What is the staging and treatment approach for a patient with breast cancer and a focal lesion involving the sternum?

Breast Cancer with Sternal Involvement: Staging and Treatment

Staging Classification

A focal lesion involving the sternum in breast cancer represents Stage IV (M1) disease, regardless of the primary tumor size or nodal status. 1 Bone metastases, including sternal involvement, are classified as distant metastases in the TNM staging system, automatically upgrading the patient to metastatic disease. 1

Diagnostic Workup for Sternal Lesions

Before initiating any treatment, comprehensive staging must be completed to identify all sites of metastatic disease, as sternal involvement frequently indicates widespread metastases. 1, 2

Required Imaging Studies

• Chest CT or chest X-ray to evaluate for pulmonary metastases 1, 2
• Abdominal ultrasound, CT, or MRI to identify hepatic or other visceral metastases 1, 2
• Bone scintigraphy with radiographic/CT/MRI confirmation of suspicious lesions 1, 2
• PET/CT scanning is particularly valuable when conventional imaging is equivocal or to confirm whether the sternal lesion is truly isolated versus part of widespread disease 1, 2

Laboratory Assessment

• Complete blood count, liver and renal function tests, alkaline phosphatase, and calcium levels 1, 2
• Tumor markers may assist in monitoring treatment response, particularly in non-measurable disease 1

Tissue Confirmation

Biopsy of the sternal lesion should be obtained when feasible to confirm metastatic disease and reassess biomarkers (ER, PR, HER2, Ki67), as receptor status can differ between primary and metastatic sites. 1 If the lesion is inaccessible for biopsy, functional imaging such as PET/CT may confirm its malignant character. 1

MRI Detection Characteristics

Breast MRI is significantly more sensitive than bone scan, PET/CT, or chest CT for detecting sternal lesions. 3 Malignant sternal lesions characteristically demonstrate rapid initial enhancement (100% of cases) and delayed washout kinetics (67% of cases) on dynamic contrast-enhanced MRI. 3 Diffuse sternal involvement (versus focal) strongly suggests malignancy (56% of malignant cases versus 0% of benign). 3

Treatment Approach for Stage IV Disease

Stage IV breast cancer with sternal metastases is treatable but not curable; treatment goals focus on prolonging survival and maintaining quality of life. 4, 5

Systemic Therapy Selection by Subtype

Hormone Receptor-Positive/HER2-Negative Disease

• First-line endocrine therapy combined with CDK4/6 inhibitors (median overall survival approximately 5 years) 5, 6
• PI3K inhibitors for PIK3CA-mutated tumors 6
• Chemotherapy reserved for endocrine-resistant or rapidly progressive disease 1, 5

HER2-Positive Disease

• Anti-HER2 targeted therapy (trastuzumab, pertuzumab) combined with chemotherapy 5, 6
• Trastuzumab emtansine or other HER2-directed agents for subsequent lines 7, 6
• Median overall survival approximately 5 years 5

Triple-Negative Disease

• Chemotherapy is the primary systemic option (median overall survival approximately 1 year) 4, 5
• Immunotherapy with checkpoint inhibitors for PD-L1-positive tumors 6
• PARP inhibitors for BRCA1/2 mutation carriers 5, 6

Bone-Directed Therapy

Zoledronic acid or other bisphosphonates should be initiated immediately for all patients with bone metastases to reduce skeletal-related events. 8 The case report demonstrates that zoledronic acid can induce calcification and hardening of osteolytic sternal lesions, improving local control. 8

Role of Local Therapy

Local treatment of the primary breast tumor may be considered in highly selected patients with limited metastatic disease and excellent response to systemic therapy, but this remains controversial. 1, 8 In the reported case, breast-conserving surgery was performed 8 months after achieving partial response with systemic therapy and zoledronic acid, followed by radiation to both the breast and sternum. 8

Radiation therapy to the sternum should be considered for symptomatic relief of pain or to prevent pathologic fracture, particularly after good systemic response. 8

Critical Clinical Pitfalls

• Never assume a sternal lesion is benign without tissue confirmation or definitive imaging characteristics, as breast MRI detects 24% of unsuspected sternal metastases that alter staging from potentially curable to Stage IV disease 3
• Do not proceed with curative-intent surgery for the primary tumor without completing full metastatic workup, as locoregional recurrence is frequently associated with distant disease 1, 2
• Sternal metastases occur more frequently with invasive lobular carcinoma (44% versus 13% for benign lesions), larger primary tumors (mean 6.4 cm versus 2.3 cm), and positive lymph nodes (78% versus 50%) 3
• Cardiac function monitoring is essential when using anthracyclines or trastuzumab-based regimens 1, 7