Differential Diagnosis for a Palpable Skin Lump

Given the clinical presentation of a 27-year-old female with a 1-month history of a palpable, non-visible, immovable skin lump on the posterior forearm, without associated symptoms or history of skin conditions, the following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Lipoma: A benign tumor composed of fat tissue. It is the most common soft tissue tumor and can present as a palpable, movable or immovable, painless lump. The location on the forearm and the size (1.5cm) are consistent with a lipoma. Lipomas are typically firm, mobile, and non-tender, but can be fixed to underlying tissues if they are deeper.

• Other Likely Diagnoses

  • Epidermoid Cyst: A benign cyst that can form in the skin, often presenting as a firm, mobile lump. They can become inflamed or infected, but in this case, the absence of pain or other symptoms makes this less likely.
  • Dermatofibroma: A benign skin growth that can appear after a minor injury, typically presenting as a firm, round, brown to purple growth. However, the patient denies any recent trauma.
  • Benign Fibrous Histiocytoma: Similar to dermatofibroma, it is a benign growth that can appear as a firm lump.

• Do Not Miss Diagnoses

  • Soft Tissue Sarcoma: Although rare, sarcomas can present as a painless lump. Given the potential severity of this diagnosis, it is crucial not to miss it, despite its low likelihood.
  • Metastatic Disease: In rare cases, a skin lump could be a metastasis from an internal malignancy. This would be extremely uncommon in a young adult without a known history of cancer but should be considered if other diagnoses are ruled out.

• Rare Diagnoses

  • Neurofibroma: A benign tumor of the nerve tissue, which can present as a lump. It is more commonly associated with neurofibromatosis type 1, but solitary neurofibromas can occur.
  • Glomus Tumor: A rare, benign tumor of the glomus body, typically presenting in the fingers or toes under the nail, but can occur elsewhere. It often presents with pain, which is not present in this case.
  • Pilomatrixoma: A rare, benign skin tumor that typically occurs in children and young adults, often on the face, neck, or arms. It can present as a firm, mobile or fixed lump.

Given the clinical presentation and the limitations of not having access to radiology or labs, a thorough physical examination and monitoring for any changes in the lump's size, consistency, or associated symptoms are recommended. If possible, referral to a dermatologist or a surgical specialist for further evaluation and potential biopsy should be considered to establish a definitive diagnosis.