What could be causing a painful lump under the skin on my forearm with no history of injury?

Painful Forearm Lump Without Injury

Start with plain radiographs (X-rays) of the forearm in anteroposterior (AP) and lateral views as your initial imaging study to evaluate this painful subcutaneous mass. 1

Initial Imaging Approach

• Radiographs are the most appropriate first-line imaging for any soft-tissue mass, as they can identify mineralization, ossification, or underlying bone involvement that may guide diagnosis and subsequent management 1
• Plain films can detect calcification patterns, cortical remodeling, or invasion that would suggest specific diagnoses or malignant potential 1
• For a superficial forearm mass, radiographs provide essential baseline information before considering advanced imaging 1

Next Steps Based on Radiograph Results

If Radiographs Show Mineralization or Bone Changes:

• CT without contrast is the optimal next study to characterize soft-tissue mineralization patterns and distinguish ossification from calcification 1
• CT excels at identifying the zonal pattern of mineralization seen in conditions like myositis ossificans 1

If Radiographs Are Normal:

• Ultrasound is the most appropriate next imaging modality for superficial soft-tissue masses on the forearm 1
• US has 94.1% sensitivity and 99.7% specificity for characterizing superficial masses, with highest accuracy for lipomas, vascular malformations, epidermoid cysts, and nerve sheath tumors 1
• US can differentiate solid from cystic lesions and assess the relationship between the mass and adjacent neurovascular structures 1

When to Consider MRI:

• MRI should be reserved for cases where US findings are atypical or inconclusive, not as an initial study 1
• MRI is most useful when the mass is deep-seated, large, or when US cannot adequately characterize the lesion 1

Differential Diagnosis Considerations

The painful nature of this mass narrows the differential significantly:

Most Common Painful Subcutaneous Lesions:

• Dermatofibroma is the most common tender cutaneous neoplasm, presenting as a dermal nodule 5-12mm in diameter that may be flesh-colored, red, brown, or tan 2
• Glomus tumor presents as a solid, painful subcutaneous nodule, though more commonly seen on the hand than forearm 3
• Angiolipoma is another painful subcutaneous tumor to consider 2
• Leiomyoma (cutaneous) can present as a painful subcutaneous mass 2

Less Common But Important:

• Neuroma or neurilemmoma can cause localized pain 2
• Eccrine spiradenoma is a painful adnexal tumor 2
• Pilomatrixoma can present as a forearm mass, though typically painless unless there is rapid growth 4

Clinical Examination Pearls

• Assess whether the mass is truly subcutaneous or dermal - dermatofibromas typically dimple when pinched laterally 5
• Note any color changes in overlying skin - sebaceous hyperplasia appears pale yellow, dermatofibromas may be brown/tan 5
• Evaluate mobility - lipomas are easily moveable under the skin, while dermatofibromas are fixed to dermis 5
• Check for tenderness to palpation - this is a key feature distinguishing painful tumors from benign asymptomatic masses 2

Common Pitfalls to Avoid

• Do not skip radiographs before ordering advanced imaging - this can lead to missed diagnoses of mineralization or bone involvement 1, 6
• Do not assume all painful masses are benign - while most tender subcutaneous tumors are benign, changing characteristics warrant biopsy 5
• Do not rely solely on clinical examination - even experienced clinicians benefit from imaging confirmation, as US has high diagnostic accuracy for superficial masses 1
• Remember that not all pathology is visible on initial imaging - clinical correlation remains essential, and symptomatic lesions may warrant excision even with normal imaging 1, 5

When to Refer or Biopsy

• Any mass with changing characteristics (size, color, pain level) should be biopsied or excised 5
• Lesions suspicious for malignancy on imaging require tissue diagnosis 1
• Persistently symptomatic lesions causing pain or functional impairment warrant excision, even if imaging suggests benign etiology 2, 5
• Simple excisional biopsy often provides both diagnosis and treatment for small painful subcutaneous tumors 2