5.3 Breast Cancer: Anatomy, Lymphatic Drainage & Radiation Considerations

Anatomy and Lymphatic Drainage

The breast overlies the pectoralis major muscle and extends roughly from the second to sixth rib vertically and from the sternal edge to the mid-axillary line horizontally, with an axillary tail of breast tissue extending into the axilla. Breast tissue is conventionally divided into four quadrants (upper outer, upper inner, lower outer, lower inner) plus the central/subareolar region; the upper outer quadrant harbors the largest volume of glandular tissue and is the most common site of primary breast cancer.

Lymphatic drainage is clinically critical because nodal status drives both staging and radiation field design. Approximately 75% of breast lymphatic drainage flows to the axillary lymph nodes, which are surgically and radiographically divided into three levels relative to the pectoralis minor muscle:

The remaining drainage travels primarily to the internal mammary lymph nodes, located along the internal thoracic vessels adjacent to the sternum, particularly from medial and central quadrant tumors. The supraclavicular nodes represent a further echelon of spread and, along with internal mammary and level III axillary involvement, are considered regional (not distant) disease requiring comprehensive regional nodal irradiation when involved or at high risk.

Radiation Treatment Approaches

Radiotherapy decisions follow the extent of surgery and nodal involvement:

• Whole breast irradiation (WBI) follows breast-conserving surgery (lumpectomy) and treats the entire remaining breast, typically with a subsequent boost to the lumpectomy cavity, using tangential beam arrangements that enter and exit lateral and medial to the breast to spare the underlying lung and heart.
• Accelerated partial breast irradiation (APBI) treats only the lumpectomy cavity with margin in carefully selected low-risk patients, shortening overall treatment time.
• Post-mastectomy chest wall irradiation treats the chest wall (and reconstructed tissue or implant, if present) and is generally added when tumors are large, margins are close or positive, or nodes are involved.
• Regional nodal irradiation (RNI) extends fields to include the supraclavicular, axillary, and/or internal mammary nodes for patients with significant nodal disease burden (commonly four or more positive axillary nodes, or fewer nodes with other high-risk features).

Because the chest wall skin itself is at risk after mastectomy, bolus material is often placed over the surgical scar and skin to bring the skin dose up to a therapeutic level, since megavoltage beams otherwise spare the skin surface (the skin-sparing effect) that mastectomy patients cannot rely on if microscopic disease could involve the dermis.

Critical Organs at Risk and Setup Implications

The heart and lungs sit immediately deep to the chest wall and breast, making cardiopulmonary sparing central to breast radiotherapy planning:

• Heart, particularly the left anterior descending (LAD) coronary artery: left-sided breast irradiation carries a small but real long-term risk of ischemic heart disease that increases with mean heart dose; this is the primary reason deep inspiration breath-hold (DIBH) techniques are used for left-sided treatment, since a full inspiration increases lung volume and physically displaces the heart posteriorly and inferiorly, away from the tangential fields.
• Ipsilateral lung: the volume of lung receiving 20 Gy or more (lung V20) correlates with radiation pneumonitis risk and is limited by careful tangent field angulation.
• Contralateral breast: scatter dose is minimized through beam angling and shielding, since patients may be young enough that secondary malignancy risk over decades is a real consideration.
• Brachial plexus: a constraint when supraclavicular fields are used, to avoid plexopathy.

Patient positioning is typically supine on a dedicated breast board with the affected arm abducted overhead to move the arm out of the tangential beam path and open up the axilla for lateral field entry; some centers use prone positioning for large-breasted patients to let breast tissue fall away from the chest wall and reduce hot spots and lung/heart dose. Reproducible setup depends on skin marks or surface-guided radiotherapy (SGRT) tracking, and for DIBH patients, a breath-hold monitoring system (spirometry-based or camera-based) confirms the patient has reached and is maintaining the correct inspiration level at each beam-on. Field-in-field or forward-planned IMRT techniques are frequently layered onto simple tangents to reduce hot spots and improve dose homogeneity, especially in larger or more pendulous breasts.

Fractionation trends have shifted meaningfully over the past decade: moderately hypofractionated whole breast irradiation (commonly 15-16 fractions rather than the traditional 25-28 fractions) is now standard for most patients based on large randomized trials showing equivalent tumor control and cosmesis with fewer visits, and ultra-hypofractionated regimens delivered in as few as five fractions are increasingly used as well. Regardless of fractionation schedule, the underlying anatomy, lymphatic drainage routes, and organ-at-risk constraints described above remain the same; only the dose-per-fraction and total number of treatment days change. Therapists should also recognize that seroma or surgical changes at the lumpectomy cavity can shift in size between simulation and the start of a boost, which is one reason boost planning is sometimes verified with updated imaging rather than relying solely on the original simulation contours.