Pathology of Urinary Bladder Squamous Papilloma

Squamous papilloma is a rare benign neoplasm seen in the bladder. It is a papillary urothelial neoplasm of low malignant potential (PUNLUMP) ^[1] composed of papillary cores with overlying histologically benign squamous epithelium. ^{[2, 3]}  It is unclear whether squamous papilloma represents the squamous counterpart of urothelial papilloma. Although there is no reported predilection for a specific location in the bladder, most cases arise in the dome, the lateral or posterior walls, ^[1] or the bladder neck. The lesion may occur in the urethra. ^[2]  Irritative symptoms and hematuria may occur. ^{[1, 2]}

There is evidence to suggest that squamous papilloma of the bladder is unrelated to human papillomavirus (HPV) infection; therefore, it seems to be unrelated to condyloma acuminatum. ^{[2, 4, 5]}  ​However, there is a case report of an elderly male who presented with nocturia and pressure during urination and was determined to have HPV infection with a low-risk subtype (HPV 6/11) associated with a urothelial carcinoma with squamous differentiation and condylomatous features. ^[6]

The risk factors for urinary bladder squamous papilloma are similar to those for other urothelial neoplasms; cigarette smoking and occupational exposure to aromatic amines are among the most important. Specific epidemiologic studies on squamous papilloma are lacking. ^{[2, 3]}

Squamous papilloma of the urinary bladder appears to have an incidence of 3 cases per 100,000 population annually, with males predominantly affected (male:female ratio of 5:1), as well as the elderly. ^[1] This tumor has been reported to comprise 25% of all bladder cancers in a Northeast state (New Hampshire) ^[7] and about 50% of US pediatric bladder neoplasms. ^[8]

Gross findings

The endoscopic appearance of squamous papilloma is identical to that of low-grade papillary neoplasms. The lesion is delicate and small or has the appearance of a polyp. ^{[2, 3]}  ​

Microscopic findings

Squamous papilloma is a benign neoplasm seen in the bladder and the urethra. Histologically, it is composed of papillary cores with overlying benign squamous epithelium (see the image below). ^{[2, 3]}

Pathology of urinary bladder squamous papilloma. Squamous papilloma of the bladder is composed of a delicate fibrovascular core covered by benign squamous epithelium.

Immunohistochemistry

No or minimal basal/parabasal p53 nuclear accumulation has been reported. Epidermal growth factor immunoreactivity has been reported to occur. ^{[2, 3, 4]}

Molecular/genetics

Squamous papilloma is a DNA-diploid lesion; it tests negative for HPV DNA. ^[2]

Tumor spread and staging

TNM stage Ta applies (ie, squamous papilloma is noninvasive). As a benign lesion, it is rarely associated with invasion or spread. ^{[1, 2, 3]}

Prognosis and predictive factors

Squamous papilloma typically has an excellent prognosis with total excision. ^[1] The disease generally follows a benign clinical course. The 10-year survival is about 95%. ^[1] However, tumors with focal high-grade carcinoma behave like other high-grade neoplasms. ^[1]

Recurrences have been reported in up to one third of squamous papillomas (and 5% of higher grade lesions recur, particularly those with high Ki-67/MIB1). ^[1] Thus, patients should be followed. ^{[1, 5]}

Some patients may develop bladder cancer in time. ^[2]  In a study that included five patients with squamous papilloma, one patient had low-grade urothelial carcinoma at cystectomy after an interval of 21 months (low-grade urothelial carcinoma preceded the diagnosis of squamous papilloma); two patients were free of lesions on follow-up biopsy, and two cases were lost to follow-up. ^[4]