Breast Cancer (Part 2): DCIS, Early-Stage and Metastatic Treatment

Continued from: Breast Cancer (Part 1): Risk, Genetics, Screening, Diagnosis and Staging

DCIS

• Definition: neoplastic epithelial proliferation confined to the ductal-lobular system (Tis, stage 0); ~20% of breast ca in the screening era. ~80% detected by mammogram (most commonly calcifications), usually asymptomatic.
• Pathology: nuclear grade (low/intermediate/high), necrosis, architecture (comedo, solid, cribriform, micropapillary, papillary), size/extent. Test ER; do NOT test HER2. No Oncotype in DCIS. Can be associated with microinvasion (≤1 mm, staged T1mi).
• Up to ~50% progress to invasive disease if untreated; ~half of local recurrences are invasive. Goal of treatment: prevent local recurrence. ~1% to 2% local recurrence after simple mastectomy; BCT + RT is higher (~8% to 10% invasive IBTR, ~15% to 20% total IBTR at 15 yrs; NSABP B-17/B-24).
• Surgery: lumpectomy + RT (preferred; margin ≥2 mm for DCIS) or mastectomy (multicentric, large extent, RT contraindication, or preference). If invasive component present, use "no ink on tumor." SLNB not routine with BCT, but do SLNB with mastectomy (feasibility lost afterward) or for large/palpable lesions.
• RT after lumpectomy: ~52% ↓ ipsilateral recurrence; no OS benefit. Conventional (5 to 6 wks) equals hypofractionated (3 to 4 wks) whole-breast RT. APBI candidates: <2.5 cm, grade I/II, margins ≥3 mm. ↑ local recurrence with margins <2 mm, large tumor, younger age, high grade, necrosis.
• Endocrine tx (5 yrs) if ER+: ↓ ipsilateral and contralateral recurrence; no OS benefit. Tamoxifen (pre- and postmenopausal, NSABP B-24: 15-yr tamoxifen ↓ invasive ipsilateral recurrence 32% (HR 0.68; 8.5% vs 10.0%, ~1.5% absolute) and contralateral (7.3% vs 10.8%, ~32%), benefit only in ER+); anastrozole vs tamoxifen postmenopausal (NSABP B-35: 10-yr breast-ca-free 93.5% vs 89.2% favoring anastrozole in patients <60). Low-dose tamoxifen 5 mg/d x 3 yrs (TAM-01) is an option if intolerant of standard 20 mg dose.
• No endocrine tx or RT after bilateral mastectomy. Consider hormone tx if unilateral mastectomy and ER+. No adjuvant chemo or HER2-directed tx even for large, high-grade DCIS.

Benign proliferative breast disease

• Atypical hyperplasia: ~30% breast ca risk over 25 yrs (not benign proliferative disease generally). BRCA1 ~72%, BRCA2 ~69% risk to age 80. ADH → excision. ALH and classic LCIS → no longer require surgical excision. No atypia → nonproliferative lesions confer little/no ↑ risk, but proliferative disease without atypia confers ~1.5 to 2x risk; flat epithelial atypia (FEA) has uncertain risk.

• Follow-up for ADH/ALH/LCIS: chemoprevention with endocrine tx (tamoxifen x5 yrs; no OS benefit) plus annual breast exam and mammo ± MRI. Pure FEA: return to routine surveillance.

Early-stage invasive: principles and local therapy

• Neoadjuvant equals adjuvant chemo for distant recurrence and mortality, but 15-yr local recurrence is higher with neoadjuvant (21.4% vs 15.9%; stresses adequate local therapy); neoadjuvant ↓ tumor size (enables BCT) and gauges chemosensitivity. pCR (no invasive ca in breast or axillary nodes) is a strong prognostic factor, especially in TNBC and HER2+.
• NSABP B-18 (preop vs postop AC) and B-27 (preop AC vs AC then docetaxel): pCR 13% with AC to 26% with AC then docetaxel; ↑ DFS/OS with pCR. Pooled analysis: pCR correlates with OS, highest in TNBC (~30%), HER2+ 30% to 50%, HR+ ~15%. TNBC or HER2+ without pCR may benefit from additional adjuvant tx.
• Endocrine and HER2-targeted tx can be given concurrently with RT; finish chemo before RT.
• Stage III (locally advanced): resectable (large tumors / multiple nodes) vs inoperable (skin/chest wall involvement, inflammatory, matted or supraclavicular nodes). Inoperable disease requires neoadjuvant systemic tx then surgery and RT.

• NSABP B-04: no OS difference between radical and total mastectomy. NSABP B-06: no OS difference between total mastectomy and lumpectomy ± RT; adjuvant RT after lumpectomy ↓ local recurrence; BCT + RT equals mastectomy without RT for local control.
• Negative margin: "no tumor on ink" for invasive ca; ≥2 mm for DCIS. Mastectomy options: total, skin-sparing, nipple-sparing.
• Relative contraindications to BCT (indications for mastectomy), "MMM CPR": multicentric disease, diffuse suspicious calcifications, multiple positive margins, unable to obtain negative margins, pregnancy (1st/2nd trimester, RT contraindicated), prior RT / connective tissue disease (e.g., SLE), informed patient preference.
• SOUND (2023) / INSEMA (2024): cN0 with negative axillary US having BCS + whole-breast RT (SOUND ≤2 cm; INSEMA cT1-2, mostly ≤2 cm): omitting SLNB non-inferior for DDFS/iDFS (primary endpoints); apply mainly to favorable biology (postmenopausal ≥50, grade 1 to 2, HR+/HER2-).

• SLNB is standard for cN0 (less morbidity than ALND). Use H&E/frozen on the SLN; do not use cytokeratin/IHC for decisions. Negative SLNB → no ALND.
• ACOSOG Z0011: cT1-2 N0 with 1 to 2 positive SLNs after BCT + whole-breast RT can OMIT ALND (no DFS/OS difference).
• No ALND for micrometastatic-only SLN (IBCSG 23-01). AMAROS: for T1-2 cN0 with positive SLN, axillary RT is non-inferior to ALND with less lymphedema.
• ALND indications: >2 positive SLN, gross/matted nodes, or positive SLN when no nodal RT is planned (1 to 2 SLN macromets can omit ALND with nodal RT per SENOMAC, or use axillary RT per AMAROS); positive SLN after neoadjuvant chemo (ycN+); inflammatory breast ca; SLN not identified.
• Post-neoadjuvant axilla: cN0 who has neoadjuvant chemo then SLNB (dual tracer, ± targeted axillary dissection): any positive SLN → ALND; negative SLN → no further surgery. Downstaging is higher in TNBC/HER2+. NSABP B-51 (RNI vs no RNI in cN1 becoming ypN0 after neoadjuvant chemo; negative) and Alliance A011202 (ALND vs axillary RT in residual positive SLN, ypN+, after neoadjuvant chemo).

• Adjuvant whole-breast RT after BCS ↓ locoregional recurrence and breast ca death. Hypofractionated WBI (40 to 42.5 Gy over 3 wks, 15 to 16 fractions) is standard; FAST-Forward 26 Gy/5 fr emerging. Tumor-bed boost if young (<50), high grade, close margins, or DCIS.
• APBI over 1 to 2 wks spares distant breast tissue. Omit RT in older patients with small node-negative ER+ tumors on endocrine tx (CALGB 9343; PRIME II: ↑ in-breast recurrence but no OS impact). RT omission is an option (not a mandate) in older patients (≥65 to 70) with small (≤3 cm) node-negative HR+ tumors on endocrine therapy; expect higher local recurrence but no OS effect.
• Regional nodal irradiation (RNI): axilla (levels I to III), supraclavicular fossa, internal mammary chain. Indicated for ≥4 node+; also for 1 to 3 node+ (esp. high-risk features, e.g., ENE+) or high-risk node-negative (NCIC MA.20, EORTC 22922).
• Post-mastectomy RT (PMRT) is beneficial when combined with adjuvant chemo: stage III, ≥4 node+, pT3N+, or residual disease after neoadjuvant chemo. PMRT is individualized for pT3N0; many with 1 to 3 positive nodes still warrant it; after neoadjuvant chemo, initial cT4 or cN2 to 3 disease generally warrants PMRT regardless of pCR, and residual nodal disease warrants PMRT.

HR+/HER2- early stage: systemic therapy

• HR+/HER2- is relatively less chemosensitive (neoadjuvant pCR ~15%), but selected patients (high genomic risk, nodal burden, younger age) still benefit from chemo; upfront surgery preferred. Tumors <0.5 cm: endocrine therapy only, no chemo. Tumors >0.5 cm warrant consideration of adjuvant tx. Chemo need is guided by Oncotype/MammaPrint (see above); when both chemo and endocrine are indicated, give chemo first. Younger age (<35) portends worse OS (5-yr 74.7% vs 85%), lowering the threshold for adjuvant tx.
• Give adjuvant chemo for any N2 (≥4 nodes); do not use Oncotype in N2. Locoregional recurrence on adjuvant ET: resect to negative margins; chemo benefits mainly ER-negative recurrence (CALOR), not ER-positive; give endocrine therapy for ER+ recurrence, switching agent if it arose on treatment.

• Endocrine tx ↓ systemic recurrence and death in all HR+ patients regardless of age, menopausal status, nodes, size, HER2, or chemo use; consider in all HR+ disease.
• Tamoxifen (pre- and postmenopausal): EBCTCG meta-analysis, 5 yrs ↓ distant recurrence ~41% and death ~34%. Extended to 10 yrs (ATLAS, aTTom): further ↓ recurrence (~3.2%) and mortality (~2.8%), benefit emerging after year 10; ↑ PE and endometrial ca (cumulative ~3.1% after 5 yrs), no ↑ stroke.
• CYP2D6: converts tamoxifen to active endoxifen. ASCO does not recommend CYP2D6 testing, but caution with strong inhibitors (fluoxetine, paroxetine, bupropion). Prefer SNRIs or citalopram/venlafaxine for vasomotor symptoms (mnemonic: tamoxifen paralyzed by "flu," take it to the CITy to VENTilate).
• Ovarian function suppression (OFS) (SOFT/TEXT): adding OFS to tamoxifen or exemestane ↑ DFS (~4.3% to 7%) vs tamoxifen alone; benefit greater with chemo or age ≤35; exemestane + OFS > tamoxifen + OFS for distant recurrence; small OS gain with OFS + tamoxifen. Low-risk stage I patients do equally well with tamoxifen alone. OFS + AI favored for higher-risk and younger women. If chemo given, use OFS + AI (rather than tamoxifen or AI alone) after chemo/RT.
• Aromatase inhibitors (postmenopausal): 3rd-gen (anastrozole, letrozole = nonsteroidal; exemestane = steroidal), comparable efficacy. Oxford meta-analysis: vs 5 yrs tamoxifen, AI ↓ recurrence ~30% proportionately vs tamoxifen while treatments differ (years 0 to 4), not cumulative 10-yr; 10-yr breast ca mortality 12.1% vs 14.2% (~15% ↓). Not effective in pre/perimenopausal women without OFS; base choice on menopausal status before treatment (chemo-induced amenorrhea can be transient). Switch to tamoxifen if AI intolerant.
• Extended endocrine tx (beyond 5 yrs): switching to or continuing an AI gives modest DFS gains (largely by preventing contralateral ca; MA.17R, NSABP B-42); reserve for higher-risk disease. BCI helps select who benefits.
• Neoadjuvant endocrine tx: acceptable in postmenopausal HR+ to enable BCT or downstage; AI superior to tamoxifen; give 4 to 6 mo; low pCR but falling Ki-67 is favorable. Investigational in premenopausal women and men.

• Adjuvant palbociclib (PALLAS, Penelope-B) did NOT improve IDFS. Benefit is with abemaciclib and ribociclib.
• monarchE (abemaciclib x2 yrs + ET): eligibility ≥4 node+, OR 1 to 3 node+ with grade 3, tumor ≥5 cm, or Ki-67 ≥20%. 5-yr IDFS 83.6% vs 76.0% (HR 0.680); FDA Oct 2021, label expanded Mar 2023 (no Ki-67 requirement). Primary OS analysis (Johnston Ann Oncol 2025, ESMO 2025; median FU 76 mo): statistically significant OS benefit, HR 0.842 (P=0.027); 7-yr OS 86.8% vs 85.0%; 7-yr IDFS 77.4% vs 70.9%.
• NATALEE (ribociclib 400 mg x3 yrs + AI): eligibility stage IIA N0 with grade 3 or grade 2 + Ki-67 ≥20%, IIA N1, IIB, III. 5-yr IDFS HR ~0.715. FDA approved Sept 17, 2024 for HR+/HER2- early BC at high recurrence risk (Hortobagyi Ann Oncol 2024; PMID 39442617). 4-yr update (Slamon, SABCS 2024, 48.8 mo FU): IDFS HR 0.715 consistent across stage II vs III and N0 vs N+; neutropenia (grade 3/4 ~44%) plus liver enzyme elevation and QT prolongation; requires CBC, LFT, and ECG monitoring.
• Practical: monarchE for very high-risk (≥4 node+); NATALEE expands to broader stage II. Choose by clinical features and tolerability.

• KEYNOTE-756 (Cardoso Nat Med 2025): pembrolizumab + neoadjuvant CT (paclitaxel then AC/EC) then adjuvant pembrolizumab x9 cycles in stage II to III HR+/HER2- grade 3. pCR 24.3% vs 15.6% (delta 8.5%, p=0.00005); benefit largest in ER-low (1% to 9%) and PD-L1+ (CPS ≥10). EFS immature; not FDA-approved for this indication.
• CheckMate 7FL (Loi Nat Med 2024): nivolumab + neoadjuvant CT in similar high-risk HR+/HER2-. pCR 24.5% vs 13.8% (delta 10.5%); stronger in PD-L1+ CPS ≥1. Terminated early; supportive, not registrational.
• Take-home: converging signal that IO adds pCR in ER-low/high-risk HR+/HER2-; role being defined, await mature EFS/OS.

• Guided by Oncotype/MammaPrint, anatomic stage, and clinical features. Common: dose-dense AC-T, or TC x4 (West German PlanB: TC x6 non-inferior to EC-T analog for 5-yr DFS in mostly HR+/N0-N1). ABC trial: dose-dense AC-T ↑ DFS in TNBC and HR+ with ≥4 node+.
• Regimen details: ddAC-T (TaxAC) = doxorubicin + cyclophosphamide q2wk x4 then weekly paclitaxel x12 (highly emetogenic, needs growth factor, cardiac and hypersensitivity risks). TC = docetaxel + cyclophosphamide q3wk x4 (shorter, no cardiotoxicity, lower leukemia risk). Doxorubicin: give 50% if tbili 1.2 to 3, 25% if 3.1 to 5; contraindicated if tbili >5 or Child-Pugh C. Cyclophosphamide has no bilirubin-based contraindication (labeled contraindications: hypersensitivity, urinary outflow obstruction); severe hepatic impairment reduces its activation, so use caution per regimen guidance.
• Adjuvant olaparib (OlympiA): germline BRCA1/2 + high-risk HER2-negative early BC (TNBC: upfront surgery requires ≥pT2 or node-positive, and after neoadjuvant chemo any residual invasive disease (non-pCR) qualifies; HR+ ≥4 positive nodes (upfront) or non-pCR with CPS-EG ≥3 (after neoadjuvant)). 3-yr IDFS 85.9% vs 77.1% (HR 0.58); 4-yr OS 89.8% vs 86.4%; FDA Mar 2022. 6-yr update (Geyer SABCS 2024): sustained IDFS; 6-yr OS 87.5% vs 83.2% (HR 0.72, p=0.009).

HER2+ early stage

• <2 cm (cT1, i.e. T1a/b/c) N0: upfront surgery acceptable. If pT1N0 → weekly paclitaxel + trastuzumab x12 (APT trial; 7-yr DFS ~93%, only ~1% distant events) then trastuzumab to complete 1 yr. If pT2 or N1 → adjuvant chemo + trastuzumab x1 yr, adding pertuzumab if node+ (APHINITY).
• ≥2 cm OR node+: neoadjuvant chemo + dual HER2 blockade. TCHP x6 (docetaxel, carboplatin, trastuzumab, pertuzumab); TRAIN-2: anthracycline-free paclitaxel/carboplatin + HP equals anthracycline-containing regimens for EFS/OS with pCR ~68% in both arms; TRYPHAENA/NeoSphere pCR ~40% to 60%. Concurrent (not sequential) trastuzumab with chemo preferred; avoid concurrent anthracycline + trastuzumab.
• Residual disease post-neoadjuvant: T-DM1 (ado-trastuzumab emtansine, KADCYLA) x14 cycles (KATHERINE): 3-yr IDFS 88.3% vs 77.0% (HR 0.50); mature OS HR 0.66 (2023). More serious AEs than trastuzumab (13% vs 8%). If HR+, give endocrine tx concurrently; RT can be concurrent. If progression on T-DM1 → T-DXd.
• DESTINY-Breast05 / DESTINY-Breast11: FDA May 15, 2026 approved T-DXd (1) post-neoadjuvant for HER2+ residual invasive disease (DB05 vs T-DM1: 3-yr IDFS 92.4% vs 83.7%, HR 0.47) and (2) neoadjuvant T-DXd followed by THP (DB11 vs ddAC-THP: pCR 67.3% vs 56.3%).
• pCR: complete adjuvant trastuzumab to 1 yr ± pertuzumab (APHINITY: 6-yr IDFS 91% vs 88%, HR 0.76; most benefit in node+ or HR-negative).
• Neratinib x1 yr (ExteNET): extended adjuvant after trastuzumab, esp. HR+ started within 1 yr; ↑ IDFS, high diarrhea (loperamide prophylaxis or labeled dose-escalation schedule).
• Trastuzumab standard duration 12 mo (HERA; 2 yrs no better and more cardiotoxic; PERSEPHONE 6 mo non-inferior in select). BCIRG 006: TCH acceptable alternative to AC then docetaxel + trastuzumab with less cardiotoxicity/leukemia. SC formulations and 6 biosimilars approved (through 2024). Cardiac monitoring q3 mo (echo/MUGA); ~18% asymptomatic EF drops.

TNBC early stage

• <2 cm AND node-negative: upfront surgery. T1a (>1 to 5 mm): observation. <1 cm: observation vs TC x4. 1 to 2 cm: TC x4.
• ≥2 cm OR node+: neoadjuvant chemo + immunotherapy. KEYNOTE-522 (NEJM 2020, OS update 2024): pembrolizumab + carboplatin/paclitaxel then AC (or EC), then adjuvant pembrolizumab x9 cycles for stage II to III. pCR 64.8% vs 51.2%; EFS improved regardless of pCR (36-mo 84.5% vs 76.8%, HR 0.63); 5-yr EFS 81.3% vs 72.3%; OS HR 0.66 (mature). FDA Jul 2021.
• Neoadjuvant platinum: adding carboplatin ↑ pCR (GeparSixto, BrighTNess) but EFS effect inconsistent across trials (CALGB 40603 no EFS benefit); optimal dose/schedule unresolved; platinum not standard adjuvant after the neoadjuvant pathway (EA1131: not non-inferior to capecitabine for residual TNBC), but PATTERN showed adjuvant paclitaxel-carboplatin improved DFS vs CEF-T after upfront surgery.
• Residual disease after neoadjuvant: if germline BRCA → adjuvant olaparib x1 yr (OlympiA). If BRCA wild-type → adjuvant capecitabine (CREATE-X: 5-yr DFS 69.8% vs 56.1%, OS 78.8% vs 70.3% in TNBC; death HR 0.52). EA1131: platinum not non-inferior to capecitabine. pCR → complete adjuvant pembrolizumab x9 cycles.
• Stage I T1aN0 (<5 mm): consider single-agent paclitaxel (limited data) or observation.

Metastatic breast cancer: general principles

• Incurable; goals are to delay progression, maintain quality of life, and prolong survival. Sequential single-agent therapy is the mainstay; combination chemo only for visceral crisis or rapid progression. OS improving ~1% to 2%/yr.
• Biopsy metastatic disease at first presentation (HR/HER2 discordance in 10% to 15%). Common first sites: bone (HR+ ~68%), lung (TNBC ~40%); brain more common in HER2+ and TNBC. Fluoroestradiol PET (FDA 2020) detects ER+ lesions. CA 27.29 / 15-3 and CEA complement imaging but not used alone; ctDNA/CTCs investigational for monitoring.

Metastatic HR+/HER2-

• CDK4/6 inhibitor + AI or fulvestrant is SOC (mechanism: block CDK4/6 phosphorylation of Rb, arresting G1 to S). Premenopausal: add OFS (e.g., goserelin). Endocrine-sensitive (relapse ≥12 mo after ET): AI or fulvestrant + CDK4/6i; endocrine-resistant (progression on or within 12 mo of AI): fulvestrant + CDK4/6i.
  • Palbociclib + letrozole (PALOMA-2): mPFS 24.8 vs 14.5 mo; OS not significantly improved.
  • Ribociclib + AI (MONALEESA-2; MONALEESA-3 + fulvestrant; MONALEESA-7 premenopausal + OFS + NSAI/tamoxifen): OS benefit confirmed (MONALEESA-7 premenopausal mOS 58.7 vs 48 mo).
  • Abemaciclib + AI (MONARCH 3): mPFS 28.1 vs 14.7 mo. MONARCH-2 (+ fulvestrant): mPFS 16.4 vs 9.3 mo, mOS 46.7 vs 37.3 mo.
• Toxicity: ribociclib → ↓↓ ANC, ↑ LFTs, QT prolongation (baseline EKG mandatory (QTcF <450 ms), repeat ~day 14 of cycle 1 then as clinically indicated; electrolytes at baseline and start of first 6 cycles); abemaciclib → diarrhea (~80%), less neutropenia; palbociclib → ↓↓ ANC (dose reduction does not reduce efficacy). Abemaciclib favored for CNS disease (BBB penetration).
• Inavolisib + palbociclib + fulvestrant (INAVO120): 1L for PIK3CA-mutant, endocrine-resistant disease (relapse during or within 12 mo of completing adjuvant ET; no prior therapy for metastatic disease). mPFS 15.0 vs 7.3 mo (HR 0.43); mOS 34.0 vs 27.0 mo (HR 0.67). FDA Oct 10, 2024 for endocrine-resistant PIK3CA-mut mBC progressing within 12 mo of adjuvant ET (Turner NEJM 2024; PMID 39476340; OS: Jhaveri NEJM 2025; PMID 40454641).

• Everolimus + exemestane (BOLERO-2): mPFS 10.6 vs 4.1 mo after nonsteroidal AI; stomatitis (steroid mouthwash prophylaxis), noninfectious pneumonitis (~12% in BOLERO-2, up to ~19% in label; not rare), can be severe/fatal. (PI3K-Akt-mTOR pathway drives endocrine resistance.)
• Alpelisib + fulvestrant (SOLAR-1): PIK3CA-mutant, mPFS 11.0 vs 5.7 mo (no benefit in non-mutated); ↑ hyperglycemia (~64% any grade, ~37% grade 3/4), rash. FDA 2019.
• Capivasertib + fulvestrant (CAPItello-291): AKT1, PIK3CA, or PTEN alterations, mPFS 7.3 vs 3.1 mo (Turner NEJM 2023; PMID 37256976). FDA Nov 16, 2023.
• Elacestrant (Orserdu) (EMERALD): oral SERD for ESR1-mutant post ≥1 line ET; mPFS 3.8 vs 1.9 mo in ESR1-mut (HR 0.55). FDA Jan 27, 2023.
• Vepdegestrant (Veppanu): oral PROTAC ER degrader; FDA May 1, 2026 for ESR1-mutated disease after ≥ 1 line of ET (VERITAC-2, ESR1m: mPFS 5.0 vs 2.1 mo vs fulvestrant, HR 0.57). Gedatolisib (Revtorpyk): IV pan-PI3K/mTOR inhibitor; FDA Jul 14, 2026 with fulvestrant ± palbociclib for PIK3CA wild-type disease after ≥ 1 line of ET (VIKTORIA-1 triplet: mPFS 9.3 vs 2.0 mo, HR 0.24).
• Imlunestrant (Inluriyo) ± abemaciclib (EMBER-3, Jhaveri NEJM 2025; PMID 39660834; FDA Sept 25, 2025 as monotherapy and Sept 18, 2026 with abemaciclib, both for ESR1-mutated disease after ≥ 1 line of ET): oral next-gen SERD post-AI (most had prior CDK4/6i). Imlunestrant mono vs SOC endocrine (ESR1-mut): mPFS 5.5 vs 3.8 mo (HR 0.62; no benefit in ESR1 wild-type). Imlunestrant + abemaciclib vs imlunestrant alone (ITT): mPFS 9.4 vs 5.5 mo (HR 0.57) regardless of ESR1 status, supporting SERD + CDK4/6i after prior CDK4/6i.
• postMONARCH (Kalinsky JCO 2025): abemaciclib + fulvestrant vs fulvestrant after progression on any prior CDK4/6i. mPFS 6.0 vs 5.3 mo (HR 0.73, p=0.017); ORR 17% vs 7%. Reinforces CDK4/6i continuation as a valid post-progression strategy.
• Camizestrant (Etcamah; next-gen oral SERD): FDA accelerated approval Sept 4, 2026 (75 mg PO daily with a CDK4/6i): SERENA-6 (Turner NEJM 2025) ctDNA-guided switch to camizestrant + CDK4/6i on emergence of ESR1 mutation during 1L AI + CDK4/6i: mPFS 16.0 vs 9.2 mo (HR 0.44). ODAC voted 6-3 against clinical benefit (Apr 30, 2026) (concern re: switching pre-progression); FDA nonetheless granted accelerated approval. SERENA-2 (vs fulvestrant) positive PFS, supportive.

• Trastuzumab deruxtecan (T-DXd) for HER2-low/ultralow: DESTINY-Breast04 (HER2-low, IHC 1+ or 2+/ISH-, post-1L chemo): mPFS 9.9 vs 5.1 mo, mOS 23.4 vs 16.8 mo (HR 0.64); FDA Aug 2022. DESTINY-Breast06 (HR+ HER2-low/ultralow, post-endocrine, chemo-naive in metastatic): mPFS 13.2 vs 8.1 mo (HR 0.62); FDA Jan 2025. Monitor for ILD/pneumonitis.
• Datopotamab deruxtecan (Datroway) (TROPION-Breast01): mPFS 6.9 vs 4.9 mo vs chemo; FDA Jan 17, 2025 for HR+/HER2- after endocrine + ≥1 chemo. ILD warning.
• Sacituzumab govitecan (Trodelvy) (TROPiCS-02): HR+ heavily pretreated, mPFS 5.5 vs 4.0 mo, mOS 14.4 vs 11.2 mo; FDA Feb 2023.
• Olaparib / talazoparib for germline BRCA1/2 (OlympiAD, EMBRACA): single-agent PARPi. Chemo: sequential single agents (anthracyclines, taxanes most active; capecitabine oral with little alopecia; eribulin OS benefit vs physician choice; gemcitabine, vinorelbine, ixabepilone, platinums esp. in BRCA-mutant).

Metastatic HER2+

• THP = taxane (docetaxel or paclitaxel) + trastuzumab + pertuzumab (CLEOPATRA): mPFS 18.5 vs 12.4 mo, mOS 57.1 vs 40.8 mo; 8-yr OS 37% vs 23%. Historical benchmark; drop the taxane after ~6 to 8 cycles and continue HP until progression. PERUSE: any taxane equivalent. MARIANNE: T-DM1 not superior to taxane + trastuzumab first-line.
• DESTINY-Breast09 (Tolaney NEJM 2025): 1L T-DXd + pertuzumab vs THP. mPFS 40.7 vs 26.9 mo (HR 0.56); ORR 87% vs 81%. FDA approved Dec 15, 2025: first new 1L SOC in HER2+ mBC in a decade; replaces THP for many. Monitor ILD.
• PATINA (Metzger-Filho ASCO 2025 plenary): 1L maintenance palbociclib + trastuzumab + endocrine (AI or fulvestrant) after induction taxane + trastuzumab + pertuzumab in HR+/HER2+ mBC. mPFS 44.3 vs 29.1 mo (HR 0.74, p=0.0074); establishes CDK4/6i maintenance in HR+/HER2+ (~50% of HER2+ mBC).

• T-DXd (DESTINY-Breast03) vs T-DM1: mPFS 28.8 vs 6.8 mo (HR 0.33); mOS 52.6 vs 42.7 mo. New 2L SOC; FDA May 2022. (DESTINY-Breast01 in ≥2 prior anti-HER2: ORR ~60%, mPFS 16.4 mo; fatal ILD ~2.2%.)
• Tucatinib + trastuzumab + capecitabine (HER2CLIMB): mPFS 7.8 vs 5.6 mo, mOS 21.9 vs 17.4 mo; 48% had brain mets at baseline (68% ↓ CNS-PFS risk). CNS-active; tucatinib → diarrhea, stomatitis, ↑ LFTs.
• T-DM1 (EMILIA, TH3RESA), neratinib + capecitabine (NALA: PFS HR 0.76 (median mPFS ~5.6 vs 5.5 mo; 8.8 vs 6.6 mo are mean/restricted-mean) vs lapatinib + cape), lapatinib + capecitabine, margetuximab + chemo (SOPHIA: mPFS 5.8 vs 4.9 mo; CD16A-158F allele may predict benefit). Continue trastuzumab beyond progression (GBG 26). ~45% of HER2+ are also HR+; add endocrine tx.
• Brain mets: tucatinib + trastuzumab + capecitabine, or T-DXd (DESTINY-Breast03/12 confirmed CNS activity, CNS ORR ~50% to 67%); SRS often integrated.

Metastatic TNBC

• Pembrolizumab + chemo for PD-L1 CPS ≥10 (KEYNOTE-355): mOS 23.0 vs 16.1 mo (HR 0.73 in CPS ≥10); FDA Nov 2020. Chemo backbone: nab-paclitaxel, paclitaxel, or gemcitabine + carboplatin. (IMpassion130 atezolizumab + nab-paclitaxel initially approved then withdrawn after IMpassion131 was negative; single-agent checkpoint inhibitors disappointing.)
• CPS <10 or not a PD-1/PD-L1 candidate: sacituzumab govitecan (ASCENT-03: mPFS 9.7 vs 6.9 mo, HR 0.62; FDA Jun 24, 2026) or datopotamab deruxtecan (TROPION-Breast02: mPFS 10.8 vs 5.6 mo, mOS 23.7 vs 18.7 mo; FDA May 22, 2026); otherwise chemo, or olaparib/talazoparib if germline BRCA.
• Sacituzumab govitecan + pembrolizumab for PD-L1 CPS ≥ 10 (ASCENT-04: mPFS 11.2 vs 7.8 mo vs chemo + pembro, HR 0.65); FDA Jun 24, 2026.

• Sacituzumab govitecan (Trodelvy) (ASCENT): mOS 12.1 vs 6.7 mo (HR 0.48), mPFS 5.6 vs 1.7 mo; FDA Apr 2020 then full Apr 2021 (≥2 prior therapies). Anti-Trop-2 conjugated to SN-38.
• T-DXd for HER2-low TNBC (DESTINY-Breast04 HR-negative cohort). PARP inhibitors for germline BRCA if not yet used. Eribulin, vinorelbine in heavily pretreated disease. Platinum (esp. BRCA-mutant): TNT trial carboplatin ORR 68% vs 33% docetaxel in germline BRCA.

Brain metastases

• Occur in ~15% of metastatic patients; highest in HER2+ and TNBC. Approach by subtype, number/size, CSF obstruction, and prognosis.
• Surgery for a single metastasis with stable/absent extracranial disease (or a large symptomatic lesion). SRS preferred for limited lesions (fewer neurocognitive effects than WBRT); WBRT for extensive disease (consider hippocampal-sparing) and ↓ intracranial recurrence but no survival gain.
• Tucatinib + trastuzumab + capecitabine (HER2CLIMB): CNS-active. T-DXd CNS activity (DESTINY-Breast03: intracranial ORR 65.7% vs 34.3% for T-DM1). Continue same systemic tx if extracranial disease is stable. Leptomeningeal disease: intrathecal chemo (methotrexate, cytarabine), CSI, or CNS-active systemic agents.

Bone-modifying agents

• ~65% to 80% of metastatic patients develop bone mets (most in HR+); risk of skeletal-related events (pain, fracture, hypercalcemia, cord compression). Breast ca cells stimulate RANKL → osteoclast activation.
• Metastatic: bisphosphonates (zoledronic acid, pamidronate) or denosumab (anti-RANKL). In 2,046 women, denosumab ↓ first-SRE risk 18% (HR 0.82) vs zoledronic acid; median time to first SRE not reached vs 26.4 mo, but no PFS/OS difference. Zoledronic acid → renal compromise; denosumab → hypocalcemia. Options: denosumab 120 mg SC q4wk; pamidronate 90 mg IV q3 to 4 wks; zoledronic acid 4 mg IV q12wk or q3 to 4 wks (monthly equals every-3-mo dosing). Calcium/vitamin D, monitor renal function; ~2% osteonecrosis of the jaw (avoid around invasive dental procedures).
• Adjuvant: adjuvant bisphosphonates in postmenopausal women ↓ distant (bone) relapse ~3.5% absolute and ↑ OS ~2.3% (meta-analysis >21,000; also AZURE benefit only if postmenopausal ≥5 yrs, ABCSG-12). Adjuvant denosumab ↑ DFS (ABCSG-18: 8-yr 80.6% vs 77.5%). Cancer Care Ontario/ASCO: consider bisphosphonates as adjuvant tx for postmenopausal (natural or induced) patients who are systemic-therapy candidates.

Inflammatory breast cancer (IBC, T4d)

• Up to ~2% of breast ca; clinical dx (rapid erythema, warmth, peau d'orange, enlargement, pain) with documented invasive ca; tumor emboli in dermal lymphatics in ~75%. HER2+ or triple-negative subtypes common; long-term OS consistently <50%.
• Trimodality mandatory: neoadjuvant anthracycline- and taxane-based chemo (add dual HER2 blockade if HER2+) → total mastectomy with level I to II ALND (no BCT, no SLNB) → post-mastectomy RT to chest wall and regional nodes (supraclavicular, infraclavicular, internal mammary). Defer reconstruction to avoid RT delay.

Special situations

• Locoregional recurrence: treat with curative intent; recurrence <5 yrs has worse prognosis. IBTR after BCT → total mastectomy (excision + reirradiation for select). Chest wall recurrence after mastectomy → excise + RT if not prior. Adjuvant chemo after local salvage benefits mainly ER-negative disease (CALOR: 5-yr DFS 69% vs 57%). IBTR ↑ distant metastasis 3- to 4-fold; systemic imaging indicated.
• Pregnancy-associated: ~1 in 3,000 pregnancies. BCS or mastectomy acceptable; blue dye discouraged (SLNB difficult). RT after delivery. Chemo (anthracycline, cyclophosphamide, taxanes; AC or FAC) from week ~14 until 3 to 4 weeks before delivery; do not give >35 wks. Trastuzumab and T-DM1 contraindicated (oligohydramnios, pulmonary hypoplasia, neonatal death). Deliver ≥37 wks if possible; G-CSF appears safe.
• Male breast ca: ~1%, almost always ER+, only ~9% HER2+, usually IDC, present later. Local/adjuvant treatment as in women. Tamoxifen is the mainstay (AI less effective due to incomplete estradiol suppression; use AI only with a GnRH analog). Palbociclib approved in men.
• Older patients: comorbidities predict worse survival, but healthy older women get the same relative chemo benefit (CALGB 49907: standard AC or CMF > capecitabine). Perform geriatric assessment. Performance status, comorbidities, and life expectancy (not age) should drive adjuvant decisions.
• Phyllodes tumor: stromal + epithelial; benign/borderline/malignant. Excision with negative margins (aim >=1 cm for borderline/malignant, not mandatory for benign); no axillary surgery and no routine adjuvant RT/chemo. Malignant phyllodes behaves like sarcoma (RT for malignant; metastatic tx as soft-tissue sarcoma; mets to lung).
• Intraductal papilloma: benign but co-presents with DCIS/ADH; excise or strongly consider excision for atypia, discordance, or symptoms; concordant papilloma without atypia can be surveilled.
• Pure tubular carcinoma: excellent outcome; BCT + SLNB then adjuvant endocrine only, no chemo.
• Angiosarcoma of the breast: ↑ with prior RT; cutaneous/subcutaneous lesions mimicking benign angioma; aggressive surgery (mastectomy for wide margins), no routine postop RT or adjuvant chemo; high recurrence, spreads to liver/lung.
• RT recall dermatitis: redness/pain after starting chemo (e.g., docetaxel, also tamoxifen); hold chemo ± steroids, then rechallenge.
• Anthracyclines and cardiomyopathy: cumulative dose matters (risk dose-dependent; >=250 mg/m2 doxorubicin = higher-risk surveillance threshold; lifetime limit ~450 to 550 mg/m2); highest risk with ongoing heart dysfunction or recent MI. Docetaxel: steroid premedication reduces hypersensitivity and fluid retention (not diarrhea/alopecia/neuropathy).

Surveillance and survivorship

• H&P every 3 to 6 mo for the first 3 yrs, then every 6 to 12 mo for the next 2 yrs; annual mammogram (start 6 to 12 mo after RT). No routine additional imaging, tumor markers, or labs in asymptomatic patients (except hereditary syndromes or prior chest RT). Most recurrences are detected between visits, so patient education on symptoms is key. Outcomes equivalent whether followed by oncology or primary care.
• Manage menopausal symptoms (venlafaxine, gabapentin, oxybutynin for hot flashes; intravaginal estrogen with caution in HR+ on AIs). Monitor bone density on AIs; monitor cardiac function after anthracyclines/trastuzumab. Encourage exercise and normal BMI (↓ recurrence risk).

High-yield breast pearls

• BRCA1 → TNBC; BRCA2 → HR+, male breast ca. BRCA1/2 are the germline biomarkers for approved PARP inhibitors, but PIK3CA, ESR1, AKT1, and PTEN direct other approved targeted therapies; PALB2 has PARPi activity (TBCRC 048).
• Z0011: cT1-2 N0 with 1 to 2 positive SLN after BCT + RT, skip ALND. AMAROS: axillary RT non-inferior to ALND. SOUND/INSEMA: in cN0 planned for BCT with favorable biology (postmenopausal ≥50, grade 1 to 2, HR+/HER2-, ≤2 cm) and negative axillary US, omit SLNB (primary endpoints DDFS/iDFS, not OS).
• Oncotype/RxPONDER: premenopausal N1 RS 0 to 25 still benefit from chemo (likely OFS effect). N2 (≥4 nodes): always chemo, no Oncotype.
• monarchE / NATALEE: adjuvant CDK4/6i (abemaciclib or ribociclib) in high-risk HR+/HER2- early stage.
• KEYNOTE-522: pembrolizumab + chemo for stage II/III TNBC, neoadjuvant + adjuvant.
• OlympiA: adjuvant olaparib x1 yr in germline BRCA1/2 high-risk early breast ca.
• KATHERINE → DESTINY-Breast05: HER2+ residual disease post-neoadjuvant; T-DM1 historically, T-DXd now FDA-approved (May 2026; IDFS HR 0.47 vs T-DM1). APT: paclitaxel + trastuzumab for small node-negative HER2+.
• CLEOPATRA → DESTINY-Breast09: 1L mHER2+ shifting from THP to T-DXd + pertuzumab. DESTINY-Breast03: T-DXd 2L (mPFS 28.8 vs 6.8 mo). DESTINY-Breast04/06: T-DXd in HER2-low/ultralow.
• INAVO120: inavolisib + palbociclib + fulvestrant 1L for PIK3CA-mut HR+ endocrine-resistant disease (relapse on or within 12 mo of adjuvant ET). CAPItello-291: capivasertib for AKT1/PIK3CA/PTEN. EMERALD: elacestrant for ESR1-mutant. SERENA-6: camizestrant (FDA accelerated approval Sept 2026; ctDNA-guided switch on emergent ESR1 mutation).
• HER2CLIMB: tucatinib + trastuzumab + capecitabine for HER2+ brain mets.
• IBC = T4d; trimodality mandatory. PRIME II / CALGB 9343: elderly T1 HR+ N0 on AI can omit RT.
• USPSTF 2024: screening starts at 40 (was 50). BI-RADS 4 or higher (>2% malignancy) needs biopsy.