The pathomorphological changes that characterize lung cancer are largely associated with the special property of the cells that form the tumor. Among these special properties inherent in cancerous tumor cells, there are: autonomy of growth, uncontrollability on the part of the influences regulating these processes, anaplasia, and persistent dedifferentiation, i.e. the loss of the ability inherent in normal cells to form specific tissue structures and produce substances specific to them, as well as the emergence of the ability to infiltrate and invasively grow with the germination and destruction of surrounding healthy tissues.
Features of pathomorphological changes are largely determined by the property of its constituent cell structures to metastasis. Metastasis is the most characteristic way of the spread of cancer cells due to separation from the main tumor in the lung. Moreover, as a result of the transfer of individual malignant cells or their complexes along the lymphatic and blood vessels, new foci of tumor growth are formed. Depending on the mechanisms of distribution and distance from the primary lesion in the lung, intraorgan, regional and distant cancer metastases are distinguished.
The growth rate, degree of invasiveness, metastatic activity of the tumor depend on its histological structure. For this reason, the morphological structure of lung cancer is always taken into account when deciding on the issue of therapeutic tactics.
An international reference center of the World Health Organization has proposed the use of a histological classification of lung cancer, which takes into account many aspects and the whole variety of tumor histogenesis and changes occurring in it. More than 20 variants of the histological structure of lung cancer were distinguished, and the formation of the so-called dimorphic and polymorphic histological types, characterized by a change in the histological structure over the course of one tumor, is often quite wide.
In practical work, they are often guided by simpler, but quite fully reflecting histogenesis of lung cancer classifications, in which three main histological types can be distinguished. It is assumed that each of them has the starting point of its development of various structures of the bronchial mucosa. Squamous – from the integumentary epithelium, adenocarcinoma – from the glandular cells, undifferentiated cancer – from the elements of the basal epithelium.
Convenient in this regard is the histological classification of lung cancer, developed by academician N.A. Kraevsky and his staff. The following histological types of lung cancer are distinguished:
I. Squamous (epidermoid) cancer: a) highly differentiated cancer; b) moderately differentiated cancer (without keratinization); c) poorly differentiated cancer.
II. Small cell cancer: a) oat cell, lymphocyte-like, spindle cell cancer; b) pleomorphic cancer.
III. Adenocarcinoma: a) highly differentiated adenocarcinoma (acinar, papillary); b) moderately differentiated adenocarcinoma (glandular solid); c) poorly differentiated adenocarcinoma (solid cancer with the formation of mucus); d) bronchioalveolar adenocarcinoma (“adenomatosis”).
IY. Large cell cancer: a) giant cell cancer; b) clear cell cancer.
Y. Mixed cancer (squamous and adenocarcinoma, adenocarcinoma and small cell, etc.).
The advantage of this classification is a fairly clear definition of the criteria for each type and subtype of tumors, which makes it possible to differentiate them in practice with great confidence, as well as compliance with the main provisions of the second edition of the “Histological classification of lung tumors” published by the World Health Organization in 1981. In practical terms, it is very advisable to distinguish three main types of lung cancer in this classification: epidermoid, undifferentiated and adenocarcinomas. Since the degree of differentiation of lung cancer is determined by the preservation of signs of epithelial tissue included in its composition, such a distribution is well founded.