Differential Diagnosis for the 9.3 mm Subcutaneous Lesion

• Single most likely diagnosis
  • Lipoma: A common, benign tumor of fat cells that can occur in the subcutaneous tissue, which would appear as an isodensity on imaging. The location in the suboccipital region and the description provided make this a plausible diagnosis.
• Other Likely diagnoses
  • Cyst (e.g., epidermoid or sebaceous cyst): These are common skin lesions that can appear in the subcutaneous tissue and might have a similar appearance on imaging.
  • Abscess or infected cyst: Although these would likely have associated clinical symptoms such as redness, warmth, and pain, they are possible considerations, especially if the patient has a history of similar infections.
  • Neurofibroma: A benign nerve sheath tumor that could present as a subcutaneous mass, though typically, these have a more characteristic appearance on imaging.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
  • Metastasis: Although less likely, a metastatic lesion from a primary cancer elsewhere in the body could present as a subcutaneous nodule. Given the potential severity of cancer, this diagnosis should not be overlooked.
  • Lymphoma: As mentioned, lymphoma could present with subcutaneous lesions, and given its potential impact on patient outcomes, it's crucial to consider and rule out with further evaluation.
  • Soft tissue sarcoma: These are rare but aggressive tumors that originate in the soft tissues, including subcutaneous fat. Early detection is critical for treatment outcomes.
• Rare diagnoses
  • Granuloma or inflammatory pseudotumor: These could present as subcutaneous masses but are less common and might be associated with specific clinical histories or systemic diseases.
  • Vascular malformations (e.g., hemangioma): Although typically diagnosed earlier in life, some vascular malformations could present in adulthood as subcutaneous lesions.