1. Introduction
Considered as a normal variant of lungs’ anatomy, the azygos lobe produced by azygos fissure containing azygos vein and four layers of pleura is of significant medical importance [1, 2]. Although it is called the right azygos lobe, the literature has also reported the left azygos lobe [3]. During the development when the precursor of azygos vein—right posterior cardinal vein—fails to normally arches over the root of the lung to join with the superior vena cava, it penetrates the apex of the right lung separating it from the rest [1]. 
According to the literature, the azygos lobe was first recognized in 1777 by Wrisberg who reported azygos lobes in both lungs of a 3-year-old boy cadaver [2]. Therefore, the azygos lobe was initially called the “lobe of Wrisberg”, but after the year 1944, Crawford described it as “azygos lobe” [4]. Azygos lobe had been investigated in cadavers, and imaging modalities reported its prevalence from 0.4% to 1.2% [5]. 
The significance of the azygos lobe has been emphasized in surgery and diagnostic radiology [1, 2, 5]. It is crucial to be aware of the manifestation of the azygos lobe in chest radiography and Computed Tomography (CT) to distinguish it from lung masses, the tuberculosis-related fistula between a lymph node and esophagus, lung abscess, etc [1, 5, 6]. Thus, the clinicians, surgeons, and anatomists should be aware of this pulmonary variant since the presence of the azygos lobe can complicate thoracic surgery or thoracoscopy by increasing the risk of vascular injury and bleeding [2, 7]. 
Regarding the significance of the azygos lobe, this study investigated its different shapes in thirteen patients and reported the dimensions and estimated volume of this curious lobe.

2. Materials and Methods
Patients and computed tomography
The chest CT scans of 789 patients (490 men and 297 women) were retrospectively analyzed for one year (2020-2021), in the Medical Imaging Center of Dr Shariati Hospital, Mahdasht City, Alborz, Iran. 
The CT imaging device was Hitachi Supria 16/32 with the power of 51 kW, the gantry bore of 75 cm, the scan range of 180 cm, a 5 MHU X-ray tube, and the minimum slice thickness of 0.675 mm. While acquiring the CT scans, patients had to stay still and hold their breath in deep inspiration to avoid motion artifacts. The CT scans of the chest with 7.5 mm thickness and 7.5 mm interval were acquired, then, the images were reconstructed with thickness and interval of 1.25 mm to analyze the azygos lobe more accurately. 

Parameters of the azygos lobe
The azygos lobe parameters are reported in mm (Figure 1) and include the Maximum Width (MW, medial to lateral), Maximum Length (ML, anterior to posterior), and Maximum Height (MH, superior to inferior) of the azygos lobe.

These parameters were measured by the internal measuring software of the CT imaging machine. Moreover, the estimated volume was calculated by the following formula and reported in mm3.

×10-3
Statistics analysis
To check the differences in the dimensions and estimated volume of the azygos lobes between males and females, normally distributed data (checked by the Kolmogorov-Smirnov test) were analyzed by independent t test using the SPSS v. 16. Data were reported as Mean±SD, and P≤0.05 was considered significant. 

3. Results
Prevalence and demographic data
The chest CT scans of 787 Iranian patients with the age range of 17 to 94 years (Mean±SD: 39.92±19.05 years) were investigated in one year (2019-2021). A total number of 13 cases (1.65%) of the azygos lobe were found. The population included 490 men and 297 women, in which nine cases (1.83%) and four cases (1.34%) of azygos lobe were observed, respectively, showing a higher prevalence in men than in women. 

Dimensions and estimated volume
Table 1 and Figure 2 represent the MW, ML, and MH of these thirteen cases.

The Mean±SD MW, ML, and MH were 47.76±19.28 mm, 60.03±15.91 mm, and 46.44±11.91 mm, respectively. However, no significant difference was detected between mean MW, ML, and MH. Also, these factors were separately investigated in men and women; in men, MW, ML, and MH were 49±21.17 mm, 63.65±16.91 mm, and 46.66±12.16 mm respectively, and in women, were 39.25±14.1 mm, 51.9±11.04 mm, and 45.93±13.12mm, respectively. Although the azygos lobe dimensions were higher in men than in women, no significant difference was detected between mean MW, ML, and MH. 
According to Table 1 and Figure 3, the mean total estimated volume was 88.831.82±80.182.03.

The estimated volume in men and women was 104.230.7±91.291.04 and 54.184.33±34.358.66, respectively. Although the estimated volume of the azygos lobe in men was higher than in women, no significant difference was detected. 

4. Discussion
Investigation of the azygos lobe in the Iranian population was previously reported neither in cadaveric studies nor imaging studies. The total incidence of the azygos lobe in the present CT imaging study was 1.65%. In men and women, it was 1.83% and 1.34%, respectively. The prevalence of the azygos lobe has been reported as 1%, 0.4%, and 1.2% in the cadaveric investigations, chest radiographs, and CT scans, respectively [1, 5, 8]. Compared with CT scans, the lower detection rate of the azygos lobe in chest radiographs can be attributed to the higher accuracy and multiplanar imaging in CT scans, which makes the detection of very thin azygos fissures possible. Also, a prevalence rate of 2.6% had been reported in imaging investigation [9].
This study reported the dimensions of azygos lobes, including MW, ML, and MH as well as the estimated volume of azygos lob. Scarce studies in the literature regarded the dimensions of the azygos lobe. One study investigated the azygos lobe in the South African cadaveric population and reported only four azygos lobes with an average MW of 4.5±0.44 cm and an average MH of 6.7±1.46 cm [2]. In our study, we reported the average MW as 4.77±1.92 cm, average ML as 6±1.59 cm, and average MH as 4.64±1.19 cm. Although the measurements of MW are close to what is reported by Rauf [2], the measurements of MH are less than what is reported in his study. Since we reported 13 cases of azygos lobe, our data can be regarded as more reliable. 
Studies report the significance of the azygos lobe since it can be considered a misdiagnosis in plain chest radiographs as a right paratracheal opacity [5], make thoracic surgeries difficult or even dangerous [5], and accompany other abnormalities, such as intrapulmonary right cephalic vein [10], esophageal atresia [11], etc. 

5. Conclusion
Regarding the azygos lobe as an important variant of lungs, this study reports the dimensions, and estimated volume of thirteen azygos lobe investigated by CT in the Iranian population. As far as the authors are concerned, no reliable and sufficient data are available about the size of azygos lobes in the literature. Data reported in this study can be of significance in diagnosis and surgical procedures.
 
Ethical Considerations
Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed about the purpose of the research and its implementation stages. They were also assured about the confidentiality of their information. They were free to leave the study whenever they wished, and if desired, the research results would be available to them.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
Conceptualization, methodology, software, validation, investigation: Mehrdad Ghorbanlou; Resources, data curation, writing – original draft preparation, writing – review & editing, visualization, supervision, project administration, and funding acquisition: Mehrdad Ghorbanlou and Fatemeh Moradi.

Conflict of interest
The authors declared no conflict of interest.

Acknowledgments
Our sincere thanks go to the supervisor (Mr Habib Rostami) and the staff of Dr Shariati Medical Imaging Center, Mahdasht City, Alborz, for their cooperation through the study. 


References

1. Al-Mnayyis A, Al-Alami Z, Altamimi N, Alawneh KZ, Aleshawi A. Azygos lobe: Prevalence of an anatomical variant and its recognition among postgraduate physicians. Diagnostics. 2020; 10(7):470. [DOI:10.3390/diagnostics10070470] [PMID] [PMCID]
2. Rauf A, Rauf WUN, Navsa N, Ashraf KT. Azygos lobe in a South African cadaveric population. Clinical Anatomy. 2012; 25(3):386-90. [DOI:10.1002/ca.21243] [PMID]
3. Takasugi JE, Godwin JD. Left azygos lobe. Radiology. 1989; 171(1):133-4. [DOI:10.1148/radiology.171.1.2928516] [PMID]
4. Crawford JH. Tomographic appearance of the azygos lobe with a description of two cases and a report of seven cases. The British Journal of Radiology. 1944; 17(202):291-322. [DOI:10.1259/0007-1285-17-202-319]
5. Akhtar J, Lal A, Martin KB, Popkin J. Azygos lobe: A rare cause of right paratracheal opacity. Respiratory Medicine Case Reports. 2018; 23:136-7. [DOI:10.1016/j.rmcr.2018.02.001] [PMID] [PMCID]
6. Munn S. Pseudoazygos lobe caused by lymph node pneumatocele. Journal of Thoracic Imaging. 2002; 17(4):310-3. [DOI:10.1097/00005382-200210000-00008] [PMID]
7. Sadikot RT, Cowen ME, Arnold AG. Spontaneous pneumothorax in a patient with an azygos lobe. Thorax. 1997; 52(6):579-80. [DOI:10.1136/thx.52.6.579] [PMID] [PMCID]
8. Felson B. The azygos lobe: Its variation in health and disease. Seminars in Roentgenology. 1989; 24(1):56-66. [DOI:10.1016/0037-198X(89)90054-0] [PMID]
9. Ndiaye A, Ndiaye NB, Ndiaye A, Diop M, Ndoye JM, Dia A. The azygos lobe: An unusual anatomical observation with pathological and surgical implications. Anatomical Science International. 2012; 87(3):174-8. [DOI:10.1007/s12565-011-0119-5] [PMID]
10. Arslan G, Cubuk M, Ozkaynak C, Sindel T, Lüleci E. Intrapulmonary right brachiocephalic vein associated with azygos lobe. Clinical Imaging. 2000; 24(2):84-5. [DOI:10.1016/S0899-7071(00)00179-0] [PMID]
11. Eradi B, Cusick E. Azygos lobe associated with esophageal atresia: A trap for the unwary. Journal of Pediatric Surgery. 2005; 40(11):e11-2. [DOI:10.1016/j.jpedsurg.2005.07.032] [PMID]