Computed tomography densitometry for discriminating between type 1 hydatid and simple hepatic cysts

Abstract

Objective

Differentiation of simple and type 1 hepatic hydatid cysts is challenging because of their similar contents and radiological appearance. We aimed to compare the density values of simple and Type 1 hepatic hydatid cysts measured on computed tomography.

Methods

Patients with histopathologically, serologically, or macroscopically confirmed Type 1 hydatid or simple hepatic cysts were included in the study. Mean density values, expressed in Hounsfield units, were measured on axial images of contrast-enhanced abdominal computed tomography scans. The results of the two groups were compared using statistical analysis, and p < 0.05 was considered statistically significant.

Results

A total of 51 Type 1 hydatid and 36 simple cysts were included in the study. The mean age of patients with hydatid cysts was significantly lower than that of patients with simple cysts (p = 0.015). The mean diameter of lesions in the hydatid group was significantly larger than that in the simple cyst group (p < 0.001). Multiple-segment extension, as well as involvement of segments I, II, III, V, VI, and VII, was more common in the hydatid group, whereas involvement of segments IV and VIII was more frequent in the simple cyst group. The mean density of hydatid cysts (9.2 ± 8.8 Hounsfield units) was slightly higher than that of simple cysts (7.3 ± 6.5 Hounsfield units), but this difference was not statistically significant (p = 0.301). The area under the receiver operating characteristic curve was 54.6%. Male gender (p = 0.026) and younger patient age (p = 0.002) were significantly associated with the presence of a hydatid cyst. Additionally, increasing lesion size positively correlated with the likelihood of a hydatid cyst diagnosis (p = 0.001).

Conclusion

Computed tomography densitometry is not a reliable tool for discriminating between Type 1 hydatid and simple hepatic cysts, consistent with previous findings for other quantitative measurements using magnetic resonance imaging and elastosonography. Additionally, younger male patients with larger cysts were more likely to have a diagnosis of hepatic hydatid cyst.

Introduction

Echinococcosis is a zoonotic infestation caused by Echinococcus granulosus, which is endemic in South America, Eastern Europe, the Middle East, and Eastern Asia. The annual incidence of the disease has been reported to be up to 50 cases per 100,000 individuals in these endemic regions. The liver is the most common site of infestation (50%–70%), although almost all organs and tissues can be involved. Serious complications such as anaphylaxis, cholangitis, cholecystitis, jaundice, abscess formation, and rupture into the peritoneum, pleura, or pericardium, and even death, can occur in untreated patients. Treatment options for hepatic hydatid cysts include antiparasitic drugs, imaging-guided percutaneous drainage, and surgical resection.^1,2

Simple hepatic cysts are homogeneous, fluid-filled, thin-walled cystic lesions lined by epithelium. They occur secondary to the developmental malformation of the ductal plate during the embryonic period. Most of these lesions do not require specific treatment; however, interventions may rarely be needed in cases of intracystic bleeding, rupture, jaundice, and portal hypertension.^3,4

With the widespread availability of imaging modalities such as ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) in routine clinical practice, the detection frequency of asymptomatic hepatic cysts has increased. Imaging modalities are highly useful for the diagnosis and staging of hepatic hydatid cysts, whereas serological examinations play only a supportive and limited role, failing to detect the disease in up to 20% of patients because of high false-negative rates. ⁵

Ultrasonography is the primary imaging modality for the diagnosis and staging of hepatic hydatid disease, with a diagnostic accuracy of up to 90%, particularly when performed by an experienced practitioner. The Gharbi (Types I–V) ⁶ or World Health Organization (CE1–5) ⁷ classifications are used to stage hepatic hydatid cysts based on their imaging features. The management of hepatic hydatid lesions depends on the grade and complication status of the lesion. Oral albendazole is preferred for CE1 and smaller CE2 lesions, whereas a combination of PAIR (percutaneous aspiration, injection, and re-aspiration) and albendazole is recommended for CE2 lesions with a diameter between 5 and 10 cm. Surgical resection may be required for CE2–CE3b lesions, whereas no treatment is necessary for inactive lesions (such as CE4–CE5), which can be monitored by US. Antiparasitic drugs decrease the frequency of complications and recurrence when administered before and after surgical or interventional procedures. ⁸

CT is an important tool for the diagnosis of hepatic hydatid disease. In particular, calcifications, solid components, and complications can be better detected using CT; it also provides superior information regarding the location and number of lesions.^9,10 However, differentiating between Type 1 hydatid and simple hepatic cysts remains a major challenge because of their overlapping imaging features, which include homogeneous fluid content without calcifications, septations, or solid components.

In this study, we aimed to compare the mean density values measured on contrast-enhanced CT images of patients with Type 1 hydatid and simple hepatic cysts, whose diagnoses were confirmed by laboratory examinations and serial CT findings.

Materials and methods

Patients

In this retrospective comparative study, patients referred to the interventional radiology department between 2020 and 2024 were screened using the hospital and radiology department databases.

The Type 1 hydatid cyst group (classified according to the Gharbi classification) included 36 consecutive patients (51 lesions) who were diagnosed following a PAIR procedure. Diagnosis was confirmed via histopathological and macroscopic examinations along with positive enzyme-linked immunosorbent assay (ELISA) results. All included patients had undergone initial hepatobiliary US and contrast-enhanced abdominal CT scans within 2 days of each other. Macroscopic findings of the hydatid lesions included a collapsed membrane and the characteristic appearance of the drained fluid (Figure 1).

Figure 1.

An axial contrast-enhanced CT image of a patient with positive ELISA and histopathological results who underwent a PAIR procedure for a Type 1 hydatid cyst in the right lobe.

CT: computed tomography; ELISA: enzyme-linked immunosorbent assay; PAIR: puncture, aspiration, injection, and re-aspiration.

The simple cyst group consisted of 19 consecutive patients (36 lesions) with a diagnosis of simple hepatic cyst. These lesions demonstrated no significant enlargement on follow-up scans (< 2 mm over a minimum follow-up duration of 2 years), yielded negative ELISA results, and had baseline hepatobiliary US and contrast-enhanced abdominal CT scans performed within a 2-day window (Figure 2).

Figure 2.

Initial (a) and follow-up (b) axial CT images of a patient with negative ELISA results obtained 3 years after the initial scan. The lesion size and extent are similar on both scans, and the cyst wall is very thin, suggesting a simple hepatic cyst.

CT: computed tomography; ELISA: enzyme-linked immunosorbent assay.

Patients with a lesion size <1 cm, intracystic calcifications, septations, or solid components were excluded from the study. Additional exclusion criteria comprised a history of hepatic surgery or interventions, previous or current use of antiparasitic drugs (including mebendazole and albendazole), and a lack of available ELISA or histopathological results.

Due to the retrospective design of the study and the complete anonymization of patient data, the requirement for informed consent was waived. The study protocol was approved by the Yüzüncü Yıl University Ethics Committee (Date: 14.07.2023, Approval No: 2023/07-08) and was conducted in accordance with the Declaration of Helsinki of 1975, as revised in 2024. This study is reported in compliance with the Standards for Reporting Diagnostic Accuracy Studies (STARD) guidelines. ¹¹

CT

All CT examinations were performed in the supine position using a 16-detector multislice CT system (Somatom Emotion 16-Slice, Siemens Healthineers, Erlangen, Germany). The scanning parameters were set at 200 mAs, 120 kVp, and a slice thickness of 3 mm.The contrast-enhanced images were acquired after a 70-s scan delay (portal venous phase) following the intravenous administration of 100–120 mL (1.5 mL/kg) of a nonionic contrast medium (Omnipaque 300 mg I/mL (Iohexol)) through a cubital vein at a flow rate of 3 mL/s. Images were acquired in a craniocaudal direction from 1–2 cm above the diaphragm to the level of the lesser trochanter during a single breath-hold.

All images were reviewed on a workstation (Syngo.via, Siemens Healthineers, Erlangen, Germany) by a radiologist with 16 years of experience in abdominal imaging who was blinded to the serological, histopathological, and macroscopic findings of the lesions.

Measurement of Hounsfield unit values

For each lesion, the three axial images displaying the largest lesion diameters were selected to measure the Hounsfield unit (HU) values. The largest possible circular region of interest (ROI) was drawn manually within the lesion on all three images, ensuring that the ROI did not extend to or include the lesion wall. The average of these three measurements was calculated and recorded as the final HU value for each lesion (Figures 3 and 4).

Figure 3.

A simple hepatic cyst with a thin wall is seen in segment II.

Figure 4.

A Type 1 hydatid cyst with a thick wall is seen in segment VI.

Statistical analysis

The Kolmogorov–Smirnov test was used to assess the normality of the distribution of continuous variables. Categorical variables were analyzed using the chi-squared test. Group comparisons for nonparametric continuous variables were performed using the Mann–Whitney U test. A receiver operating characteristic (ROC) curve analysis was conducted to evaluate the diagnostic performance of HU values in discriminating between simple and Type 1 hepatic hydatid cysts. Furthermore, a binary logistic regression analysis using the enter method was performed to examine the associations of independent variables. Because only a few variables were considered in this study, a univariate analysis was omitted. Age, gender, lesion size, and HU values were instead directly entered into the multivariable analysis, consistent with previous similar studies. Continuous data are presented as mean ± SD. Statistical analyses were performed using Statistical Package for the Social Sciences (SPSS) software (IBM SPSS Statistics for Windows, Version 21.0; IBM Corp., Armonk, NY, USA). A p-value <0.05 was considered statistically significant.

Results

Among the 87 consecutive hepatic cysts included in this study, 51 lesions (in 36 patients) were classified as Type 1 hydatid cysts and 36 lesions (in 19 patients) as simple cysts. The mean age of the entire patient cohort was 54.7 years (range, 5–90 years). Patients in the hydatid cyst group (34.8 ± 23.1 years) were significantly younger than those in the simple cyst group (62.3 ± 14.7 years; p = 0.015) (Table 1).

Table 1.

Demographic and lesion characteristics of the two groups.

	Hydatid cyst (n = 51)	Simple cyst (n = 36)	p
Age (years, mean ± SD)	34.8 ± 23.1	62.3 ± 14.7	0 . 015
Gender (M/F)	14/22	9/10	0.541
Diameter (mm, mean ± SD)	55.9 ± 30.0	25.6 ± 12.4	<0.001
HU value (mean ± SD)	9.2 ± 8.8	7.3 ± 6.5	0.301

Bold values denote statistically significance (<0.05).

HU: Hounsfield unit.

The mean maximum diameter of hydatid cysts (55.9 ± 30.0 mm) was statistically significantly larger than that of simple cysts (25.6 ± 12.4 mm; p < 0.001). The male-to-female ratio was lower in the hydatid cyst group (0.64) than in the simple cyst group (0.90), but this difference was not statistically significant (p = 0.541) (Table 1)

The mean density value of hydatid cysts was slightly higher than that of simple cysts, but the difference did not reach statistical significance (p = 0.301). Furthermore, no significant correlation was observed between cyst diameter and HU measurements (p > 0.05) (Table 1).

In the hydatid cyst group, isolated right lobe involvement was detected in 58.8% of cases, isolated left lobe involvement in 39.2%, and bilobar involvement (segments IV, VII, and VIII) in one lesion (2.0%). In the simple cyst group, isolated right lobe involvement was observed in 50% of the lesions (50.0%), and no bilobar extension was noted. The most commonly involved segments were segments VII (35.3%) and II (31.4%) in the hydatid group, compared with segments VIII (27.8%) and IV (25.0%) in the simple cyst group (Table 2).

Table 2.

Radiological and laboratory characteristics of the two groups.

	Hydatid cyst (n = 51)		Simple cyst (n = 36)
	N	%	N	%
Lobar extension
Right	30	58.8	18	50.0
Left	20	39.2	18	50.0
Bilateral	1	2.0	0	0.0
Segment extension
One segment	32	62.7	34	94.4
Two or more segmentsa	19	37.3	2	5.6
Segmental involvement
I	0	0.0	0	0.0
II	16	31.4	7	19.4
III	6	11.8	3	8.3
IV	6	11.8	9	25.0
V	7	13.7	0	0.0
VI	15	29.4	4	11.1
VII	18	35.3	5	13.9
VIII	6	11.8	10	27.8
HU value
≤0	3	5.9	2	5.6
1–5	19	37.3	14	38.9
6–10	11	21.6	11	30.6
11–14	8	15.7	4	11.1
15–19	5	9.8	1	2.8
≥20	5	9.8	4	11.1
Laboratory
Eosinophilia (>500/ μL)	4/31	12.9	0/13	0.0
Elevated AST (>40 U/L)	4/28	14.3	1/12	8.3
Elevated ALT (>45 U/L)b	3/28	10.7	1/12	8.3
Hyperbilirubinemia (>0.3 mg/dL)	1/28	3.6	0/12	0.0
Leukocytosis (>11,000 /µL)	6/31	19.4	2/13	15.4

^a15 two and 4 three segments in hydatid and two segments in simple cyst groups.

^b > 45 U/L for women and >55 U/L for men.

ALT: alanine aminotransferase; AST: aspartate aminotransferase; HU: Hounsfield unit.

Regarding laboratory findings, eosinophilia was present in 12.9% of patients and hyperbilirubinemia in 3.6% of patients in the Type 1 hydatid cyst group, whereas neither was observed in the simple cyst group. Additionally, elevated transaminase levels and leukocytosis were more frequently detected in patients with Type 1 hydatid cysts (Table 2).

The optimal cutoff value for the HU measurement was determined to be 8.5, yielding a sensitivity of 43.1% and a specificity of 72.2% (p = 0.62). The area under the ROC curve was 54.6% (Figure 5).

Figure 5.

ROC analysis for HU values.

HU: Hounsfield unit; ROC: receiver operating characteristic.

Demographic variables (age and gender) and radiological variables (lesion size and HU value) were selected for inclusion in the final multivariate model based on previous literature. Binary logistic regression analysis revealed that lesion size, age, and gender were independently and significantly associated with cyst classification. An increase in lesion size significantly increased the likelihood of a hydatid cyst diagnosis (p < 0.001), whereas the likelihood of a hydatid cyst significantly decreased with advancing age (p = 0.002). Furthermore, male gender was significantly associated with the presence of hydatid cysts; the likelihood of a hydatid cyst diagnosis was approximately 4.2 times higher in men than in women (p = 0.026). In contrast, HU values demonstrated no independent or significant association with cyst classification (p = 0.907) (Table 3).

Table 3.

Results of the binary logistic regression analysis.

Variables	OR	95% CI for OR	p
Gender	4.168	[1.190–14.597]	0.026
Age	0.941	[0.906–0.978]	0.002
Size	1.081	[1.039–1.126]	0.001
HU	0.995	[0.916–1.081]	0.907
Constant	0.229		0.316

CI: confidence interval; HU: Hounsfield unit; OR: odds ratio.

Discussion

In the hydatid group, 61.1% (22/36) of the patients were women, compared with 52.6% (10/19) in the simple cyst group. This finding is consistent with the literature, which reports that both hydatid and simple hepatic cysts are more common in women.^12,13 Patients with Type 1 hepatic hydatid cysts (34.8 ± 23.1 years) were statistically significantly younger than those with simple cysts (62.3 ± 14.7 years; p = 0.015). Furthermore, the likelihood of hydatid cyst occurrence significantly decreased with advancing age (p = 0.002). This age discrepancy is likely due to the fact that hydatid cysts tend to grow over time, eventually leading to symptoms and complications such as compression, jaundice, or pain. Additionally, screening examinations may detect lesions in asymptomatic relatives of patients with hydatidosis. ¹⁴ In contrast, simple cysts rarely demonstrate marked enlargement over time and frequently remain asymptomatic. Consequently, simple cysts are usually detected incidentally in older individuals and typically do not require treatment. ¹⁵ Another plausible explanation is that younger individuals tend to spend more time outdoors, particularly in rural areas where the risk of exposure to parasite-contaminated material is higher, whereas older individuals tend to remain at home more frequently.

The mean diameter of the lesions in the hydatid cyst group (55.9 ± 30.0 mm) was significantly larger than that in the simple cyst group (25.6  ± 12.4 mm; p < 0.001). A significant association was also observed between larger lesion size and an increased likelihood of a hydatid cyst diagnosis (p < 0.001). This finding aligns with a previous study comparing the diffusion-weighted imaging (DWI) features of hepatic hydatid cysts (7.49  ± 2.95 cm) and simple cysts (2.59  ± 1.23 cm). ¹⁶ We attribute this difference to the very slow-growing nature of simple cysts, which tend to remain stable in size, whereas Type 1 hydatid cysts enlarge continuously unless they rupture or calcify. Although hydatid cysts tend to be larger, it must be kept in mind that large simple cysts and small or early-stage hydatid cysts can yield overlapping and misleading results. Thus, although larger size supports a diagnosis of a hydatid cyst over a simple cyst, it remains an unreliable finding on its own. Serial comparisons of lesion sizes with previous imaging studies may provide greater diagnostic utility.

In the present study, half of the lesions in the simple cyst group and approximately 59% in the hydatid cyst group were located in the right lobe. A prior study reported that roughly 80% of hydatid cysts occur in the right hepatic lobe. ¹⁷ Similarly, Gomez et al. reported right lobe involvement in 30 out of 53 patients (56.6%) with simple hepatic cysts. ¹⁸ Our results regarding lobar distribution are therefore consistent with the existing literature. In one patient within the hydatid cyst group, the lesion was located in segments IVA and VIII, demonstrating bilobar extension. No bilobar extension was detected in the simple cyst group. Multisegment extension involving two segments was noted in 15 (29.4%) lesions in the hydatid group and 2 (5.6%) lesions in the simple cyst group. Extension into three segments was detected in 4 (7.8%) hydatid cysts, whereas none was seen among the simple cysts. Involvement of two or more segments is frequently reported for hydatid cysts in the literature. ¹⁹ The higher frequency of multisegment extension in the hydatid cyst group may be attributed to their continuous enlargement over time; the thick, resilient cyst wall combined with increased intracystic pressure leads to the compression of neighboring liver parenchyma, resulting in indentation or extension into adjacent segments. Because of this aggressive growth pattern, hydatid cysts can eventually extend into the intraperitoneal, intrapleural, and even pericardial spaces. ²⁰

The most commonly involved segments were segments VII (35.3%) and II (31.4%) in the hydatid group, compared with segments VIII (27.8%) and IV (25.0%) in the simple cyst group. All segment involvements were more common in the hydatid cyst group, except for segments VIII and IV. It has been previously described that segments VII, VIII, and VI are the most frequently involved, whereas segment I is the least involved in hepatic hydatid disease. ¹⁹ Consistent with that study, no segment I involvement was observed in either the hydatid or simple cyst groups in our cohort.

In this study, the mean HU value of Type 1 hydatid cysts (9.2 ± 8.8 HU) was slightly higher than that of simple cysts (7.3 ± 6.5 HU), but the difference was not statistically significant (p > 0.05). Yan et al. reported the mean density value of Type 1 hydatid cysts to be between 14 and 20 HU. ²¹ In contrast to their findings, the majority of lesions in both the Type 1 hydatid (58.8%) and simple hepatic cyst (69.4%) groups in our study had a density between 1 and 10 HU. When stratified into attenuation categories (<0, 1–5, 6–10, 11–14, 14–19, and >20 HU), the most frequent density range was 1–5 HU for both Type 1 hydatid (37.3%) and simple hepatic cysts (38.9%). We believe that the nonsignificant difference in mean HU values, as well as the similar distribution across HU categories, stems from their shared homogeneous cystic content, lacking calcifications, solid components, or septations. Furthermore, intracystic hemorrhage or infection, which can alter cyst HU values, can occur in both Type 1 hydatid and simple hepatic cysts.^22–25 The slightly higher mean HU value in the hydatid cyst group might be explained by the presence of hydatid sand. Although this preliminary study with a relatively small sample size indicates that CT densitometry fails to discriminate between Type 1 hydatid and simple hepatic cysts, this finding should be validated by age- and gender-matched multicenter studies with larger cohorts.

Durmaz et al. compared the stiffness of 22 simple and 51 Type 1 hydatid cysts using US elastography. They reported a mean shear wave velocity (SWV) of 2.6 ± 0.96 m/s (range, 0.96–4.25) for simple cysts and 2.8 ± 1.69 m/s (range, 0.66–4.84) for hydatid cysts, stating that there was no statistically significant difference in SWV values between the two groups. ²⁶

There are also conflicting results in the literature regarding the utility of DWI for differentiating between simple and Type 1 hepatic hydatid cysts. Inan et al. compared the apparent diffusion coefficient (ADC) values of 43 simple and 26 Type 1 hepatic hydatid cysts using a b-value of 1000 s/mm², concluding that the ADC values of Type 1 hydatid cysts were significantly lower than those of simple cysts. ²⁷ Similarly, Sonmez et al. evaluated the ADC values of 22 simple and 16 Type 1 hepatic hydatid cysts and reported significantly lower ADC values for Type 1 hydatid cysts at a b-value of 1000 s/mm². ²⁸ Conversely, Oruc et al. (10 Type 1 hydatid cysts, 5 simple cysts) ²⁹ and Aksoy et al. (6 Type 1 hydatid cysts, 36 simple cysts) ³⁰ reported no statistically significant difference in ADC values between simple and Type 1 hydatid hepatic cysts. A more recent study with a larger patient population (50 Type 1 hydatid cysts, 45 simple cysts) also suggested a nonsignificant difference. ¹⁶

In the present study, hypereosinophilia was detected in four patients and hyperbilirubinemia in one patient within the Type 1 hydatid cyst group, whereas neither was observed in the simple cyst group. Similarly, elevated transaminase levels and leukocytosis were more common among patients with Type 1 hydatid cysts. These findings are generally consistent with the literature. ³¹ However, some studies have reported higher frequencies of hypereosinophilia (up to one-third of cases) and hyperbilirubinemia (up to 20%) in hydatid cyst patients. ³² We attribute these discrepancies to the fact that we exclusively included noncomplicated Type 1 hydatid cysts, which typically elicit a relatively milder immune response and involve fewer biliary duct complications.

Based on our results, HU measurement is not a highly useful tool for differentiating between simple and Type 1 hepatic hydatid cysts because of their highly similar fluid content. These findings should be further validated by studies focusing on simple versus Type 1 hydatid cyst differentiation in other organs, such as the kidney and spleen. Given the limitations of quantitative cystic content measurements across CT, MRI, and elastosonography, future studies focusing on high-resolution, magnified assessments of the cyst wall structure may provide a solution to this diagnostic challenge.

The major limitation of this study is its relatively small sample size. However, we strictly included patients in the hydatid group who had concurrent positive ELISA results, had characteristic US/CT images, and underwent a PAIR procedure. Similarly, all patients in the simple cyst group presented with negative ELISA results and definitive follow-up scans. These stringent inclusion criteria inevitably restricted our sample size. As another limitation, the CT densitometry measurements were performed by a single researcher.

Conclusion

There is no statistically significant difference in the mean attenuation values on CT between Type 1 hepatic hydatid and simple hepatic cysts. Because of their highly similar fluid content, the majority of both cyst types exhibit density values ranging between 1 and 10 HU. Given the limitations of quantitative measurements across CT densitometry, elastosonography, and DWI-MRI, qualitative imaging findings such as a thick cyst wall, multisegment extension, and bilobar involvement should raise a strong suspicion of a Type 1 hydatid cyst rather than a simple hepatic cyst.