Epithelial dysplasia in lichenplanus

Epithelial dysplasia in lichenplanus Girish HC* Sanjay Murgod**Savita JK*** *M.D.S, Professor & Head, **M.D.S, Professor,***M.D.S, Senior Lecturer,Dept of Oral Pathology, Raja Rajeswari Dental College & Hospital, Bangalore, India. Email: girishhc@rediffmail.com Abstract: The presence of dysplastic features in oral Lichen planus is a well-known and documented feature. A retrospective study was carried out to correlate the dysplastic features with age, gender, site of the lesion and also the grades of dysplasia encountered. A significant correlation between the above mentioned factors was noticed which was comparable to other similar studies. However, the author suggests more longitudinal studies to differentiate the dysplastic features with regards to prognostic value. Keywords: Oral lichen planus, epithelial dysplasia Introduction: Lichen planus is a relatively common chronic dermatological disease that often affects the oral mucosa. The strange name of this condition was provided by British Physician Erasmus Wilson in 1869. Lichens are primitive plants composed of algae and fungi. The term planus in Latin is for flat and Wilson probably thought that the skin lesions looked similar enough to the lichens growing on the rocks to merit this designation (1). Lichen planus is a chronic disease lasting for 2-4 years affecting the oral cavity from 0.1 to about 4% of the individuals. Oral lichen planus generally is a disease of the middle age and elderly, and the female to male ratio is about 2:1 (2). However, different opinions have been expressed about the premalignant potential of oral lichen planus (3, 4). The possible premalignant potential of oral lichen planus is the subject of contrasting views (5). The dermatologic literature documents a number of cases of histologically proven or clinically suspected skin lichen planus cases in which carcinoma subsequently developed. Serial Listing: Print-ISSN (2229-4112) Online-ISSN (2229-4120) Bibliographic Listing: Index Copernicus. EBSCO Publishing Database. In a report of the World Health organization, it is stated that there remains considerable uncertainty about the frequency of cancer arising in oral lichen planus. In the same report, it was mentioned that some epithelial dysplasia may be seen in lichen planus. However, there is a valid question whether carcinomatous degeneration is part of the natural evolution of the lesion or due to extrinsic factors (4). Shafer et al (6), Regezi Joseph and Sciubba James J (7), Eversole Lewis R (8) reported the characteristic histological features of oral lichen planus as hyperkeratosis, presence of civatte bodies, liquefaction degeneration of basal layer, saw tooth rete ridges and a band of inflammatory cell infiltrate below the basement membrane in the connective tissue. However, McDonald and Rennie J S (3) in their study on oral epithelial atypia in denture induced hyperplasia, lichen planus and squamous cell carcinoma found that lichen planus cases showed irregular epithelial stratification, loss of polarity of cells and pleomorphic cells and nuclei. Jong De W.F.B (5) et al in their study on epithelial dysplasia in oral lichen planus found 87% of cases had more than one layer having a basaloid appearance. Other common factors included nuclear hyperchromatism (60%), enlarged nucleoli (45%), loss of polarity of basal cells (33%) and keratinization of single cells or cell groups in the spinous layer in 27% of cases. However, Sigrgesson and Lindelof (6) found no increased risk of malignancy in skin lichen planus but found an increased risk of 0.4% in oral lichen planus. Therefore people diagnosed with oral lichen planus appear to be at a greater risk of developing oral cancer than in general population. Therefore, it was thought fit to study the prevalence of dysplastic feature in oral lichen planus. A study was designed to retrospectively review all the cases of oral lichen planus and evaluate the changes. Aims and objectives: The aim of the present study was to study the following parameters of the disease and note the correlation among the related parameters, if any; 1. Age of the patient and gender predilection. 2. Most frequented site of the lesion. 3. Evaluation of dysplastic changes. Objectives of the study 1. To suggest views on whether lichen planus is a begnin or premalignant lesion. 2. To know the prognosis of these lesions. 19

TABLE 1: AGE AND SEX WISE DISTRIBUTION OF ORAL LICHENPLANUS AGE (YRS) MALES (%) FEMALES (%) TOTAL (%) 16-25 12 (24) 2 (4) 14 (28) 26-35 5 (10) 9 (18) 14 (28) 36-45 8 (16) 2 (4) 10 (20) 46-55 0 (0) 1 (2) 1 (2) 56-65 3 (6) 4 (8) 7 (14) 66-75 4 (8) - 4 (8) TOTAL 32 (64) 18 (36) 50 TABLE 2: SITE WISE DISTRIBUTION OF ORAL LICHEN PLANUS SITE NO. OF CASES PERCENTAGE Buccal Mucosa 34 68 Tongue 12 24 Labial Mucosa 4 8 Palate 4 8 Gingiva 4 8 Floor of mouth 1 2 TABLE 3: DYSPLASTIC FEATURES PRESENT IN ORAL LICHEN PLANUS DYSPLASTIC FEATURES PERCENTAGE Presence of more than one layer having basaloid appearance 32 Enlarged nucleoli 22 Increased nuclear-cytoplasmic ratio 12 Increased number of mitotic figures 6 Mitotic figures in superficial layer 2 TABLE 4: GRADES OF DYSPLASIA-DISTRIBUTION IN ORAL LICHEN PLANUS GRADES NO. OF CASES PERCENTAGE 0 28 56 1 21 42 2 1 2 3 - - TOTAL 50 100 TABLE 5: DISTRIBUTION OF MEAN GRADES OF EPITHELIAL DYSPLASIA BY GENDER IN ORAL LICHEN PLANUS GRADES OF OCCURRENCE IN OCCURRENCE IN DYSPLASIA MEN (%) WOMEN (%) TOTAL (%) 0 15 (30) 13 (26) 28 (56) 1 16 (32) 5 (10) 21 (42) 2 1 (2) 0 1 (2) 3 0 0 0 TOTAL 32 18 50 (100) 20

TABLE 6: DISTRIBUTION OF MEAN GRADES OF EPITHELIAL DYSPLASIA BY AGE IN ORAL LICHEN PLANUS GRADES OF DYSPLASIA AGE GROUP 0 (%) 1 (%) 2 (%) 3 (%) PROPORTION OF PATIENTS WITH ANY GRADES OF DYSPLASIA 16-25 7 (14%) 7 (14%) 0 0 7 (14%) 26-35 6 (12%) 6 (12%) 0 0 6 (12%) 36-45 5 (10%) 5 (10%) 1 (2%) 0 6 (12%) 46-55 1 (2%) 0 0 0 0 56-65 8 (16%) 0 0 0 0 66-75 1 (2%) 3 (6%) 0 0 3 (6%) TOTAL 28 (56%) 21 (42%) 1 (2%) TABLE 7: DISTRIBUTION OF GRADES OF EPITHELIAL DYSPLASIA BY SITE IN ORAL LICHEN PLANUS GRADES SITE 0 (%) 1 (%) 2 (%) 3 (%) TOTAL (%) Buccal 18 (38) 15 (30) 1 (2) 0 34 (68) mucosa Tongue 10 (20) 2 (4) 0 0 12 (24) Labial mucosa 1 (2) 3 (6) 0 0 4 (8) Palate 3 (6) 1 (2) 0 0 4 (8) Gingiva 2 (4) 2 (4) 0 0 4 (8) Floor of mouth 0 1 (2) 0 0 1 (2) Materials and methods: This retrospective study was undertaken by retrieving the records and the paraffin blocks of the confirmed cases of oral lichen planus from the Department of Oral Pathology and Microbiology, Bapuji Dental College and Hospital, Davangere. The paraffin blocks of oral lichen planus were sorted out and cut sections were prepared and stained with Hematoxylin and Eosin. Totally, 50 blocks and records were included in the study. Methodology: The cases of oral lichen planus involved in this study were retrieved from the files of Department of Oral Pathology and Microbiology, Bapuji Dental College and Hospital, Davangere The relevant clinical details and the histopathological report were recorded. The nature of dysplastic features to be evaluated was formatted in a proforma and the study was done by criteria given by Jong et al (5). The Hematoxylin and Eosin stained sections were studied under 5X, 10X and 45 X magnifications for evaluation of dysplastic features, namely 1. Loss of polarity of basal cells. 2. Basal cell hyperplasia. 3. Increased nuclear cytoplasmic ratio. 4. Drop shaped rete pegs. 5. Irregular epithelial stratification. 6. Increased number of mitotic figures. 7. Abnormal mitotic figures. 8. Cellular pleomorphism. 9. Nuclear hyperchromatism. 10. Enlarged nucleoli. 11. Reduction of cellular cohesion. 12. Keratinization of single cells or cell groups in spinous layer. The grading of the dysplastic features was done according to the method followed by Odukoya O et al (9). The grading is as follows Grade 0: No feature of dysplasia seen. Grade I: One or two features of dysplasia seen. Grade II: Three or four features of dysplasia seen. Grade III: More than four features of dysplasia seen. Results and observations: A total number of 50 oral lichen planus cases have been retrieved from the files of the Department of Oral Pathology, Bapuji Dental College & Hospital, Davangere. 21

J. Adv Dental Research ORIGINAL RESEARCH There were 32 (64%) males and 18 (36%) females in the study with the age ranging from 17 to 75 years and a mean age of 38.68 years (Table 1). 80 70 60 50 40 30 20 10 0 No of Cases Percentage Mitotic figure in superficial layer increased nno. Of mitotic figures Increased nuclearcytoplasmic ratio Enlarged nucleoli Presence of >1 layer having basaloid appearance GRAPH 1: SITEWISE DISTRIBUTION PLANUS OF ORAL LICHEN Buccal mucosa was the most common site affected with 34 (68%) cases. The tongue was involved in 12 (12%) of the subjects while palate, gingival and labial mucosa each had 4 (8%) distribution. The floor of the mouth was affected only in 1 (2%) of the subjects (Table 2, Graph 1). Out of 50 lichen planus cases 22 (44%) showed dysplastic features and 28 (56%) did not show them.the dysplastic features found in the present study were in the form of basilar hyperplasia 32% (Photographs 1 & 2), enlarged nucleoli 22% (Photographh 3) and increased nuclear cytoplasmic ratio in 12% (Photograph 4 & 5). This was followed by increased number of mitotic figures in 6% and mitotic figures confined tosuperficial layer in 2% (Photograph 6) (Table 3, Graph 3). Among the 50 oral lichen planus cases Grade I dysplastic features were observed in 21 (42%) and Grade II in 1 (2%) while Grade III were absent (Table 4, Graph 2). 0 10 20 30 40 Graph 3: Dysplastic features in oral lichen planus Figure 1- Basilar Hyperplasia(40x). Grade 1, 42% Grade 2, 2% Grade 3, 0% Grade 0, 56% Grade 0 Grade 1 Grade 2 Grade 3 Figure 2- Basilar Hyperplasia(40x). Graph 2: Grades Of Dysplasia

Figure 3-Enlarged Nucleoli(40x) In the present study 22 (44%) showed dysplastic features out of which 17 (34%) were males and 5 (10%) were females. Grade I dysplastic features were seen in 16 (32%) of males and 5 (10%) of females. Grade II dysplasia was seen in only 1 (2%) of male patients (Table 5). A maximum of 7 (14%) Grade I dysplastic features was found in 16-25 year age group followed by 6 (12%) in 26-35, 5 (10%) in 36-45, 3 (6%) in 66-75 age groups and 1 (2%) of grade II in 36-45 age group (Table 6) No dysplasia was seen in 18 (36%) of buccal mucosa, 10 (20%) on tongue, 1 (2%) of labial mucosa, 3 (6%) of palate and 2 (4%) of gingiva. Grade I dysplasia was noted in 15 (30%) of buccal mucosa, 2 (4%) of tongue, 3 (6%) of labial mucosa, 1 (2%) of palate, 2 (4%) of gingiva and 1 (2%) of the floor of the mouth. Grade II dysplasia was noted in 1 (2%) of the left buccal mucosa (Table 7) Figure 4- Altered Nuclear cytoplasmic Ratio (40x) Discussion: Lichen planus is a relatively common chronic dermatological disease that often affects the oral mucosa (1). The prognostic significance of an individual lesion is difficult to determine, and none of the currently available molecular markers have proved to be prognostically significant and none have yet been evaluated in large prospective studies. At present therefore, the gold standard for the assessment of oral potentially malignant lesions is microscopic evaluation of haematoxylin and eosin stained sections for the presence of architectural and cytological changes, which are generally referred to as epithelial dysplasia (10). The possible premalignant potential of oral lichen planus is the subject of contrasting views (5). In a report of World Health Organization, it was stated that there was considerable uncertainty about the frequency of cancer arising in oral lichen planus. However, there was a valid question whether carcinomatous degeneration was a part of the natural evolution of the lesion or was due to extrinsic factors (4). Shafer et al (6), Regezi, Joseph and Sciubba, James (7), Eversole Lewis (8), Prabhu SR (11), Hedberg et al (12), have reported the histopathology of oral lichen planus as typical with the following features without mentioning the presence of dysplastic features. 1. The surface epithelium may show varying degrees of hyperorthokaratosis or hyperparakeratosis with thickening of granular layer and with intracellular edema of spinous cells. 2. Civatte bodies, hyaline bodies, fibrillar bodies are often present in the epithelium. 3. Liquefaction degeneration of the basal layer. 4. Replacement of basal layer by eosinophilic coagulum. 5. Saw-toothed rete ridges. 6. Infiltration of lymphocytes and plasma cells into the subepithelial layer of connective tissue. 7. Focal separation of epithelium and connective tissue. 8. Dilated vessels in the connective tissue Nevertheless McDonald and Rennie (3) in their study on oral epithelial atypia in denture induced hyperplasia, lichen planus and squamous cell papilloma found that lichen planus cases showed irregular epithelial stratification, loss of polarity and pleomorphic cells and nuclei. Jong De et al (5) evaluated epithelial dysplasia in oral lichen planus and observed that 87% of cases had more than one cell layer having basaloid appearance followed by 60% with nuclear hyperchromatism 45% with enlarged nucleoli, 15% basilar hyperplasia and 12% nuclear hyperchromatism. They added that these cases had an increased risk of cancer development and follow up should be carried out. Odukoya et al (9) studied epithelial dysplasia in oral lichen planus and reported enlarged nucleoli in 49%, hyperplasia of basal layer and nuclear hyperchromatism in 15% and increased number of mitotic figures in 11%. Because of the above stated contrasting and interesting features a study was designed to retrospectively review 50 histologically confirmed cases of oral lichen planus and evaluate the dysplastic features with correlation to age, gender and site of the lesion. In the present study the age of the subjects ranged from 17 to 75 years. This is in accordance with 13 to 78 years of McCarthy and Shklar (13), 19 to 81 years of De Jong et al (5) and 17 to 81 years of Allen Carl et al (14). It was observed in the present study that there were 32 (64%) males and 18 (36%) females showing a male predominance with a ratio of 1.7:1. This finding is in agreement with the gender incidence of 2.1:1 observed in the study of Mehta et al (15). 23

The male predominance of the present study is contradictory to the observations made by Cawson (16), Lacy et al (17), Neville et al (1) and Scully et al (2) who have reported a female predilection. Even though the male predominance in the study is contradictory to the above authors, it appears to be less significant since Laufer and Kuffer (16), Shafer et al (6), Regezi and Sciubba (18), Rose and Kaye (19) have reported no dramatic sex predilection in their study. Buccal mucosa was the most common site affected with 34 (68%) followed by tongue 12 (24%), gingiva, palate, labial mucosa each 4 (8%) and floor of mouth 1 (2%) of cases. The above findings are consistent with 90% of buccal mucosa, 27% tongue and 7% on palate of Tydesly (1974), 90% on buccal mucosa and 28% on tongue of Axell and Rendquist (18). Batsakis et al (20) and Gorsky (21) observed that most cases occurred on buccal mucosa followed by tongue, gingiva and lips. In the present study 56% showed no dysplastic features, while 42% showed grade I and 2% grade II dysplasia. Grade II dysplasia was not observed. The study by Odukoya et al (9) revealed no dysplastic features in 32%, Grade I in 57%, Grade II in 9% and Grade III in 2%. The difference in grades could be attributed to their methodology of multiple sections and evaluation of dysplastic features by more than one investigator. Basilar hyperplasia of 32% was the most common dysplastic feature observed in this study. This figure is contradictory to that of 87% reported by De Jong et al (5). This variation could be attributed to their higher sample size, study from two different places namely Amsterdam and Budapest, and surveying of multiple sections when compared to single section of this study. Enlarged nucleoli was the next common feature observed in 22% of cases in this study. This is very much low when compared to 45% of De Jong et al (5) and 49% of Odukoya (9). The marked disparity could have been due to differences in methodology. In the present study random single section was surveyed when the above authors have followed a different methodology wherein they have surveyed multiple selected sections, as also a different grading system of dysplasia. They have selected those sections which showed most dysplastic features. The other dysplastic features like increased nuclear cytoplasmic ratio of 12%, abnormal mitotic figures of 6% and mitotic figures only in the superficial layer in 2% were found in this study. These figures are compatible with that of 9%, 5% and 1% respectively of De Jong et al (5). In the present study 44% dysplastic features and 56% without dysplastic features were observed. Therefore it could be inferred that dysplastic features also could be observed in Lichen planus. This inference is in agreement with the authors as stated in the beginning. However some studies have reported a lack of dysplastic features but report the presence of other features such as hyperplasia, hyperkeratosis, acanthosis and the presence of necrotic keratinocytes (22). In the present study Grade I dysplasia was observed in 7 (14%) of 16-25, 6 (12%) of 26-35, 6 (12%) of 36-45 and 3 (6%) of 66-75 years age group. Grade II dysplasia was present only in 1 (2%) of the 36-45 year age group. It appears that grade I and II dysplastic features to be more in the younger and middle age group respectively than in older age. Grade I dysplasia was observed in 16 (32%) males and 5 (10%) in females. Grade II dysplasia was noted in 1 (2%) of males. The study shows a strong predilection for dysplastic features to occur in males but was not statistically significant. Odukoyo et al (9) observed 78.8% of males and 62.7% of females with any grade of dysplasia. This disparity may be due to the larger sample size, significantly higher number of female subjects in their study which included 67% females and 33% males and a different methodology in grading of dysplasia. The cheek was the most common site affected 15 (30%) with grade I dysplasia followed by 3 (6%) on labial mucosa. These figures are compatible with 28% and 1% of Odukoya et al (9) who observed in 28% and 1% on labial mucosa. This was followed by 4% on tongue and 3% on floor of the mouth in the present study which is compatible with 3% incidence at each site in the study of Odukoya et al (9). However Odukoya et al (9) observed Grade I dysplasia in 19% of gingiva whereas it was 2% in this study. This variation may be due to involvement of gingiva in more number of cases (33%) in their study by oral lichen planus. Grade II dysplasia was observed only in 2% in this study on the buccal mucosa and is in agreement with 2% incidence of Odukoya et al (9). Studies have also been conducted with regard to the relation between candida and dysplastic lesions such as lichen planus but did not report an association between the two (23). Morphometric analysis of cells in oral lichen planus has revealed an increase in cell dimension compared to other white lesions (24). In conclusion, this study reveals that dysplastic features of mild degree could be present often than moderate to severe degree. The histopathologic diagnosis of oral lichen planus, mainly when differentiated against epithelial dysplasia, is quite difficult, as some cell disorders indicative of malignant disease such as increased nuclearcytoplasmic ratio, nuclear hyperchromatism, and irregular chromatin distribution may be seen in either of the lesions (25). Whether these dysplastic features are benign as observed in denture hyperplasia, squamous cell papilloma (McDonald and Rennie (3) Darrier s disease (Shafer (6), Neville (1), Keratoacanthoma (Shafer (6), Regezi and Sciubba (7) and Neville (1) and hereditary intraepithelial dyskeratosis (Shafer (6), Regezi and Sciubba (7) and Neville (1) or potentially a premalignant change is uncertain. Lichen planus as a lesion at risk for malignant change has been disproved by some studies though the authors have suggested that this be confirmed by the use of other genetic markers (26). While the malignant transformation potential of oral lichen planus is not confirmed, the fact that its cell proliferation rate is inferior to epithelial dysplasia s and oral squamous cell carcinoma s might explain why its malignant transformation potential is lower than epithelial dysplasia s (27). 24

Future studies are directed towards advanced methods for cytological and histological evaluation like micronucleus test, computer aided studies, stereological techniques, histochemical stains, enzyme histochemistry, immunohistochemistry, studies like surface antigens, intracellular components/products, basement membrane zone, stromal changes. In situ hybridization RNA/DNA TM and SEM studies for viral, fungal infection and advanced methods for functional evaluation studies like cellular proliferation, DNA histograms, DNA cytophotometry, analysis of immune status, biochemical studies of tissue homogenates, body fluids and analysis of circulating cell products (28) on a larger sample of longitudinal study to ascertain whether these dysplastic features are potentially premalignant or not. In the absence of conclusive evidence it is suggested that chemoprevention could be tried in those cases which shows dysplastic features followed by a biopsy to study whether such epithelium has reverted to normalcy on intervention with the treatment. Future longitudinal studies with advanced morphological and functional investigations are necessary to differentiate the dysplastic features of lichen planus from benign or potentially premalignant one. References: 1. Neville B et al: Oral and Maxillofacial Pathology. First edition, WB Saunders Company. 1995:252-53, 572-76. 2. Scully C et al. Update on oral Lichen planus: Etiopathogenesis and management. 1988 Crit Rev Oral Biol Med; 9(1):86-122. 3. MacDonald DG &Rennie JS. Oral epithelial atypia in denture induced hyperplasia, Lichen planus and squamous cell papilloma. 1975. Int Journal of Oral Surgery; 4: 40-45. 4. Kaurgars E George &Svirsty A John. An update on the dysplastic/carcinomatous transformation of oral lichen planus. 1982. Journal of Oral Medicine; 37(3): 75-79. 5. Jong De WFB et al. Epithelial dysplasia in oral lichen planus. 1984. Int J of Oral Surg; 13: 221-225. 6. Shafer William G, Hine Maynard K and Levy Barnet M. A text book of Oral Pathology. Fourth edition, WB Saunders company, 1983: 808-814 7. Regezi Joseph A &Sciubba James J. Oral Pathology; Clinico-pathologic correlation. WB Saunders company, 1989: 105-110 8. Eversole Lewis R. Clinical outline of Oral Pathology; Diagnosis and Treatment, Second edition 9. OdukoyaOnatolu et al. A histological study of epithelial dysplasia in oral lichen planus. 1985. Arch Dermatol; 121: 1132-1136. 10. Paul M. Speight. Update on Oral Epithelial Dysplasia and Progression to Cancer. 2007. Head and Neck Pathol (2007) 1:61 66. 11. Prabhu SR. Oral disease in the tropics. Oxford Community Press, 1993: 422-423 12. Hedberg N. A semi quantitative assessment of histopathology of oral lichen planus. 1986. J Oral Pathol; 15: 268-272. 13. McCarthy L, Philip &Shklar Gerald. Diseases of the oral mucosa. 1980. Second edition, Lea and Febiger, Philadelphia: 203-224. 14. Allen Carl M. Relation of stress and anxiety to oral lichen planus. 1986. Oral Surg, Oral Med, Oral Path; 61: 44-46. 15. Kovesi G &Banoczy J. Follow up studies in oral lichen planus. 1973. Int J Oral Surg; 2(1): 13-19. 16. Scully C & El Kom. Lichen planus: Review and update on pathogenesis. 1985. J Oral Pathol; 14: 431-458. 17. Lacy MF et al. A theory of pathogenesis. 1983. Oral Surgery; 56(5): 521-526. 18. RundquistAxell T. Oral lichen planus-a demographic study. 1987. Community Dent Oral Epidemiol; 15: 25-56 19. Rose LF & Kaye D. Internal Medicine for Dentistry. 1990. The CV Mosby company, second edition, 845-848. 20. Batsakis JG et al. Lichen planus and lichenoid lesions of the oral cavity. 1994. Annals of Otology, Rhinology and Laryngology; 103(6): 495-497. 21. Gosky M et al. Clinical characteristics and treatment of patients with oral lichen planus in Israel. 1996. Oral Surg, Oral Med, Oral Path; 82(6): 644-649. 22. Francisca Fernandez-Gonzalez et al. Histopathological findings in oral lichen planus and their correlation with the clinical manifestations. Med Oral Patol Oral Cir Bucal. 2010 Aug. 23. Spolidorio LC, Martins VR, Nogueira RD, Spolidorio DM. PesquiOdontol Bras. The frequency of Candida sp in biopsies of oral mucosal lesions. 2003 Jan-Mar;17(1):89-93. 24. Virdi M, Sachdev A, Gupta A &AggarwalK : Lichen Planus Or Lichenoid Dysplasia : Is It Premalignant!. The Internet Journal of Head and Neck Surgery. 2010 Volume 4 Number 2 25. Sousa FA, Paradella TC, Brandão AA, Rosa LE. Braz J Otorhinolaryngol. Oral lichen planus versus epithelial dysplasia: difficulties in diagnosis. 2009 Sep-Oct;75(5):716-20. 26. Zhang L, Michelsen C, Cheng X, Zeng T, Priddy R, Rosin MP. Molecular analysis of oral lichen planus. Apremalignant lesion? Am J Pathol. 1997 Aug; 151(2):323-7. 27. De Sousa FA, Paradella TC, Carvalho YR, Rosa LE. Med Oral Patol Oral Cir Bucal. Comparative analysis of cell proliferation ratio in oral lichen planus, epithelial dysplasia and oral squamous cell carcinoma. 2009 Nov 1;14(11):563-7. 28. Burkhardt A. Advanced methods of evaluation of premalignant lesions and carcinomas of the oral mucosa. 1985. J Oral Path; 14: 751-778 25

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