A kidney lesion is a general term describing an abnormal area, growth, or mass found on or within the kidney tissue. This finding indicates that a segment of the organ deviates from its normal appearance. The word “lesion” is broad and does not automatically mean cancer, as many such abnormalities are benign, or non-cancerous. A medical evaluation is necessary to determine the specific nature of the lesion and its potential risk.
Classification of Kidney Lesions
Kidney lesions are categorized into two structural types: cystic and solid. Cystic lesions, often called cysts, are fluid-filled sacs and represent the majority of all kidney lesions. Simple cysts are the most common type, characterized by thin, smooth walls and clear fluid, and are almost universally benign, requiring no treatment. Complex cysts may feature thicker walls, internal divisions called septa, or calcifications, which increase the suspicion of malignancy.
Solid lesions are masses composed of tissue rather than fluid and include both tumors and non-cancerous growths. These masses carry a higher potential for malignancy and are treated with urgency until proven otherwise. The most common malignant solid lesion is renal cell carcinoma (RCC), which accounts for the vast majority of kidney cancers. Some solid lesions are benign, such as angiomyolipomas, which are characterized by the presence of fat, blood vessels, and smooth muscle tissue, allowing for a non-cancerous diagnosis on imaging.
How Lesions Are Identified
Most kidney lesions are discovered incidentally when a patient undergoes imaging for an unrelated medical issue, such as a back injury or a gallbladder problem. This is the most common method of detection and often leads to the finding of small, localized masses that have not caused any symptoms. The increasing use of advanced imaging technologies like ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) has resulted in more asymptomatic lesions being found.
In a smaller number of cases, lesions are discovered because they cause noticeable symptoms. These symptoms can include blood in the urine, unexplained flank pain between the ribs and hip, or a palpable mass in the abdomen. Symptomatic discoveries are generally less common, and the presence of such signs prompts immediate diagnostic imaging to characterize the abnormality.
Assessing Malignancy Risk
The criteria used to evaluate a lesion depend on whether it is cystic or solid, with physicians using specific imaging characteristics to assign a risk level. For cystic lesions, the Bosniak Classification System is used to grade complexity from category I to IV, correlating with the likelihood of cancer. Category I cysts are simple fluid-filled sacs with a near-zero malignancy risk. Category IIF lesions show minimal complexity, such as thin septa or slight wall thickening, warranting active surveillance with follow-up imaging.
Lesions classified as Bosniak Category III are indeterminate, featuring thick or irregular walls and septa, and have a malignancy rate of approximately 50%, often requiring intervention. Category IV lesions are highly suspicious, showing enhancing solid components within the cyst, and have a malignancy risk approaching 100%.
For solid lesions, characteristics like size, growth rate, and enhancement are the primary indicators of potential cancer. Masses larger than four centimeters are more likely to be malignant. A solid mass that “enhances,” or lights up after the injection of a contrast dye during imaging, is treated as cancerous until proven otherwise.
Management Options
Once a lesion’s risk is assessed, the management strategy is tailored to the individual patient and the characteristics of the mass. Active surveillance is a common initial approach for small, low-risk solid masses (typically those less than four centimeters) and for Bosniak IIF cysts. This strategy involves monitoring the lesion with periodic follow-up imaging, such as CT or MRI, to track changes in size or appearance.
For smaller, localized tumors, physicians may recommend minimally invasive interventions to destroy cancerous cells while preserving healthy kidney tissue. These ablative techniques include cryoablation, which uses extreme cold to freeze the tumor, or radiofrequency ablation, which uses heat. Surgical removal is the traditional approach for larger, higher-risk, or malignant masses. This can involve a partial nephrectomy, where only the tumor and a margin of tissue are removed, or a radical nephrectomy, which entails removing the entire kidney, reserved for very large or complex tumors.