SIMPLEX VIN

Simplex (differentiated) VIN:
an underappreciated threat

By William R. Hart, MD

Simplex (differentiated) vulvar intraepithelial neoplasia-probably the immediate precursor of most HPV-negative invasive squamous-cell vulvar carcinomas-is easily missed or misdiagnosed. The women who are most at risk are those with vulvar dystrophy or a prior cancer.

Experts have long known that untreated, vulvar intraepithelial neoplasia (VIN) may lead to invasive vulvar carcinoma. Less well known is that simplex VIN-a subtler form that chiefly targets postmenopausal women-probably poses a greater threat for progression than does the classic form of VIN.

My goal here is to first briefly contrast the features of the two major types of VIN-simplex (differentiated) VIN versus classic (bowenoid) VIN. I will then update our understanding of VIN, sharing insights from our research-which include a clinicopathologic study of women with simplex VIN in which we also analyzed HPV and p53 expression. Ideally, as clinicians and pathologists become more aware of simplex VIN, they will be more likely to diagnose the disease in women at risk.

Although researchers first identified this highly differentiated form of VIN over four decades ago, simplex VIN has often been overlooked or misdiagnosed.^1-3 Today, fortunately, after many reclassifications of VIN, detours, and missteps,^4-8 simplex (differentiated) VIN is widely accepted as a distinctive type of VIN that differs both clinically and pathologically from classic VIN.^9,10 Moreover, current evidence points to it having a greater role in the pathogenesis of most invasive vulvar squamous cell carcinomas (SCC) than does classic VIN-even though the latter is diagnosed far more often.

Simplex VIN is by definition a high-grade lesion

Initially, only high-grade intraepithelial squamous cell lesions were diagnosed as VIN. Later the histologic spectrum of VIN was expanded to include mild and moderate grades of dysplasia, resulting in a three-tiered grading system (VIN I, II, III) similar to the schema for cervical intraepithelial neoplasia (CIN). An analogy between CIN and VIN, however, is problematic. Unlike CIN I-a very common cervical lesion-VIN I lesions are very infrequent.

A retrospective study of patients with mild vulvar atypia accrued over 10 years underscores this point. Of 21 cases that were eligible for a diagnosis of VIN I, only two met the criteria for VIN.¹¹ A genuine VIN I lesion is nearly always associated with higher grades of VIN. The vast majority of VIN lesions are grade II or III, and any distinction between these two grades is highly subjective and of no proven significance. A two-tier system of low-grade VIN (VIN I) and high-grade VIN (VIN II and III) is probably more useful and reproducible. While grading is common for classic VIN, it is not applicable to simplex (differentiated) VIN. In current classifications, simplex VIN is by definition a high-grade lesion, in spite of the orderly maturation of the epidermis.^9,10

Characteristics of classic VIN

Because classic VIN has been well studied and most gynecologists are aware of its clinicopathologic features, I'll just touch on its salient aspects as a basis for comparison with simplex VIN. Synonyms for classic VIN include bowenoid VIN, undifferentiated VIN, and Bowen's disease. Accounting for at least 90% of cases of VIN, classic VIN is the easiest to recognize due to its striking histologic and cytologic abnormalities.¹²

Even at the low levels of magnification shown in Figure 1, the epidermis appears disorganized and is immediately recognized as neoplastic. The intraepithelial cells have hyperchromatic nuclei with indistinct cytoplasm and numerous-often abnormal-mitotic figures. In addition, superficial koilocytotic cells are seen in variable numbers. In contrast to the striking pleomorphism with multinucleated cells and papillary surface found in the "warty" variant, the "basaloid" variant displays a homogeneous monotonous cellular composition. But mixtures of these two patterns are common. HPV DNA is present in 53% to 90% of classic VIN cases, irrespective of pattern.^13-16 Of the several different HPV types demonstrated by DNA hybridization, HPV-16 has emerged as the predominant one.^13-18

Classic VIN occurs over a wide age range, but has a predilection for relatively young women, often in their 30s and 40s. Among these patients, condyloma acuminatum, a history of sexually transmitted diseases, human immunodeficiency virus-positivity, and cigarette smoking are common. Four in 10 affected women will have multifocal involvement of the vulva, while 18% to 52% display multicentric squamous neoplasia involving the cervix or vagina.¹⁹ Only about 3% to 10% of treated women with classic VIN go on to develop invasive SCC.^8,20,21 Although a much higher progression rate has been found in a small series of untreated patients, spontaneous regression may also occur.^21-23 Women over age 40 and those who are immunosuppressed are at greatest risk for progression to invasive SCC. Seemingly at lowest risk are young women, especially those having multifocal small papular pigmented lesions (corresponding to the clinical picture of so-called bowenoid papulosis). These patients are most likely to experience spontaneous regression, particularly if they are pregnant or postpartum at the time of diagnosis.²⁴

Distinguishing features of simplex (differentiated) VIN

Accounting for only between 2% and 10% of biopsy specimens with VIN, simplex VIN is most often discovered adjacent to typical invasive SCC during microscopic examination of vulvectomy specimens.^12,16 We recently reported our experience with this type of VIN in a comprehensive clinicopathologic study.²⁵

Simplex VIN displays important clinical and pathologic features that differ from classic VIN. It characteristically occurs in postmenopausal women, with the average age being 67 years-more than two or three decades older than women with classic VIN.^1,25 In addition, about one patient in every four has a documented history of cigarette smoking. In contrast to classic VIN, multicentric lower-geni-tal-tract squamous neoplasia is not a feature.^1,25 It seems that simplex VIN is more likely to progress into invasive SCC than is classic VIN. Also unlike classic VIN, HPV DNA is almost always absent in simplex VIN and in the invasive carcinomas associated with it.²⁵

Simplex lesions are less bulky and subtler. Simplex VIN lesions typically range from 0.5 to as much as 3.5 cm in dimension and are less bulky than classic VIN lesions.²⁵ Moreover, simplex VIN often appears as discrete or ill-defined gray-white lesions with a roughened surface or a raised thickened white plaque (Figure 2), and some are discrete elevated nodules. The lesions may be multifocal, both grossly and microscopically. In most but not all cases, simplex VIN is associated with dystrophic vulvar lesions, particularly lichen sclerosus and squamous hyperplasia.²⁵ In fact, when VIN arises in a patient with vulvar dystrophy, it's almost always of the simplex type. We therefore advise biopsying any irregularly thickened area in a patient with vulvar dystrophy to determine whether simplex VIN or early invasive carcinoma has developed.

Histologically, simplex VIN is a dramatically subtler lesion than is classic VIN. Hindrances to its recognition have been the high degree of cellular differentiation combined with an absence of widespread architectural disarray, nuclear pleomorphism, and diffuse nuclear atypia. It's easy to see how pathologists can mistake it for nonspecific squamous hyperplasia or acanthosis. Typically, the epidermis is thickened and has a prominent parakeratotic surface plaque (Figures 3 and 4). The rete ridges are often elongated and branched. In spite of the relatively orderly appearance of epidermal maturation, most of the epidermis is composed of abnormal keratinocytes, even in the superficial epidermis. The neoplastic keratinocytes are conspicuously enlarged. Cytoplasm is abundant and densely eosinophilic, indicating premature differentiation or keratinization (Figure 5). In addition, intercellular bridges are usually very prominent, and typically, the nuclei are large, vesicular, and have macronucleoli. A few cells may be binucleated or multinucleated.

These abnormal keratinocytes-the hallmark of simplex VIN-typically extend deeply into the lower epidermis and the rete ridges, where they may encroach on the basal cells or abut against the dermis (Figure 6). Whorls of abnormally differentiated keratinocytes, some forming keratin pearls, often are seen in these areas (Figure 7). Scattered mitotic figures are most common in the basilar areas, but mitoses occasionally are found into the upper levels of the epidermis. The basal cells usually have irregular hyperchromatic nuclei, sometimes called basilar atypia (Figure 6). Occasional cases of VIN have mixed simplex and classic features.

When simplex VIN invades the dermis, small clusters or nodules of enlarged eosinophilic keratinocytes appear to explode from the basilar epidermis or from elongated rete ridges (Figure 8). In superficial biopsies, distinguishing simplex VIN from early invasive SCC may be very difficult. Foci of microinvasion are not uncommon.^23,25 In our series, almost 60% of patients either developed or had an antecedent invasive SCC, all of which were HPV-negative and were of the typical keratinizing type.²⁵ The cytologic similarities between the large abnormal eosinophilic keratinocytes of simplex VIN and the cells of well-differentiated keratinizing SCC are striking. In one study of nuclear DNA content in vulvar neoplasia, comparable nuclear size and stem cell modal values were found in a case of simplex VIN and the keratinizing SCC associated with it.²⁶

Differential diagnosis. The principal differential diagnosis of simplex (differentiated) VIN is squamous hyperplasia, earlier known as hyperplastic dystrophy without atypia. It's defined as a benign acanthotic proliferation of epidermal keratinocytes that cannot be attributed to a specific dermatologic disorder such as psoriasis or lichen planus. Squamous hyperplasia occurs in 21% to 54% of patients undergoing biopsy for vulvar dystrophy.³ Women with lichen sclerosus are most likely to develop squamous hyperplasia (Figure 9).

In our experience, more than one in three patients with vulvar lichen sclerosus have superimposed squamous hyperplasia, and squamous hyperplasia with lichen sclerosus is found histologically in 83% of cases of simplex VIN.^25,27 Some dermatopathologists insist that most cases of vulvar squamous hyperplasia are actually examples of lichen simplex chronicus ("neurodermatitis"), resulting from chronic scratching induced by the pruritus of lichen sclerosus. Regardless, it may be challenging at times for pathologists to distinguish the histologic changes of simplex VIN from those of squamous hyperplasia or other acanthotic proliferative epidermal lesions.

The value of immunostaining. In difficult diagnostic cases, immunohistochemical staining for p53 protein may be of value.²⁵ In response to DNA-damaging agents, the role of the tu-mor suppressor gene p53 is to encode a nuclear protein (wild-type p53 protein) that induces cell cycle arrest of damaged cells to allow repair-or when DNA damage is beyond repair, to stimulate apoptosis (programmed cell death). But when somatic mutation of the p53 gene takes place, dysfunctional mutant p53 protein results and p53 loses its role in cell cycle regulation and tumor suppression. Mutant p53 protein has a longer half-life and is more stable than wild-type p53 protein, allowing it to accumulate in the nucleus (overexpression) and become detectable by immunohistochemical staining methods. Diffuse, strong immunostaining is usually considered circumstantial evidence that the p53 tumor suppressor gene has mutated. Although upregulation or stabilization of wild-type p53 can also result in stainable p53 protein, the intensity and extent of staining is usually less. In our series of patients with simplex VIN,²⁵ we found overexpression of p53 protein in more than 83% of cases, with a high labeling index (LI) of the basilar cells that exceeded 90% in two thirds of the subjects. Suprabasilar extension of p53-positive cells occurred in all positive cases (Figure 10). In sharp contrast, the LI of accompanying squamous hyperplasia and lichen sclerosus was substantially lower and suprabasilar extension of p53-positive cells was absent.

The pathogenesis of invasive vulvar SCC

Invasive vulvar SCCs can be subdivided into those that are HPV-positive and those that are HPV-negative. A minority are HPV-positive (about one fifth to one third of all vulvar SCC).^28,29 The pathogenesis of these HPV-positive carcinomas is thought to be analogous to that of invasive cervical cancer. In that disease, infection with oncogenic HPV produces viral proteins that bind to wild-type p53 protein, causing rapid degradation and loss of its tumor suppressor function. HPV-positive vulvar tumors often have basaloid and warty histologic patterns that resemble the HPV-positive classic VIN lesions from which they're believed to arise, although some are typical keratinizing carcinomas. Classic VIN is often found in the adjacent epidermis.

Unlike cervical cancers, however, the vast majority of invasive SCC of the vulva are HPV-negative.²⁸ Generally, they appear in postmenopausal women at a more advanced age than do viral-associated carcinomas. Classic VIN is usually not found in the adjacent skin. Rather, these vulvar cancers are more often accompanied by squamous hyperplasia, lichen sclerosus, and simplex (differentiated) VIN.^25,28 Morphologically, they usually are of the typical keratinizing type. One study documented p53 mutations in 45% of HPV-negative invasive vulvar SCC,³⁰ while another study found p53 mutations or overexpression of p53 protein-or both-in 75% of typical SCC carcinomas.³¹ Taken together with our HPV and p53 data on simplex VIN, these findings indicate that simplex VIN is a likely precursor of HPV-negative vulvar invasive SCC and that lichen sclerosus and squamous hyperplasia may also play a role.²⁵

To explain these observations, an "itch-scratch" hypothesis for the pathogenesis of HPV-negative vulvar carcinomas associated with lichen sclerosus was recently proposed.³² According to this hypothesis, local irritants that cause scratching tend to induce lichen sclerosus in women who have the appropriate immunogenetic profile. (Women who do not fit that profile, on the other hand, develop lichen simplex chronicus or "neurodermatitis.") The pruritic nature of lichen sclerosus leads to vicious cycles of itching and scratching. With continued scratching, superimposed lichen simplex chronicus initially develops. Eventually, squamous hyperplasia supervenes. With further progression, squamous hyperplasia becomes exaggerated and simplex VIN ensues, eventually progressing to HPV-negative invasive SCC.³² Other theories of vulvar carcinogenesis focusing on the inflammatory and dermal scarring of vulvar lichen sclerosus also implicate simplex VIN as a potential precursor of vulvar SCC.³³

While clinically silent and apparently undiagnosed lichen sclerosus is frequently found adjacent to invasive SCC in surgical resection specimens, both prospective and retrospective studies of vulvar lichen sclerosus show that fewer than 5% progress to invasive SCC.^27,33 Only rarely do patients being followed for vulvar lichen sclerosus develop invasive vulvar SCC. On the other hand, a cumulative risk of 14.8% (compared with 0.06% in the general female population) and a relative risk of 246.6 were found in a longitudinal cohort study of patients with histologically confirmed vulvar lichen sclerosus.³⁴ Some investigators postulate that effective treatment of lichen sclerosus might lessen the likelihood of progression to SCC, but this is unproved. Other events must occur in a very small portion of patients with lichen sclerosus before transformation to invasive cancer develops. It's likely that simplex VIN-or other closely related highly differentiated epidermal proliferative lesions with even subtler morphologic changes-precedes progression to invasive SCC, possibly following mutation of the p53 tumor suppressor gene or other critical molecular genetic perturbations.

Roadblocks to detecting simplex VIN

If simplex VIN is the immediate precursor of most vulvar invasive SCC, why is it so infrequently diagnosed? There are several possible explanations.²⁵ First, underdiagnosis of simplex VIN continues to be a problem. We have seen it mistaken for squamous hyperplasia and chronic vulvitis. More importantly, perhaps, is the likelihood of a limited window of opportunity-that simplex VIN has a relatively brief intraepithelial phase before it progresses to invasive carcinoma.^1,25 When superficially and minimally invasive SCC are carefully studied, the overlying epidermal lesion often resembles simplex VIN, and it may be difficult to determine in some simplex VIN lesions whether "early" dermal invasion has already occurred.²⁵ Thus, simplex VIN may not be detected before it progresses to invasive carcinoma and obscures the precursor lesion. The disease is most likely to be discovered in women who are under close surveillance because of a prior cancer or a dystrophic vulvar condition such as lichen sclerosus. These patients are more likely to be helped in time because they're more apt to undergo early biopsy of the small vulvar lesions in which simplex VIN resides. With increasing clinician awareness of simplex VIN's likely role as a precursor of invasive SCC, my hope is that far more women will be diagnosed before progression takes place.

Key points

Acknowledgment

The author thanks Mrs. Patricia Matkovic for her secretarial assistance in the preparation of this manuscript.

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33. Carlson JA, Ambros R, Malfetano J, et al. Vulvar lichen sclerosus and squamous cell carcinoma: a cohort, case control, and investigational study with historical perspective; implications for chronic inflammation and sclerosis in the development of neoplasia. Hum Pathol. 1998;29:932-948.

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Dr. Hart is Chairman, Division of Pathology and Laboratory Medicine, The Cleveland Clinic Foundation, Cleveland, Ohio.

William Hart. Simplex (differentiated) VIN: an underappreciated threat.

Contemporary Ob/Gyn

Apr. 1, 2003;48:59-83.