Figs. 6.9 6.10 ). Cancers lack well-defined capsules. There are instances in which a slowly growing malignant tumor deceptively appears to be encased by the stroma of the surrounding host tissue, but microscopic examination reveals tiny crablike feet penetrating the margin and infiltrating adjacent structures. This infiltrative mode of growth makes it necessary to remove a wide margin of surrounding normal tissue when surgical excision of a malignant tumor is attempted. Surgical pathologists carefully examine the margins of resected tumors to ensure that they are devoid of cancer cells (clean margins).
Metastasis is defined by the spread of a tumor to sites that are physically discontinuous with the primary tumor and unequivocally marks a tumor as malignant, as by definition benign neoplasms do not metastasize. The invasiveness of cancers permits them to penetrate into blood vessels, lymphatics, and body cavities, providing opportunities for spread (
Fig. 6.11 ). Overall, approximately 30% of patients with newly diagnosed solid tumors (excluding skin cancers other than melanomas) present with
Fig. 6.10 Microscopicviewofbreastcarcinomaseenin
Fig.6.9
Fig.6.8 ).Notetheabsenceofawell-definedcapsule.(Courtesy of Dr. Susan Lester, Brigham and Women’s Hospital, Boston, Massachusetts.) clinically evident metastases. An additional 20% have occult (hidden) metastases at the time of diagnosis.
In general, the more anaplastic and the larger the primary neoplasm, the more likely is metastatic spread, but as with most rules there are exceptions. Extremely small cancers have been known to metastasize; conversely, some large and ominous-looking lesions may not. While all malignant tumors can metastasize, some do so very infrequently. For example, basal cell carcinomas of the skin and most primary tumors of the central nervous system are highly locally invasive but rarely metastasize. It is evident then that the properties of local invasion and metastasis are sometimes separable.
A special circumstance involves so-called “blood cancers”, the leukemias and lymphomas. These tumors are derived from blood-forming cells that normally have the capacity to enter the bloodstream and travel to distant sites; as a result, with only rare exceptions, leukemias and lymphomas are taken to be disseminated diseases at diagnosis and are always considered to be malignant.
Malignant neoplasms disseminate by one of three pathways: (1) seeding within body cavities, (2) lymphatic spread, or (3) hematogenous spread. Spread by seeding
Fig. 6.11 Aliverstuddedwithmetastaticcancer.
http://ebooksmedicine.net occurs when neoplasms invade a natural body cavity. This mode of dissemination is particularly characteristic of cancers of the ovary, which often cover the peritoneal surfaces widely. The implants literally may glaze all peritoneal surfaces and yet not invade the underlying tissues. Here is an instance where the ability to reimplant and grow at sites distant from the primary tumor seems to be separable from the capacity to invade. Neoplasms of the central nervous system, such as a medulloblastoma or ependymoma, may penetrate the cerebral ventricles and be carried by the cerebrospinal fluid to reimplant on the meningeal surfaces, either within the brain or in the spinal cord.