Pancreatic cancer is among the top five malignancies that cause fatalities around the world. Early detection of these masses may make curative treatment possible, but unfortunately these malignancies are often quite advanced at the time of presentation and, therefore, easily visualized with ultrasound. Clinical signs associated with pancreatic cancer include progressive obstructive jaundice, an abdominal mass or migratory thrombophlebitis [Trousseau’s sign]. Elevated levels of CEA and CA 19-9 may also be noted.
Transabdominal ultrasound is often the initial modality that is used for screening and may allow for adequate diagnosis. The other promising applications of ultrasound include endoscopic ultrasound [EUS] and laparoscopic ultrasound. EUS detects small tumors [less than 2 cm] not seen by conventional ultrasound and sometimes even difficult to visualize by CT. Laparoscopic ultrasound can detect extremely small hepatic and peritoneal metastases, not otherwise detected by other modalities and hence change the prognosis. However a complete staging with ultrasound may not be possible. Other modalities like CT [especially helical CT with dual phase contrast] and MR [with MRCP] are used to map the vascular anatomy and determine the extent of the disease before considering surgery.
Pancreatic cancers can arise from the exocrine portion as well as the endocrine part. Ninety five percent of the cancers are adenocarcinomas and majority of these arise in the pancreatic head.
Ultrasound features of malignancy in the pancreatic head:
- Focal enlargement or a change in the contour of the pancreas
- Presence of a hypoechoic mass
- Abrupt cut off of a duct
- Dilatation of the biliary and pancreatic ductal system due to back pressure changes
- Vascular invasion
- Extrapancreatic spread to the surrounding tissues
- Metastasis to nodes or liver
If the mass is very small [< 2 cm], direct visualization of the mass may not be possible by ultrasound and the presence of the other indirect pointers such as ductal dilatation should be relied upon to indicate a malignant process and alternative imaging with CT or MR may be necessary for detection.
Recognition of vessel invasion by the malignant process is critical as this makes the patient inoperable. Duplex Doppler sonography aids in the detection of the peri-vascular involvement and may obviate the need for more expensive and invasive modalities. According to a study by Angeli, et al, absence of contact or a short contiguity [<2 cm] between the tumor and peri-pancreatic vessels was considered to be a sign of resectability. Inoperability was suggested by a long contiguity of contact [>2 cm], encasement, compression or thrombosis of the vessels. They concluded that, when color Doppler is used in the initial evaluation of pancreatic malignancy, it can improve the selection of patients for further diagnostic procedures or surgical exploration.
Pseudotumorous [focal mass forming] pancreatitis is the most challenging and closest differential for pancreatic malignancy. In some cases, fine needle aspiration and cytology will be required to differentiate between these entities.
Features that can be used to distinguish between the two entities on imaging are:
- The pattern of dilatation of the main pancreatic duct [MPD]- MPD dilatation due to pancreatic malignancy is more regular versus the kind seen in chronic pancreatitis which tends to be more irregular and beaded. In addition, research using MRI has shown that a mass with a smoothly narrowed duct penetrating through ("duct penetrating" sign) is more likely to be due to pancreatitis than cancer, which often completely obstructs the duct.
- The "double duct" sign that is simultaneous dilatation of both the common bile duct and the pancreatic duct also favors malignancy.
- Vascularity of the mass - a malignant mass per se is relatively avascular, while according to certain research studies, a mass due to focal pancreatitis may be either isovascular or hypervascular as compared to the pancreatic parenchyma [this is better demonstrated by contrast enhanced CT].
In summary, although contrast enhanced CT and in some cases EUS are needed to completely and accurately stage a pancreatic malignancy, ultrasound does have a role to play as a screening modality and in advanced cases, may be the only modality employed to diagnose the condition.