Kidney metastases: symptoms and detection methods

Renal metastases are secondary lesions of the renal parenchyma by tumor cells from another organ. They are rare in clinical practice, but in autopsy series, the incidence of renal involvement in advanced tumors reaches approximately 2.36-12.6%. Such lesions are detected significantly less frequently during intravital imaging, often incidentally, during staging of the primary cancer. The most common sites of metastasis are the lungs, mammary gland, gastrointestinal tract, and melanoma. [1]

The clinical picture is often subtle: symptoms are absent for long periods, and as the lesion grows, macro- or microhematuria, dull pain in the lower back, and, less commonly, fever appear. On CT, metastases are often less exophytic, multiple, bilateral, and hypovascular than those of primary kidney cancer, which aids in differential diagnosis but does not eliminate the possibility of error—an accurate diagnosis is usually confirmed by biopsy. [2]

Treatment tactics depend primarily on the primary tumor and its extent. For a single (oligometastatic) kidney lesion, local treatment (organ-preserving resection, nephrectomy, ablation, or stereotactic radiotherapy) is considered alongside systemic therapy tailored to the primary tumor. For multiple lesions, the emphasis shifts to systemic approaches, with the possible addition of local control in the event of bleeding symptoms or the threat of loss of function. [3]

The choice of strategy requires multidisciplinary discussion (oncology, urology, radiology, pathology, radiation oncology). Patient selection for active local management is based on the number and size of metastases, control of the primary lesion, the absence of extrarenal metastases, and functional status. Such approaches can prolong survival in carefully selected cases. [4]

Code according to ICD-10 and ICD-11

In ICD-10, secondary malignant tumors of the kidney and pelvis are coded in block C79.0, specifying the side: C79.01 (right), C79.02 (left), C79.00 (unspecified). If necessary, the primary localization (e.g., lung cancer, breast cancer, etc.) and associated complications (bleeding, obstruction) are additionally coded. The correct sequence of codes reflects the current clinical task (treatment of metastasis or primary tumor). [5]

ICD-11 has a separate section for metastases: "Malignant neoplasm metastases." For the kidney and renal pelvis, code 2E00 "Malignant neoplasm metastasis in kidney or renal pelvis" is used, with post-coordination of anatomy (left/right/bilateral) and, if necessary, an indication of the primary tumor. It is important to distinguish a secondary tumor (2E00) from primary kidney cancer (e.g., 2C90.0 for renal cell carcinoma). [6]

Table 1. Examples of coding of kidney metastases

System	Code	Name	Comment
ICD-10	C79.0	Secondary malignant neoplasm of the kidney and pelvis	Basic class code C79
ICD-10	C79.01 / C79.02 / C79.00	Metastasis of the right/left/unspecified kidney	Clarification of the party
ICD-11	2E00	Metastasis of a malignant neoplasm to the kidney or renal pelvis	Add expanders: side, topography
ICD-11	2C90.0	Renal cell carcinoma (primary)	Not to be confused with 2E00 (metastasis)
ICD-11	X… (extension)	Anatomy/Laterality Expanders	Post-coordination in ICD-11 [7]

Epidemiology

According to retrospective series and reviews, renal metastases are rarely detected during life, but in autopsy studies, the incidence is approximately 2.36-12.6% in patients who died from widespread solid tumors. Clinically diagnosed cases represent the "tip of the iceberg," as many foci are small and asymptomatic. [8]

A large US center analyzed 151 patients with renal metastases over approximately 30 years of follow-up, highlighting the rarity of the diagnosis even in specialized institutions. The average patient age was between 50 and 60 years, and a significant proportion had multiple lesions. These data are comparable with later smaller series. [9]

The most common primary sources are lung cancer (up to 40-60% of cases), colorectal cancer, esophageal and gastric tumors, breast cancer and melanoma; much less common are the thyroid gland, ovaries, and lymphomas. In one modern series, the proportion of primary pulmonary tumors was 60%. [10]

Most patients have no specific complaints, and renal lesions are discovered during staging or during treatment of the primary cancer. This requires caution when interpreting a "new" renal mass in a cancer patient—primary renal cancer is more common, but metastasis cannot be ruled out. [11]

Table 2. Frequent sources of kidney metastases (according to clinical series)

Primary tumor	Estimated proportion of kidney metastases
Lung cancer	~40-60%
Colorectal cancer	~5-10%
Esophagus/stomach	~5-10%
Breast cancer	~5-10%
Melanoma	~5-10%
Others (thyroid, ovaries, lymphomas, etc.)	<10% each group

Reasons

The main route of tumor cell entry into the kidney is hematogenous embolism, which is logical given the organ's high perfusion and the arterial "influx" of metastatic cells. The kidneys filter a large volume of blood and have a dense capillary network, creating conditions for the settling of circulating tumor cells. [12]

The biological characteristics of primary tumors (invasiveness, angiogenesis, molecular drivers) determine their ability to metastasize and organotropism. For example, melanoma and lung cancer are prone to multiple hematogenous metastases, which increases the risk of secondary lesions in the kidneys in late stages. [13]

The kidney microenvironment (rich vasculature, specific growth factors) can create a "fertile ground" for the engraftment of individual clones—this explains the rare "tumor-in-tumor" phenomenon, when a metastasis colonizes the tissue of a primary renal cell carcinoma. Although this is casuistry, it highlights the complexity of the metastatic process. [14]

At the individual level, the likelihood of secondary kidney damage increases with the duration and progression of the primary cancer, as well as in the presence of hematogenous metastases to other organs. The molecular characteristics of the primary tumor and the effectiveness of systemic therapy are important. [15]

Risk factors

Risk factors include aggressive primary tumors with high metastatic activity (e.g., non-small cell and small cell lung cancer, melanoma), as well as late stages with existing distant metastases. In these conditions, the likelihood of detecting lesions in the kidneys is higher. [16]

A long interval from initial diagnosis to the appearance of new lesions (months to years) does not exclude renal metastases: in some series, periods exceeding 100 months have been recorded. Therefore, the appearance of a "new" mass in the kidney in a patient with a history of cancer always requires oncological considerations. [17]

Multiple lesions in different organs, high metastatic volume, and poor functional status increase the risk of an unfavorable outcome and influence the choice of palliative-oriented tactics. Conversely, control of the primary tumor and a single renal lesion increase the chances of local intervention. [18]

Iatrogenic factors (e.g., delayed staging, untimely imaging in the presence of new symptoms) can lead to late detection of metastases and loss of the window of opportunity for local control. Regular surveillance according to disease-specific standards reduces these risks. [19]

Pathogenesis

The metastatic cascade includes cell detachment from the primary tumor, intravasation, survival in the bloodstream, embolization into the renal microvasculature, extravasation, and colonization of the parenchyma. Molecular mechanisms of adhesion, immune evasion, and stromal remodeling are active at each stage. [20]

In the kidney, metastases are most often localized cortically and tend to be multiple and bilateral. Limited vascularization compared to renal cell carcinoma explains the weaker contrast enhancement on computed tomography/magnetic resonance imaging. [21]

The rate of growth and the tendency to necrosis depend on the biology of the primary cancer and the therapy being administered. Immunotherapy and targeted therapy can slow progression or lead to partial regression, but in resistant cases, renal lesions often persist as part of the overall disease. [22]

The rare phenomenon of "tumor-in-tumor" (metastasis to existing renal cancer tissue) highlights the role of the host tumor microenvironment and the "niche" for circulating cells. Clinically, this complicates image interpretation and increases the need for histological verification. [23]

Symptoms

There may be no complaints for a long time; the lesion is discovered "accidentally" on staging CT/PET-CT. As the lesion increases in size, microhematuria/macrohematuria, dull pain in the lower back, and, rarely, subfebrile temperature are possible. Symptoms are nonspecific and require imaging. [24]

In some patients, metastasis presents with sudden macrohematuria or pain, which is associated with necrosis/hemorrhage at the site. In these cases, rapid local control (embolization, SLT/ablation, surgery) alongside systemic therapy is essential. [25]

Bilateral multiple lesions sometimes lead to a decrease in glomerular filtration, which limits the use of contrast agents and a number of drugs - the treatment plan is adapted to the nephrological status. [26]

It should be remembered that in a cancer patient, any “new” mass in the kidney may be either a metastasis or primary kidney cancer; clinically, it is impossible to distinguish between them, so imaging and (often) biopsy are decisive. [27]

Classification, forms and stages

In practice, the following are distinguished: 1) solitary metastases to the kidney; 2) multiple/bilateral; 3) metastases combined with other distant foci; 4) rare “collisional” cases (metastasis against the background of primary kidney cancer). The goals and scope of local treatment depend on this gradation. [28]

In terms of the source of the primary tumor, the lungs predominate, followed by colorectal cancer, esophagus/stomach, mammary gland, and melanoma; in lymphomas, parenchymatous-interstitial lesions are common, requiring different diagnostics and therapy. [29]

Based on visualization, hypovascular (more typical for metastases) and hypervascular foci (more common for renal cell carcinoma) are conventionally distinguished, however, this rule is incomplete - the final decision is based on histology. [30]

Based on the clinical course: asymptomatic incidental findings, symptomatic lesions without a threat to function, symptomatic lesions with a risk of bleeding/obstruction and secondary renal failure. This assessment helps to prioritize, from observation to urgent local control. [31]

Table 3. Practical classification for choosing tactics

Sign	Options	Impact on tactics
Number of foci	Solitary / multiple / bilateral	From local to systemic treatment as volume increases
Primary tumor control	Controlled/progressing	Selection for metastasectomy is possible with control
Symptoms	No/there is pain/hematuria	Local control for symptoms
Renal function resource	Maintained / Reduced	Choice of contrast, medications, and surgical scope

Complications and consequences

The main complications are bleeding (macrohematuria), posthemorrhagic anemia, pain syndrome, ureteropelvic junction obstruction with hydronephrosis, and decreased renal function. These events impair the tolerability of systemic therapy and quality of life. [32]

In bilateral multiple lesions or after extensive nephrectomy, a decrease in the glomerular filtration rate is possible, limiting the use of contrast agents and some targeted drugs; therefore, organ-preserving techniques and nephroprotection are of high value. [33]

Untreated renal metastases typically reflect the overall aggressiveness of the disease and are associated with a poor prognosis, especially when combined with extrarenal lesions. However, in the oligometastatic group, localized therapies can improve control and symptom management. [34]

Rare complications of interventions include bleeding after biopsy or ablation, damage to the collecting system, radiation nephritis after stereotactic radiotherapy - the risk of these is reduced by adherence to modern protocols. [35]

When to see a doctor

Oncology patients should immediately report to their physician the development of macrohematuria, increasing dull pain in the lower back/flank, or a drop in hemoglobin without an obvious cause. These symptoms require imaging of the urinary tract. [36]

During routine oncological monitoring, a “new” renal mass on CT/MRI is a reason for in-depth diagnostics, even if the kidneys were previously “clean”: it is possible that primary renal cancer or metastasis may develop. [37]

If you are receiving nephrotoxic drugs or contrast studies, it is important to report symptoms of dehydration, decreased urine output, and edema - this helps to tailor treatment and protect the kidneys. [38]

In the case of a controlled primary tumor and a solitary lesion in the kidney, it is important to discuss local options in a timely manner - the windows for metastasectomy, ablation or SLT are limited by the progression of extrarenal lesions. [39]

Diagnostics

Step 1. Clinic and laboratory. Collection of complaints (hematuria, pain), urinalysis (red blood cells), complete blood count (anemia), creatinine, and calculation of SCF. These data determine the urgency and choice of contrast/imaging method. [40]

Step 2. Imaging. The first line is a contrast-enhanced abdominal CT scan using a multiphase protocol; metastases usually enhance contrast less significantly and may be multiple, bilateral, cortical, and endophytic. MRI is useful in cases of contraindications to iodinated contrast and for clarifying the nature of hypovascular lesions. PET-CT assists with systemic staging and sometimes detects lesions not visible on CT. [41]

Step 3. Biopsy. Percutaneous needle biopsy under ultrasound/CT navigation is a key step in distinguishing metastasis from primary renal cancer and choosing systemic therapy; diagnostic accuracy is high, complications are rare. It is especially indicated in cases of atypical presentation or when deciding on systemic therapy without surgery. [42]

Step 4. Staging and consultation. Assessment of extrarenal lesions, functional status, and risk of complications (bleeding, obstruction). A multidisciplinary consultation selects an individual strategy, ranging from systemic therapy with lesion observation to local intervention. [43]

Table 4. Signs of metastasis versus primary kidney cancer

Sign	Kidney metastasis	Primary kidney cancer
Number and distribution	Often multiple, bilateral, cortical	Most often solitary, unilateral, exophytic
Contrast	Usually weaker than the parenchyma (hypovascular)	Often hypervascular (especially clear cell)
Growth form	Endophytic, less pronounced exophyticity	Exophytic growth is common
Diagnostic tactics	A biopsy is required for confirmation.	Biopsy is indicated selectively

Differential diagnosis

Metastases must be differentiated primarily from primary renal cell carcinoma (PRCC). PRCC is often solitary, more readily enhances contrast, and has exophytic growth; however, imaging "rules" are not absolute, and biopsy remains the standard when in doubt. [44]

Hypovascular infectious and inflammatory foci (e.g., abscesses) can mimic metastases; clinical presentation, laboratory inflammatory activity, and antibiotic response help differentiate these. If necessary, a targeted biopsy is performed. [45]

Lymphoproliferative processes (lymphoma) often produce diffuse infiltrates without pronounced necrosis; the key is the context of the primary disease and morphology. PET-CT may be useful in controversial cases. [46]

Incidental benign findings (low-fat angiomyolipoma, complex cysts) also fall within the spectrum of “masks” and require careful interpretation using modern CT/MRI protocols and, if necessary, verification. [47]

Table 5. Differential diagnosis of a “new” mass in the kidney in an oncology patient

Possible diagnosis	"Hinting" signs	What helps to confirm
Metastasis	Multiple, hypovascular, bilateral	Biopsy, relationship to primary cancer
PPKR	Solitary, hypervascular, exophytic	Biopsy/typical CT image
Lymphoma	Diffuse infiltrates	PET-CT, biopsy
Infection/abscess	Fever, leukocytosis, dynamics on ABT	Clinic + MRI/CT, puncture

Treatment

The basic principle: treat systemic disease based on the primary tumor profile, and consider renal lesions(s) for local control as indicated. For multiple and symptom-less metastases, systemic therapy is the priority; for solitary lesions and a controlled primary tumor, we discuss local methods that potentially improve outcomes. [48]

Nephrectomy/partial nephrectomy is an option for solitary peripheral lesions of sufficient size in patients with good functional status and control of the primary lesion. In a series of 35 cases, surgery improved survival in patients without other metastases (37 versus 18 months). Risks include bleeding and parenchymal loss; benefits include symptom control and histological verification. [49]

Radical nephrectomy is considered for central/large lesions, debilitating hematuria, or failure to preserve the organ. However, in the era of effective systemic therapy, the burden on renal function and delayed initiation of drug therapy limit the indications—the decision is strictly individual. [50]

Percutaneous ablations (radiofrequency, cryoablation, microwave) are a minimally invasive method for local control of small solitary lesions or as a palliative measure for symptoms. Advantages include parenchymal sparing and a short recovery period; limitations include the size/location of the lesion and its proximity to the renal pelvis. [51]

Stereotactic radiotherapy (SBRT) is a modern, noninvasive alternative to surgery/ablation in selected patients. Current series demonstrate high local control and acceptable toxicity, including in combination with immune therapy (synergy/abscopal effects are discussed). Dose and fractionation are selected based on the proximity of critical structures and the initial SCF. [52]

Renal artery embolization is a rapid method for relieving massive hematuria or preparing for surgery in high-risk patients. It can be used as a standalone palliative measure when surgery/SLT is not possible. The decision is made in consultation with an endovascular radiologist. [53]

Systemic therapy is selected based on the primary tumor nosology (checkpoint inhibitors, targeted therapy, chemotherapy, hormonal therapy, and combinations thereof). The goal is to control the overall disease and reduce metastatic volume and symptoms. If the systemic response is effective, it is advisable to re-discuss local options for residual lesions. [54]

The role of biopsy over local methods is difficult to overestimate: the results determine the choice of systemic treatment and help avoid unnecessary nephrectomy in cases of metastatic disease. Current data confirm the high accuracy and safety of percutaneous biopsy with a low rate of complications. [55]

Active surveillance is possible in patients with a small, asymptomatic lesion, high surgical risk, and priority for systemic therapy; regular CT/MRI assessment of the lesion size and vascularity is performed, as well as monitoring of renal function. If growth/symptoms occur, the strategy is revised. [56]

Multidisciplinary management is the standard: a urologist, oncologist, radiologist, radiation oncologist, and pathologist jointly determine the sequence of systemic and local steps. This approach minimizes delays in systemic therapy, preserves renal function, and improves symptom control. [57]

Table 6. Comparison of local methods for monitoring foci in the kidney

Method	Advantages	Restrictions	Typical patient
Partial nephrectomy	Radical control, histology, parenchyma preservation	Invasiveness, bleeding	Solitary peripheral lesion, good status
Nephrectomy	Rapid control of large/central lesions, hematuria	Loss of function, delay in systemic therapy	Large symptomatic lesion, not suitable for organ preservation.
Ablation (RFA/cryo/MVA)	Minimal invasion, fast recovery period	Size/location, risk of damage to the renal pelvis	Small solitary lesion, high surgical risk
SLT (SBRT)	Non-invasive, high local control	Dose planning, risk of radiation nephritis	Lesions unsuitable for surgery, palliative purpose
Embolization	Rapid hemostasis	Risk of post-embolization syndrome	Massive hematuria/preparation for surgery

Prevention

There is no specific primary prevention of kidney metastases - prevention is limited to early detection and treatment of the primary tumor according to current standards (lung cancer screening in risk groups, mammography, colorectal screening, etc.). This reduces the likelihood of a metastatic stage. [58]

Secondary prevention involves high-quality staging and regular monitoring in accordance with nosological guidelines (CT/MRI, PET-CT as indicated). Early detection of solitary lesions increases the chances of local control while preserving renal function. [59]

Nephroprotection during cancer treatment (monitoring of SCF, caution with nephrotoxic drugs and contrast agents, hydration) reduces the risk of iatrogenic complications and allows for wider use of local techniques and drug options. [60]

Lifestyle (smoking cessation, weight control, physical activity) improves the overall prognosis in cancer patients and the tolerability of therapy, although there is no direct evidence of a reduction in the risk of renal metastases. [61]

Forecast

Prognosis is determined primarily by the biology and stage of the primary tumor, as well as the total metastatic volume. In retrospective series, the benefit of renal surgery is noticeable in selected patients without other distant lesions, whereas in advanced disease, the benefits of local intervention are limited. [62]

Multivariate models show worse outcomes in patients with multiple metastases compared to solitary ones, as well as in patients with poor functional status. These factors are taken into account when choosing treatment intensity and goals (radical vs. palliative). [63]

SLT and modern ablations provide high local control with acceptable toxicity in patients unsuitable for surgery, preserving function and avoiding major surgery. Combinations with immune therapy are being actively studied. [64]

In general, with proper selection and the "systemic therapy → local control" sequence, it is possible to prolong relapse-free intervals and reduce symptom burden while maintaining quality of life. Individualization is the key principle. [65]

FAQ

Is a kidney metastasis always a death sentence?
No. It's a manifestation of a systemic disease, but in some patients with a solitary lesion and a controlled primary tumor, local treatments (surgery, ablation, SLT) provide good control and symptomatic relief. The decision is individual. [66]

How can one differentiate metastasis from primary kidney cancer?
CT/MRI shows that metastases are usually hypovascular, multiple, and bilateral, but this is not a rule. The "gold standard" when in doubt is percutaneous biopsy, which has high accuracy and a low risk of complications. [67]

Is surgery always necessary?
No. In the presence of multiple metastases and active systemic disease, drug therapy is the priority, while the renal lesion is treated locally if symptomatic or if function is compromised. Surgery is appropriate for solitary lesions, good status, and control of the primary cancer. [68]

Is there a place for stereotactic radiotherapy?
Yes. For selected patients, SRT provides high local control and can be combined with immune therapy. It is an option for those who are not suitable for surgery or ablation. [69]