Inverted papilloma is a rare benign urothelial neoplasm of the bladder, constituting 1-2% of all bladder tumors. As the name implies, the urothelial epithelium shows an inverted growth pattern into the stroma rather than papillary growth toward the surface. It is 5-10 times more common in men than women and most frequently diagnosed between ages 50-70. Usually located at the bladder base or trigone; less commonly found on lateral walls and near ureteral orifices. On imaging, it appears as a smooth-margined, pedunculated or sessile polypoid intraluminal lesion — the surface is smooth (does not show frond architecture unlike papillary tumors). On CT it shows homogeneous enhancement; on MRI intermediate signal on T2 and homogeneous enhancement on contrast-enhanced T1. Histologically, two subtypes are described — trabecular and glandular: trabecular type is more common showing well-defined stromal invagination; glandular type contains cystic areas and glandular structures. Inverted papilloma is considered benign but carries 1-6% recurrence risk; rarely may coexist with urothelial carcinoma or show malignant transformation (under 1%). Treatment is complete transurethral resection (TUR-BT) and minimal follow-up is sufficient in low-risk cases.
Age Range
40-80
Peak Age
60
Gender
Male predominant
Prevalence
Rare
The formation mechanism of inverted papilloma is not fully understood but two theories predominate: (1) chronic irritation/inflammation theory — chronic irritants such as recurrent UTI, catheterization, or bladder outlet obstruction trigger urothelial proliferation and epithelial growth is directed into the stroma rather than the surface (inverted pattern); (2) developmental theory — activation of embryological urogenital sinus remnants produces non-neoplastic proliferation. Histopathologically, in normal urothelial papilloma the epithelium forms papillary projections toward the lumen (exophytic), but in inverted papilloma the epithelium folds inward toward the lamina propria (endophytic) → creates orderly interlocking urothelial trabeculae. This inverted growth pattern is important in imaging correlation: polypoid mass with smooth surface contour → characteristic frond (sea anemone-like) architecture of papillary carcinoma is ABSENT → surface smoothness is a distinguishing finding for inverted papilloma. Since the lesion is well-vascularized it shows homogeneous enhancement on contrast-enhanced CT — however this enhancement pattern is not distinguishing from urothelial carcinoma. On MRI, T2 signal depends on lesion cellularity: inverted papilloma has moderate cellularity with orderly cellular architecture showing intermediate T2 signal → similar to muscle; high-grade urothelial carcinoma shows more heterogeneous signal. On DWI, inverted papilloma may show mild-to-moderate diffusion restriction but less restriction is expected compared to high-grade carcinoma. Imaging differences between trabecular and glandular subtypes are minimal and histological diagnosis is mandatory.
The main distinguishing feature of inverted papilloma on imaging is smooth surface contour — the inverted (endophytic) growth pattern does not form papillary projections at the surface. Papillary urothelial carcinoma grows exophytically showing characteristic frond (sea anemone/cauliflower-like) architecture. This surface difference is recognizable on CT, MRI, and US and is the strongest imaging clue favoring inverted papilloma.
Appears as a smooth-margined, homogeneously enhancing polypoid lesion in the bladder lumen on contrast-enhanced CT. The lesion surface is smooth — characteristic frond (finger-like projections) architecture of papillary carcinoma is not seen. May be pedunculated (stalked) or sessile (broad-based). Size is usually <3 cm. Enhancement is homogeneous and moderate-to-prominent (50-80 HU increase). No invasion findings — bladder wall muscle layer is preserved, perivesical fat is clean. The lesion is usually located at the bladder base, trigone, or near urethral orifices.
Report Sentence
A smooth-margined, homogeneously enhancing polypoid lesion measuring __ mm is seen at the bladder base; compatible with benign urothelial neoplasm including inverted papilloma, histopathological confirmation via TUR-BT is recommended.
Appears as an intermediate signal intensity (close to muscle signal), smooth-margined polypoid lesion in the bladder lumen on T2-weighted MRI. The lesion showing intermediate signal against hyperintense urine background is easily distinguished. The lesion shows homogeneous signal — heterogeneity, necrosis, or cystic change is atypical for inverted papilloma and should raise malignancy concern. The peduncle (stalk) may be visible as a thin hypointense structure on T2. Homogeneous enhancement is seen on contrast-enhanced T1 — enhancement pattern cannot be reliably distinguished from urothelial carcinoma. Bladder wall muscle layer (muscularis propria) is hypointense on T2 and should be preserved — muscle layer disruption suggests invasion.
Report Sentence
An intermediate signal intensity, smooth-margined polypoid lesion is seen in the bladder lumen on T2; its homogeneous structure and smooth surface are compatible with benign lesion including inverted papilloma.
Inverted papilloma may show mild-to-moderate diffusion restriction on DWI — mild signal increase at high b-value and moderately low signal on ADC map. ADC values are generally in the >1.0 x 10⁻³ mm²/s range. High-grade urothelial carcinoma shows marked diffusion restriction (ADC <0.8-0.9 x 10⁻³ mm²/s). This ADC difference may help differentiate inverted papilloma from high-grade carcinoma but does not reliably distinguish from low-grade carcinoma. DWI findings are not diagnostic alone — should always be evaluated together with conventional MRI findings and clinical correlation.
Report Sentence
Mild diffusion restriction is seen in the lesion on DWI with ADC value of __ x 10⁻³ mm²/s; this finding is compatible with low-grade benign lesion but histopathological confirmation is recommended.
Appears as a smooth-margined, solid, hypoechoic or isoechoic intraluminal mass in the bladder lumen on B-mode US. The lesion surface is smooth — different from the irregular/frond surface seen in papillary tumors. In pedunculated lesions, a thin stalk may be visible and the lesion may move slightly with bladder filling. Doppler US demonstrates vascularity within the lesion — this finding is important for differentiating from clot (clot is avascular). Differentiation from intraluminal debris/sediment: inverted papilloma stays at same location with positional change (wall-attached), debris moves gravity-dependently.
Report Sentence
A smooth-surfaced solid polypoid lesion measuring __ mm with internal vascularity is seen in the bladder lumen; compatible with bladder neoplasm including inverted papilloma, cystoscopy and TUR-BT are recommended.
Appears as a smooth-margined filling defect in the opacified bladder lumen on delayed phase of CT urography. The lesion shows enhancement but creates a filling defect pattern by contrast with surrounding opaque urine. Smooth surface contour distinguishes from irregular/frond surface of papillary carcinoma. In stalked lesions, the stalk (peduncle) may be visible as a thin line. This phase provides the best evaluation of lesion morphology and relationship to the bladder lumen.
Report Sentence
A smooth-margined filling defect is seen in the bladder lumen on delayed phase CT urography; smooth surface morphology without papillary frond architecture is compatible with benign lesion including inverted papilloma.
Color Doppler US demonstrates internal vascularity within the lesion — usually a single feeder vessel or diffuse central flow pattern is seen. Papillary carcinoma shows multiple parallel vascular structures reflecting central vessels of each frond; this pattern is absent in inverted papilloma — vascularity is more homogeneous and central. Also important for differentiating from clot: clot is completely avascular (Doppler negative), inverted papilloma shows internal vascularity (Doppler positive). RI is generally 0.5-0.7, compatible with benign vascularity pattern.
Report Sentence
Internal vascularity is demonstrated in the lesion on Doppler US without frond-based vascularity pattern; compatible with benign solid lesion.
Criteria
Most common type; interlocking urothelial trabeculae showing well-defined stromal invagination; minimal atypia; low recurrence rate (1-3%).
Distinct Features
Appears as homogeneous solid lesion on imaging. Shows homogeneous enhancement and homogeneous T2 signal on CT and MRI. Cystic areas are typically absent.
Criteria
Less common; contains cystic areas and glandular structures; may show histological overlap with cystic adenoma; recurrence rate slightly higher than trabecular type.
Distinct Features
May appear more heterogeneous than trabecular type on imaging due to small cystic areas. Cystic areas show hyperintense signal on T2. ADC value may be higher due to cystic areas.
Criteria
Very rare (under 1%); urothelial carcinoma development on background of inverted papilloma; heterogeneity, necrosis, irregular margin, and invasion findings may be seen.
Distinct Features
Heterogeneous enhancement, irregular margin, muscle layer disruption, or perivesical invasion findings should be considered deviation from inverted papilloma on imaging. Urgent TUR-BT and histopathological evaluation are mandatory.
Distinguishing Feature
Urothelial carcinoma usually shows irregular/lobulated/frond surface, heterogeneous enhancement, broad base, and invasion findings. Inverted papilloma is smooth-surfaced, homogeneous, and non-invasive. However, low-grade papillary neoplasm may also appear smooth — histological differentiation is mandatory.
Distinguishing Feature
Leiomyoma is a submucosal/intramural solid lesion showing low signal on T2 (smooth muscle); inverted papilloma is an intraluminal polypoid lesion showing intermediate T2 signal. Leiomyoma is usually more round/oval and broad-based.
Distinguishing Feature
Paraganglioma shows markedly hypervascular enhancement (intense in arterial phase) and may be functional (catecholamine secretion — hypertensive crisis during micturition). Inverted papilloma shows moderate enhancement and is non-functional.
Distinguishing Feature
Endometriosis shows hyperintense signal on T1 (hemorrhagic content) and is intramurally located within the bladder wall; inverted papilloma is intraluminal polypoid and isointense on T1. Endometriosis is seen in women with menstruation-related symptoms.
Urgency
routineManagement
surgicalBiopsy
NeededFollow-up
12-monthInverted papilloma is a benign lesion treated with complete transurethral resection (TUR-BT). Histopathological examination is mandatory — definitive diagnosis cannot be made by imaging and differentiation from low-grade urothelial neoplasm, PUNLMP (papillary urothelial neoplasm of low malignant potential), and low-grade carcinoma can only be made pathologically. Recurrence rate after complete resection is low (1-6%). Cystoscopy at 3-6 month intervals in the first 12 months, then annual cystoscopy follow-up is recommended — however minimal follow-up is also acceptable for isolated, completely resected inverted papilloma. Due to risk of accompanying urothelial carcinoma (0.5-2%), all bladder mucosa should be evaluated cystoscopically. More aggressive follow-up and extensive sampling should be considered in multifocal or recurrent cases.
Inverted papilloma is a benign tumor treated by transurethral resection (TURBT). Recurrence rate is low (1-4%). However, it may rarely coexist with urothelial carcinoma (2-6%), so histological confirmation by biopsy is required. Long-term cystoscopic follow-up is recommended. Prognosis is excellent. Imaging differentiation from papillary urothelial carcinoma can be difficult, and definitive diagnosis is made by pathological examination.