Carcinoma in situ: Dysplasia or Neoplasia?
When cells have lost control of their physiologic processes and grow uncontrollably, they are considered cancerous. Formally, these growths are termed "carcinomas" or "neoplasms." If the basement membrane of an epithelial cancer cell is still intact, then the carcinoma is considered a carcinoma in situ, or "cancer in place."
Carcinomas in situ remain localized and confined to the epithelium; they have not invaded into sub-surface tissues. They will not metastasize (spread) unless they progress to become cancer (via further DNA mutations). However, even though carcinomas in situ are not malignant and do not invade and destroy, they are at very high risk for transforming into invasive carcinoma/cancer. Thus, carcinomas in situ can be considered precursor forms of cancer; if left untreated for long enough, they will likely become malignant neoplasms.
This liminal description of carcinomas in situ subjects carcinomas in situ to dual interpretations. In one sense, carcinomas in situ are neoplastic cells (neoplasms) that proliferate in their normal habitat. In another sense, they are "high-grade dysplasias."
Bottom line: carcinoma in situs are either high-grade dysplasias or early, non-malignant neoplasms.
Dysplasia
Dysplasia refers to any disordered growth, maturation, or differentiation of an epithelium that is reversible if its growth-driving factors are eliminated. Generally speaking, dysplasia involves growth in quantity and spread of immature cells, with a corresponding decrease in location and number of properly differentiated cells.
Dysplasia is often the earliest form of pre-cancerous lesion recognizable in a pap smear or in a biopsy by a pathologist, and it can be low grade or high grade. The risk of a low-grade dysplasia transforming into high-grade dysplasia (and eventually cancer) is low, but high grade dysplasias represent a more advanced progression towards malignant transformation. In most organs, "high-grade dysplasia" is synonymous with "carcinoma in situ," and if this occurs, then the risk of cancer is high.
Neoplasias
When an entire region of epithelium is dysplastic (and no normal epithelial cells are present), then the growth is termed a neoplasia. Neoplasia occurs when cells have lost control of their physiologic processes. Neoplastic cells grow uncontrollably, and once a neoplasm has started, it is not reversible. Neoplastic epithelium is termed carcinoma, and when people talk about cancer or "carcinoma," they are usually referring to neoplasms (tumors).
Note: The term "tumor" is often used synonymously with neoplasm, but a "tumor" can refer to any uncontrolled cellular growth, whether it is inflammatory, hemodynamic, or neoplastic in origin. Additionally, not all neoplasms are malignant. Benign neoplasms include: uterine fibroids and melanocytic nevi (skin moles). These neoplasms are not cancerous.