What is the cause of a small palpable bump on my left forearm?

Evaluation of a Small Palpable Bump on the Left Forearm

Begin with ultrasound imaging as the first-line diagnostic test for any superficial soft-tissue palpable lesion on the forearm. 1

Initial Diagnostic Approach

Ultrasound is the primary imaging modality for evaluating superficial soft-tissue masses because it provides high spatial resolution in superficial layers and can confidently diagnose many lesions with characteristic features without further workup. 1 A comprehensive ultrasound examination should include grayscale, color-Doppler, power-Doppler, spectral-Doppler, and potentially elastography to fully characterize the lesion. 1

Key Clinical Information to Document

Before imaging, document the following specific characteristics:

• Size and precise location of the bump in centimeters 2
• Duration of presence and any changes in size over time 3
• Associated symptoms including pain, skin changes, or functional limitations 2
• Consistency on palpation (soft, firm, mobile, fixed) 2
• History of trauma to the area 4

Common Benign Diagnoses by Location

For forearm lesions specifically, the most common benign soft-tissue tumors include:

• Lipomas and lipoma variants (16% of all benign soft-tissue tumors) 5
• Fibrous histiocytoma (13% of benign lesions) 5
• Hemangiomas (8% of benign tumors) 5
• Neurofibromas (5% of benign lesions) 5

Approximately 80% of benign soft-tissue tumors fall into seven diagnostic categories, making the differential diagnosis more manageable than the large number of pathologic possibilities might suggest. 5

When to Proceed Beyond Ultrasound

If ultrasound shows indeterminate features or findings suspicious for malignancy, proceed to MRI for further characterization. 1 MRI without contrast is the preferred advanced imaging modality for soft-tissue lesions when ultrasound is inconclusive. 4

If ultrasound demonstrates clearly benign features (such as a simple lipoma, ganglion cyst, or other characteristic benign lesion), no further imaging is required and clinical follow-up is appropriate. 1

Biopsy Indications

Core needle biopsy is superior to fine-needle aspiration for tissue diagnosis, providing better sensitivity, specificity, and histological grading. 2

Proceed directly to image-guided core biopsy when:

• Ultrasound features are suspicious for malignancy (irregular margins, heterogeneous echotexture, increased vascularity) 1
• The lesion is indeterminate on imaging and cannot be confidently characterized 1
• Patient anxiety is significant and definitive diagnosis would provide reassurance 2
• The lesion is rapidly growing or changing in character 3

Critical Pitfalls to Avoid

Do not skip ultrasound and proceed directly to biopsy without imaging characterization, as many lesions can be diagnosed confidently by ultrasound alone, avoiding unnecessary invasive procedures. 1

Do not rely on clinical examination alone to determine whether a lesion requires biopsy, as physical examination cannot reliably distinguish benign from malignant masses. 2

Do not delay evaluation if there are concerning features such as rapid growth, firm/hard consistency, or fixation to underlying structures, as these may indicate malignancy. 2

Special Consideration for Rare Malignancies

While uncommon, Merkel cell carcinoma can present as a small red lesion on the forearm and may initially appear benign. 3 Any lesion that persists, grows, or has atypical features on ultrasound warrants tissue diagnosis to exclude malignancy. 3