Fetus in fetu: CT appearance - a case report and review of the literature

Fetus in fetu: CT appearance - a case report and review of the literature | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Case Report Fetus in fetu : CT appearance - a case report and review of the literature Roger Guy Pilo Ndibo, Anne Marie Grâce Belobo Eyebe, Orel Kelvin Ndouandju Saha, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6797653/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Fetus in fetu is an extremely rare congenital anomaly characterized by the presence of a malformed, non-viable fetus within the body of its host twin. This case report describes a 5-month-old female infant presenting with abdominal distension since birth. Clinical examination revealed a firm right flank mass, prompting imaging evaluation. Abdominal ultrasound identified a non-specific subhepatic mass, while contrast-enhanced CT scan demonstrated a well-circumscribed retroperitoneal mass (109 × 79 × 96 mm) containing organized bony structures resembling long bones, ribs, and vertebral calcifications, alongside fatty and fluid components. These findings strongly suggested fetus in fetu , with retroperitoneal teratoma considered as a differential diagnosis. Surgical excision confirmed the diagnosis, revealing a fetal-like mass with histopathological evidence of tissues from all three germ layers. The patient recovered without complications, highlighting the importance of imaging, particularly CT scan, in preoperative diagnosis. Nuclear Medicine & Medical Imaging Fetus in fetu CT scan abdominal mass retroperitoneal case report Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Fetus in fetu is an extremely rare benign anomaly that occurs secondary to abnormal embryogenesis, in which a non-viable fetus is located within another normally developing fetus [ 1 ]. It is a very rare condition with an incidence of 1/500,000 live births [ 1 , 2 ]. It can occur in several parts of the body, but most commonly in the abdomen, with 80% of those cases being in the retroperitoneal cavity [ 1 , 3 ]. This is an unusual cause of retroperitoneal abdominal mass in infants. On CT scan, it is usually a well-circumscribed, calcified abdominal mass with clearly visible bony components, particularly the spine and long bones of the limbs, which allows a preoperative diagnosis to be made [ 2 ]. The aim of this study is therefore to describe the CT features of a case observed at the Western Radiology and Medical Imaging Centre in Bafoussam (Cameroon) and to review the literature. Case description Patient information The baby was female, 5 months old, with no documented prenatal ultrasound findings, and presented with abdominal bloating since birth. The mother, aged 35, had no significant medical history. Clinical findings On clinical examination, a firm mass was palpated in the right flank, with no other associated abnormalities. Further imaging was therefore recommended. Diagnostic assessment Initially, an abdominal ultrasound showed a non-specific subhepatic and pre-renal abdominal mass (Fig. 1 ). Then, the patient was referred to us for an abdominal CT scan to better characterize the abnormality and make a diagnosis. Examination without and after injection of iodinated contrast medium at portal time revealed a well-circumscribed right subhepatic mass measuring 109 x 79 x 96 mm, i.e. a volume of 413.32 ml, pushing back the right liver and kidney while preserving their contours (Fig. 2 ), probably suggesting a retroperitoneal origin. The mass was heterogeneous, with well-defined bony structures corresponding to the long bones of the limbs (Fig. 3 ) and ribs (Fig. 4 ), a shapeless calcified mass that may correspond to the vertebral column (Fig. 5 ), fluid and fatty areas. Initially, an abdominal ultrasound showed a non-specific subhepatic and pre-renal abdominal mass (Fig. 1 ). Subsequently, the patient was referred to us for an abdominal CT scan to better characterize the abnormality and establish a diagnosis. The examination, performed without and after injection of iodinated contrast medium at portal phase, revealed a well-circumscribed right subhepatic mass measuring 109×79×96 mm, with a volume of 413.32 mL. This mass displaced the right liver and kidney while preserving their contours (Fig. 2 ), likely suggesting a retroperitoneal origin. The mass was heterogeneous, containing well-defined bony structures corresponding to the long bones of the limbs (Fig. 3 ) and ribs (Fig. 4 ), a shapeless calcified mass possibly corresponding to the vertebral column (Fig. 5 ), and fluid and fatty areas. Diagnosis We concluded that it was a complex cystic mass, likely in the right retroperitoneal region, with well-defined bony structures, suggestive of a fetus in fetu . A retroperitoneal teratoma was considered as a differential diagnosis. Consultation and surgical management were recommended to perform an excision and a possible anatomopathological assessment. Therapeutic interventions A laparotomy was performed one month later, resulting in the resection of a right retroperitoneal fetal-looking mass with visible membranes and hair (Fig. 6 ). On macroscopic examination, the mass was composed of hairy, cheesy material and structures resembling limb buds. On cutting section, it showed fatty, cartilaginous, and bony areas. Microscopically, there were elements from all three germ layers, with no evidence of immature elements. A final diagnosis of fetus in fetu was given based on histopathological findings. Follow-up and outcome of interventions The patient tolerated the operation well, and there were no complications. To date, she has been discharged from the hospital, and there have been no postoperative complications. Patient’s parent perspective "The child is doing very well; it is anticipated that she will achieve normal growth without any sequelae." Discussion Fetus in fetu is an extremely rare congenital anomaly where a malformed fetus develops inside the body of its twin [ 4 ]. This condition occurs in approximately 1 in 500,000 live births, with fewer than 200 cases documented in medical literature [ 1 , 2 , 5 , 6 ]. While most cases are detected in infants and children, reports in adults are exceedingly rare. For instance, an exceptional case was documented in a 17-year-old woman [ 5 ]. Fetus in fetu arises secondary to abnormal embryogenesis in a monochorionic diamniotic pregnancy [ 1 ]. Its etiopathogenesis is explained by two main theories: (1) the parasitic twin theory, which postulates that a malformed twin develops within its normal host twin. This parasitic twin is often unable to survive independently and relies entirely on the host for vascularization and nutrients via a shared blood supply, leading to its generally atrophied and malformed state; and (2) the defective implantation theory, which hypothesizes that an embryo can implant abnormally into the mesenchyme of its twin instead of the uterine wall. This can result in the development of a fetal mass inside the host twin, exhibiting fetus-like structures, including organs and limbs [ 7 ]. The primary location for fetus in fetu is the retroperitoneal region, though it can also be found in other areas such as the abdomen, skull, or even the scrotum [ 2 , 4 ]. Diagnosis of fetus in fetu relies on key elements observed during imaging, particularly CT scans. The main diagnostic criteria include: (1) the presence of a vertebral axis, characterized by a central structure resembling a vertebral column, often surrounded by tissues that may include limbs and other organs; (2) the appearance of a fetus, showing outlines of limbs, muscle tissue, or organs reminiscent of fetal anatomy; and (3) heterogeneous density on CT scan, where the mass typically appears with varying densities, indicating the presence of different tissue types (bone, muscle, fat, etc.) [ 7 ]. Lord stated in 1954 that the presence of a vertebral column, extremities, and appropriately located organs constitute the basic diagnosis of fetus in fetu [ 5 , 8 , 9 ]. These criteria remain valid today [ 5 ]. In our case, we observed non-specific calcification possibly corresponding to a spine, well-formed long bones characteristic of limb buds, and rib-like structures within a heterogeneous mass containing various tissue types (bone, fat). This CT appearance is described in the literature as enabling a pre-surgical diagnosis, which was corroborated by the surgical and anatomopathological findings. Similar observations have been reported by Kumar et al. in one case [ 5 ] and by Karaman et al. and Ragab et al. in two cases [ 3 , 10 ]. Furthermore, in 1974, when computed tomography was still developing, Grosfeld was able to demonstrate a well-organized vertebral column on the right flank of the abdomen with clearly identified long bones on a standard X-ray, subsequently confirming the diagnosis through surgical and anatomopathological findings [ 4 ]. However, other pathological conditions may present with similar features. The primary differential diagnosis for fetus in fetu is teratoma [ 1 , 3 ]. Often confused with fetus in fetu , a teratoma can also appear as a heterogeneous mass with calcifications. Some authors propose that fetus in fetu is, in fact, a well-differentiated and highly organized teratoma [ 8 ]. Meconium pseudocyst can also be considered in the differential diagnosis, despite its distinct presentation, as can certain complex vascular malformations that may mimic the appearance of fetus in fetu [ 10 ]. Although the diagnosis in our case was histologically proven, the CT features were of significant importance in the diagnostic process. The main treatment for fetus in fetu is surgical excision. Complete removal of the mass is curative and generally without major complications [ 6 ]. Conclusion CT diagnosis of the fetus in fetu is based on the identification of specific morphological features, in particular the vertebral axis and characteristic bony structures. Retroperitoneal location is the most common, and it is important to confirm the diagnosis in pathology to differentiate it from an organized differentiated teratoma. Abbreviations CT Computed tomography Declarations Author Contribution G.R.P.N. wrote the first draft of the manuscript, G.R.P.N., A.M.G.B.E., O.K.N.S., R.F.B.D., S.S.K., N-M.N., and F.Z.L.C. critically reviewed and revised successive drafts of the manuscript. All authors read and approved the final manuscript. Ethical Approval Statement Informed consent was obtained in writing from the patient's legal guardian. Consent for Publication Informed consent was obtained in writing from the patient's legal guardian to publish this case report and any accompanying images. Availability of data and materials Data sharing does not apply to this article as no dataset was generated or analyzed during the current study. Competing interests All authors declare no conflict of interest. Funding Source This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors. References Ji Y, Chen S, Zhong L, Jiang X, Jin S, Kong F et al (2014) Fetus in fetu: two case reports and literature review. BMC Pediatr 14:88 Patankar T, Fatterpekar GM, Prasad S, Maniyar A, Mukherji SK (2000) Fetus in Fetu: CT Appearance—Report of Two Cases. Radiology 214(3):735–737 Karaman I, Erdogan D, Ozalevli S, Karaman A, Cavusoglu Yh A, Mk et al (2008) Fetus in fetu: A report of two cases. J Indian Assoc Pediatr Surg 13(1):30 Grosfeld JL, Stepita DS, Nance WE, Palmer CG (1974) Fetus-in-fetu: An ususual cause for abdominal mass in infancy. Ann Surg 180(1):80–84 Kumar A, Paswan SS, Kumar B, Kumar P (2019) Fetus in fetu in an adult woman. BMJ Case Rep 12(8):e230835 Sharma A, Goyal A, Sharma S (2012) Fetus in fetu: A rare case report. J Res Med Sci Off J Isfahan Univ Med Sci 17(5):491–494 Hadhri R, Bellalah A (2019) Retroperitoneal fetus in fetu . Pan Afr Med J 34:78 Hopkins KL, Dickson PK, Ball TI, Ricketts RR, O’Shea PA, Abramowsky CR (1997) Fetus-in-fetu with malignant recurrence. J Pediatr Surg 32(10):1476–1479 Federici S, Prestipino M, Domenichelli V, Antonellini C, Sciutti R, Dòmini R (2001) Fetus in fetu: report of an additional, well-developed case. Pediatr Surg Int 17(5–6):483–485 Ragab M, Abdelhakeem ON, Mansour O, Gad M, Hussein HA (2021) Fetus in fetu: two case reports from North African country. Egypt Pediatr Assoc Gaz 69(1):2 Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6797653","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Case Report","associatedPublications":[],"authors":[{"id":464881314,"identity":"1951c41b-40ef-4955-b6c0-ad802e379066","order_by":0,"name":"Roger Guy Pilo 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mass\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6797653/v1/129155d33fe3e36c82ae41ce.png"},{"id":83905565,"identity":"cc2af38f-1163-43b3-95cc-d6c1196c0d8a","added_by":"auto","created_at":"2025-06-04 10:15:24","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":60731,"visible":true,"origin":"","legend":"\u003cp\u003eCT axial section of the abdomen showing a mass probably retroperitoneal, preserving the contours of the liver and right kidney.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6797653/v1/5209594c95d899ac62321a08.png"},{"id":83906148,"identity":"3d6e0f29-2b78-4f7c-9cf2-77453d5baa8d","added_by":"auto","created_at":"2025-06-04 10:23:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":61391,"visible":true,"origin":"","legend":"\u003cp\u003eCT axial section of the abdomen showing long bones within the mass.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6797653/v1/3799921d6557bddbc93da4cb.png"},{"id":83905573,"identity":"9bc2d1d8-ee62-48f6-ad0c-437601bc0767","added_by":"auto","created_at":"2025-06-04 10:15:24","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":61244,"visible":true,"origin":"","legend":"\u003cp\u003eCT axial section of the abdomen showing ribs within the mass.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6797653/v1/5ce897e36725e25520bd7d86.png"},{"id":83905566,"identity":"82fed698-f9e3-4809-b7bc-c54d3027976d","added_by":"auto","created_at":"2025-06-04 10:15:24","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":56775,"visible":true,"origin":"","legend":"\u003cp\u003eCT axial section of the abdomen showing non-specific calcification within the mass.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6797653/v1/3affc51d31dbe678ca50842a.png"},{"id":83905569,"identity":"9eace404-5da5-44fb-8844-be3b084a6de7","added_by":"auto","created_at":"2025-06-04 10:15:24","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":106624,"visible":true,"origin":"","legend":"\u003cp\u003eIntra-operative view of the excised mass.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6797653/v1/fb092156108115d3b177bb14.png"},{"id":83906410,"identity":"2e441990-b647-47fa-a4ef-724131dd2047","added_by":"auto","created_at":"2025-06-04 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It is a very rare condition with an incidence of 1/500,000 live births [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. It can occur in several parts of the body, but most commonly in the abdomen, with 80% of those cases being in the retroperitoneal cavity [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. This is an unusual cause of retroperitoneal abdominal mass in infants. On CT scan, it is usually a well-circumscribed, calcified abdominal mass with clearly visible bony components, particularly the spine and long bones of the limbs, which allows a preoperative diagnosis to be made [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The aim of this study is therefore to describe the CT features of a case observed at the Western Radiology and Medical Imaging Centre in Bafoussam (Cameroon) and to review the literature.\u003c/p\u003e"},{"header":"Case description","content":"\u003cp\u003e \u003cb\u003ePatient information\u003c/b\u003e The baby was female, 5 months old, with no documented prenatal ultrasound findings, and presented with abdominal bloating since birth. The mother, aged 35, had no significant medical history.\u003c/p\u003e \u003cp\u003e \u003cb\u003eClinical findings\u003c/b\u003e On clinical examination, a firm mass was palpated in the right flank, with no other associated abnormalities. Further imaging was therefore recommended.\u003c/p\u003e \u003cp\u003e \u003cb\u003eDiagnostic assessment\u003c/b\u003e Initially, an abdominal ultrasound showed a non-specific subhepatic and pre-renal abdominal mass (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Then, the patient was referred to us for an abdominal CT scan to better characterize the abnormality and make a diagnosis. Examination without and after injection of iodinated contrast medium at portal time revealed a well-circumscribed right subhepatic mass measuring 109 x 79 x 96 mm, i.e. a volume of 413.32 ml, pushing back the right liver and kidney while preserving their contours (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), probably suggesting a retroperitoneal origin. The mass was heterogeneous, with well-defined bony structures corresponding to the long bones of the limbs (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and ribs (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), a shapeless calcified mass that may correspond to the vertebral column (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), fluid and fatty areas.\u003c/p\u003e \u003cp\u003eInitially, an abdominal ultrasound showed a non-specific subhepatic and pre-renal abdominal mass (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Subsequently, the patient was referred to us for an abdominal CT scan to better characterize the abnormality and establish a diagnosis. The examination, performed without and after injection of iodinated contrast medium at portal phase, revealed a well-circumscribed right subhepatic mass measuring 109\u0026times;79\u0026times;96 mm, with a volume of 413.32 mL. This mass displaced the right liver and kidney while preserving their contours (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), likely suggesting a retroperitoneal origin. The mass was heterogeneous, containing well-defined bony structures corresponding to the long bones of the limbs (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and ribs (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), a shapeless calcified mass possibly corresponding to the vertebral column (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), and fluid and fatty areas.\u003c/p\u003e \u003cp\u003e \u003cb\u003eDiagnosis\u003c/b\u003e We concluded that it was a complex cystic mass, likely in the right retroperitoneal region, with well-defined bony structures, suggestive of a \u003cem\u003efetus in fetu\u003c/em\u003e. A retroperitoneal teratoma was considered as a differential diagnosis. Consultation and surgical management were recommended to perform an excision and a possible anatomopathological assessment.\u003c/p\u003e \u003cp\u003e \u003cb\u003eTherapeutic interventions\u003c/b\u003e A laparotomy was performed one month later, resulting in the resection of a right retroperitoneal fetal-looking mass with visible membranes and hair (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). On macroscopic examination, the mass was composed of hairy, cheesy material and structures resembling limb buds. On cutting section, it showed fatty, cartilaginous, and bony areas. Microscopically, there were elements from all three germ layers, with no evidence of immature elements. A final diagnosis of \u003cem\u003efetus in fetu\u003c/em\u003e was given based on histopathological findings.\u003c/p\u003e\u003cp\u003e \u003cb\u003eFollow-up and outcome of interventions\u003c/b\u003e The patient tolerated the operation well, and there were no complications. To date, she has been discharged from the hospital, and there have been no postoperative complications.\u003c/p\u003e \u003cp\u003e \u003cb\u003ePatient\u0026rsquo;s parent perspective\u003c/b\u003e \"The child is doing very well; it is anticipated that she will achieve normal growth without any sequelae.\"\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e \u003cem\u003eFetus in fetu\u003c/em\u003e is an extremely rare congenital anomaly where a malformed fetus develops inside the body of its twin [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. This condition occurs in approximately 1 in 500,000 live births, with fewer than 200 cases documented in medical literature [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. While most cases are detected in infants and children, reports in adults are exceedingly rare. For instance, an exceptional case was documented in a 17-year-old woman [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cem\u003eFetus in fetu\u003c/em\u003e arises secondary to abnormal embryogenesis in a monochorionic diamniotic pregnancy [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Its etiopathogenesis is explained by two main theories: (1) the parasitic twin theory, which postulates that a malformed twin develops within its normal host twin. This parasitic twin is often unable to survive independently and relies entirely on the host for vascularization and nutrients via a shared blood supply, leading to its generally atrophied and malformed state; and (2) the defective implantation theory, which hypothesizes that an embryo can implant abnormally into the mesenchyme of its twin instead of the uterine wall. This can result in the development of a fetal mass inside the host twin, exhibiting fetus-like structures, including organs and limbs [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe primary location for \u003cem\u003efetus in fetu\u003c/em\u003e is the retroperitoneal region, though it can also be found in other areas such as the abdomen, skull, or even the scrotum [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDiagnosis of \u003cem\u003efetus in fetu\u003c/em\u003e relies on key elements observed during imaging, particularly CT scans. The main diagnostic criteria include: (1) the presence of a vertebral axis, characterized by a central structure resembling a vertebral column, often surrounded by tissues that may include limbs and other organs; (2) the appearance of a fetus, showing outlines of limbs, muscle tissue, or organs reminiscent of fetal anatomy; and (3) heterogeneous density on CT scan, where the mass typically appears with varying densities, indicating the presence of different tissue types (bone, muscle, fat, etc.) [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eLord stated in 1954 that the presence of a vertebral column, extremities, and appropriately located organs constitute the basic diagnosis of \u003cem\u003efetus in fetu\u003c/em\u003e [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. These criteria remain valid today [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. In our case, we observed non-specific calcification possibly corresponding to a spine, well-formed long bones characteristic of limb buds, and rib-like structures within a heterogeneous mass containing various tissue types (bone, fat). This CT appearance is described in the literature as enabling a pre-surgical diagnosis, which was corroborated by the surgical and anatomopathological findings. Similar observations have been reported by Kumar et al. in one case [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] and by Karaman et al. and Ragab et al. in two cases [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Furthermore, in 1974, when computed tomography was still developing, Grosfeld was able to demonstrate a well-organized vertebral column on the right flank of the abdomen with clearly identified long bones on a standard X-ray, subsequently confirming the diagnosis through surgical and anatomopathological findings [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, other pathological conditions may present with similar features. The primary differential diagnosis for \u003cem\u003efetus in fetu\u003c/em\u003e is teratoma [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Often confused with \u003cem\u003efetus in fetu\u003c/em\u003e, a teratoma can also appear as a heterogeneous mass with calcifications. Some authors propose that \u003cem\u003efetus in fetu\u003c/em\u003e is, in fact, a well-differentiated and highly organized teratoma [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Meconium pseudocyst can also be considered in the differential diagnosis, despite its distinct presentation, as can certain complex vascular malformations that may mimic the appearance of \u003cem\u003efetus in fetu\u003c/em\u003e [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Although the diagnosis in our case was histologically proven, the CT features were of significant importance in the diagnostic process.\u003c/p\u003e \u003cp\u003eThe main treatment for \u003cem\u003efetus in fetu\u003c/em\u003e is surgical excision. Complete removal of the mass is curative and generally without major complications [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eCT diagnosis of the fetus in fetu is based on the identification of specific morphological features, in particular the vertebral axis and characteristic bony structures. Retroperitoneal location is the most common, and it is important to confirm the diagnosis in pathology to differentiate it from an organized differentiated teratoma.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCT\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eComputed tomography\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e G.R.P.N. wrote the first draft of the manuscript, G.R.P.N., A.M.G.B.E., O.K.N.S., R.F.B.D., S.S.K., N-M.N., and F.Z.L.C. critically reviewed and revised successive drafts of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval Statement\u0026nbsp;\u003c/strong\u003eInformed consent was obtained in writing from the patient\u0026apos;s legal guardian.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u0026nbsp;\u003c/strong\u003eInformed consent was obtained in writing from the patient\u0026apos;s legal guardian to publish this case report and any accompanying images.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e Data sharing does not apply to this article as no dataset was generated or analyzed during the current study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003eAll authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding Source\u003c/strong\u003e This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eJi Y, Chen S, Zhong L, Jiang X, Jin S, Kong F et al (2014) Fetus in fetu: two case reports and literature review. BMC Pediatr 14:88\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePatankar T, Fatterpekar GM, Prasad S, Maniyar A, Mukherji SK (2000) Fetus in Fetu: CT Appearance\u0026mdash;Report of Two Cases. Radiology 214(3):735\u0026ndash;737\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKaraman I, Erdogan D, Ozalevli S, Karaman A, Cavusoglu Yh A, Mk et al (2008) Fetus in fetu: A report of two cases. J Indian Assoc Pediatr Surg 13(1):30\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGrosfeld JL, Stepita DS, Nance WE, Palmer CG (1974) Fetus-in-fetu: An ususual cause for abdominal mass in infancy. Ann Surg 180(1):80\u0026ndash;84\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKumar A, Paswan SS, Kumar B, Kumar P (2019) Fetus in fetu in an adult woman. BMJ Case Rep 12(8):e230835\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSharma A, Goyal A, Sharma S (2012) Fetus in fetu: A rare case report. J Res Med Sci Off J Isfahan Univ Med Sci 17(5):491\u0026ndash;494\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHadhri R, Bellalah A (2019) Retroperitoneal \u003cem\u003efetus in fetu\u003c/em\u003e. Pan Afr Med J 34:78\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHopkins KL, Dickson PK, Ball TI, Ricketts RR, O\u0026rsquo;Shea PA, Abramowsky CR (1997) Fetus-in-fetu with malignant recurrence. J Pediatr Surg 32(10):1476\u0026ndash;1479\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFederici S, Prestipino M, Domenichelli V, Antonellini C, Sciutti R, D\u0026ograve;mini R (2001) Fetus in fetu: report of an additional, well-developed case. Pediatr Surg Int 17(5\u0026ndash;6):483\u0026ndash;485\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRagab M, Abdelhakeem ON, Mansour O, Gad M, Hussein HA (2021) Fetus in fetu: two case reports from North African country. Egypt Pediatr Assoc Gaz 69(1):2\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Fetus in fetu, CT scan, abdominal mass, retroperitoneal, case report","lastPublishedDoi":"10.21203/rs.3.rs-6797653/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6797653/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFetus in fetu is an extremely rare congenital anomaly characterized by the presence of a malformed, non-viable fetus within the body of its host twin. This case report describes a 5-month-old female infant presenting with abdominal distension since birth. Clinical examination revealed a firm right flank mass, prompting imaging evaluation. Abdominal ultrasound identified a non-specific subhepatic mass, while contrast-enhanced CT scan demonstrated a well-circumscribed retroperitoneal mass (109 \u0026times; 79 \u0026times; 96 mm) containing organized bony structures resembling long bones, ribs, and vertebral calcifications, alongside fatty and fluid components. These findings strongly suggested \u003cem\u003efetus in fetu\u003c/em\u003e, with retroperitoneal teratoma considered as a differential diagnosis. Surgical excision confirmed the diagnosis, revealing a fetal-like mass with histopathological evidence of tissues from all three germ layers. The patient recovered without complications, highlighting the importance of imaging, particularly CT scan, in preoperative diagnosis.\u003c/p\u003e","manuscriptTitle":"Fetus in fetu: CT appearance - a case report and review of the literature","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-04 10:15:20","doi":"10.21203/rs.3.rs-6797653/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"19627bf4-180e-420e-b8a4-5c56b946215d","owner":[],"postedDate":"June 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":49352751,"name":"Nuclear Medicine \u0026 Medical Imaging"}],"tags":[],"updatedAt":"2025-06-04T10:15:20+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-04 10:15:20","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6797653","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6797653","identity":"rs-6797653","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}