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As I mentioned, one of the most common

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of the benign neoplasms of the sinonasal

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cavity is the inverted papilloma.

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Inverted papilloma is one of the three

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types of Schneiderian papillomas.

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We have oncocytic, exophytic, and inverted papillomas.

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The inverted papilloma is the most common,

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representing about 70% of Schneiderian papillomas.

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Inverted refers to the histopathologic feature of the

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squamoid-appearing cells growing inward

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rather than outward from the epithelial lining.

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One of the features of the inverted papilloma

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on CT scan is that it's a lesion that

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generally grows from the medial wall of

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the maxillary sinus into the nasal cavity,

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but it may have this little bony bar

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from which it arises. On CT scanning,

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it may also show calcification, so the features of

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inverted papilloma on CT: growth through the medial

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wall of the maxillary antrum into the nasal cavity,

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calcified matrix sometimes, and this little bony

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bar, which may be the site where it is arising

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from. On MR, it also has relatively characteristic

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features known as the cerebriform appearance.

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By that, it means that there's this heterogeneity

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to the signal intensity, both on T2-weighted scanning,

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as well as on post-gadolinium enhanced scanning,

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in which it kind of looks like brain tissue in its—

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the way it has maybe gray matter and gyri,

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and sulci on the T2-weighted scan, and even

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on the post-gadolinium enhanced scan.

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So these features, again, growing through the

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medial wall of the maxillary antrum into the

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nasal cavity, darkish signal in T2-weighted scan—

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so therefore, not an inflammatory polyp,

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but dark signal, which may imply neoplasm.

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One of the features of inverted papilloma

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is that there may be coexistent

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squamous cell carcinoma. That occurs in about

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15 to 20% of patients with inverted papillomas.

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Unfortunately, the signal intensity of the

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squamous cell carcinoma and the signal intensity

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of the native inverted papilloma are similar,

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and it's hard to define when an inverted

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papilloma has cancer within it versus doesn't.

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That's usually made on histopathologic resection.

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Here's another example

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on CT scan. sYou see the mass that's growing

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along the medial border of the maxillary

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antrum, and then into the nasal cavity.

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It has some calcified matrix to it,

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and as you can see, an expansile lesion,

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another feature of inverted papilloma.

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The second most common location for

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an inverted papilloma is along the nasal

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septum. So in that situation, it may

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erode the nasal septum. Again, usually

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has some hyperdense calcified matrix

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to it, and squamous cell carcinoma—

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again, 15 to 20% rate.

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This is a tumor that is resected in its

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entirety with good margins around because

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they want to make sure that they get any

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squamous cell carcinoma if it is coexistent.

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As I mentioned, there is another

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entity known as the oncocytic papilloma.

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This is a tumor that occurs in the posterior

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ethmoid and nasal septal region as well.

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Its characteristic feature is that it may be

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slightly bright on a pre-GAD T1-weighted scan,

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which is unusual for an inverted papilloma,

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unless we're dealing with the matrix of

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the calcification of an inverted papilloma.

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Let's just reiterate the differences

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between an inflammatory polyp versus an

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inverted papilloma on T2-weighted imaging.

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On the left, we have an inflammatory polyp,

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which is typically very bright on the T2-weighted

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signal intensity and does not have very much

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in the way of lower signal intensity within it.

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These lesions generally will have a high

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ADC value—apparent diffusion coefficient—

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on the DWI scan and therefore

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are not bright on the DWI image.

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Contrast that with our inverted

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papilloma with the cerebriform

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look to it—sort of like the brain tissue

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here and brain tissue in the sinus—but it's

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darker in signal intensity on T2-weighted scan.

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And because of the hypercellular nature

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of this lesion, it does generally

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have lower ADC values. As I mentioned,

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there is that 15% to 20% coexistence of

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squamous cell carcinoma in inverted papillomas.

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You do not see that with squamous cell carcinoma

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in things such as an antrochoanal or antromeatal polyp.