Anechoic formation in the kidney: possible causes

The term "anechoic lesion" is an ultrasound description, not a diagnosis. It means that the lesion lacks internal echoes (appears "black"), typically has a thin, smooth wall, and exhibits posterior acoustic enhancement. This combination of features is typical of simple renal cysts , which are benign in the vast majority of cases and require no treatment. [1]

However, not every "black spot" on ultrasound is harmless. Sometimes, the cavity contains thin septa, a thickened wall, calcifications, or suspected blood/pus (in which case the lesion is no longer completely anechoic). CT/MRI may also reveal contrast enhancement of the septa or soft tissue areas, which classifies the lesion as a complex cyst with a different risk and different management. The assessment of "complexity" is now standardized according to the Bosniak classification v2019. [2]

Where do such findings come from? It's most often an age-related phenomenon: simple cysts are detected in 7-10% of people during routine ultrasounds, and up to 30-35% in the elderly. They grow slowly, rarely cause complications, and usually become a "background" in your medical history. Infected/hemorrhaging cysts, parasitic cysts, and solid cystic tumors are a different story—it's important to recognize them and not confuse them with "simple" ones. [3]

Therefore, the algorithm is always as follows: (1) ensure that the ultrasound criteria for “simplicity” are met; (2) if there is even the slightest “complexity”, confirm using contrast-enhanced methods (CT/MRI or contrast-enhanced ultrasound - CEUS); (3) if necessary, use Bosniak v2019 for risk stratification and choice of observation or treatment. [4]

Code according to ICD-10 and ICD-11

For acquired simple renal cysts, ICD-10 uses code N28.1 "Renal cyst, acquired." An important exception: congenital cystic diseases (autosomal dominant/recessive polycystic disease, multicystic dysplasia) are coded in a separate block Q61.*; tumors - block C64/D30.0, etc. [5]

In ICD-11, the category GB80.0 "Simple renal cyst" (section "Non-genetic unclassifiable cystic diseases of the kidney") is used for simple cysts. Post-coordination can be used to add lateralization (right/left/bilateral) and other specifications (e.g., symptomatology). "Complex" lesions suspected of being a tumor are coded according to oncology sections. [6]

Table 1. How to code common scenarios

Situation	ICD-10	ICD-11
Simple renal cyst (acquired)	N28.1	GB80.0 (with post-coordination by side)
Congenital cystic diseases	Q61.*	GB7* / GB8* (corresponding branches)
Cystic tumor/suspected RCC	C64/D30.0 (depending on the situation)	Oncological branches (with clarification of localization)

Epidemiology

Simple cysts are one of the most common findings on ultrasound and CT scans of the abdomen in adults. Their prevalence increases with age: from approximately 6-10% in 20-40-year-olds to 30-36% after age 70. In population-based samples, "silent" cysts are recorded at 7-14% during medical examinations, and among older individuals, the rate is up to 31.9%. Men are affected somewhat more frequently. [7]

Most simple cysts cause no symptoms and remain stable for decades. Long-term observations show that some cysts enlarge over time, others remain unchanged, and only a small percentage cause pain, infection, bleeding, or compression of the renal pelvis. Overall, the risk of complications is low. [8]

"Complex" cystic masses (Bosniak ≥IIF) are less common, but it is in this group that the main oncologic risk lies. According to the Bosniak v2019 systematic reviews: I-II - almost always benign; IIF - require observation; III - intermediate risk; IV - high risk of malignancy, tactics similar to those for solid tumors. [9]

Reasons

Simple cysts are benign epithelial cavities that likely arise from small tubules with localized outflow obstruction and microdilation. This explains their frequent cortical location and slow growth. The connection with lifestyle is secondary: aging and microtrauma to the parenchyma are key factors. [10]

A cyst can become "complex" due to hemorrhage (hemorrhagic cyst), infection (wall thickening, septa), or calcification of the wall following long-standing processes. Parasitic cysts (e.g., echinococcus) form multi-chambered structures with "daughter vesicles," which do not at all fall under the "simple anechoic" pattern. [11]

A separate class are cystic renal tumors: these are not “cysts,” but tumors with cystic components (multilocular cystic neoplasia, RCC with cystic degeneration, etc.). On ultrasound, they are often not completely anechoic, and on CT/MRI, they demonstrate enhanced septa/nodular walls. [12]

Risk factors

Age is a well-documented factor. The likelihood and number of cysts increases with age. Arterial hypertension, diabetes, and metabolic syndrome are indirectly associated, but the cause-and-effect relationships are ambiguous (some studies find correlations, others do not). [13]

Cysts are more often discovered incidentally in people who undergo frequent imaging (e.g., for the back/abdomen)—this is a protective selection, not a "true" risk. Rare familial syndromes (tuberous sclerosis, VHL, etc.) produce specific cysts/tumors and are managed according to specialized protocols. [14]

Table 2. What really increases the likelihood of a "cyst on ultrasound"

Factor	Comment
Age >50 years	The main predictor of simple cysts
Male gender	Slightly more often than in women
Metabolic conditions	The connections are contradictory (yes/no in different series)
Random visualization	More findings with more research

Pathogenesis

A simple cyst is a thin-walled cavity (<2 mm) lined by a single-layer epithelium, filled with clear fluid; on ultrasound, it is completely anechoic with posterior enhancement and without septa or solid components. This is a classic ( Bosnian I ). Any abnormalities (septa, thickenings, calcifications) make it "not so simple." [15]

When blood enters the cavity (trauma, spontaneous rupture of a vessel), the signal becomes heterogeneous; during infection, the walls thicken, internal echoes and sometimes gas appear. Therefore, "absolute anechoicity" is an important, but not the only, criterion for benignity: the absence of enhancement of septa/walls on contrast studies is also required. [16]

The Bosniak v2019 system formalizes the morphology of cystic masses based on the enhancement of septa/walls and nodular components on CT/MRI (and in some centers, on CEUS). This reduces interpretative variability and helps avoid overtreatment of benign cysts, keeping activity in categories III-IV. [17]

Symptoms

Most simple cysts cause no symptoms. Sometimes, if they are large (5-10 cm or more), dull pain in the side, a feeling of heaviness, and occasional microhematuria may occur. A cyst that obstructs the calyces can cause pyelectasis and occasional discomfort. [18]

Complications (infection/hemorrhage/rupture) are accompanied by sudden pain, fever, severe hematuria, and sometimes a drop in hemoglobin. Such situations require urgent diagnosis. If, in the presence of fever and pain, an ultrasound reveals a "no longer anechoic" cavity with a thickened wall, this is an argument in favor of a complicated cyst. [19]

Forms and stages

Ultrasound criteria for a simple cyst include: round/oval shape, anechoic contents, thin, smooth wall (<2 mm), clear margins, posterior acoustic enhancement, and absence of septa/nodes/vascularization. Such cysts do not require observation. [20]

If septa (1-3 thin) and barely noticeable calcifications without enhancement appear, this corresponds to the least complex variants (in CT logic, Bosniak II). Observation is not required. Bosniak IIF (multiple thin septa, minimal thickening) requires dynamic observation (see below). Bosniak III-IV are a reason to discuss surgical/ablation treatment. [21]

Table 3. Ultrasound versus “complexity” (very simplified)

Ultrasound signs	Interpretation
Anechoic cavity + thin wall + posterior enhancement, no septa	Simple cyst (Bosniak I). Observation is not required.
1-3 thin septa, no blood flow, smooth wall	Minimal difficulty (≈ Bosniak II). Usually unsupervised.
Lots of partitions/minimal thickening, but no reinforcement	Suspected Bosniak IIF → observation according to schedule.
Nodules, thickened/enhanced septa or wall	Bosniak III-IV: discussion of surgery/ablation.

Complications and consequences

Rare but possible complications of simple cysts include hemorrhage into the cavity, infection, rupture, and compression of the renal pelvis (reversible obstruction). Associations with hypertension and even erythrocytosis have been described, but the causality is controversial. In most cases, the prognosis is favorable. [22]

The main "negative" scenario arises not from simple cysts, but from cystic tumors: then the risks correspond to localized renal cancer and are managed according to onco-urological guidelines (EAU/AUA). Here, correct primary stratification (Bosniak) is crucial. [23]

When to see a doctor

• If a cyst is detected for the first time and it is unclear whether it meets the “simple” criteria, you need a doctor who can read ultrasound and, if necessary, prescribe a CT/MRI or CEUS.
• If you experience pain, fever, or blood in your urine, seek immediate medical attention. These are signs of infection/hemorrhage/rupture.
• If the cyst is large and there is a feeling of pressure/frequent urge to urinate, discuss decompression (puncture with sclerotherapy or laparoscopic decortication).
• If the lesion is converted to Bosniak IIF/III/IV, a follow-up or treatment plan with a urologist/radiologist is required. [24]

Diagnostics

The basic tool is ultrasound. It distinguishes an anechoic simple cyst (thin wall, posterior enhancement) from a lesion with septa/echo content. Doppler ultrasound helps visualize vascular tissue (there should be no blood in the septum for a cyst). If the image is less than ideal, contrast-enhanced techniques are used. [25]

Contrast-CT/MRI is the gold standard for stratification according to Bosniak v2019: it evaluates enhancement of walls/septa/nodular areas. The 2019 version refined the criteria, increasing specificity and interobserver agreement, leading to a reduction in the number of benign cysts being treated. [26]

CEUS (contrast-enhanced ultrasound) is a modern alternative for those who do not want iodine/gadolinium and for clarifying the internal architecture of the cyst: it clearly demonstrates microenhancement of septa and helps differentiate complex cysts from necrotic tumors. In a number of European centers, CEUS is a standard step for "indeterminate" ultrasound findings. [27]

Test results for an uncomplicated cyst are often normal. In complications, leukocytosis/elevated C-reactive protein may occur, while hemorrhage may cause a drop in hemorrhage. Urine from a simple cyst without communication with the renal pelvis may be unremarkable; in infection, pyuria/bacteriuria may be present. [28]

Table 4. What do different methods provide?

Method	Best use	What excludes/confirms
Ultrasound (B-mode + Doppler)	Screening and monitoring of simple cysts	Classic "simple" cyst; gross complexity
CEUS	Unclear ultrasound, contraindications for CT/MRI	Micro-reinforcement of partitions/walls
CT with contrast	Standard for Bosniak	Enhancement, calcifications, fat/density
MRI with contrast	Alternative to CT	Thin septa/nodes, no iodine
Laboratory	In case of complications	Infection, blood loss

Differential diagnosis

Simple cyst vs. hemorrhagic cyst/abscess: with blood/purulence, the contents become non-anechoic, internal echoes appear, and the wall thickens; with contrast, there may be moderate wall enhancement (inflammation). Clinical signs (pain, fever) are helpful. [29]

Cystic tumor: presence of septal/nodular enhancement on CT/MRI/CEUS; often heterogeneity, calcifications, and wall irregularities. This classifies the lesion as Bosniak III-IV and requires onco-urological management (resection/ablation or active surveillance in selected cases). [30]

Parasitic cyst (echinococcus): multilocular, "daughter" cysts, specific membranes – ultrasound image is different, often with calcification of the walls. Requires an infectious approach. Pyelectasis/calicectasis can mimic a cyst – connection with the renal pelvis and Doppler are important. [31]

Treatment

If the cyst is "simple" (Bosniak I-II) and asymptomatic, no treatment is necessary. No "preventive" procedures, diets, or antibiotics are recommended. Follow-up is only necessary if the doctor has doubts about the criteria or if the cyst is very large and could potentially interfere (for example, with a planned pregnancy or surgery on adjacent organs). [32]

Surveillance for Bosniak IIF. Current reviews and the AJR recommend follow-up imaging at 6 months, 12 months, and then annually for up to 5 years (CT/MRI; in some centers, CEUS). The goal is to detect category upgrades early. If stability persists for several years, surveillance can be discontinued. [33]

When are simple cysts treated? Symptoms (persistent pain, pressure), infection, hemorrhage, or hydronephrosis due to compression. Two main approaches are available:

• Ultrasound-guided percutaneous aspiration with sclerotherapy (usually with ethanol; modern sclerosants are also used). Pros: minimally invasive, outpatient procedure. Cons: higher risk of recurrence than with surgery; repeat treatment is sometimes necessary.
• Laparoscopic/robot-assisted decortication (cyst decapping): has the lowest recurrence rate and is suitable for large/recurrent cysts or combined procedures. The choice depends on the size, location, experience of the center, and your preferences. [34]

What is done for "complex" cysts (Bosniak III-IV)? The tactics are similar to those for localized renal tumors:

• Nephron-sparing resection (partial nephrectomy) is the standard for resectable peripheral lesions.
• Thermal ablation (radiofrequency/microwave/cryo) is an option for selected patients and small lesions where surgery is not desirable.
• Active surveillance is possible in patients with high surgical risk, small tumor volume, and stable dynamics. The decision is made by the oncology team, based on the EAU/AUA. [35]

The role of CEUS and new technologies. CEUS allows for the avoidance of unnecessary CT/MRI scans and better visualization of low-level enhancement. The ACR also notes the benefit of dual-energy CT and CEUS for indeterminate lesions. This improves accuracy and reduces dose and contrast loading. [36]

Table 5. When to do what (tactical summary)

Scenario	What to do	Comment
Classic simple cyst (ultrasound criteria met)	Nothing	Without supervision
"A little more complicated" on ultrasound	CEUS or CT/MRI	Eliminate amplification
Bosniak IIF	Observation for 6, 12 months, then annually up to 5 years	If stable, remove from control.
Symptomatic large simple cyst	Aspiration + sclero or laparoscopic decortication	Select by size/location/experience
Bosniak III-IV	Resection/ablation/observation in selected patients	According to EAU/AUA

Prevention

There's no specific "prevention" for simple cysts—they're not a lifestyle choice, but an age-related phenomenon. However, you can prevent complications: don't ignore prolonged pain/fever, stay hydrated (important for the entire urinary tract), and promptly treat urinary tract infections. [37]

Avoid self-medication with antibiotics or NSAIDs for unexplained flank pain—diagnosis is key. For large cysts that interfere with sports or work, consider minimally invasive decompression rather than a long, drawn-out "I'll just have to wait it out." [38]

Forecast

For simple cysts (Bosniak I-II), the prognosis is excellent: the risk of malignancy is virtually zero, complications are rare, and quality of life is not affected. Most patients forget about the finding altogether. [39]

For Bosniak IIF, the risk of malignancy is low, and proper monitoring allows for timely response to progression. For III-IV, onco-urological logic applies: modern partial nephrectomy and ablations provide high oncological and functional outcomes with timely treatment. [40]

FAQ

1) Is an "anechoic cyst" cancerous?
No. A classic simple cyst is benign and does not undergo malignancy. Danger is assessed not by the word "anechoic," but by the enhancement of the septa/wall on contrast studies (Bosniak). [41]

2) Should a simple cyst be observed?
No, if all ultrasound criteria for simplicity are met. Observation is prescribed for categories IIF and higher. [42]

3) Which is better: puncture or "removal of the cap"?
For large symptomatic cysts, aspiration and sclerotherapy are less invasive, but the risk of recurrence is higher. Laparoscopic decortication has a lower recurrence rate, but it is a surgical procedure. The choice is individual. [43]

4) Is it possible to do without CT/MRI?
Sometimes, yes: CEUS characterizes cysts well and is indicated if there are contraindications to iodine/gadolinium. The decision is made by the ultrasound specialist. [44]

5) How often should Bosniak IIF be monitored?
Usually 6 months → 12 months → annually for up to 5 years (CT/MRI, sometimes CEUS). If stability is achieved, monitoring is stopped. [45]

Table 6. Red flags: when to see a doctor immediately

Symptom/situation	Why is this important?
Sudden severe pain in the side + blood in the urine	Possible hemorrhage/rupture of the cyst
Fever, chills, lower back pain	Infected cyst/pyelonephritis
Severe weakness, drop in Hb	Possible intraosseous bleeding
Ultrasound is no longer "perfectly anechoic"	Possibility of a "complex" cyst - CT/MRI/CEUS is needed

Table 7. Mini-cheat sheet for Bosniak v2019 (for patients)

Category	What does it mean?	Tactics
I-II	Benign cysts	Without supervision
IIF	Low risk, but not "perfect"	Control 6, 12 months, then annually up to 5 years
III	Intermediate risk	Surgery/ablation or observation in selected patients
IV	High risk	Surgery/ablation according to oncoprotocols