ICD-10 Code D38.4 – Neoplasm of uncertain behavior of thymus (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code D38.4 – Neoplasm of uncertain behavior of thymus
What it is
D38.4 identifies a thymic neoplasm whose behavior cannot be determined from available evidence. Use it when the thymus is the site and the pathology or documentation does not confirm benign or malignant behavior.
Clinical signs
Clinical features vary; refer to documentation. Thymic masses may be found on imaging or during evaluation of chest symptoms, but the code itself is based on uncertain tumor behavior, not a specific symptom pattern.
When to use this code
Use D38.4 when the provider documents a thymic tumor, mass, or neoplasm with uncertain behavior, borderline behavior, or indeterminate malignant potential. It is appropriate when pathology has not established malignancy and the thymus is clearly the primary site. Check documentation if the report only says “mass” without tumor behavior.
Do not use for
Do not use this code for confirmed malignant thymic neoplasms or clearly benign thymic lesions. If the record points to another mediastinal site or a nonspecific chest mass, code the documented site instead.
Coding tip
Verify that both the thymus location and uncertain behavior are documented before assigning D38.4.