CT: computed tomography; MRI: magnetic resonance imaging.
* Classic, "fat-rich" AMLs (determined by imaging) do not undergo malignant transformation and are therefore low risk. By contrast, "fat-poor" or "fat-invisible" AMLs may contain epithelioid cells and sometimes undergo malignant transformation. Such lesions should be biopsied. Features consistent with a high risk for malignant transformation include ≥70% epithelioid cells, vascular invasion, ≥2 mitotic figures per 10 high-power fields, atypical mitotic figures, and necrosis.
¶ Stability of the AML size is present if there is no appreciable increase in the AML size (eg, annual growth <2.5 mm) over at least five years of follow-up. If an AML is >6 cm on the initial imaging study (ie, at the time of diagnosis), documentation of stability should be assessed with imaging every four to six months for the first year.
Δ These indications for intervention in patients with small sporadic AMLs are not absolute. As an example, in patients with a high-risk occupation, a plan to intervene would be the result of shared decision-making with the patient and, often, a multidisciplinary team of clinicians.
◊ After an initial CT or MRI, surveillance is typically performed with the same modality at 6 and 12 months and then yearly for five years. Thereafter, if the lesion size is stable, kidney ultrasound can be performed every three years.