Lung carcinoma is the leading cause of increases in the morbidity and mortality rates of malignant diseases worldwide. Adenocarcinoma has been the most common histological type in the last decades due to: changes in the tobacco industry, smoking habits and the use of immunohistochemistry. Among more than half of patients, lung adenocarcinoma is diagnosed in an advanced stage of the disease. The discovery of mutations in epidermal growth factor receptor (EGFR) in lung adenocarcinoma is a major advancement in molecular pathology and a new approach to the treatment of these patients. Patients with EGFR mutated lung adenocarcinoma receive a targeted therapy (Tyrosine Kinase Inhibitors-TKI) which leads to improvements in disease prognosis and quality of life. Real-time polymerase chain reaction (PCR) is the most widely used and most reliable method since it requires a minimum amount of starting material and allows the amplification of the desired DNA segment up to a billion times. In this way, deletions in exon 19 are detected in approximately 90% of cases, more often in women, non-smokers and in the territory of Asia. The following may be used for EGFR testing: fresh tissue, fast-frozen tissue, tissue molded into paraffin blocks after fixation in formalin and cytological material obtained by scraping from glass tiles. Tissue processed by decalcination, acid treated or heavy metal treated tissue should be avoided. Although surgical samples represent the golden standard in determining EGFR mutations, the results obtained are compatible with the results obtained by bronchoscopic biopsy and thus eliminate the need for invasive diagnostic procedures. Bronchoscopy is an invasive diagnostic method, whose objectives are to diagnose lung tumors, determine the endoscopic spread of the disease and assess tumor operability. The presence of a tumor may be indicated by a different bronchoscopic aspect of the endobronial mucosa. The sensitivity and specificity of this method depends on: bronchologist’s skills, endoscopic findings, the number of biopsy samples, the professional competence of pathologist-cytologist and the obtained tumor amount. The tumor amount is generally small and depends on the histological type, endoscopic findings, sampling technique and the presence of other cells. It is recommended to take three to five biopsy samples, used for diagnosing but also for molecular testing. Targeted therapy is applied based on the obtained results. Given that biopsy samples molded in paraffin are cut into multiple histological sections, and that the tumor amount decreases, it is necessary to minimize the “consumption”. The concentration of isolated DNA does not differ among patients with wt EGFR and mutated EGFR adenocarcinoma. To date, there has been no consensus regarding the number of tumor cells necessary to determine EGFR mutations, and it is recommended to take samples with a minimum of 200 to 400 tumor cells. Invalid results obtained by using the PCR method are most commonly the result of a small number of preserved tumor cells in a biopsy sample. Blood and necrosis may be limiting factors for molecular testing, but not exclusion factors for the same. Bronchoscopic biopsy sample is adequate for the determination of EGFR mutations because the majority of biopsy samples have more than 100 tumor cells, the difference between the concentration of isolated DNA in EGFR mutated and wt EGFR adenocarcinomas is not statistically significant, EGFR mutations are also detected in samples with a small number of tumor cells when using highly sensitive tests.

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