Radiation and chemotherapy can be given before or after breast cancer resection. As a part of the AJCC and CAP breast cancer staging protocols, in cases where neoadjuvant chemotherapy has been given, the pathologist should evaluate and mention the effects of the regimen after resection.
The problem that pathologists commonly face is that these therapies can cause the normal breast tissue and any breast cancer present to undergo some funky changes. Like, these changes can look REALLLYYYY ugly! As you can imagine, these can make it hard to distinguish from residual breast cancer.
Common Histologic Changes in Radiation Induced Atypia
Sclerosed TDLUs with scattered enlarged atypical epithelial cells
Concentric fibrosis often surrounds the acini
Atypical cells have vacuolated eosinophilic cytoplasm, enlarged hyperchromatic or vesicular nuclei and inconspicuous nucleoli
Glandular atrophy & dilated capillaries
Mild vascular endothelial atypia & hypercellularity. NOTE: Vascular channels are still well defined and do NOT invade into the surrounding stroma! (Contrast to angiosarcoma where vessels invade the stroma)
Clues that Favor Radiation-Changes over cancer
Atypical cells are NOT uniformly occupying the lobular acini
Lobular pattern of acini is maintained
Spindled myoepithelial cells are still present!
Most acini are atrophic
No evidence/rare cellular stratification or mitotic activity seen
Histologic Findings Indicating a Good Treatment Response
Little to no viable tumor cells remaining
Fibrosis in the site of the tumor
Keep in mind, a desmoplastic stromal response can be seen in normal breast tissue in response to tumor!