite different from that of meibomian secretions. The main lipids in human sebum are triglycerides, wax esters, and squalene. In contrast, meibomian lipids are composed of sterols and wax esters, with only minor amounts of triglycerides, hydrocarbons, polar lipids, and free fatty acids. In addition, the chains of meibomian lipids are longer and contain unique fatty acids and alcohols. Cutaneous sebum was shown to disrupt the tear film, suggesting that the unique composition of meibomian gland lipids is important to the maintenance of the tear film and likely prevents the spread of cutaneous sebum on to the ocular surface.
Clinical background
The diagnosis of sebaceous cell carcinoma is difficult as it often masquerades as more common processes. This can lead to critical delay in the diagnosis and contribute to the morbidity and mortality associated with the disease. Therefore, understanding the demographics and risk factors as well as recognizing the key clinical features of sebaceous cell carcinoma may allow prompt diagnosis and therapy, to reduce mortality and ocular morbidity.
Demographics
Sebaceous cell carcinoma mostly affects older adults with an estimated mean age at diagnosis between 63 and 77 years. However, it may occur at a much younger age in people with prior history of facial irradiation. Asia and the Indian subcontinent have a high incidence of sebaceous cell carcinoma. In North America, sebaceous cell carcinoma is primarily seen in people of European descent. A possible association with the autosomal–dominant Muir–Torre syndrome with mutations in the mismatch repair genes hMLH-1 and hMLH-2 has been shown in some cases.
Clinical presentation
Theoretically, sebaceous cell carcinoma can occur anywhere in the body where sebaceous glands are found. However, the ocular adnexa is by far the most common location for this neoplasm, with a vast majority occurring in the meibomian glands and fewer developing in the pilosebaceous glands of Zeis and caruncle.
The most common presentation of sebaceous cell carcinoma is a solitary lid nodule with yellowish discoloration and madarosis, a key clinical feature differentiating it from more common benign lesions such as a chalazion or hordeolum. A recurrent chalazion in an older patient should raise the suspicion for sebaceous cell carcinoma. Madarosis is not a requisite, however ( Figure 52.1 ), so a high level of suspicion must be maintained in the proper clinical setting.
The second most common pattern for sebaceous cell carcinoma development is a diffuse pattern with unilateral lid thickening and reactive inflammation which is often mistaken for blepharitis ( Figure 52.2 ). Refractory and unilateral cases should raise suspicion for sebaceous cell carcinoma. A recent discussion of the varied clinical presentations of sebaceous cell carcinoma and the differential diagnosis has been published.
Pathology
The histopathologic patterns of sebaceous cell carcinoma vary among tumors, making the disease challenging to diagnose ( Box 52.2 ). However, there are certain characteristics that should be looked for in making the diagnosis. Sebaceous carcinoma cells are pleomorphic: they commonly exhibit enlarged nuclei and basophilic cytoplasm that is foamy in appearance due to the presence of fat. Mitotic figures, often with unusual appearance, are common. In well-differentiated carcinomas, vacuolization is common ( Figure 52.3 ), and a comedo pattern can often be seen, showing the tumor cells attempting to reiterate the normal holocrine architecture of sebaceous glands ( Figure 52.4 ). Poorly differentiated carcinomas have large cells, greater pleomorphism, higher mitotic rates, and disorganized architectures ( Figure 52.5 ). Intraepithelial spread, also called pagetoid invasion – an important hallmark of sebaceous cell carcinoma – is known to occur in 44–80% of cases ( Figure 52.6 ). It is characterized by invasion of tumor cells, individually or in clusters, within the epithelium of the conjunctiva and skin, often eliciting subepithelial chronic inflammation. Pagetoid invasion often results in skip lesions in which normal epithelial tissue may be found between nests of tumor cells, raising the need for map biopsies to sample the ocular surface.
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The histopathologic patterns of sebaceous cell carcinoma vary among tumors
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The level of differentiation appears to correlate well with the aggressiveness of the tumor
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Given the likelihood of pagetoid invasion, map biopsies of the skin and conjunctiva are essential to determining the extent of the disease and treatment options
The level of differentiation appears to correlate well with the aggressiveness of the tumor: well-differentiated tumors are typically less aggressive than poorly differentiated tumors. The cytologic appearance of low-grade sebaceous cells is characterized by a cytoplasm containing many vacuoles, little nuclear pleomorphism, and rare mitoses ( Figures 52.3 and 52.4 ). High-grade tumors are more intensely basophilic, exhibiting fewer cytoplasmic vesicles, prominent nucleoli, and more mitotic figures ( Figure 52.5 ). Unfortunately, both well- and poorly differentiated sebaceous tumor cells can be found within the same tumor ( Figures 52.3–52.5 are from the same patient as shown in Figure 52.1 ). A recent publication attempted to classify histological patterns of sebaceous cell carcinoma into lobular, comedocarcinoma, papillary, and mixed. However, the clinical value of such a classification is unclear. In addition, the term seboapocrine carcinoma has been proposed to describe sebaceous tumors with focal glandular pattern of apocrine glands (decapitation secretion). Such a variety of architectures and tumor configurations highlights the likelihood that these tumors arise from stem cells associated with sebaceous and pilosebaceous glands (see below), but have not been shown to alter prognosis.
The pathologic diagnosis of poorly differentiated sebaceous cell carcinomas can be difficult because epidermal and sebaceous cells are derived from a common precursor and may display sebaceous and epidermal characteristics. For instance, sebaceous cell carcinomas may show areas with keratin pearls, intracellular bridges, and dyskeratosis, leading to a misdiagnosis as a poorly differentiated squamous cell carcinoma. In addition, there are spindle cell and basaloid variants of sebaceous cell carcinoma which mimic spindle cell squamous cell carcinoma and basal cell carcinoma, respectively. The roles of the Wnt and Notch signaling systems and of the transcriptional regulator Lef-1 in cell type specification will be discussed below.
Given the difficulty in correctly identifying poorly differentiated sebaceous cell carcinomas, several immunohistochemical markers and special stains have been used to aid in diagnosis. Like normal sebaceous cells, the tumor cells contain lipid, which stains red with the oil red-O stain, a histochemical stain which has been performed with frozen sections of tumors for many years. Immunohistochemistry staining for human milk fat globule-1 (HMFG1) and epithelial membrane antigen (EMA) has also been used as these antigens are strongly expressed on sebaceous cells. Other studies have demonstrated that low-molecular-weight cytokeratins such as Cam5.2 and anti breast carcinoma-associated antigen-225 (BRST-1) are expressed in sebaceous cell carcinoma, but not in basal or squamous cell carcinomas, and may be useful in differentiating these tumors.
Regional metastasis to preauricular, parotid, submandibular, and cervical lymph nodes is known to occur in approximately 30% of cases. Distant metastases of sebaceous cell carcinoma are rare, occurring in the lungs, liver, brain, and bone. There are current studies on the usefulness of sentinel lymph node biopsy for assessing regional metastasis, followed by treatment with local lymph node dissection and/or adjuvant chemotherapy. In the largest study to date, 10 patients with sebaceous cell carcinoma underwent sentinel lymph node biopsy and two of the 10 demonstrated microscopic evidence of tumor metastasis. It remains to be determined whether detection and treatment of lymph node micrometastases will alter the clinical course of the disease.
Disease management
The initial management of sebaceous cell carcinoma depends on several factors, including the index of clinical suspicion and the size of the tumor ( Box 52.2 ). Small tumors for which there is high clinical suspicion should be excised with margin control as the first intervention. In contrast, a full-thickness biopsy of the eyelid is considered a better approach for the initial assessment of large lesions requiring extensive eyelid reconstruction following excision. At the present time there is still considerable controversy as to whether Mohs micrographic surgery, serial excisions with frozen section control, or serial excisions with permanent section control are most effective. Based on retrospective studies, there is a suggestion that, in experienced hands, traditional excision with permanent section control of the margins provides the best chance of avoiding recurrence. With pagetoid spread, wide surgical margins are advisable, although the ideal width of clear margins is controversial.
Given the likelihood of pagetoid invasion, map biopsies of the skin and conjunctiva are essential to determining the extent of the disease and treatment options. Historically, extensively positive map biopsies have been an indication for orbital exenteration. However, the development of new surgical techniques and materials for reconstructing the eyelid and conjunctiva following extensive excision have prompted more localized tumor excision. In addition to surgery, a number of adjuvant techniques have been employed to supplement surgical excision or to treat local recurrences. Cryotherapy has been used with success as an adjuvant to surgical excision to treat pagetoid invasion of the conjunctiva. More recently, topical mitomycin C, an alkylating agent that inhibits DNA synthesis, has been advocated as an adjuvant agent to treat pagetoid invasion on the conjunctiva and cornea in a small number of patients. Further studies are needed to determine the efficacy of these treatments, particularly as pagetoid invasion may extend into adnexal structures, including the lacrimal gland ducts and drainage system.
Orbital exenteration remains the definitive treatment in cases of extensive conjunctival involvement and where there is orbital invasion without evidence of metastases. In patients with orbital extension who are unable or unwilling to undergo orbital exenteration or who have advanced disease and are seeking palliative measures, irradiation with at least 55 Gy of radiation has been used with some success. However, the use of radiotherapy for this neoplasm is controversial and surgical excision is preferred. Finally, some authors advocate brachytherapy, in which a radioactive plaque is inserted close to the tumor and delivers 150 Gy directly to the area as an alternative to orbital exenteration while protecting much of the surrounding tissue from additional exposure. Additional studies will be required to assess its efficacy.
Pathophysiology
The pilosebaceous gland, the bulge, and the role of hair follicle stem cells
The skin and its appendages are critical for animal survival. Among its many biological functions, skin protects animals from dehydration, radiation, trauma, temperature changes, and microbial infections. The adult skin is composed of varied groups of cells from diverse embryologic origins. The surface ectoderm forms a layer of progenitor cells that goes on to form stratified epidermis, hair follicles, as well as sebaceous and apocrine glands. The mesoderm contributes the collagen-producing fibroblasts of the dermis, the skin vasculature, the erector pili muscles of the hair follicles, the subcutaneous fat, and immune cells. Neural crest cells contribute melanocytes, sensory nerve endings, and the dermis of the head and face.
Sebaceous cell carcinomas of the ocular adnexa can derive from both pilosebaceous glands of Zeis and the specialized sebaceous glands of the eyelid margin – the meibomian glands ( Box 52.3 ). Our understanding of the biology of the pilosebaceous gland is significantly greater than that of the meibomian glands, but this understanding can be extrapolated to shed some light on the origins, genetics, and cell biology of eyelid sebaceous carcinomas in general. This extrapolation is based on the self-renewal properties that the holocrine cells of all sebaceous glands share with the progenitor cells of the hair follicle and skin.
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Normal holocrine function requires the presence of a multipotent stem cell population that serves to regenerate the cells of holocrine glands
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These populations of multipotent stem cells are responsible for the malignant degeneration that results in eyelid cancer
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Signaling pathways are involved in the malignant process, including the Hedgehog, Notch, and Wnt pathways
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The association between Lef-1 mutations and sebaceous hyperplasia and tumorigenesis is intriguing because of the apparent dual roles of Lef-1 in activating squamous cell differentiation and concurrently serving as an activator of tumor suppressor genes such as p53 and p21
The epidermis and hair follicles are renewed throughout life. Hair follicle renewal is achieved through a cycle comprising a growth phase (anagen), a regression phase (catagen), and a resting phase (telogen). This renewal depends on the presence of progenitor stem cells in an area referred to as “the bulge” in both rodents and humans. The bulge region contains undifferentiated stem cells that maintain a high proliferative capacity and multipotency (similar to intestinal, corneal, and other clustered stem cell populations; Figure 52.7 ). It should be noted that the stem cell population of the bulge may have been experimentally overestimated by label-retaining techniques, since rigorous studies of stem cell behavior and label retention revealed that this presumed stem cell characteristic is not universal. On the other hand, the multipotency of epidermal stem cells is well established, and use of epidermal stem cells for generating cells with embryonic pluripotency has been published.
The morphology of the bulge is different between mouse and human hair follicles. The murine bulge is a discrete protuberance of the outer root sheath, while the human bulge is in fact just a subtle swelling. However, the biology of the human and murine epidermal and bulge stem cells appears to be quite similar. The adult skin epithelium is composed of the pilosebaceous unit and the surrounding interfollicular epidermis (IFE) and its associated apocrine glands. The IFE relies on its own source of progenitor cells to provide for tissue renewal in the absence of injury, while the pilosebaceous bulge contains multipotent stem cells that are activated at the start of each new hair cycle, at the time of injury, and as needed to supply the holocrine cells of the sebaceous gland. During the hair cycle, bulge stem cells are stimulated to migrate out of the stem cell niche, proliferate, and differentiate into the various cell types of the pilosebaceous unit. Although the bulge stem cells are relatively quiescent, they can also be induced to migrate and proliferate by mitogenic stimuli such as phorbol esters (12-O-tetradecanoylphorbol-13-acetate (TPA)). Bulge stem cells appear to be in continuous flux throughout the growth phase of the hair cycle, migrating from the bulge along the basal layer of the outer root sheath where they proliferate and differentiate. The ability of these stem cells to differentiate into multiple cell types of the epidermis, sebaceous glands, and hair follicles has been shown by elegant in vivo and in vitro experiments, including transplantation experiments and clonal analysis. In addition, even when bulge cells detach from the basal lamina and undergo early commitment to the hair follicle lineage, the process is reversible, at least in vitro.
Genetic and molecular regulation of sebaceous cell carcinoma
Genetic profiling of bulge stem cells using microarray analysis has identified many genes which are expressed at higher levels within this population. Interestingly, 14% of these genes are also expressed at higher levels in other stem cell types such as hematopoietic, neuronal, and embryonic stem cells. The most interesting of these are genes that belong to the Wnt-β-catenin and the transforming growth factor-β/bone morphogenic protein (BMP) genetic signaling pathways. In addition, microarray analysis demonstrated decreased expression of many genes that inhibit proliferation in bulge stem cells, consistent with their relatively quiescent state. Both the Wnt and the BMP signaling pathways play very important roles in hair follicle morphogenesis and cycling. Hence, upregulation of Wnt and BMP regulatory components in bulge stem cells strongly suggests that these pathways are critical for proper stem cell function.
The Wnt/β-catenin signaling pathway is conserved throughout the eukaryotic kingdom and has repeatedly been shown to be critical to embryonic and postnatal development. It is often referred to as the canonical Wnt pathway and is involved in a variety of human cancers ( Figure 52.8 ). An effector of intercellular adhesion, β-catenin is usually stabilized at the plasma membrane through association with cadherin at adherens junctions via armadillo repeats. Under basal conditions, free cytoplasmic β-catenin is rapidly degraded by the proteosome in a ubiquitin-dependent manner. β-catenin is normally bound by two scaffolding proteins, adenomatous polyposis coli (APC) and axin, which leads to the phosphorylation and ubiquitination of β-catenin, resulting in the subsequent proteosomal degradation. Wnts are a large family of cysteine-rich secreted glycoproteins which bind members of the frizzled family of serpentine receptors and a member of the low-density lipoprotein receptor family, Lrp5/6. The binding of Wnt to a frizzled receptor inactivates axin by a mechanism that may involve the binding of disheveled. This inactivates the phosphorylation and ubiquitination of β-catenin, leading to β-catenin stabilization. Stable, free β-catenin is then translocated to the nucleus, where it binds the N-termini of DNA-binding transcription factors of the T-cell factor/leukocyte enhancer factor (Tcf/Lef) family.
