Postmastectomy irradiation: Rationale for treatment field selection

The goal of postmastectomy irradiation is to eliminate residual viable tumor in tissue remaining after standard mastectomy. Because this subclinical disease is, by definition, not detectable by current technology, the choice of patients and treatment volumes for postmastectomy irradiation must be inferred from a variety of data sources. The absolute risk of locoregional recurrence is related to the stage of disease, the extent of lymphatic involvement, and other treatment received. Patterns of failure analyses consistently identify the chest wall as the most important target for treatment with radiation therapy in high-risk patients. When patients with multiple locoregional sites of recurrence are included, the chest wall may be involved in as many as 60% to 80% of patients. The second most common place for locoregional failure is the undissected lymphatics of the paraclavicular region. The cumulative probability of failure in this region ranges from 10% to 35% of the patients treated for locoregional recurrence. Microscopic tumor metastases in the internal mammary chain are theorized to represent a potential source for distant metastases. Each of the prospective trials of postmastectomy irradiation that have shown survival benefit included the internal mammary chain within their target volume. Nonetheless, local failure in the internal mammary nodes is an uncommon finding. Similarly, after a level I and II axillary dissection, axillary failure is a minor component of local recurrence risk, and it is probable that only a subset of patients may benefit from axillary irradiation.