The CD4+ T cell count often is useful in narrowing the differential diagnosis. As a rule of thumb, bacterial and tubercular infections occur with mildly suppressed CD4+ counts (more than 200 cells/µL); Pneumocystis pneumonia usually occurs at CD4+ T cell counts below 200 cells/µL, while CMV and M. avium complex infections are uncommon until the very late stages of the disease (CD4+ T cell counts below 50 cells/µL).
Finally, an important point is that pulmonary disease in HIV-infected individuals may result from more than one cause, and that even common pathogens may be responsible for disease with atypical manifestations.
Roughly 95% of primary lung tumors are carcinomas; the remaining 5% span a miscellaneous group that includes carcinoids, mesenchymal malignancies (e.g., fibrosarcomas, leiomyomas), lymphomas, and a few benign lesions. The most common benign tumor is a spherical, small (1 to 4 cm), discrete “hamartoma” that often shows up as a so-called “coin lesion” on chest imaging. It consists mainly of mature cartilage admixed with fat, fibrous tissue, and blood vessels in various proportions. Clonal cytogenetic abnormalities have been demonstrated, indicating that it is actually a benign neoplasm and that the name hamartoma (which implies a developmental anomaly) is a misnomer.
Carcinoma of the lung is the most important cause of cancer-related deaths in industrialized countries. It has long held this position among males in the United States, accounting for about one-third of cancer deaths in men, and has become the leading cause of cancer deaths in women as well. American Cancer Society estimates for 2016 include approximately 221,200 new cases of lung cancer and 158,040 deaths. The incidence among males is gradually decreasing, but it continues to increase among females, with more women dying each year from lung cancer than from breast cancers since 1987. These sobering http://ebooksmedicine.net
Table 13.6 Histologic Classification of Malignant Epithelial Lung Tumors (2015 WHO Classification, Simplified Version)
AdenocarcinomaAcinar,papillary,micropapillary,solid,lepidicpredominant,mucinous statistics reflect the fact that the incidence of smoking in women increased markedly over the past half century. The peak incidence of lung cancer is in individuals in their fifties and sixties. At diagnosis, more than 50% of patients already have distant metastases, while an additional one-fourth have disease in the regional lymph nodes. The prognosis remains dismal: the 5-year survival rate for all stages of lung cancer combined is about 16%, a figure that has not changed much over the last 35 years; even with disease localized to the lung, the 5-year survival rate is only 45%.
The four major histologic types of carcinomas of the lung are adenocarcinoma, squamous cell carcinoma, small cell carcinoma (a subtype of neuroendocrine carcinoma), and large cell carcinoma (
Table 13.6 ). In some cases, there is a combination of histologic patterns (e.g., small cell carcinoma and adenocarcinoma). Squamous cell and small cell carcinomas have the strongest association with smoking, but there is also an association with adenocarcinoma. In fact, possibly because of changes in smoking patterns in the United States, adenocarcinoma has replaced squamous cell carcinoma as the most common primary lung tumor in recent years. Adenocarcinomas also are by far the most common primary tumors arising in women, in never-smokers, and in individuals younger than 45 years of age.