Todani Classification of Choledochal Cysts: Types I to V

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Key facts for practice and radiology board exams

The Todani classification sorts a choledochal cyst by where the bile duct is dilated. Type I is a fusiform or saccular cyst of the extrahepatic duct and is the commonest. Type II is a true diverticulum, type III a choledochocele inside the duodenal wall, type IV cysts of both the intrahepatic and extrahepatic ducts, and type V the intrahepatic ducts alone, which is Caroli disease.

Most type I and type IV cysts carry an anomalous pancreaticobiliary junction, a union of the ducts outside the duodenal wall that leaves a long common channel. There is no official millimetre threshold: the channel measures about 14 mm on MRCP in maljunction against about 7 mm in a merely high confluence [5][6].
A choledochal cyst follows bile duct signal and attenuation on every modality and does not enhance. Wall thickening, enhancement or a mural nodule means superadded infection or carcinoma, not a simple cyst.
Types I and IV are treated by complete excision with hepaticojejunostomy. Internal drainage is obsolete: malignancy was still found in 16.4% of patients after internal drainage [8].
Report the Todani type and the entity name. A choledochocele is managed endoscopically and Caroli disease by liver resection or transplantation, so the type number alone does not tell the surgeon what to do.

In this article

Key facts for practice and radiology board exams
Todani classification of choledochal cysts
Why types III and V sit awkwardly in the Todani scheme
Anomalous pancreaticobiliary junction and the long common channel
Imaging of choledochal cysts
Frequently asked questions
References and further reading

Todani classification of choledochal cysts

Todani and colleagues published the scheme in 1977 as a modification of the Alonso-Lej classification, adding the intrahepatic types that cholangiography had begun to show [1]. Five types are recognised. The subtype letters describe the shape and extent of the dilatation, not a different disease.

Type	What is dilated	Share of cases [2]	Imaging clue	Standard operation
I (Ia, Ib, Ic)	Extrahepatic bile duct. Ia saccular, Ib focal segmental, Ic fusiform	50 to 80%	Cystic structure at the porta continuous with the common bile duct and separate from the gallbladder	Complete cyst excision with Roux-en-Y hepaticojejunostomy
II	True diverticulum of the extrahepatic duct on a narrow neck	2%	Saccular outpouching with a stalk; the rest of the duct is normal calibre	Diverticulectomy, duct preserved
III	Intraduodenal segment of the common bile duct, a choledochocele	1.4 to 4.5%	Cystic mass inside the duodenal wall bulging into the second part, filling on MRCP	Endoscopic sphincterotomy or unroofing
IVA	Intrahepatic and extrahepatic ducts	15 to 35% (IVA and IVB together)	Multiple communicating cysts; look for a stricture at the hilum	Extrahepatic excision with hepaticojejunostomy, plus hepatectomy for lobar disease
IVB	Multiple extrahepatic cysts only	part of the type IV share	Beaded extrahepatic duct with normal intrahepatic ducts	Extrahepatic excision with hepaticojejunostomy
V	Intrahepatic ducts only, that is Caroli disease	20%	Saccular intrahepatic dilatation with the central dot sign	Segmental resection if localised, transplantation if diffuse

Todani classification of choledochal cysts: anatomy, relative frequency, the finding that identifies each type, and the operation it implies.

Type I is often quoted higher than it should be. The reported range is 50 to 80% [2], and modern adult series sit inside it at 64 to 65% [8]. The one subtype distinction that changes the operation is IVA against IVB, because intrahepatic involvement may need a hepatectomy on top of the extrahepatic excision.

Why types III and V sit awkwardly in the Todani scheme

Types III and V are grouped with the rest because the duct is dilated, not because the disease is the same. Visser and colleagues called the standard classification confusing and misleading, argued that Caroli disease and the choledochocele are unrelated to choledochal cyst and should no longer be thought of as subtypes, and pointed out that even the split between type I and type IV is arbitrary [3]. The objection is practical, not academic.

Choledochocele, type III. Many are lined by duodenal rather than biliary mucosa, it is typically not associated with a pancreaticobiliary maljunction, and it carries a lower risk of malignancy than the other types [4]. It is treated endoscopically.
Caroli disease, type V. A ductal plate malformation on the autosomal recessive polycystic kidney disease spectrum, not a consequence of pancreatic reflux. The extrahepatic duct is normal, so it never gets an extrahepatic excision.
Type II is a focal structural anomaly with a normal duct beyond it, which is why a diverticulectomy is enough.

So give the Todani type, because that is how the surgical literature is indexed, and then name the entity. Type V on its own reads as a choledochal cyst needing extrahepatic surgery when what you mean is Caroli disease.

Anomalous pancreaticobiliary junction and the long common channel

An anomalous pancreaticobiliary junction, also called pancreaticobiliary maljunction, is a union of the pancreatic duct and common bile duct outside the duodenal wall, beyond the reach of the sphincter of Oddi. Pancreatic juice refluxes freely into the bile duct, and the duct dilates. It is present in about 83% of adults with a type I cyst [9].

How long does the common channel have to be? The honest answer is that there is no official number. The 2013 criteria of the Japanese Study Group on Pancreaticobiliary Maljunction require an abnormally long common channel or an abnormal union to be shown on direct cholangiography, MRCP, endoscopic ultrasound or drip infusion cholangiography CT, and they state no millimetre threshold at all [5]. The widely repeated figures of 15 mm in adults and 5 mm in children are not in them. What has actually been measured is this:

Common channel	Direct cholangiography	MRCP
Pancreaticobiliary maljunction, 184 patients	16.2 plus or minus 6.9 mm	13.9 plus or minus 6.2 mm
High confluence of pancreaticobiliary ducts, 22 patients	7.7 plus or minus 1.5 mm	6.6 plus or minus 1.4 mm
Best cut-off to separate the two	not applicable	9 mm

Measured common channel length in maljunction against a high confluence of the pancreaticobiliary ducts [6].

MRCP reads the channel about 2 mm shorter than direct cholangiography, so do not reject the diagnosis on a borderline MRCP number. At or below 9 mm on MRCP, direct cholangiography is needed to confirm it [6].
A short channel is not a get-out either. The criteria explicitly ask you to confirm, by direct cholangiography, that the papillary sphincter does not reach the junction when the channel is relatively short [5].

The point most often missed: a maljunction without a dilated duct is still a cancer risk, and the cancer is usually in the gallbladder. In the Japanese nationwide survey, biliary cancer occurred in 21.6% of adults with congenital biliary dilatation, of which 62.3% were gallbladder cancer and 32.1% bile duct cancer. In maljunction without biliary dilatation the cancer rate was higher still at 42.4%, and 88.1% of those cancers were in the gallbladder [7]. Prophylactic surgery is recommended once maljunction is established, although the same authors note that the strategy for maljunction without biliary dilatation remains controversial [7]. A normal calibre duct on your MRCP does not make the finding benign, and the gallbladder is the organ to scrutinise.

Imaging of choledochal cysts

MRCP is the test of choice. It shows the extent of the cyst, proves that it communicates with the biliary tree, and demonstrates the common channel in the same study. The single most useful rule is that a choledochal cyst follows bile duct signal and attenuation on every modality: anechoic on ultrasound, fluid attenuation on CT, T1 hypointense and T2 hyperintense, with no wall enhancement.

T2 fat saturated MRI image of a choledochal cyst — T2 fat saturated MRI: the cyst follows bile duct signal and does not enhance.

Ultrasound. Anechoic cystic lesion continuous with the biliary tree and separate from the gallbladder. That separation is what distinguishes it from a duplicated or distended gallbladder.
CT. Non-enhancing fluid attenuation structure at the porta contiguous with the duct; an intramural duodenal cyst in type III; multiple communicating cysts in types IV and V. CT is usually where an unsuspected cyst is first seen.
MRCP. Map the proximal and distal extent, the hilar confluence, any stricture, the segments involved, and the length of the common channel. Hepatobiliary contrast agents or hepatobiliary scintigraphy prove communication when MRCP is equivocal.
Red flags for carcinoma. Irregular or nodular wall thickening, mural enhancement, an enhancing intraluminal nodule, restricted diffusion, or a new stricture. Suspect cholangiocarcinoma, and look for the double duct sign if the pancreatic duct is dilated too.

MRCP showing a choledochocele, a type III choledochal cyst — MRCP: a choledochocele, the type III cyst, filling inside the duodenal wall.

Choledochocele radiology case — Choledochocele bulging into the second part of the duodenum.

What else looks like this

Mimic	What separates it from a choledochal cyst
Obstructive biliary dilatation	The duct tapers to a cause at the distal end and the dilatation is proportionate; a type I cyst is disproportionately dilated with a normal duct beyond it
Duplicated or distended gallbladder	Continuous with the cystic duct, not with the common bile duct; trace the connection
Pancreatic pseudocyst	History of pancreatitis, no communication with the bile duct, and it displaces rather than replaces the duct
Cystic biliary atresia in infancy	A small hilar cyst with non-dilated intrahepatic ducts, an abnormal or absent gallbladder and no visible distal duct; a choledochal cyst is larger with a patent duct [10]
Hepatic or peribiliary cyst	No communication on MRCP or on hepatobiliary contrast imaging

Lesions mistaken for a choledochal cyst, and the discriminator for each.

The complications are worth carrying as the 3Cs: calculi, carcinoma and cyst rupture with biliary peritonitis, to which cholangitis and pancreatitis can be added. The overall malignancy risk is usually quoted as 10 to 15% and rises steeply with age, from about 2.3% at 20 to 30 years to 75% at 70 to 80 years [2]. A systematic review of 1337 adults in the West found 10.9% [8], and a modern series of 100 adults with type I cysts found cholangiocarcinoma in 6%, five of those six having been diagnosed in childhood and never resected or only bypassed [9]. That age gradient is the whole argument for excising a cyst when it is found rather than watching it.

Frequently asked questions

What is the Todani classification of choledochal cysts?

The Todani classification divides choledochal cysts into five types by the site of biliary dilatation. Type I is dilatation of the extrahepatic bile duct and accounts for 50 to 80 percent of cases, with subtypes Ia saccular, Ib focal segmental and Ic fusiform. Type II is a true diverticulum of the extrahepatic duct. Type III is a choledochocele of the intraduodenal segment. Type IV involves multiple cysts, IVA both intrahepatic and extrahepatic and IVB extrahepatic only. Type V is dilatation of the intrahepatic ducts alone, that is Caroli disease.

How long is the common channel in an anomalous pancreaticobiliary junction?

There is no official millimetre cut-off. The 2013 Japanese diagnostic criteria require an abnormally long common channel or an abnormal union of the ducts to be demonstrated on imaging, and state no threshold value. In practice the measured channel in maljunction averages 16.2 mm on direct cholangiography and 13.9 mm on MRCP, against 7.7 mm and 6.6 mm in a merely high confluence of the pancreaticobiliary ducts. The best MRCP cut-off is 9 mm, and at or below 9 mm on MRCP direct cholangiography is needed to confirm the diagnosis.

What is a choledochocele and why is it treated differently?

A choledochocele is Todani type III, a cystic dilatation of the intraduodenal portion of the common bile duct that bulges into the second part of the duodenum. Many are lined by duodenal rather than biliary mucosa, it is typically not associated with a pancreaticobiliary maljunction, and it carries a lower risk of malignancy than the other types. It usually presents with pancreatitis or obstruction and is managed endoscopically by sphincterotomy or unroofing rather than by bile duct excision.

What is the risk of cancer in a choledochal cyst?

The overall risk of biliary cancer is usually quoted as 10 to 15 percent and it climbs steeply with age, from about 2.3 percent in patients aged 20 to 30 years to 75 percent in patients aged 70 to 80 years. A systematic review of 1337 adults in Western populations found a combined malignancy rate of 10.9 percent, cholangiocarcinoma being the commonest tumour. A modern cohort of 100 adults with type I cysts found cholangiocarcinoma in 6 percent, and in 5 of those 6 the cyst had been diagnosed in childhood and never resected or only bypassed.

Why is a choledochal cyst excised rather than drained?

Because the cyst epithelium itself is the cancer risk, and an internal drainage procedure leaves it in place. In the Western systematic review, malignancy was still found in 16.4 percent of patients who had undergone internal drainage and in 18.5 percent of those who had a previous resection, so drainage does not remove the risk and even excision does not abolish it. The standard operation for types I and IV is complete excision of the extrahepatic duct with a Roux-en-Y hepaticojejunostomy, with hepatectomy added for lobar intrahepatic disease.

References and further reading

[1] Todani T, Watanabe Y, Narusue M, et al. Congenital bile duct cysts: Classification, operative procedures, and review of thirty-seven cases including cancer arising from choledochal cyst. Am J Surg. 1977;134(2):263-269. PMID: 889044.
[2] Singham J, Yoshida EM, Scudamore CH. Choledochal cysts: part 1 of 3: classification and pathogenesis. Can J Surg. 2009;52(5):434-440. PMID: 19865581.
[3] Visser BC, Suh I, Way LW, et al. Congenital choledochal cysts in adults. Arch Surg. 2004;139(8):855-860. PMID: 15302695.
[4] Law R, Topazian M. Diagnosis and treatment of choledochoceles. Clin Gastroenterol Hepatol. 2014;12(2):196-203. PMID: 23660418.
[5] Kamisawa T, Ando H, Hamada Y, et al. Diagnostic criteria for pancreaticobiliary maljunction 2013. J Hepatobiliary Pancreat Sci. 2014;21(3):159-161. PMID: 24307541.
[6] Itokawa F, Kamisawa T, Nakano T, et al. Exploring the length of the common channel of pancreaticobiliary maljunction on magnetic resonance cholangiopancreatography. J Hepatobiliary Pancreat Sci. 2015;22(1):68-73. PMID: 25234051.
[7] Kamisawa T, Kuruma S, Tabata T, et al. Pancreaticobiliary maljunction and biliary cancer. J Gastroenterol. 2015;50(3):273-279. PMID: 25404143.
[8] Bloomfield GC, Nigam A, Calvo IG, et al. Characteristics and malignancy rates of adult patients diagnosed with choledochal cyst in the West: a systematic review. J Gastrointest Surg. 2024;28(1):77-87. PMID: 38353080.
[9] Walia S, Saad M, Butler J, et al. Type I choledochal cysts in adults: natural history and implications for management. Surg Endosc. 2026;40(9):7827-7835. PMID: 42432235.
[10] He F, Guan X, Yang B, Liu H. Key postnatal magnetic resonance characteristics for differentiating cystic biliary atresia from choledochal cyst. Eur Radiol. 2024;34(11):7471-7480. PMID: 38753194.