Gallbladder polyp

A gallbladder polyp is defined as any elevated lesion of the mucosal surface of the gallbladder, and as such includes a variety of both benign and malignant entities. 

• benign polyps - 95% of all polyps
  • cholesterol polyps: > 50% of all polyps ^3,7
  • adenoma: ~ 30%, possibly premalignant ⁴
  • inflammatory polyps
  • adenomyomatosis
  • other rare entities (see benign tumours and tumour like lesions of the gallbladder)
• malignant polyps: 5% of all polyps
  • adenocarcinoma: ~ 90% of malignant polyps
  • other rare entities including
    • metastases to gallbladder
    • squamous cell carcinoma
    • angiosarcoma

The remainder of this article discusses general concepts of management of gallbladder polyps and focus on imaging characteristics of cholesterol polyps.

Epidemiology

Gallbladder polyps are relatively frequent, seen in up to 5% of the population ^1,7. Over 90% are benign, and the majority are cholesterol polyps.

Cholesterol polyps are most frequently identified in patients between 40 and 50 years of age, and are more common in women (F:M,  2.9:1) ³.

Clinical presentation

Typically gallbladder polyps are incidentally found on upper abdominal imaging, in patients with upper abdominal discomfort. In most instances however the polyps themselves are thought to be asymptomatic ^1,3.

Patients with Peutz-Jeghers syndrome have an increased prevalence of adenomas within the gallbladder.

Pathology

Clearly, as a wide variety of entities appear as polyps, histology is equally variable.

Cholesterol polyps appear as yellow lobulated and often pedunculated masses.  Histologically they are composed of lipid-laden macrophages, covered with normal epithelium ³.

Radiographic features

In most instances predicting histology based purely on imaging is not possible, with the possible exception of cholesterol polyps in some instances (see below), and thus features that are predictive of benign vs malignant disease should be noted (see benign vs malignant features of gallbladder polyps) ^1,6-7. Additionally, as adenomas are considered pre-malignant, surgical management is warranted, and thus the important imaging distinction is between a cholesterol polyp and a solid (and thus most likely neoplastic (benign or malignant) lesion) ⁷.

Overall size is probably the most useful indicator of malignancy, with polyps over 10mm in diameter having a malignancy rate of 37-88% ³.

Ultrasound

Ultrasound is the initial investigation of choice, and is able in most instances to separate cholesterol polyps from those requiring treatment. General features of gallbladder polyps are a non shadowing polypoid ingrowth into gallbladder lumen, which is usually immobile unless there is a relatively long pedunculated component.

Endoscopic ultrasound is useful in further assessing polyps as it is able to generate higher resolution images ^7-8.

Features of cholesterol polyp include ⁸:

• small size 
  • > 90% are less than 10 mm
  • most are less than 5 mm
• echogenicity varies with size
  • small polyps are echogenic but non-shadowing
  • larger cholesterol polyps tend to be hypoechoic
• morphology
  • small polyps are adherent to the wall and a smooth
  • larger lesions tend to be pedunculated and granular in outline

Adenomas on the other hand tend to be larger, solitary, more often sessile with internal vascularity and of intermediate echogenicity. It is not possible to distinguish an adenoma from an adenocarcinoma ^6-9.

CT

CT is often unable to detect small lesions. Larger polyps will appear as soft tissue density projections into the lumen of the bladder, and demonstrate enhancement similar to that of the rest of the gallbladder. More intense enhancement should be viewed with suspicion, as it is more commonly associated with malignancy.

Treatment and prognosis

Recommendations as to follow-up of small polyps (<10mm) varies from author to author. Some suggest that polyps that are less than 5 mm in size are almost always cholesterol polyps, and thus require no follow-up ⁶ whereas others suggest yearly follow-up with ultrasound ⁹. Polyps that are 5-10 mm in size warrant follow-up to ensure no interval growth. Follow-up interval varies from 3 to 6 months ^6,9.

Larger polyps or polypoid masses (>10mm) usually require cholecystectomy ⁹.

Differential diagnosis

The differential for a gallbladder polyp is limited, and includes ⁶:

• gallstones
  • usually mobile, but may be adherent
  • usually casts and acoustic shaddow
• biliary sludge