What are the treatment options for a lump on my arm that appeared after a fall 7 months ago?

Evaluation and Management of a Persistent Arm Lump Following Trauma

For a lump on your arm that has persisted for 7 months after a fall, you should undergo ultrasound imaging as the first diagnostic step, followed by MRI if the ultrasound shows concerning features or if the mass is deep-seated. 1, 2

Initial Diagnostic Approach

Ultrasound is the most appropriate first-line imaging modality for evaluating your persistent arm lump, with diagnostic accuracy of 86.87-94.1% sensitivity and 95.95-99.7% specificity for characterizing soft tissue masses. 2 This imaging should be performed within 2 weeks given the chronicity of your symptoms. 1

Key Clinical Context

The most likely diagnosis for a soft tissue mass on the arm is a benign lipoma, which represents the vast majority of limb masses seen in clinical practice. 3 However, the 7-month persistence following trauma requires systematic evaluation to exclude other etiologies, including:

• Post-traumatic hematoma organization (most common after significant trauma)
• Benign lipoma (most common soft tissue mass overall)
• Atypical lipomatous tumor (if deep-seated or >5 cm)
• Soft tissue sarcoma (rare but must be excluded)

1, 2, 3

Ultrasound Interpretation

Benign Features on Ultrasound

If your ultrasound demonstrates the following characteristics, the mass is likely benign and you can be reassured:

• Well-circumscribed, hyperechoic or isoechoic appearance compared to surrounding fat
• Thin, curved echogenic lines within an encapsulated mass
• Minimal to no internal vascularity on Doppler examination
• No acoustic shadowing

2

Red Flags Requiring Advanced Imaging

You should proceed directly to MRI if ultrasound reveals any of these concerning features:

• Size larger than 5 cm
• Deep location (beneath the fascia)
• Atypical ultrasound appearance
• Significant internal vascularity
• Rapid growth or change in size
• Associated pain or tenderness beyond what would be expected

1, 2

When MRI is Indicated

MRI is the preferred advanced imaging modality and should be obtained if:

• The ultrasound findings are uncertain or atypical 1
• The mass is deep-seated (beneath the muscle fascia) 2
• The mass is larger than 5 cm 1, 2
• Clinical concern persists despite benign-appearing ultrasound 1

MRI can differentiate benign lipomas from atypical lipomatous tumors in up to 69% of cases and provides the most accurate characterization of extremity masses. 2, 3

Critical Pitfall to Avoid

Do not assume a post-traumatic lump is simply a resolving hematoma after 7 months. While trauma can draw attention to a pre-existing mass, the most common presentation of a soft tissue sarcoma is a painless enlarging mass, and the median size at diagnosis remains over 9 cm due to delayed recognition. 1 The 7-month timeframe makes a simple hematoma unlikely and warrants definitive imaging evaluation.

Management Algorithm

1. Obtain ultrasound imaging within 2 weeks 1
2. If ultrasound shows typical benign features (lipoma): Clinical follow-up is appropriate, particularly if the mass is superficial and <5 cm 2
3. If ultrasound shows red flags or is uncertain: Proceed to MRI 1, 2
4. If MRI suggests malignancy or atypical features: Referral to a specialist sarcoma multidisciplinary team for image-guided biopsy before any surgical intervention 1

Special Consideration for Deep or Lower Extremity Masses

Ultrasound is considerably less accurate for deep masses compared to superficial ones, and all deep-seated masses or those in the lower limb should raise concern for atypical lipomatous tumors, even if they appear benign on ultrasound. 2 In these cases, MRI should be obtained regardless of ultrasound appearance.