Abstract
Field cancerization was first described in 1953 as histologically altered epithelium surrounding tumor samples taken from the upper aerodigestive tract. Since then, the term has been used to describe multiple patches of pre-malignant disease, a higher-than-expected prevalence of multiple local second primary tumors, and the presence of synchronous distant tumors within the upper aerodigestive tract. Molecular techniques such as karyotype analysis, microsatellite analysis, p53 mutation screening, and X-chromosome inactivation studies have further refined the relationship among these lesions. While there are differences in the techniques used to identify the clonal origins of the lesions, these studies indicate that there is often lateral clonal spread of pre-malignant or malignant disease, and a significant portion of local second primary tumors are in fact genetically related. Distant second primary tumors found in the esophagus are often not related to concurrent head and neck cancer, whereas synchronous squamous lung tumors with a head and neck primary are often, in fact, metastases, rather than independently arising malignancies. These observations help to explain the high incidence of recurrent disease, despite excision or other therapy—pre-malignant or malignant clones often have the ability to migrate and persist outside of the field of treatment. Therefore, alternative means of prevention or therapy that can affect the entire head and neck region may be of benefit to such patients. Future studies will further refine the relationship among these lesions and perhaps identify key molecular alterations to be used as targets for gene therapy.
Introduction
The term field cancerization was first coined by Slaughter et al. in 1953 (Slaughter et al., 1953). The investigators examined pathology slides from 783 patients with head and neck cancer in an effort to understand the gross changes found in epithelia surrounding these tumors and explain their clinical behavior. It was discovered that all of the epithelium beyond the boundaries of tumor possessed histologic changes, and 88/783 (11%) of patients were found to have more than one independent area of malignancy. The conclusion drawn was that the mucosa of the head and neck had undergone a change, perhaps due to carcinogen exposure, and was therefore more susceptible to the development of many foci of malignant transformation. At the time of this study, there was no molecular basis for the observation. In fact, this study was performed the same year that Watson and Crick published their seminal article on the three-dimensional structure of DNA (Watson and Crick, 1953). However, many investigators have since attempted to use recent molecular techniques to elucidate the mechanism that underlies the clinical phenomenon of field cancerization.
While Slaughter did not describe a molecular foundation for his observations, the term has taken on a slightly different meaning in today’s literature. This phrase has been used to describe three phenomena: (1) a wide field of aerodigestive mucosa that tends to be affected by pre-malignant disease, (2) the frequent occurrence of multiple primary tumors in epithelial areas affected by widespread pre-malignant disease, and (3) the possibility of distant related primary tumors in the upper aerodigestive tract. The initial major debate has centered on the clonal relationship of large patches of affected mucosa and second primary tumors. These clinical expressions of a “field cancerization” phenotype could come about as a result of either independent molecular events affecting multiple cells separately, or as a molecular event in a single clonal progenitor that gives rise to this phenomenon, via mechanisms of widespread clonal expansion or an alternative means of undergoing lateral spread across the mucosa of the upper aerodigestive tract. Of course, these two mechanisms may not be mutually exclusive, and may be simultaneous and/or complementary events.
In this review, we will begin by discussing the concept of clonality and the molecular techniques used by researchers to elucidate clonal relationships between cell populations. A discussion of the theory of field cancerization as it pertains to head and neck squamous cell carcinoma (HNSCC) and distant second primary tumors of the aerodigestive tract will follow. We will then focus on the occurrence of multiple foci of pre-malignant or malignant disease in the upper aerodigestive tract and explore the different studies that have attempted to look at the question of clonality in these lesions. We will then put this into the context of the genetic progression involved in the development of head and neck squamous cell carcinoma (HNSCC) and discuss some of the possible therapeutic interventions in this disease. Finally, future areas of research that have impact on these observations will be highlighted.
Clonality & Molecular Methods of Determining Clonality
The idea of clonality has formed the basis for the way researchers view cancer and its development (Fialkow, 1976; Nowell, 1976). A single cell, altered by inactivation of a tumor suppressor gene(s) and/or activation of an oncogene(s), will gain a growth advantage and expand to form a clonal mass of cells, or tumor (Fearon and Vogelstein, 1990). In more practical terms, this is a dynamic process. Genetic alterations are acquired in a cumulative manner, often resulting in a progression through a continuum of benign, pre-malignant, and malignant neoplastic states. Throughout this process, successive waves of clonal outgrowth may occur, as a cell population increases in aggressiveness and in its selective growth advantage. While it is widely accepted that the cells within this spectrum of progression are all genetically related, there is variability in the degree of relationship between cells that have diverged early on the pathway to malignancy. To make matters more complex, some tumors have inherent defects in the ability of cells to maintain consistency in their genetic content. Depending on the degree of genetic relationship between two neoplastic processes, the ease of demonstrating a clonal relationship between them can vary widely.
The common theme in all of the techniques utilized to pinpoint a clonal relationship is identification of early, shared genetic alterations that are unique to the lesions and not found elsewhere in normal tissue. Thus, these molecular patterns form a type of DNA fingerprint. The difficulty remains that not all of the genetic mutations involved in cancer development are known, and these alterations are cumulative and may change as the cancer progresses. Therefore, one must trace back and assess the earliest known genetic alterations to find a common ancestry. Even then, a negative result does not necessarily disprove clonality—rather, it could be a limitation on the assay itself, or a lack of information about the initial mutations involved in cancer progression for a particular tumor type. Thus, clonality is often easier to prove than to disprove.
Investigators have performed statistical analyses looking at the incidence of second primary tumors in an attempt to “tease out” the associations between tumor types. One such study demonstrated that there was a greater-than-expected correlation between head and neck cancers and esophageal and lung cancers, giving epidemiological support to field cancerization (Begg et al., 1995). Furthermore, there was an unexpectedly high rate of bladder and kidney cancer associated with head and neck cancer. The association between head and neck cancer and bladder tumors, while fairly infrequent, does indicate that some second primary tumors, especially those located in non-contiguous epithelial systems, are unlikely to be clonally related. Rather, they are indicators of the degree of carcinogen exposure or general organism-wide susceptibility to carcinogenesis. However, this would support the concept that a group of carcinogens can simultaneously affect multiple organ epithelia when metabolized systemically.
An early cytogenetic technique used to determine clonality is karyotype analysis. This method involves looking grossly at the chromosomes arrested in metaphase between the two lesions and comparing their appearance, searching for clues such as ploidy and characteristic chromosomal breaks or re-arrangements. However, this is a rather insensitive method of detection, and since the cells may be constantly changing on a sub-microscopic basis, it is difficult to draw conclusions based only on such large-scale alterations.
Another common method used by many investigators is the use of p53 mutations. p53 is the most widely mutated gene across all cancer types (Hollstein et al., 1991) and has been shown to be important in the regulation of apoptosis and many other pathways. In head and neck cancer, approximately half of tumors have been found to contain some form of p53 mutation (Boyle et al., 1993; Kropveld et al., 1999). Alterations in this gene are thought to be relatively early events in HNSCC and therefore have been widely utilized as a method of determining clonality. However, there are many different forms of p53 mutations and polymorphisms, which make it a tedious assay to perform, since direct sequencing ought to be used for every sample. The presence of an identical p53 mutation is usually understood to indicate a clonal relationship, since there is no single, dominant mutation found in HNSCC—rather, over 100 different mutations have been described. Furthermore, since not all head and neck tumors contain mutations in p53, this assay cannot be used in all cases. However, p53 mutations are among the most reliable markers for head and neck cancer (van Houten et al., 2000) and have been used in a variety of studies to determine clonality.
X-chromosome inactivation is a method that capitalizes on the fact that, in females, each cell has a different pattern of inactivation of genes on the X-chromosome as the cells differentiate and mature. Therefore, clones can be identified on the basis of their X-chromosome methylation pattern. While this is a reliable method of detection, it is again limited in that it can be used only for female patients. However, inactivation of genes on the X-chromosome by methylation occurs early in the development of the fetus, and a different X-chromosome inactivation pattern has a high likelihood of excluding a clonal relationship between two neoplasms.
Finally, microsatellite alterations have been widely utilized to determine clonality between lesions. Microsatellites are tandem-repeat sequences found usually in non-coding regions that are scattered randomly throughout the genome. They have been well-studied and mapped onto different chromosomal loci, and therefore are useful for the mapping of genes relevant to cancer and other processes. Loss of allelic material adjacent to microsatellite markers, known as loss of heterozygosity (LOH), is a marker that can be used to characterize lesions by means of a simple, Polymerase chain reaction (PCR) -based technique. A head and neck genetic progression model has been developed by this method (Califano et al., 1996), outlining a temporal pathway for the accumulation of genetic alterations as defined by microsatellite loss. Therefore, markers that are altered early in the progression of HNSCC are utilized to maximize the probability of identifying common genetic patterns. Again, since no one marker is 100% affected in HNSCC, a panel of makers needs to be used. In addition, utilization of a single microsatellite to determine clonality within a set of tumors is less than ideal, since a single marker may be identically altered in unrelated tumors. Overall, this is a very effective method of demonstrating clonality, though the inclusion of a panel of markers is very important. Furthermore, not all of the loci have been identified in different cancer types, and there may always be additional markers that precede the known alterations.
In summary, there are many different molecular methods of demonstrating clonal relationships between samples, each with its own benefits and downfalls. The primary limitation of any one technique is that accurate determination of a clonal relationship relies on the use of multiple genetic alterations, usually found by a variety of techniques. Therefore, studies which rely on analysis of a single gene or alteration to determine clonality are inherently flawed, although sequencing analysis that demonstrates a single, identical point mutation in two tumors indicates a high likelihood of clonal relationship. In fact, it has been demonstrated that there is genetic variability within a single primary tumor, indicating that clonal relationships are a matter of degree, rather than a binary determination. However, most studies have a fairly high threshold for determination of a clonal relationship between two neoplasms, and those that are reported as clonal tend to be highly related.
The timing of genetic alterations also poses a difficult question. These studies are a snapshot in the history of the lesion and do not reflect the unique pathway that any one lesion followed to become a malignancy. Therefore, the ideal genetic test would be one that could identify rare alterations that: (1) did not occur by chance, (2) happened early in the progression to malignancy, and (3) occurred 100% of the time. Since such a gold standard has not yet been discovered or may not exist, our determination of relatedness should include both early and late known alterations that are assayed by a combination of different methods.
Distant Second Lesions
Because of the common conduit connecting the oral cavity, lungs, and esophagus, there is a similar exposure pathway to the mucosa from environmental carcinogens. As such, Slaughter’s observation of frequent synchronous or metachronous tumors in the aerodigestive tract is expected, based on elevated risk from carcinogen exposure alone. However, in the cases of synchronous tumors separated by large distances, it is of interest if these tumors arose as a result of independent events or from the same progenitor clone that subsequently migrated.
Patients with head and neck squamous cell carcinomas and concurrent esophageal squamous cell lesions have been studied for the relationship between the two tumors. One study looking at 16 such patients demonstrated, by the use of microsatellite markers, that the lesions were not clonally related in 14 of the patients (Califano et al., 1999a). However, two of the 16 patients had lesions that demonstrated clonal relatedness, one migrating a distance of 40 cm. Therefore, it is generally assumed that esophageal lesions in conjunction with head and neck squamous cell carcinoma represent two separate primary tumors rather than metastases.
A different study examined the question of synchronous lung tumors and their relationship to HNSCC (Leong et al., 1998). Samples from 16 patients with HNSCC and a concurrent solitary lung lesion were tested by microsatellite analysis. Ten of 16 samples (63%) demonstrated concordant patterns of loss at all loci tested, suggesting that the majority of solitary lung lesions were in fact metastases, rather than separate primary tumors. Another study looked at second primary tumors of the upper aerodigestive tract after primary HNSCC (Chung et al., 1993) by p53 mutation analysis. Although this study did not find a high rate of clonally related second primary tumors, it relied solely on a single genetic alteration when tumors were compared.
Therefore, the distance between two malignancies does not necessarily predict clonality. As a general rule, distant, peripheral, solitary, squamous lung lesions in conjunction with HNSCC are thought to be metastases, and concurrent esophageal tumors are thought to be separate primary tumors. While the probability of synchronous aerodigestive tract tumors remains high with environmental exposure, the relationship between them is often predicted by anatomic subset, rather than distance.
Pre-malignant Disease in the Upper Aerodigestive Tract
The upper aerodigestive tract is a common site of pre-malignant disease that is easily detectable and biopsied. It is well-known that a subset of these pre-malignant lesions will often develop into true malignancy and are thus treated by excisional biopsy. One study looking at oral cavity dysplasias demonstrated that from 6.6 to 36% of pre-malignant lesions transformed into invasive squamous cell carcinoma (Lumerman et al., 1995), while another large study discovered a 17.5% incidence of progression of oral cavity dysplasias to invasive carcinoma over an average time of 8.1 years (Silverman et al., 1984). Therefore, many studies have been performed on these precursor lesions to assess for clonality early in the progression to malignancy.
A genetic progression model for pre-malignant head and neck cancer was put forth involving microsatellite markers at ten commonly altered chromosomal loci (Califano et al., 1996). The finding that there was a stepwise progression of allelic loss as the lesions increased in histopathologic severity provided an explanation of how fields of altered mucosa could grow and develop. A single precursor cell with an early alteration could spread superficially, and as these daughter cells accumulate more alterations, they develop altered histologic findings until finally becoming malignant. While this study strongly supports the idea of clonal expansion, there may be limits to the extent to which clones can migrate.
Another study focused on five patients with recurrent pre-malignant lesions at the same anatomic site over time, from 1 to 144 months (Califano et al., 2000). The lesions were biopsied, and microsatellite analysis was performed at ten loci known to show loss of heterozygosity early on in cancer development. All of the serial biopsies performed demonstrated identical patterns of allelic loss at multiple loci, suggesting that they did indeed arise from the same clone. One of the patients had a lesion that migrated several centimeters, suggesting that lateral spread is feasible in the oral cavity.
Many studies have looked at pre-malignant disease or normal mucosa either surrounding the primary tumor or remaining after resection. Field cancerization would predict the presence of foci of pre-malignant change surrounding a malignancy. However, there are conflicting reports as to whether these foci are clonally related to the primary malignancy. In the gastrointestinal tract, there are data suggesting that lateral migration of altered clones occurs through crypt fission (Garcia et al., 1999). A similar mechanism involving cell adhesion mutations that would allow for both local and distant spread of pre-cancerous cells could exist in the head and neck as well. This concept does fit with the observation that second primary tumors after an index HNSCC are usually second HNSCC, indicating that proximity to an initial primary tumor confers a higher risk of subsequent malignant transformation.
Different molecular techniques have been used that favor evidence of multicentric disease that developed independently. Investigators (Lydiatt et al., 1998) used microsatellite analysis in 21 patients with oral cavity and oropharyngeal tumors to examine the relationship between the malignancy and the adjacent dysplasia and normal tissue. The investigators noted that 9p deletions occurred earlier than 3p deletions, and there was about a 50% concordance between 9p loss in both the normal mucosa and dysplasias when compared with the tumors. There were two cases of discordant loss at 9p21, and the rest of the samples were unclear. Ultimately, the study demonstrates that the selection of markers is essential—had only 3p markers been used, no clonal relationship would have been identified, whereas the 9p markers suggested clonality between dysplasias and normal mucosa adjacent to tumors in approximately half of the test cases.
Other studies using microsatellite analysis have demonstrated a 43% concordance between tumors and adjacent pre-malignant lesions (Jang et al., 2001). Using a karyotype analysis to assess aneuploidy, investigators noted that all of the synchronous pre-malignant and malignant lesions displayed discordant molecular patterns, implying a multifocal origin (Ai et al., 2001). However, it should be noted that karyotype analysis may be limited by its lack of sensitivity.
These studies all suggest that mucosa does not necessarily need to undergo histologic change to possess molecular alterations. Much of the periphery around the malignancy (histologically normal or pre-malignant lesions) was found to contain genetic alterations. One interesting study looked at 28 patients with HNSCC and assayed five non-contiguous regions of macroscopically normal appearing mucosa using microsatellite markers (Tabor et al., 2001). The investigators noted that ten of 28 patients had evidence of pre-malignant disease, despite the normal-appearing mucosa. The size of the affected field varied, though in seven out of ten patients tested, the field extended beyond the surgical margins. The expansion of the affected field could account for the high rate of local recurrence. Similar conclusions were drawn in a different study that examined the predictive value of microsatellite markers for progression (Partridge et al., 2000b). Thirty-nine cases of oral cavity dysplasias that subsequently progressed into tumors were noted to have a higher chance of development if LOH was noted at two or more loci. The carcinomas were noted at contiguous and distant sites, suggesting that either the clones migrated, or the field of affected mucosa extended beyond the boundaries of the dysplastic lesion. This idea of clonal migration has also been shown in the esophagus, whereby clones arising from a field of Barrett’s disease have been shown to migrate and develop into pre-malignant and malignant disease (Raskind et al., 1992).
Synchronous and Metachronous Primary Tumors
In 1932, Warren and Gates performed an extensive review of the literature and autopsy results and put forth guidelines to distinguish between separate primary tumors and metastases. Using Billroth’s postulates as a basis, the authors reported that each tumor must be malignant, must arise in different locations, and must not be a metastasis from the other primary tumor. However, with the added insight of molecular data, it is clear that these principles are not so easy to define.
In addition to examining the clonal relationship between pre-malignant lesions and tumors, many studies using the molecular techniques we have already discussed have been performed looking at synchronous tumors in the upper aerodigestive tract. The X-chromosome inactivation assay was utilized to examine eight female patients with multiple HNSCCs (Bedi et al., 1996). Ultimately, the researchers demonstrated that four of eight patients had the same pattern of inactivation in their tumors. The remaining four patients could not be assayed due to X-chromosome deletion, or a lack of informativity. Thus, clonality could be neither proved nor disproved in these patients.
Karyotype analysis and fluorescence in situ hybridization (FISH) performed by one group (Worsham et al., 1995) also suggested a common clonal ancestor in a patient with synchronous tumors located in the anterior floor of the mouth and pyriform sinus, both of which recurred. The study indicated that all four of the tumors (tumors and recurrences) were clonally related.
In another study, p53 mutations were tested in 12 patients who developed recurrences and metastases. Nine of these patients had conclusive results confirmed by DNA sequencing, of whom four patients were confirmed to have recurrences related to the initial primary tumor (Gasparotto et al., 1995). The other recurrences demonstrated a different mutation pattern in p53, which does not necessarily exclude a clonal relationship.
However, other studies looking at p53 alterations do not support the theory of clonality in field cancerization. Immunohistochemistry and sequencing of a limited region of p53 looking for point mutations were performed in another series on 17 patients with 41 separate primary tumors (Ribeiro et al., 1996). These patients with metachronous and synchronous tumors demonstrated a complete discordance between the tumors in the same patients, suggesting that there was no clonal relationship between the two. However, it must be stressed once again that p53 mutations may not be the earliest lesion, and therefore the potential remains that other genetic alterations may have occurred prior to the acquisition of p53 point mutations.
Microsatellite analysis has also been used to examine the clonal relationship between second primary tumors. In a Japanese study, 100 pre-cancerous and cancerous lesions from 26 patients were tested with seven markers known to demonstrate early alterations (Jang et al., 2001). Concordant losses were detected in only nine of 63 (14%) lesions from 4/22 (18%) patients who first presented with pre-malignant disease. However, in 2/4 patients who presented with invasive cancer, the subsequent lesions demonstrated clonal relationships. This study seems to suggest that the more aggressive lesions have a higher likelihood of clonal spread. Similar results were noted in a different study (Scholes et al., 1998), where synchronous tumors from five patients were assayed for microsatellite loss. Three of five patients demonstrated concordant loss patterns, while two of five were discordant.
In summary, the molecular basis behind field cancerization has been well-described, with many genetic alterations having been identified adjacent to pre-malignant and malignant lesions. A significant proportion of multiple synchronous or recurrent lesions appear to be clonally related, supporting the idea of local clonal spread. Again, due to the limitations of our knowledge and the assays used, it is impossible to determine the exact fraction of lesions that are clonally related. However, the evidence remains strong that transformed cells have the ability to migrate laterally, leading to a high incidence of multiple synchronous and metachronous lesions.
A new classification method of second primary tumors was recently proposed (Braakhuis et al., 2002), to account for the information we have gained from molecular studies. In the past, these lesions were distinguished as being distinct simply by an arbitrary distance, often 1.5 or 2.0 cm apart. The tumors were also classified by the time to recurrence: If a tumor recurred at the same anatomic site, then some investigators believed that, for it to be considered a second primary tumor, at least three years had to have elapsed between detection of the tumors. These somewhat arbitrary distinctions have been refined by molecular techniques that can identify relationships between lesions. Therefore, the authors suggest a different designation—"second field tumors" (SFT)—for those lesions that are anatomically distinct but demonstrate genetic similarities. For those tumors that arise in the same anatomic location post-resection, SFTs can be identified as well. Thus, true second primaries would be those lesions that did not share any genetic similarity and therefore likely arose as a result of independent events.
Implications for Therapy
The theory of field cancerization suggests that there is an increased likelihood of concurrent or future disease in patients with head and neck lesions. Therefore, it is incumbent upon the care providers to be more diligent about screening and directed biopsies in these patients. High-risk patients (those who smoke and drink alcohol) should see their dentists and primary care physicians frequently for exams. However, the challenges of early detection of pre-malignant and early malignant disease remain daunting, since many HNSCC remain asymptomatic or clinically undetected until advanced stages, even with adequate, routine care.
The controversy between lateral spread of clones vs. multiple foci of independent alterations does not currently affect the surgical and medical management of these pre-malignant and malignant lesions. However, detection and therapy based on molecular techniques depend on an answer to this question. For example, if a molecular therapy capitalizes on the presence of a particular gene alteration, would this be sufficient to cure a patient, or would there be other mutations present in an adjacent field that would not be affected by such a focused treatment?
The presence of altered fields of mucosa remaining beyond the limits of resection has been shown both histologically and on a molecular basis. Initial studies performed demonstrated that p53 mutations noted in histologically normal margins could be detected, and in fact, there was a higher incidence of local recurrence in those patients with known mutations in the altered margins (Brennan et al., 1995; Partridge et al., 2000a). The histologically benign mucosa often can progress to further pre-malignant or malignant disease (Califano et al., 1999b, 2000). Microsatellite alteration has been shown to be predictive of malignant progression (Partridge et al., 2000b; Rosin et al., 2000). Larger studies are currently under way to examine more definitively this question of margin analysis and p53 mutations. Molecular techniques may aid in the analysis of surgical margins. In the future, the presence of altered clones at mucosal margins may be an indication for more aggressive therapy, including chemopreventive or radiotherapy, to treat altered clonal patches that are unable to be detected grossly and are beyond the initial scope of surgical excision.
The issue of whether those with an extensive, visible mucosal field defect are more likely to benefit from chemotherapy, radiotherapy, or chemoprevention is a complex one. Current management is often site-specific: Recurrent oral pre-malignant disease is often treated by surgical excision, whereas diffuse high-grade pre-malignant changes in the laryngeal mucosa are frequently treated with radiotherapy. Determination of the role for these and other treatment modalities for clinically occult, clonally altered patches of epithelium is likely to be a difficult issue, since treatment of mucosa with widespread visible alterations is already challenging.
Screening studies that take advantage of the known genetic alterations may also become routine, especially for those people in a high-risk group (smokers and drinkers). Treatment would need to be directed at correcting gene defects in a wide field, at all stages of progression. It is difficult to determine at this time whether current chemotherapy or radiotherapy techniques are warranted in such patients with early-stage disease. On the one hand, such aggressive treatment would offer treatment of a larger field than solely the primary site of disease. On the other hand, many patients might needlessly receive therapy that is not without morbidity, and future treatment modalities for recurrence of disease would be limited.
Further studies need to be performed to elucidate the mechanisms behind lateral clonal spread. We simply do not know exactly what factors control the ability of certain cells to migrate. Cell adhesion genes may be altered, or there may simply be a mass effect due to the growth advantage conferred on these affected clones by other genetic alterations. Identification of these genes would provide additional targets for therapy and the prevention of metastasis/migration.
Chemoprevention
Whether they are clonally related or not, it is clear that there are wide fields of mucosa that undergo genetic alterations in patients. It would not be feasible to remove all of the areas with molecular alterations surgically. Thus, using the knowledge gained from molecular studies, researchers have attempted to come up with protective measures that could render the mucosa less sensitive to DNA alterations. Patients at risk could be treated to prevent the development of disease, and patients with pre-malignant lesions could have them reversed or halted. And finally, chemoprevention could be used to prevent the recurrence of cancer after surgery.
There have been several proposed compounds thought to be potential chemotherapeutic agents, but perhaps the most widely studied compound in the upper aerodigestive tract has been 13-cis retinoic acid. This family of chemicals has been shown to play a role in the differentiation, development, and growth of epithelial cells (Armstrong and Meyskens, 2000). 13-cis retinoic acid has been shown to up-regulate the retinoic acid receptor-β, leading to a good clinical response in head and neck pre-malignant lesions (Lotan et al., 1995). High doses of 13-cis retinoic acid led to a regression in oral cavity leukoplakias as compared with placebo (Hong et al., 1986), as well as the prevention of second primary tumors (Hong et al., 1990). However, an additional study noted that, despite clinical regression of pre-malignant lesions, genetic alterations in mucosal fields remain unchanged (Mao et al., 1998). This implies that definitive therapy for genetically altered fields of mucosa will ultimately consist of targeted ablation of altered clonal populations, repair of genetic damage in affected cells, or ongoing treatment with chemopreventive agents that will continue for years or decades.
While the focus of clinical trials for chemoprevention agents has been on the use of retinoid-based compounds, the toxicity (conjunctivitis, mucositis, dry skin, hypertriglyceridemia, and malaise [Lippman et al., 1987]) of this drug at higher doses may limit its utility. Other compounds, such as cyclooxygenase-2 (COX-2) inhibitors, are being studied as chemopreventive agents because of a known increase in COX-2 expression in patients with head and neck cancer as well as in normal epithelium adjacent to tumors (Chan et al., 1999).
Summary
"Field cancerization" has signified various concepts since the initial usage of the term 50 years ago. What began as a description of simple histologic changes has been refined to include the molecular basis behind the relationships between multiple pre-malignant and malignant lesions. The molecular techniques utilized are helpful in establishing clonal relationships between the multiple lesions, though disproving clonality is not always straightforward.
These molecular techniques have been used to confirm that most synchronous esophageal lesions are not related, and most solitary squamous cell carcinoma lung nodules associated with HNSCC are often metastases, rather than second primary tumors. Pre-malignant disease adjacent to a tumor is often related to the original tumor. Several published reports on synchronous malignancies have used different modalities for assessing clonality. Overall, it appears that significant fractions of the synchronous tumors are related.
The concept of field cancerization, or a mucosa rendered more susceptible to tumor formation, has led to the notion of chemoprevention, whereby a compound could be given to prevent tumor recurrence, prevent the progression of pre-malignant disease into malignancy, or retard tumor progression. Because surgical excision of the entire affected mucosa is not feasible, chemical compounds delivered systemically or topically can provide a wider range of coverage. Retinoids and other compounds have been studied extensively with promising results, but the side-effects and efficacy are still being studied. Newer, promising agents are being tested, but the persistent genetic alterations found in clonal patches that give rise to the clinical appearance of widespread mucosal alteration may ultimately be successfully treated by elimination of genetic alterations or the cells that carry them.
Future studies on the mechanism(s) behind lateral clonal spread may provide additional targets for molecular intervention. Furthermore, as the earliest genetic events behind the progression to malignancy are discovered and mapped, the relationships between these lesions will become clearer and indicate that an even higher proportion of synchronous tumors are genetically related than is currently thought. Thus, the theory of field cancerization will continue to evolve as the molecular information is gathered and refined. The persistence of genetically altered fields of cells in patients exhibiting this phenomenon provides a useful mechanism for this challenging clinical problem.