This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The malignant growths of the breast are either sarcomata or carcinomata.
Sarcomata originate from the fibrous stroma of the breast surrounding the ducts and acini. As it increases in size it may irritate the glandular structure and obstruct the ducts, thus forming cysts which may be quite large. Such a growth has been called a cystic sarcoma. It also shows itself as a single tumor, which may be large but solid. The lymph-nodes are rarely affected. The disease when it wanders from the seat of the primary growth shows itself in some of the internal organs. It is disseminated by the blood and not by the lymphatics.
Carcinomata originate from the epithelium lining the ducts and acini. For our purposes we may divide them into two classes, those that grow into the ducts (intracanalicular) and those that break through the ducts and invade the surrounding tissues, of these scirrhus is the type.
Intracanalicular growths have by many authors been considered nonmalignant on account of the rarity of their producing general infection. They grow at times rapidly and produce tumors of considerable size. On section they contain many cysts and into these cysts, which are derived from the dilated milk-ducts, protrude outgrowths from the walls. Sometimes the cavity of the cyst has its liquid contents replaced by the solid tumor which has grown into it. A discharge of bloody serum from the nipple is common in these growths.
Scirrhus is the ordinary form of cancer of the breast. It starts in the epithelial structures of the gland, breaks through the basement membrane and involves the structures immediately adjacent to it, and is disseminated more widely by the lymphatics. Paget's disease is a true carcinoma which begins as an eczema or ulceration around the nipple and later becomes disseminated.
Carcinoma follows the gland structure, and readily involves the pectoral fascia covering the pectoralis major muscle. Anteriorly, the gland structure in places follows the ligaments of Cooper to the skin above, hence the frequency with which the skin is involved.
The scirrhus variety does not involve the ducts in the same manner as does the intracanalicular variety, hence bloody discharges from the nipple are not so common as in that affection. The disease, when affecting the region of the nipple, has been considered more dangerous because of the greater development of the lymphatics, particularly the subareolar plexus of Sappey, at that point.
Carcinomatous disease extends especially by way of the lymphatics. These follow the fibrous and canalicular structure, therefore on section the cancerous tissue can be seen extending like roots into the surrounding gland. This tissue shrinks, contracts, and becomes harder as the disease progresses, that is why retraction of the nipple and dimpling of the skin occurs. The most free lymphatic drainage occurs toward the axilla, not toward the mediastinal nodes. The first nodes to show infection are those lying along the edge of the pectoralis major muscle about the level of the third rib. Later, the nodes at the anterior edge of the scapula accompanying the subscapular artery become involved, or those along the axillary vessels. Still later those along the subclavian vessels may be enlarged and may be felt above the clavicle and farther inward behind the sternomastoid muscle low down.
In rare instances the disease may be carried superficially to the subclavian nodes in the infraclavicular triangle between the deltoid and pectoralis major muscles. Should the disease spread, it may be carried by the lymphatics to the opposite breast directly across the median line. If it involves the lymphatics of the chest-wall generally there is produced the brawny condition of the skin called cancer "en cuirasse" of Velpeau already alluded to. A cancerous nodule beyond the edge of the pectoralis major muscle is not necessarily an enlarged node, but may be due to the involvement of one of the cusps of the gland, which sometimes extend even into the axilla.