Principles of Radiation Therapy for Invasive Breast Cancer

Treatment Modalities and Dosing Strategies

Radiation dosing is no longer "one size fits all." Depending on the surgical approach and tumor characteristics, different regimens are utilized.

Whole Breast Irradiation (WBI) and Hypofractionation

For patients undergoing breast-conserving surgery, WBI is standard. Current trends favor hypofractionated dosing, which uses larger doses per fraction over a shorter total number of days. A common regimen is $40–42.5\text{ Gy}$ delivered in $15–16$ fractions. Furthermore, ultra-hypofractionated regimens, such as $28.5\text{ Gy}$ in $5$ fractions, are supported by clinical data and offer significant convenience to patients.

Tumor Bed Boost

In many cases, a "boost" is required to provide a higher dose to the area where the tumor was originally located (the tumor bed). This is particularly recommended for high-risk patients. This can be delivered as a Simultaneous Integrated Boost (SIB), where the boost dose is delivered at the same time as the whole breast dose, or sequentially.

Post-Mastectomy Radiation Therapy (PMRT) and Regional Nodal Irradiation (RNI)

For patients who have undergone a mastectomy, PMRT targets the chest wall and the mastectomy scar. If there is skin involvement or positive superficial margins, a bolus (a tissue-equivalent material placed on the skin) is used to ensure the radiation dose reaches the surface effectively.

If the cancer has spread to the lymph nodes, Regional Nodal Irradiation (RNI) is indicated. This targets the supraclavicular, infraclavicular, internal mammary, and axillary nodes.

Advanced Delivery Techniques

To achieve the complex dose distributions required to spare OARs while covering irregular target volumes, advanced delivery technologies are used:

1. Intensity-Modulated RT (IMRT) and Volumetric Modulated Arc Therapy (VMAT): These techniques allow for the modulation of the radiation beam intensity. VMAT, in particular, delivers radiation in a continuous arc around the patient, which can lead to more efficient treatment times and better dose conformity.
2. Stereotactic Body RT (SBRT): While SBRT provides highly concentrated doses, it is currently used primarily for local control or pain relief in oligometastatic disease. Current data do not support the use of SBRT to extend overall survival (OS) or progression-free survival (PFS) in the metastatic setting.

Quality Assurance and Clinical Integration

The final step in ensuring treatment efficacy is rigorous quality assurance. Clinicians utilize Dose-Volume Histograms (DVHs) to assess the dose distribution. A DVH is a graphical representation that shows how much of a specific volume (e.g., the heart or the tumor) receives a certain dose of radiation.

$$\text{Dose to Volume} = \int_{0}^{V} D(v) \, dv$$

By analyzing the DVH, physicists and oncologists can ensure that the target receives the prescribed dose while the OARs remain below their toxicity thresholds.

Exam Tip: If asked about the primary purpose of a DVH, remember it is to evaluate the trade-off between target coverage and OAR sparing.

Finally, radiation therapy must be integrated with systemic treatments. For example, radiation often follows chemotherapy, but certain regimens like CMF may be given concurrently. When managing endocrine therapy, clinicians may choose to delay its start until RT is complete to avoid compounding side effects.