What are the differential diagnoses for a painless, enlarging sternal mass five years after a modified radical mastectomy?

Differential Diagnoses for Non-Tender Sternal Enlargement 5 Years Post Modified Radical Mastectomy

The most critical differential to rule out is locoregional recurrence—either parasternal lymph node recurrence or sternal metastasis—and ultrasound should be the immediate first-line imaging modality to distinguish between these entities and guide tissue diagnosis. 1, 2

Primary Differential Considerations

1. Parasternal Lymph Node Recurrence (Most Common)

• Parasternal recurrence accounts for 59.1% of sternal region masses in breast cancer patients, far exceeding true sternal metastases (27.8%), making this the leading differential diagnosis. 2
• This presentation is particularly associated with primary tumors located in the inner quadrants of the breast. 2
• Locoregional recurrence occurs at a constant rate of 1-2% per year from years 2 through 8 post-mastectomy, placing this patient squarely in the high-risk window. 1
• Most local recurrences after mastectomy present as clinically detectable chest wall masses, and the painless nature does not exclude malignancy. 3

2. Sternal Bone Metastasis

• True sternal metastases represent 27.8% of sternal region masses and are rarely solitary—they typically occur with multiple skeletal lesions elsewhere. 2
• When sternal metastasis is present, it usually manifests as a small soft tissue tumor with underlying bone involvement. 2
• Invasion into the sternum indicates poor prognosis but can be resected in carefully selected patients with excellent outcomes. 3, 4

3. Combined Parasternal Recurrence with Secondary Sternal Invasion

• 5.2% of cases demonstrate both parasternal soft tissue recurrence and secondary sternal involvement, representing an intermediate presentation. 2

4. Benign Post-Surgical Changes

• Fat necrosis is the most common benign finding in the early postoperative period but can occur years later. 3
• Non-tumorous changes account for only 6.1% of sternal region masses in breast cancer patients with clinical suspicion. 2
• Seroma, hematoma, or chronic inflammatory changes are possible but less likely 5 years post-surgery. 3

Immediate Diagnostic Approach

First-Line Imaging

• Ultrasound is the initial imaging modality of choice for any palpable chest wall mass post-mastectomy, including sternal region abnormalities. 1, 2
• Ultrasound reliably distinguishes parasternal lymph node recurrence (soft tissue mass anterior to the sternum) from true sternal metastasis (bone involvement with small soft tissue component). 2
• Ultrasound is superior to chest X-ray (often false-negative) and bone scan (positive only with sternal invasion or skeletal metastases) for this indication. 2

Advanced Imaging When Indicated

• MRI with T1-weighted fat-suppressed gadolinium sequences is superior to CT for identifying chest wall and parasternal tumor involvement. 1
• PET-CT demonstrates 91% sensitivity and 97% specificity for sternal involvement when infection or tumor recurrence is suspected. 1
• CT is useful for pretreatment planning and ruling out distant metastatic disease but not as the initial diagnostic test. 3

Tissue Diagnosis

• Biopsy is mandatory for any suspicious sternal region mass to confirm recurrence versus benign etiology and to assess hormone receptor and HER2 status. 3, 2
• Ultrasound guidance facilitates safe tissue sampling of parasternal masses. 1

Critical Risk Factors to Assess

High-Risk Features for Recurrence

• Node-positive disease at initial diagnosis (11% recurrence rate at 5 years versus 6.7% for node-negative). 3
• High-grade tumor, lymphovascular invasion, non-luminal tumor type at original diagnosis. 3
• Tumor size >5 cm and positive surgical margins at initial surgery. 3
• Young age (<35-40 years) at initial diagnosis. 3

Imaging Pitfalls

• Bone scans are unreliable for isolated parasternal recurrence without sternal invasion—they will be negative in pure soft tissue recurrence. 2
• Chest X-rays are frequently false-negative and should not be used to exclude sternal region pathology. 2
• Do not rely on the absence of pain to exclude malignancy—most recurrences are painless. 3

Prognostic Implications

• Isolated locoregional recurrence without distant metastases can be treated with curative intent through surgical resection, radiation therapy, and systemic therapy. 3, 5
• Parasternal recurrence typically presents without multiple skeletal lesions, offering better prognosis than true sternal metastasis. 2
• Late recurrences (even up to 40 years post-mastectomy) have been documented, though 5 years falls within the peak recurrence window. 6
• Invasion into thoracic structures indicates poor prognosis but aggressive local therapy may still achieve long disease-free survival. 3, 4