8.1.1 Breast Anatomy
Localized regional radiotherapy for breast cancer generally requires irradiation of the breast/thoracic wall，axillary fossa，supraclavicular and internal mammary lymphatic drainage areas.The design of the irradiation field requires not only reference to surgical methods，tumor staging，tumor characteristics and failure modes，but also to each patient anatomy.Understanding the anatomy of the breast and lymphatic drainage areas helps to properly map the target area and perform radiotherapy.
The mammary gland is attached to the surface of the pectoralis major fascia，mostly between the 2nd and 6th anterior ribs，the inner boundary is the sternal border，and the outside reaches the anterior or medial line.The outer upper pole can extend to the armpit to form the appendix of the breast.The inner side of the breast is 2/3 on the surface of the pectoralis major and the outer third is on the surface of the anterior serratus.A small part of the breast tissue can exceed the above range，reach the lower edge of the clavicle，and release the anterior sheath of the rectus abdominis，which reaches the midline of the body and reaches the leading edge of the latissimus dorsi.The junction of the lower edge of the breast and the surface of the trunk is called the underlying breast fold.The outer upper quadrant of the breast has the largest glandular tissue and is the most common site of breast cancer.
The mammary gland is located between the superficial and deep layers of the subcutaneous superficial fascia.The superficial layer of the superficial fascia is located deep in the dermis and is a fat-rich connective tissue；the deep layer of the superficial fascia is located deep in the mammary gland and is connected with the loose connective tissue between the superficial layers of the pectoralis major fascia.The superficial fascia extends into the mammary gland to form a fibrous septum（called the breast suspensory ligament or Cooper ligament），one end connected to the skin and one end connected to the pectoral fascia，which supports the mammary gland.The superficial and deep layers of the superficial fascia and the mammary gland enclosed in it are the main resection areas of radical mastectomy，and are also the main content of total breast irradiation after breast conserving surgery.The mammary gland consistsof a ductal glandular tissue（substantial），fibrous connective tissue（interstitial）surrounding the glandular tissue，and adipose tissue.The mammary gland is composed of the lactiferous duct，the mammary gland，and the acinus.Adult breasts have 1520 glandular lobes，arranged radially around the nipple.The lymphatic system of the breast includes lymphatic vessels in the breast and lymphatic vessels and regional lymph nodes from the breast.The lymphatic vessels in the breast are composed of the capillary lymphatic network of the skin and mammary gland.The lymphatic vessels of the breast skin are located in the dermis，with shallow and deep layers of capillary lymphatic network.The superficial lymphatic network has extensive communication with the superficial lymphatic network of the surrounding skin.When the tumor invades the blocked lymphatics，the cancer cells It can be reversed through the superficial lymphatic network，invading the contralateral breast，contralateral axillary lymph nodes or chest and abdomen back skin.The mammary gland is mainly drained to the axillary，internal milk，and supraclavicular lymph nodes，and can also be drained to the intermuscular lymph nodes（Rotter's lymph nodes）.
The armpit is a fat，connective tissue cavity.The inner wall of the sacral cavity is the anterior serratus muscle.The anterior wall is the chest and small muscles.The posterior wall is the muscle and fascia of the subscapular region.The axillary lymph nodes are arranged along the axillary nerve vessels.There are two methods for grouping according to the location，the extent of lymphatic storage，and clinical needs：1.anatomical grouping（pre-group，posterior group，lateral group，central group，and tip group）.The anterior group receives the mammary gland lymph；the posterior group receives the lymph in the back，scapular and posterior wall；the lateral group receives the lymph of the upper limb；the central group receives the anterior，posterior，lateral，and partial lymph nodes that directly receive the mammary gland；Subclavian lymph nodes，（arranged along the proximal end of the iliac vein）the anterior，posterior，lateral，central and intermuscular lymph nodes of the axilla，and the lymph nodes in the upper part of the breast；2.clinical grouping（level Ⅰ，level Ⅱ and levelⅢ），level Ⅰ（low armpit）is located outside the pectoralis minor；level Ⅱ（median armpit）is located deep in the pectoralis minor（including the intermuscular lymph nodes）；level Ⅲ（the crest or subclavian）is located inside the pectoralis minor，that is，the cluster of lymph nodes（Figure 8-01）.Axillary lymph node metastasis in breast cancer is mostly from level i to level Ⅱ，and then to level Ⅲ，jump metastasis is rare，at about 3%-3.8%.
The internal mammary lymph nodes are located in the sternal intercostal space and are arranged along the internal thoracic arteriovenous layer of the extrapleural fat layer，mainly in the 1～3 intercostal space（Fig.8-02）.The inner mammary lymph nodes and their lymphatic vessels are also commonly referred to as the inner breast lymphoid chain.The output lymphatic vessels of the internal mammary lymph nodes were injected into the supraclavicular lymph nodes，or the right lymphatic duct was injected into the right side，and the thoracic duct was injected into the left side or directly into the jugular vein.
The supraclavicular lymph nodes contain most of the lymphatic drainage of the apex group and the internal mammary lymph nodes.They are located in the connective tissue above the clavicle and deep in the platysma.The deep side is the scalene muscle，the inner boundary is the internal jugular vein，and the outside is the trapezius muscle.The upper boundary is the level of the ring membrane and the lower boundary is the subclavian vein.The output lymphatic vessels of the supraclavicular lymph nodes andthe deep cervical lymph node output tube synthesize the neck stem，sink the human thoracic duct or the right lymphatic duct，or directly inject into the jugular vein.The vicinity of the junction of the internal jugular vein and the subclavian vein is the predilection site for lymph node metastasis（Fig.8-03）.(https://www.daowen.com)
Figure 8-01 CT shows right Ⅰ-Ⅲ horizontal axillary lymph node metastasis
Figure 8-02 CT shows left breast lymph node metastasis
Figure 8-03 CT shows right supraclavicular lymph node metastasis