Breast Cancer ·

03 — Chemotherapy & Radiation

BREAST CANCER SERIES

03 — Chemotherapy & Radiation

Systemic control and local eradication of breast cancer

Breast cancer treatment is rarely based on a single intervention. Surgery removes known macroscopic disease, while systemic therapies address cancer cells that may exist elsewhere in the body and radiation therapy provides highly localized treatment to tissue at risk of residual microscopic disease. The choice and sequence of treatment depend on tumour stage, lymph-node status, histologic grade, estrogen/progesterone receptor status, HER2 expression, genomic risk, response to treatment, and the patient's overall health. (Cancer.gov)

Anatomical orientation: the breast contains lobes, lobules and ducts within fatty and connective tissue. Lymphatic drainage is particularly important because tumour cells can migrate from the breast to regional lymph nodes, especially the axillary, internal mammary and supraclavicular regions. (NCBI)

01 — Chemotherapy: treating cancer systemically

Chemotherapy is systemic therapy. Unlike surgery or radiation, which are directed primarily at defined anatomical regions, chemotherapy circulates through the bloodstream and can reach malignant cells throughout the body. Its fundamental action is to kill cancer cells or interfere with their ability to divide. (Cancer.gov)

How chemotherapy works

Many conventional chemotherapeutic agents exploit characteristics of rapidly proliferating cells. Depending on the drug, they may:

damage DNA;
inhibit DNA replication;
interfere with mitotic spindle formation;
block nucleotide synthesis;
generate cellular stress or other forms of lethal damage;
prevent successful cell division.

Cancer cells are particularly vulnerable because many proliferate rapidly and have abnormal mechanisms for repairing cellular damage.

However, chemotherapy is not completely selective for cancer cells. Normal tissues containing rapidly dividing cells—such as bone marrow, gastrointestinal epithelium and hair follicles—can also be affected. This accounts for characteristic toxicities such as neutropenia, nausea, mucosal injury, fatigue and alopecia. (Cancer.gov)

When is chemotherapy used?

Chemotherapy can be administered before surgery, after surgery, or for recurrent/metastatic disease.

Neoadjuvant chemotherapy is administered before surgery. It can reduce tumour volume, permit breast-conserving surgery in selected patients, provide an early measure of tumour sensitivity to treatment, and treat potential systemic micrometastatic disease before definitive surgery. (Cancer.gov)

Adjuvant chemotherapy is administered after surgery. Its purpose is to eliminate microscopic malignant cells that remain after removal of the primary tumour and thereby reduce the probability of distant recurrence. (Cancer.gov)

Chemotherapy is not automatically required for every breast cancer. Modern treatment selection incorporates tumour biology and, for selected hormone-receptor-positive/HER2-negative cancers, genomic assays that estimate the likelihood of benefit from chemotherapy. (Cancer.gov)

Major chemotherapy classes

Breast cancer regimens may include combinations of:

Anthracyclines — e.g., doxorubicin, epirubicin
Taxanes — paclitaxel, docetaxel
Cyclophosphamide
Platinum agents — particularly relevant to selected triple-negative cancers
Fluoropyrimidines — such as 5-fluorouracil or capecitabine
Other agents selected according to tumour subtype and treatment setting. (Cancer.gov)

Chemotherapy may also be combined with targeted therapy or immunotherapy when tumour biology indicates that these approaches are appropriate—for example, anti-HER2 therapy for HER2-positive disease or pembrolizumab-containing treatment in selected triple-negative breast cancers. (Cancer.gov)

02 — Radiation Therapy: precision treatment of the tumour bed

Radiation therapy is fundamentally different from chemotherapy.

Chemotherapy travels through the body. Radiation is directed toward defined anatomical targets.

Radiotherapy uses high-energy radiation, commonly photons produced by a linear accelerator, to create molecular damage within irradiated tissue. The principal therapeutic mechanism is DNA damage. When cancer-cell DNA is damaged beyond effective repair, the cell loses its capacity to reproduce and ultimately dies. (Cancer.gov)

Radiation is generally delivered in multiple fractions rather than one large exposure. Fractionation allows normal tissues to repair sublethal damage between treatments while exploiting differences in repair and repopulation between malignant and normal cells. (Cancer.gov)

03 — Where is radiation directed?

Depending on the clinical situation, radiation may target:

Breast tissue
After breast-conserving surgery, radiation commonly treats the remaining breast because microscopic tumour cells may remain outside the surgical cavity.

Tumour bed
A focused additional dose, known as a boost, may be directed toward the region where the tumour was removed in selected patients.

Chest wall
After mastectomy, radiation may be directed at the chest wall or reconstructed breast when the risk of locoregional recurrence warrants treatment.

Regional lymphatics
Radiation may include axillary, supraclavicular/infraclavicular and internal mammary lymph-node regions when these areas are considered at risk. (PubMed)

The 2025 ASTRO/ASCO/SSO postmastectomy guideline recommends postmastectomy radiation for most patients with node-positive disease and selected patients with node-negative disease, with treatment decisions individualized according to tumour and treatment characteristics. (PubMed)

04 — Radiation technology: targeting while protecting normal tissue

Modern breast radiotherapy is a form of image-guided, three-dimensional treatment planning.

A planning CT creates an anatomical map of:

Target volume → breast/chest wall → lymph nodes → heart → lungs → surrounding tissues

The radiation oncologist and medical physicist then design beam arrangements that deliver the prescribed dose to the target while minimizing exposure to organs at risk.

Common techniques include:

3D conformal radiation therapy (3D-CRT)
Intensity-modulated radiation therapy (IMRT)
Image-guided radiation therapy (IGRT)
Partial-breast irradiation in appropriately selected patients
Deep-inspiration breath hold (DIBH), particularly useful for reducing cardiac exposure during treatment of some left-sided breast cancers. (Cancer.gov)

ASTRO's current guidance recognizes partial-breast irradiation as an appropriate alternative to whole-breast irradiation for selected patients with early-stage disease and favourable clinical and tumour characteristics. (Astro)

05 — Chemotherapy versus radiation

Characteristic	Chemotherapy	Radiation therapy
Treatment type	Systemic	Local/regional
Primary target	Cancer cells throughout the body	Defined tumour/tissue region
Delivery	IV, oral or other systemic routes	External beam or internal radiation
Main cellular effect	Cytotoxicity / inhibition of proliferation	DNA damage
Major purpose	Reduce systemic and distant recurrence risk	Reduce local/regional recurrence
Before surgery	Neoadjuvant chemotherapy	Less commonly used; selected investigational/clinical situations
After surgery	Adjuvant systemic treatment	Adjuvant breast/chest-wall ± nodal treatment
Major normal tissues affected	Bone marrow, gastrointestinal tract, hair follicles	Skin, subcutaneous tissue, lung and potentially heart depending on treatment field

The two modalities therefore address different biological problems. Chemotherapy is concerned primarily with microscopic disease that may have escaped the breast; radiation is concerned primarily with malignant cells remaining within or near the original anatomical site.

06 — Why treatment is often sequenced

A simplified treatment pathway can look like:

Diagnosis → Biomarker testing/staging → Systemic therapy when indicated → Surgery → Radiation when indicated → Long-term systemic therapy when indicated

But the actual sequence varies.

For example:

Large/high-risk tumour
→ neoadjuvant systemic therapy
→ surgery
→ pathology evaluates treatment response
→ radiation if indicated
→ additional systemic therapy based on residual disease and tumour biology

Alternatively:

Small, favourable tumour
→ breast-conserving surgery
→ radiation
→ endocrine therapy when hormone-receptor positive

The objective is not simply to administer more treatment. It is to select the appropriate intensity of treatment for the biological risk of the cancer.

07 — Effects on normal tissue

The therapeutic window of cancer treatment depends upon exploiting differences between malignant and normal tissue.

Chemotherapy

Because chemotherapy reaches the entire body, systemic toxicities may include:

fatigue;
neutropenia and infection risk;
anemia;
thrombocytopenia;
nausea and vomiting;
mucositis;
alopecia;
peripheral neuropathy;
organ-specific toxicities depending on the drug.

Some adverse effects are temporary, while others can persist.

Radiation

Radiation-related effects depend strongly on the irradiated volume and dose.

Common effects include:

skin erythema and irritation;
breast or chest-wall tenderness;
fatigue;
edema;
later tissue fibrosis or changes in skin/subcutaneous tissue.

Less common but clinically important late effects can involve the lung, heart or reconstructed breast, which is why contemporary planning emphasizes dose optimization and normal-tissue sparing. (Cancer.gov)

08 — Radiation and breast reconstruction

Radiation becomes particularly important when reconstruction follows mastectomy.

Radiation can alter the biological environment of reconstructed tissue by producing inflammation and, over time, fibrosis, vascular changes and tissue contraction. These effects can influence the appearance, softness and long-term behaviour of an implant or autologous reconstruction.

Therefore, when mastectomy and reconstruction are being planned, the potential need for postmastectomy radiation should be considered before the reconstruction strategy is finalized.

Modern postmastectomy guidelines specifically include the reconstructed breast within the radiation target when PMRT is indicated. (PubMed)

09 — The biological principle

At its most fundamental level, breast cancer treatment is an exercise in controlling tumour biology at two spatial scales:

SYSTEMIC SCALE

Chemotherapy ± targeted therapy ± immunotherapy

↓

Cancer cells potentially distributed beyond the primary breast tumour are exposed to treatment.

LOCAL SCALE

Surgery + radiation

↓

The known primary tumour and the anatomical tissues at risk for residual microscopic disease are physically removed or irradiated.

This distinction explains why surgery alone may not be sufficient for biologically aggressive breast cancers—and why systemic therapy and radiation are not interchangeable.

Conclusion

Chemotherapy and radiation represent two fundamentally different but complementary approaches to breast cancer control. Chemotherapy is systemic, using pharmacologic agents to eliminate or suppress malignant cells throughout the body. Radiation is local and regional, using precisely delivered ionizing radiation to damage the DNA of cancer cells remaining within the breast, chest wall or regional lymphatic pathways.

Modern breast oncology increasingly treats the tumour according to its molecular phenotype, anatomical stage and response to therapy, rather than applying one treatment sequence to every patient. Advances in genomic testing, targeted drugs, immunotherapy, radiation planning, hypofractionation and normal-tissue protection have made treatment increasingly individualized.

The central objective remains straightforward: eradicate malignant disease while preserving as much normal tissue and function as possible.

Scientific References

National Cancer Institute. Chemotherapy to Treat Cancer. NCI — Chemotherapy to Treat Cancer
National Cancer Institute. Chemotherapy for Breast Cancer. NCI — Breast Cancer Chemotherapy
National Cancer Institute. Radiation for Breast Cancer. NCI — Radiation for Breast Cancer
National Cancer Institute. Radiation Therapy to Treat Cancer. NCI — Radiation Therapy
Jimenez RB, et al. Postmastectomy Radiation Therapy: An ASTRO-ASCO-SSO Clinical Practice Guideline. Journal of Clinical Oncology. 2025. (PubMed)
ASTRO/ASCO/SSO. Postmastectomy Radiation Therapy Clinical Practice Guideline. 2025. ASTRO — Postmastectomy Radiation Therapy Guideline
Smith BD, et al. Accelerated Partial Breast Irradiation Consensus Statement. ASTRO. ASTRO — Partial Breast Irradiation Guideline
Burstein HJ, et al. Adjuvant Endocrine and Chemotherapy for Early Breast Cancer. ASCO clinical practice guidance. (PubMed)

Educational article for the URIBHO Breast Cancer Series; individual treatment decisions require assessment by the patient's oncology team.