Evaluation of Sudden-Onset Dorsal Forearm Soft Tissue Mass
Begin with plain radiographs immediately, followed by ultrasound if the mass is superficial, as this two-step approach identifies the vast majority of benign lesions and guides appropriate next steps. 1, 2
Initial Imaging Algorithm
Step 1: Plain Radiographs (Always First)
- Radiographs are mandatory as the initial study for any soft tissue mass, even though they may seem unrewarding at first glance 1, 2
- Radiographs demonstrate positive findings in 62% of soft tissue masses, including:
- Can be diagnostic for specific entities like myositis ossificans (showing characteristic peripheral ossification pattern), phleboliths in hemangiomas, or osteocartilaginous masses 1
- Rules out underlying bone pathology that may present as a soft tissue mass 2
Step 2: Ultrasound (For Superficial Masses)
- Ultrasound is highly appropriate for superficial dorsal forearm masses with 94.1% sensitivity and 99.7% specificity 1, 2
- Particularly useful for differentiating:
- For suspected lipomas, look for: minimal acoustic shadowing, minimal vascularity, and curved echogenic lines within an encapsulated mass 1, 3
Critical Red Flags Requiring Advanced Imaging
If any of the following are present, proceed directly to MRI with and without contrast:
- Mass is deep to the fascia 1, 4
- Size >5 cm 1, 4
- Firm consistency (firmer than surrounding muscle) 4
- Atypical ultrasound features (heterogeneous, increased vascularity, irregular borders) 1, 2
- Rapid growth over 2 days (highly concerning for aggressive process) 4, 5
When to Obtain MRI
- MRI with and without contrast is indicated when ultrasound is inconclusive or shows concerning features 2, 6, 7
- MRI provides superior soft tissue characterization and is essential for preoperative planning 2, 6
- However, MRI should not be the initial study due to its inability to adequately identify mineralization 1
Tissue Diagnosis
Indications for Biopsy
Any mass that cannot be definitively characterized as benign on imaging requires tissue diagnosis before definitive treatment 1, 2, 8
Biopsy Technique
- Core needle biopsy (>16G) with multiple samples is the standard approach 1, 2, 8
- Excisional biopsy is acceptable only for superficial lesions <5 cm where imaging strongly suggests benignity 1, 2
- Critical: The biopsy tract must be planned so it can be removed during definitive surgery if malignancy is found 1, 2, 8
- Consider tattooing the biopsy entrance point 1
Urgent Referral Criteria
Refer immediately to a sarcoma multidisciplinary team if: 1, 8
- Any unexplained deep mass 1
- Superficial mass >5 cm 1
- Imaging features suggestive of sarcoma (irregular margins, heterogeneous enhancement, mass effect) 8
- Rapid growth pattern (appearing within 2 days is extremely concerning) 4, 5
Common Pitfalls to Avoid
- Never assume all masses are lipomas—physical examination alone is only 85% accurate 1, 2, 3
- Avoid "whoops surgery" (excision without proper imaging and planning), which necessitates wider re-excision and increases morbidity 5
- Do not rely solely on ultrasound for deep masses—accuracy drops precipitously for non-superficial lesions 1
- Do not skip radiographs even if ultrasound is planned—radiographs provide unique information about calcification and bone involvement 1, 2
Special Consideration for 2-Day Onset
A mass appearing within 2 days suggests:
- Hematoma (most likely, especially with trauma history) 7
- Acute inflammatory process or abscess 4
- Rapidly growing malignancy (rare but must be excluded) 4, 5
- Accessory muscle becoming symptomatic (rare) 9
The rapid onset warrants expedited evaluation with both radiographs and ultrasound within days, not weeks, to distinguish benign from potentially aggressive processes 8, 4.