Differential Diagnosis for a Large Firm Growing Mass on Left Upper Back Near Neck/Shoulder

Single Most Likely Diagnosis

• Lipoma: A benign tumor composed of fat tissue, which can grow to be quite large and is often found in the subcutaneous tissue. The location near the neck/shoulder and the description of the mass as firm and growing could align with a lipoma, especially if it has been present for some time and has recently increased in size.

Other Likely Diagnoses

• Cyst: Such as a sebaceous cyst or an epidermoid cyst, which can grow and become firm, especially if they become inflamed or infected.
• Fibroma: A benign tumor of fibrous connective tissue, which can present as a firm, growing mass.
• Hematoma or Seroma: Although less likely if the mass has been growing over time without a history of trauma, a hematoma (collection of blood outside of blood vessels) or seroma (collection of fluid) could present as a firm mass if it becomes organized and fibrotic over time.

Do Not Miss Diagnoses

• Malignant Tumor: Such as a Sarcoma (e.g., liposarcoma, fibrosarcoma) or a Metastatic Tumor from another primary site. Although less common, these diagnoses are critical to rule out due to their potential for serious morbidity and mortality.
• Infected or Inflamed Lesion: An Abscess or a Cellulitis that has become localized and firm could mimic a growing mass, especially if there's an underlying cause like an infected cyst or foreign body.
• Lymphoma: Although more commonly associated with lymph node enlargement, certain types of lymphoma could present with a mass in this location.

Rare Diagnoses

• Neurofibroma: A benign nerve sheath tumor that can occur anywhere in the body, including the neck and shoulder region, and can grow to be quite large.
• Desmoid Tumor: A rare, benign but locally aggressive tumor that can occur in any part of the body, including the neck and shoulder area.
• Pilomatrixoma: A rare, benign skin tumor that originates from the hair follicle matrix and can present as a firm, growing mass.

Each of these diagnoses would require a thorough clinical evaluation, including history, physical examination, and potentially imaging studies (like MRI or CT scans) and biopsy for definitive diagnosis. The ICD code would depend on the final diagnosis after these evaluations.