Oral premalignancies and squamous cell carcinoma

Oral Premalignancies and Squamous Cell Carcinoma SOL SILVERMAN JR, MA, DDS PHILIP B. SUGERMAN, BDS, PhD, FRACDS, FDSRCS, FFOPRCPA Five-year survival rates for oral squamous cell carcinoma (SCC) are poor. Even in developed countries with the advances made in surgery, radiation, and chemotherapy, the overall survival for oral cancer approximates only 50%. As a result, therapy is usually aggressive in attempts to improve survival; however, when this is accomplished, morbidity is increased, which tends to lower significantly the quality of life. Explanations for this bleak picture are primarily due to delays in diagnosis, and subsequently a large number of advanced-staged tumors. This compromises surgical tumor margins, increases regional metastases, and reduces the effectiveness of radiation due to biologic factors related to tumor size. Therefore, because of delays in diagnosis, prevention becomes a crucial priority. Prevention, if possible, involves controlling the known risk factors of tobacco and alcohol consumption. Another important factor in reducing the occurrences of oral cancers is the recognition and management of established precancerous lesions.

The Cause of Oral Cancer Although the cause of oral cancer is unknown, malignant transformation results from genetic damage. There is increased risk of oral cancer associated with exposure to genetic mutagens in tobacco, alcohol, and betel quid.1,2 Gene mutations have been detected in oral SCC in chromosomes 3p, 8p, 9p, 13q (retinoblastoma [Rb] tumor suppressor gene), 17p (p53 tumor suppressor gene), 18q (deleted in colon cancer [DCC] tumor suppressor gene), and 21q.3 Under normal circumstances, the p53 tumor suppressor protein will detect DNA damage and halt progression through the cell cycle. In some instances, p53 will trigger apoptosis (programmed cell death) in cells with genetic damage.4 If the p53 gene is itself mutated, however, the protection offered by the p53 tumor suppressor protein against DNA damage is lost. Mutated p53 will not detect DNA From the Department of Stomatology, School of Dentistry, University of California, San Francisco, California USA; and Department of Oral Biology and Pathology, School of Dentistry, The University of Queensland, St. Lucia, Queensland, Australia. Address correspondence to Sol Silverman Jr., DDS, Department of Oral Medicine, School of Dentistry, University of California, San Francisco, Box 0422, 521 Parnassus Avenue, San Francisco, CA 04143 USA. © 2000 by Elsevier Science Inc. All rights reserved. 655 Avenue of the Americas, New York, NY 10010

damage and cells will continue to divide, passing on their gene mutations to subsequent generations. Mutation of the p53 tumor suppressor gene is the most common genetic lesion in human neoplasia.5 It therefore seems likely that cancer results from a deactivating mutation of the p53 tumor suppressor gene and a simultaneous activating mutation of a growth-promoting gene. If such gene mutations occur in an oral keratinocyte, oral cancer may result. In support of this hypothesis, the p53 tumor suppressor gene was found to be mutated in up to 80% of oral cancers.6 –9 Mutation of p53 may precede10 or accompany11 the transition from oral precancer to SCC. In oral SCC, p53 expression correlates with a history of heavy smoking12 and is associated with increased epithelial cell proliferation.13,14 The genetic hypothesis predicts a role for hyperactive oncogenes (growth-promoting genes) in oral carcinogenesis. Both growth and division of a normal cell are controlled largely by its surroundings.15 External growth-stimulatory signals, originating from neighboring cells or the circulation, interact with receptors on the cell surface. A cascade of proteins transmits the growth signal from the cell membrane to the nucleus. The cell responds to the growth-stimulatory signal by activating its synthetic machinery, copying DNA and dividing. Oncogenes encode many of the signal-transmitting proteins via which cells respond to external growth signals. Normal cells, with normal oncogenes, will not commit themselves to another round of DNA replication and cell division without stimulation from such external signals. With oncogene mutation, however, the mutant oncoprotein may send a growth-stimulatory signal to the nucleus, regardless of events taking place in the cell’s surroundings. The subsequent autonomous proliferation of mutant oncogene-bearing cell results in tumor formation.15 The ras oncoprotein lies on the internal aspect of the cell membrane and is involved in the transmission of the epidermal growth factor (EGF) growth-stimulatory signal to the nucleus. Overexpression of ras oncoprotein transmits a growth message to the cell nucleus, even in the absence of binding between EGF and its receptor on the cell surface.16 Many studies have shown ras oncogene mutation and ras oncoprotein overexpression in oral SCC.17–21 Furthermore, oral premalignant lesions 0738-081X/00/$–see front matter PII S0738-081X(00)00146-2

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and SCC may be associated with upregulated EGF receptor expression.22–25 Oral cancer may therefore result from mutation of an oncogene in the EGF pathway. The mutant oncoprotein would send a continuous growth signal to the nucleus, resulting in continuous oral keratinocyte proliferation and tumor development. As discussed, such an oncogene mutation would normally be detected by the p53 tumor suppressor protein and the cell cycle halted; however, oral keratinocytes with simultaneous oncogene and p53 mutations would proliferate to form oral cancers. Infection of oral keratinocytes with human papillomavirus (HPV) may be involved in the development of oral SCC in some patients. A role for HPV in oral SCC is supported by findings of HPV in tumor tissue and by studies showing that HPV immortalizes oral keratinocytes in vitro.4 The E6 and E7 genes of HPV16 and HPV18 encode proteins that are involved in the direct destruction of the p53 and Rb tumor suppressor proteins, respectively.4

Field Cancerization Some oral cancer patients develop SCC over a broad area of the oral mucosa, with multiple lesions arising simultaneously or over a period of time. The high incidence of second primary cancers in patients with oral SCC was first reported in 1953.26 The investigators proposed the term “field cancerization” to describe this phenomenon, and subsequent reports have confirmed their observations.27,28 Approximately 2–3% of oral cancer patients develop a second primary cancer each year after removal of the primary tumor29,30 and 90% of recurrences manifest within 2 years of initial treatment.31 With advances in therapy, more patients survive initial tumors. Hence, the incidence of second primary oral cancers is expected to rise.32 Unfortunately, tumor recurrence or a second primary tumor has a significant adverse effect on survival of oral cancer.33,34 Hence, the identification of a predictive marker for second primary oral cancers would have significant prognostic and patient management implications. The mechanisms of “field cancerization” are unknown, although “field changes” (molecular changes throughout the oral mucosa of oral cancer patients) may predispose to the development of multiple primary cancers. Three basic hypotheses were recently provided32 to explain the development of multiple primary oral cancers. First, a large region of the oral mucosa may be exposed to the etiological agent(s) that causes independent transformation of multiple epithelial cells at separate sites. A single etiological agent acting at different sites would cause multiple separate cancers with identical genetic defects, each arising as a separate clone within the oral mucosa. Subsequent genetic modifications (due to spontaneous mutation or continued expo-

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sure to exogenous mutagens) may render the separate clones genetically distinct. Different etiological agents acting at different sites would cause multiple separate cancers with different genetic defects, each arising as a separate clone within the oral mucosa.32 Second, a combination of etiological agents may result in the transformation of a single oral epithelial cell. The expanding clone of cancer cells might spread through the oral mucosa via local tissue spread, regional blood vessels, seeding via the saliva into a mucosal erosion or seeding due to the trauma of surgery.32,35 This would give rise to geographically distinct but genetically identical cancers. Subsequent genetic modifications (due to spontaneous mutation or continued exposure to exogenous mutagens) may mask the clonal origin of tumors at different sites.32 Third, a tumor may have a paracrine effect on the adjacent oral mucosa, making it more susceptible to the development of oral cancer. Studies have shown reduced cytoplasmic area,36 alterations in keratin expression,37 upregulated EGF receptor expression,25 and p53 mutation38 in histologically normal epithelium adjacent to oral cancers. The clinical significance of p53 mutation within the normal mucosa of oral cancer patients is unclear. Some reports suggest an association with the development of second primary cancers39 while others find no such association.40,41 Tumors may secrete tumor inhibitory factors such as inhibitors of neovascularization.42 Removal of the primary tumor would remove these inhibitors of cancer development and hence promote second primary tumor formation.32 Alternatively, tumors may secrete promoters of apoptosis. Removal of the primary tumor would reduce the level of apoptosis in adjacent tissue and hence promote second primary tumor formation.32

Oral Premalignancies Oral premalignancies can be best characterized as lesions in which there is a risk for uncontrolled cellular growth and transformation into cancer, followed by the disruption of normal functioning tissues. This pathologic process of oral premalignancies primarily affects the stratified squamous epithelium that lines the entire oral cavity and oropharynx. The most frequent clinical manifestations are the leukoplakias that are due to the biochemical process of hyperkeratosis. A sound scientific explanation between the association of excess keratin formation and malignant transformation is unknown; the relationship is based upon epidemiologic findings of an excess occurrence of carcinomas in these individuals compared with the general population (Fig 1).43– 45 Hyperkeratosis leads to a white patch or plaque that cannot be scraped from the mucosal surface. Another high-risk lesion is the red-appearing patch (erythroplakia or erythroplasia). Often there is a com-

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Figure 1. Squamous cell carcinoma occurring in a long-standing area of leukoplakia of the lateral tongue. The previously benign hyperkeratotic lesion had also been symptomatic.

Figure 3. Symptomatic erythroplakia developed in an asymptomatic buccal leukoplakia after 12 years. A biopsy from the newly developed red area of the posterior buccal mucosa showed squamous cell carcinoma.

bined white and red appearance (erythroleukoplakia). In these instances, it is more likely that dysplasia or carcinoma will occur as compared with the homogeneous white patch. In most cases, prediction of the transformation to malignancy is inaccurate. Therefore, cellular markers are being developed and tested to improve the accuracy of identifying those lesions that are most likely to become cancerous. Molecular genetics will also contribute eventually to this assessment. Therefore, at the present time, we are limited to the recognition of risk factors to help formulate clinical assessments and management.

degrees of dysplasia, as well as carcinoma in situ; however, transformation rates following the microscopic recognition of dysplasia have been significantly high. Because of that risk, excision is usually indicated. Although regression of dysplasia can occur, it is unusual. A red-appearing lesion may also be a manifestation of epithelial dysplasia, or even carcinoma.45,49 Therefore, a mucosal change of this nature must either disappear or be microscopically evaluated. As already cited, a red component to a clinically white lesion increases the possibility of dysplasia and carcinoma (Fig 3). Therefore, this appearance increases the risk of transformation and must be investigated. Because leukoplakias are usually not associated with discomfort, symptomatic lesions might indicate a transformation and should be recognized as a risk factor. The clinical appearance and behavior of a leukoplakic lesion can at times signal an increased risk. A form of leukoplakia, referred to as proliferative verrucous leukoplakia, has demonstrated a high risk for malignant transformation (Fig 4A,B).50 Thus, a more aggressive treatment approach is indicated. There may be an association with human papillomavirus type 16 that may account for this behavior. Candida species are often associated with leukoplakia. Their role is uncertain. Candida is capable of producing carcinogenic nitrosoamines through biochemical tissue reactions. Additionally, candidal hyphae are often seen in leukoplakic microscopic sections. Though the association is not clear, candidal infection must be considered as a potential risk. Smoking has been associated with hyperkeratosis and leukoplakia; however, a paradox exists: in those patients with oral leukoplakia who do not smoke, transformation risks appear to be higher! Moreover, though smokeless tobacco is associated with an increased risk for developing carcinoma,

Risk Factors for Premalignancies Probably the most accurate prediction of transformation is based upon microscopic epithelial dysplasia (Fig 2).46 – 48 Frequently, there is some disagreement on the

Figure 2. Microscopic transition from benign hyperkeratosis (right) to severe epithelial dysplasia (left).

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Figure 4. Proliferative verrucous leukoplakia. (A) The tongue lesion showed microscopic areas of dysplasia. (B) The palatal lesion showed areas of squamous cell carcinoma.

the primary factor is the long-time usage. Cessation of the habit will usually lead to disappearance of an associated epithelial change. The most common association with oral cancer is aging, which is also true for the leukoplakias. This makes biologic sense; the very sensitive homeostatic mechanism controlling epithelial growth is influenced by the behavior of oncogenes, which, in turn, appear to be responsive to time-related exposures to viruses and to chemical/physical agents. It must be remembered, even if a patient with leukoplakia does not demonstrate any of the above risk factors, transformation to malignancy may occur. Therefore, long-term follow-up of all leukoplakic lesions is mandatory.

Diagnosis of Premalignant Lesions The diagnosis of premalignant lesions depends upon clinical suspicion and microscopic assessment following biopsy(s). Obviously, disappearance of a lesion

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helps rule out malignancy and premalignancy at that time. Often, biopsy is delayed because of patient or clinician choice, and/or the institution of other treatment to rule out infection or local irritation; however, these reasons should not delay a definitive diagnosis for more than 3 to 4 weeks. Sometimes the extensiveness of a lesion complicates the most appropriate area to biopsy in order to rule out dysplasia or carcinoma. Therefore, other supplements to clinical judgment (not substitutes) are helpful in both accelerating the biopsy and selecting the most appropriate area to sample. The most useful, because of accuracy, low cost, quickness, simplicity, and noninvasive nature, is the application of toluidine blue dye.51 This involves applying a 1% aqueous solution of toluidine blue to the lesion, rinsing with water, applying a 1% solution of acetic acid, rinsing with water, and observing any binding. In our experience, the accuracy has exceeded 90%. The probable mechanism is the affinity/binding of toluidine blue with DNA and sulfated mucopolysaccharides, both of which are selectively high in dysplastic and malignant oral epithelium. Exfoliative cytology has been both helpful and accurate, utilizing a new brush biopsy technique. The question often arises as to the timing of rebiopsy once a precancerous lesion has been initially evaluated. First, if there is evidence of dysplasia, both removal and follow-up are recommended. If there is no dysplasia and a lesion is not removed, then at least periodic follow-up is essential. This involves a thorough clinical examination as well as toluidine blue application. If there is any change in signs and/or symptoms, then rebiopsy is indicated. Changes include dye uptake, clinical features of spread or proliferation, the development of a red/erythematous component, erosion or ulceration, or discomfort/pain. Any of these characteristics may indicate dysplasia, a worsening of existing dysplasia, or transformation to carcinoma.

Management of Premalignant Lesions As indicated, a thorough initial evaluation of signs and symptoms is essential, which includes a biopsy and subsequent follow-up. This is true for follow-up even if what appears to be a precancerous lesion disappears. Treatment involves surgical removal. Margins often create a problem and explain recurrences; this is because epithelial cells that both clinically and microscopically appear normal may have molecular pathology that reestablishes the pathologic process. Biologic markers can help diminish this problem. Laser techniques have helped improve our surgical approaches and ultimate control. Chemoprevention utilizing vitamin A analogues (retinoids) and other antioxidant vitamins and nutrients (beta carotene, vitamins C and E) have not been effective in well-designed, prospective stud-

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ies.52 Problems in chemoprevention involve toxicities and recurrences. Eventual effectiveness will follow when we learn more about and are able to coordinate dosages, regimens, and patient profiles.53 In the meantime, a helpful nutritional approach would be the daily intake of a diet rich in fruits and vegetables.54 In summary, recognition and control of premalignant lesions is an effective approach to reducing the occurrence, and thus the morbidity and mortality, of oral cancer.

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15. 16.

Acknowledgments Philip Sugerman is supported by a National Health and Medical Research Council (NHMRC, Australia) C. J. Martin Postdoctoral Fellowship.

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