Squamous lesions of the esophagus encompass a spectrum of disorders ranging from reactive changes and benign papilloma to squamous dysplasia and squamous cell carcinoma, which may pose diagnostic challenges especially in superficial biopsies.
To provide a review on the typical features of squamous neoplasia in the esophagus, with an emphasis on the key diagnostic features as well as differential diagnosis from mimicking lesions.
Data sources include published peer-reviewed literature and personal experiences of the authors.
Accurate diagnosis of squamous neoplasia requires adherence to established diagnostic criteria, attention to subtle histologic features, and correlation with clinical and imaging findings. In difficult cases, multiple biopsies may be necessary to reach a definitive diagnosis.
In the Western world, Barrett dysplasia and adenocarcinoma arising in the setting of Barrett esophagus are the most common neoplastic lesions in the esophagus, which far outnumber squamous neoplasia. As such, there is much less attention to squamous neoplasia in the esophagus, which often poses a diagnostic challenge to gastrointestinal pathologists. Squamous disorders of the esophagus encompass a spectrum of lesions, which include esophagitis with reactive epithelium, squamous papilloma, squamous dysplasia, and squamous cell carcinoma (SCC). This review discusses the neoplastic squamous lesions in the esophagus, with an emphasis on the pathologic diagnosis and differential diagnosis.
Esophageal squamous papilloma is a rare esophageal lesion that was first described by Adler et al1 in 1959. The prevalence of squamous papilloma in esophagogastroduodenoscopy has been reported to vary from 0.01% to 0.43%.2 It is mostly seen in middle-aged males,2 though it may be present in the pediatric or elderly population.3 In a US series of low-incidence population, 58% of squamous papillomas are located in the lower esophagus.4 In the high-incidence areas, like China, squamous papillomas are more often located in the middle esophagus.3
The pathogenesis of squamous papilloma is not well understood. Two etiologic mechanisms have been proposed. The first one is chronic mucosal irritation due to chemical and mechanical factors, such as reflux disease, minor trauma, alcohol, and smoking.2 The second mechanism is related to human papilloma virus (HPV) infection, which is however not consistently identified in squamous cell papilloma.2,4,5 In the literature, the prevalence of HPV-positive esophageal squamous papilloma varies between 0% and 87.5%. Lavergne and de Villiers6 used the polymerase chain reaction technique and found that 63.6% of squamous papillomas harbored HPV, and more than half of them had HPV-6. Bohn et al7 found that HPV was present in 80% of the squamous papillomas and in most of the cases they were also the low-risk types. Studies have failed to conclusively document the role of high-risk HPV (such as types 16 and 18) in esophageal squamous papilloma by either polymerase chain reaction or in situ hybridization.2,8
Patients with squamous papilloma are usually asymptomatic and the papilloma is discovered as an incidental finding during esophagogastroduodenoscopy.9 Giant squamous papillomas are known to occur and can present with such symptoms as dysphagia and coughing.10 During endoscopy, squamous papilloma often shows a single, small (usually <0.5 cm), whitish, and elevated sessile lesion or with vessels crossing on a wart-like surface.3 Endoscopic appearance is usually characteristic but not pathognomonic. Hence, a firm diagnosis depends on histologic findings and differentiation from other entities.
The characteristic histologic features of squamous papilloma include fibrovascular core with multiple finger-like projections covered by hyperplastic squamous epithelium, sometimes with koilocytosis (squamous cells with perinuclear halo, cytoplasmic thickening, and irregular nuclei)11 (Figure 1, B). The proliferation patterns of squamous epithelium are most often exophytic, but they can also be endophytic or spiked with rare dysplasia.3,4 The overlying squamous epithelium generally has an orderly maturation, resembling normal esophagus (Figure 1, A), though mild atypia can be seen in the spiked papilloma. Inflammation is sparse. The differential diagnoses include acanthosis, hypergranulosis (epidermoid metaplasia, see details below and in Figure 1, C), hyperkeratosis, fibrovascular polyp, and well-differentiated SCC.2,3 Pseudoepitheliomatous hyperplasia (Figure 1, D) may occasionally enter into the differential diagnosis for squamous papilloma, though its main differential is SCC (see details below).
Squamous papilloma is generally cured by endoscopic resection and its malignant potential is negligible. Unlike anogenital region where condyloma is considered a premalignant lesion, the progression of esophageal squamous papilloma to SCC and its association with high-risk HPV has not been conclusively proven.11 Even in “high-incidence” areas the potential malignant progression could also be attributed to environmental factors present in those areas.6 Rarely squamous dysplasia or even SCC has been reported arising from squamous papilloma; however, it is unclear whether some of the reported cases actually represented well-differentiated SCC with papillary growth to start.2,11
Squamous dysplasia is considered a precursor lesion of esophageal SCC.12 In developing countries, the prevalence of squamous dysplasia is highest in Iran (4%) and China (Taihang Mountain, 20% range), similar to SCC.13 The risk factors for esophageal squamous dysplasia have been studied in the high-incidence Taihang Mountain region in China via questionnaires. These analyses found significantly increased risk of squamous dysplasia among persons who had a positive family history of cancer, high systolic blood pressure, used a heating stove without a chimney, lost more but not all of their teeth, and had high serum 25-hydroxyl vitamin D. The association with HPV was inconsistent. The risk factors were found to be concordant for both dysplasia and SCC, except smoking (not a risk factor for dysplasia) and serum pepsinogens (not a risk factor for SCC).13
The classification and histologic criteria for the diagnosis of squamous dysplasia has evolved over the years. Dawsey et al14,15 described, for squamous dysplasia, there were nuclear atypia, loss of normal cell polarity, and abnormal tissue maturation, without invasion through the basement membrane. Squamous dysplasia was traditionally separated into the following 3 progressive grades: mild, moderate, and severe dysplasia. Wang et al16 suggested that the increasing grade of dysplasia increases the chance of progression to esophageal SCC, with a relative risk of 2.9 for mild dysplasia, 9.8 for moderate dysplasia, and 28.3 for severe dysplasia, respectively. Analogous to squamous dysplasia in other organs, such as anorectal region, in mild dysplasia the abnormalities were confined to the lower third of the epithelium; in moderate dysplasia they were present in the middle third of the epithelium; and in severe dysplasia, the upper third of the epithelium was involved. Full thickness involvement of the epithelium without maturation at the surface (defined as carcinoma in situ) was included under severe dysplasia. Dysplasia not otherwise specified was diagnosed when dysplasia was present but could not be graded accurately because of poor tissue orientation or another artifact.
Recently, there is a trend to consolidate squamous dysplasia into a binary grading system similar to cytology specimen, though this is not uniform across all organ systems (Table 1). For noninvasive lower anogenital lesions, which are mostly related to HPV infection, the recommended terminology includes low-grade squamous intraepithelial lesion and high-grade squamous intraepithelial lesion, where the lesion that was formerly diagnosed as moderate dysplasia is now generally classified as high-grade squamous intraepithelial lesion.17–19 In the oral mucosa, squamous dysplasia was previously graded as mild, moderate, and severe dysplasia. The current World Health Organization (WHO) classification recommends a 2-tiered system (low and high grades). The grading criteria are rather complicated and based on a combination of multiple cytologic and architectural changes.20 The larynx also employs a 2-tiered system, with low-grade dysplasia showing mild cytologic atypia limited to the lower half of the squamous epithelium, and high-grade dysplasia showing cytologic atypia extending to the upper half of the epithelium or marked cytologic atypia. Atypical mitosis necessitates the designation of high-grade dysplasia.21 A 4-tiered system is still used to grade bronchial precursor lesions (mild, moderate, severe dysplasia, and carcinoma in situ).22
The 2010 WHO classification of digestive system tumors recommended the binary system and separated esophageal squamous precursor lesions into low- and high-grade intraepithelial neoplasia.23 This classification is largely retained and further defined in the 2019 new WHO classification of digestive tumors (5th edition), where “dysplasia” is a preferred term over “intraepithelial neoplasia.”24 Dysplastic changes include architectural and cytologic abnormalities. Examples of the former include cellular disorganization/loss of polarity and downward growth of the epithelium, while the latter includes hyperchromasia, increased nuclear to cytoplasmic ratio, and mitotic activity.23 In low-grade dysplasia, the abnormalities are confined to the lower half of the squamous epithelium and there is only mild cytologic atypia (Figure 2, B; for comparison reactive change is shown in Figure 2, A). High-grade dysplasia is diagnosed when more than half of the squamous epithelium is involved or when severe cytologic dysplasia is present regardless of the extent of the epithelial involvement (Figures 2, C and D).24 This classification system is similar to the WHO classification of squamous dysplasia in the larynx.21 The WHO classification acknowledges the presence of other systems in grading squamous dysplasia, and in a binary system, severe dysplasia and carcinoma in situ are included in the rubric of high-grade dysplasia and may have the same clinical implications.23
The main diagnostic challenge is to separate squamous dysplasia from various benign reactive squamous lesions. Normal squamous epithelium is stratified, with orderly maturation, preserved polarity, and abundant clear or eosinophilic cytoplasm (Figure 1, A); scattered lymphocytes and compressed nuclear fragments (squiggle cells) were occasionally seen in the epithelium. The lamina propria, if present, commonly contains a few scattered mononuclear inflammatory cells.13–15 Reactive change in esophagus, such as reflux esophagitis, often shows basal cell hyperplasia with basal zone thickness greater than 15% of the total epithelial thickness, and may be accompanied by elongation of lamina propria papillae or other abnormality.13–15 The cytologic features are rather regular with maturation (Figure 2, A). Acanthosis is defined as portion of epithelium measured from basal lamina to surface greater than or equal to 0.5-mm thick without evidence of esophagitis, squamous dysplasia, or carcinoma.14,15
Esophageal epidermoid metaplasia is a related condition. It predominantly affects middle-aged to elderly men, and is commonly seen in the middle to distal esophagus. It has a strong association with tobacco smoking and alcohol intake and is considered a precursor to SCC. Endoscopically, it is seen as a distinct leukoplakia. Histologically, the lesion is sharply demarcated from the uninvolved esophageal mucosa (Figure 1, C). The squamous mucosa is undulated with flattening of rete ridges. The basal cell layer is thickened and the mid-zone shows moderate acanthosis and a prominent granular layer. The squamous mucosa is surmounted by varying degrees of compact hyperorthokeratosis. A layer of parakeratosis is often present between the hyperorthokeratosis and hypergranulosis.25 Recognition of this entity warrants a strict surveillance for squamous dysplasia and SCC.
Several biomarkers, such as Ki67 and P16, were found useful for the diagnosis of squamous dysplasia in the uterine cervix and the oral cavity; however, no such marker is in use for esophageal squamous dysplasia.26 Goran et al27 showed that scanning electron microscope is useful for differentiating dysplasia from reactive changes by loss of intercellular adhesiveness. Molecular changes in premalignant lesions have not been studied in detail. Studies that use immunohistochemical stains provide the most widely available data to evaluate the biology of these lesions. P53 has been shown to be overexpressed in squamous dysplasia compared with normal esophagus.13 Other proteins with increased expression in squamous dysplasia include CD44, transforming growth factor beta, proliferating cell nuclear antigen, Fas-associated death domain protein, CDC25B, fascin, cytokeratin 14 (CK14), laminin subunit gamma-2, SPARC, P15, and protein patch hemolog,13 though their usefulness in the routine diagnosis of squamous dysplasia is unknown.
Follow-up of the untreated patients in the high-risk Chinese population with squamous dysplasia (based on the 3-tiered system) showed progression to clinically diagnosed SCC in 5% of mild, 27% of moderate, and 65% of severe dysplasia after 3.5 years; and 24% of mild, 50% of moderate, and 74% of severe dysplasia after 13.5 years of follow-up, respectively.14–16 Based on the available data, severe dysplasia generally requires prompt treatment, moderate dysplasia can be treated or followed at periodic intervals, while mild dysplasia may be followed up at longer intervals.13 Screening for dysplasia and esophageal cancer was also proposed, similar to cervical dysplasia. However, studies in this area are inadequate and supporting data are lacking.13 The main treatment for squamous dysplasia includes endoscopic excision and ablation. The 2 major types of excision are endoscopic mucosal resection using the “cap” or “banding” method, and endoscopic submucosal dissection.13–15 Ablation includes multipolar electrocoagulation, argon plasma coagulation, and radiofrequency ablation.13,28
SQUAMOUS CELL CARCINOMA
Esophageal cancer is the eighth most common cancer and sixth most common in terms of cancer-causing deaths, with an estimated 482 000 new cases and 407 000 deaths in 2008.13 Adenocarcinoma and SCC are the 2 major histologic types of esophageal cancer. Although adenocarcinoma is much more common in the United States, SCC continues to predominate worldwide.29,30
Esophageal SCC occurs most frequently in the middle third of the esophagus, followed by the lower and upper thirds.31 Risk factors for esophageal SCC include genetic factors, tobacco use, alcohol, hot beverages, preserved food, and other medical conditions, such as esophageal achalasia and Plummer-Vinson syndrome.32 SCC comprises 2 groups based on the geographic location and etiologies. In developed countries, SCC is more related to alcohol and smoking, while in developing countries it is related to undernutrition, thermal damage, opium, polycyclic aromatic hydrocarbons, and tobacco exposure only plays a minor role.13 HPV has been reported as one of the less common causative factors for esophageal SCC. Worldwide HPV infection rates in esophageal SCC range from 11.7% to 38.9%, with wide geographic variation.33 In “high-incidence” areas, such as China and South Africa, a spectrum of HPV types were recovered in approximately one third of SCC cases.6 In the “low-incidence” areas, such as Europe and the United States, the association with HPV is less obvious, and the reported prevalence is around 10%.33 For high-risk HPV, 1 analysis reported an infection rate of 11.7% for HPV-16 and 1.8% for HPV-18, respectively.34
Frequently, squamous dysplasia serves as a precursor lesion of SCC. Invasive esophageal SCC is defined as uncontrolled proliferation of neoplastic squamous epithelial cells penetrating the basement membrane and invading into and/or beyond the lamina propria. Tumor cells show polypoid, irregular nests/sheets, or solid growth pattern. Keratinization, inflammatory cell infiltration, and desmoplasia are variably present. Cytologic presentations are similar to high-grade squamous dysplasia/SCC in situ as described previously, such as lack of maturation, pleomorphism, increased nuclear to cytoplasmic ratio, increased mitosis, hyperchromatic nuclei, and irregular nuclear contour. A 3-tiered grading system is commonly used for the grading of esophageal SCC based on its resemblance to normal squamous epithelium. Grade I is defined as well-differentiated SCC. The tumor cells have abundant eosinophilic cytoplasm, nuclear atypia and mitotic rate are low, and keratinization is often present. Grade III is defined as poorly differentiated SCC with basaloid tumor cells arranged in nests or sheets, mitosis and nuclear atypia are frequent, and keratinization is minimal. Moderately differentiated or grade II SCC shows features intermediate between Grades I and III, where cytologic atypia and mitosis are present and well-formed keratin pearls are rare.30
The diagnosis of esophageal SCC can be challenging, especially when only small-sized superficial biopsy specimens are available, and cytologic dysplasia may be subtle and difficult to appreciate. The differential diagnoses for well-differentiated SCC include reactive changes such as esophagitis, squamous dysplasia, squamous papilloma, pseudoepitheliomatous hyperplasia (Figure 1, D), and other poorly differentiated epithelioid neoplasms. Attention to subtle features, such as complex papillary architecture, pushing invasive growth, bulbous projection, keratin pearl in rete ridges, irregular tumoral island, and early desmoplasia at the epithelial/stromal interface, often help clinch the diagnosis of SCC (Figures 3, A and D). Clinical correlation is also helpful, where SCC often presents as a mass lesion (often >3 cm). If SCC is suspected clinically and initial biopsy is nondiagnostic, repeat biopsy or surgical excision is often necessary.
Pseudoepitheliomatous hyperplasia is an important differential diagnosis for SCC. Pseudoepitheliomatous hyperplasia shows squamous proliferation, but lacks cytologic atypia and invasive growth (Figure 1, D). It may be a primary lesion, or secondary to granular cell tumor, ulceration, and other squamous injury. In a small, unoriented biopsy however, it is very challenging to make this diagnosis. Communications between the clinician and pathologist are often helpful. Squamous papilloma is generally a small and incidental lesion, with papillary growth devoid of cytological dysplasia, as described previously (Figure 1, B). Compared with SCC, squamous papilloma often shows less prominent inflammation and/or parakeratosis. Metastatic SCC in the esophagus is rare. Coexisting squamous dysplasia especially high-grade dysplasia favors a diagnosis of primary esophageal SCC. Poorly differentiated SCC can be difficult to distinguish from poorly differentiated adenocarcinoma, poorly differentiated neuroendocrine carcinoma, undifferentiated carcinoma, and sometimes melanoma or lymphoma. Immunohistochemical studies are often necessary for an accurate diagnosis. Adenocarcinoma is frequently positive for CK7 and negative for squamous cell markers, such as CK5/6, P63, and P40. Neuroendocrine carcinoma often shows nested growth with prominent necrosis, and is positive for neuroendocrine markers, such as synaptophysin, chromogranin, and CD56.35,36 Undifferentiated carcinoma shows no evident line of differentiation with negative immunohistochemistry for squamous, glandular, and neuroendocrine markers. Melanoma is generally positive for S100, SRY-box transcription factor 10, Mart-1, and HMB45, while negative for keratin. Lymphoma (mostly diffuse large B-cell lymphoma) is usually positive for CD45 and B-cell markers.
As mentioned previously, HPV is not considered a major etiologic factor for squamous dysplasia and SCC in the esophagus, and currently P16 overexpression plays no role in its diagnosis. Hu et al5 proved that TP53 is a primary target for inactivation in esophageal SCC and plays a critical role in its pathogenesis. Of the 56 cases they studied, approximately 96% had at least 1 genetic alteration, including TP53 mutation (77%), allelic loss within the TP53 gene (73%), and/or loss of heterozygosity at the TP53 microsatellite marker (80%); 75% had 2 or more such alterations, including 59% with both a point mutation and an iatrogenic allelic loss.5
SQUAMOUS CELL CARCINOMA VARIANTS
In the new WHO classification of digestive system tumors, verrucous carcinoma, spindle cell carcinoma, and basaloid SCC are recognized as variants of esophageal SCC.30 Verrucous carcinoma is a rare variant of esophagus SCC, which is more likely to involve the distal esophagus.37, 38 The risk factors include tobacco, alcohol, inflammatory disorders such as reflux esophagitis, and HPV (HPV-51 was detected in 1 case report).37,39–46 Clinically, the tumor grows slowly and often presents as an exophytic warty projecting mass under endoscopy.40 Histologically, verrucous carcinoma shows a polypoid, well-differentiated, and keratinizing SCC with pushing rather than infiltrating border. Cytologic atypia and mitoses are minimal. Horizontal spreading of the tumor is often more prominent than vertical growth. Metastasis, including regional lymph node metastasis, is rare.38 Due to its well-differentiated cytologic features and acanthoma-like growth pattern, superficial biopsies frequently lead to the diagnosis of inflammation with reactive changes. In most reported cases of verrucous carcinoma, multiple biopsies were performed before a definitive diagnosis was rendered.38,40,47 Interestingly, coexisting fungal infection was discovered in a couple of cases.48,49 Verrucous carcinoma should be differentiated from benign lesions, such as papilloma and low-grade squamous dysplasia, which are confined within the epithelium. Despite its slow growth and low metastasis rate, recurrence of verrucous carcinoma is not uncommon and the prognosis is not very promising, possibly because of the late onset of symptoms and difficulty in early diagnosis.
Spindle cell SCC typically appears grossly as a large polypoid mass. It has been variably called sarcomatoid SCC, pseudosarcoma, and carcinosarcoma in some literatures due to the spindled mesenchymal-like component. Histologically, tumor cells have biphasic appearance with mixed squamous and spindle tumor cells. Differentiation into other mesenchymal components, such as osseous, chondroid, and muscular tissues, occasionally occurs.50,51 Squamous cell markers, such as cytokeratin CK5/6 and P40, are usually positive in squamous epithelial component. The spindle cells frequently express vimentin.52,53 Other mesenchymal markers, such as desmin, may be positive depending on the line of mesenchymal differentiation.52,53 The squamous component is essential for the diagnosis of spindle cell SCC. Either component, however, can be absent in a small biopsy specimen. If no squamous epithelial component is found after thorough examination of the entire tumor, the diagnosis of sarcoma should be considered. Another differential diagnosis is conventional SCC with dense desmoplasia. The major difference is the fibroblasts in desmoplastic changes are benign and reactive while the spindle cells in spindle cell SCC are malignant with atypia and possible mitosis. Although spindle cell SCC tends to have regional lymph node metastasis, the overall prognosis is better than conventional SCC.53 Patients with spindle cell SCC are more likely to be diagnosed at an earlier stage.53 The possible reason is a polypoid mass in the esophageal lumen may cause obstructive symptoms, prompting patient- seeking clinical intervention.
Unlike its counterpart in the head and neck area, basaloid SCC of esophagus is not associated with HPV infection.54 This variant of SCC is identified mostly in males.54,55 The endoscopic and gross appearance is similar to conventional esophageal SCC.56 Histologically, the tumor consists predominantly of basaloid cells with high nuclear-to-cytoplasmic ratio and scant cytoplasm. Nuclear atypia and mitosis are frequently present. The tumor cells are arranged in large nests, cribriform, microcyst, or solid sheets. Comedo necrosis can be seen sometimes in the center of the tumor nests. The basaloid component may be absent in superficial biopsies, leading to a diagnosis of conventional SCC or squamous dysplasia.57 Basaloid SCC is clinically aggressive and most patients present at later stages compared with conventional esophageal SCC,57,58 though when stratified by stage, its prognosis is statistically similar to conventional SCC. The cribriform or small cystic clear space in tumor nests can mimic adenoid cystic carcinoma histologically. In fact, basaloid SCC has been mistakenly diagnosed as adenoid cystic carcinoma historically.55 Adenoid cystic carcinoma has a biphasic pattern and consists of a mixture of myoepithelial cells and epithelial cells. Luminal epithelial cells are positive for CD117 (c-Kit) and myoepithelial cells can be highlighted by P63. Adenoid cystic carcinoma in the esophagus tends to have a better prognosis than basaloid SCC,55 thus an accurate diagnosis is important for patient management and prognostication. Other differential diagnoses of this variant include poorly differentiated neuroendocrine carcinoma, poorly differentiated adenocarcinoma, and undifferentiated carcinoma.
Squamous cell differentiation may be present in several other types of esophageal cancers. Adenosquamous carcinoma has a mixture of distinct malignant squamous and glandular components. No intermediate cells should be present. The definitive diagnosis is reached on surgical excision specimens for most cases. Either component is often absent on endoscopic biopsies, resulting in a diagnosis of SCC or adenocarcinoma.59,60 Mucoepidermoid carcinoma is thought to originate from the submucosal glands. Similar to its counterpart in the salivary gland, the tumor is composed of mucin-producing cells intermixed with nests of squamoid cells. The presence of intermediate cells is essential for the diagnosis. On superficial biopsy, it is often misdiagnosed as SCC or adenosquamous carcinoma.59 Whether the mastermind-like transcriptional coactivator 2 translocation characteristic of salivary gland mucoepidermoid carcinoma is also present in esophageal mucoepidermoid carcinoma is unclear.61 Both adenosquamous carcinoma and mucoepidermoid carcinoma of the esophagus show aggressive clinical feature with poor prognosis, comparable to poorly differentiated SCC,59,60,62 though the survival of mucoepidermoid carcinoma has improved during the past 2 decades.63
In summary, the pathogenesis of esophageal SCC is thought to follow a dysplasia-carcinoma sequence, and HPV is not a major causative factor in this process. SCC and its variants can be difficult to diagnose in superficial biopsies. Attention to the presence of squamous dysplasia, subtle features of invasion, and correlation with clinical and imaging findings can be helpful. In challenging cases, repeat biopsies are often necessary for a definitive diagnosis. When the tumor contains more than 1 component, discrepancies between biopsy diagnosis and final diagnosis in surgical excision frequently occur.
The authors have no relevant financial interest in the products or companies described in this article.
This work is part of the proceeding of the sixth Princeton Integrated Pathology Symposium (PIPS VI), which was held on April 7, 2019.