In this post, I’ll cover the choroid. I’ll start with histology, then address some practical points for retinoblastoma reporting. This is my final retinoblastoma post for the moment – following this I’ll be back to my usual teaching blog schedule.
Choroid histology
This is a high power view of normal choroid, with the vitreous to the left and sclera to the right. From left to right, there is the monolayer of retinal pigment epithelium, Bruch’s membrane, the choriocapillaris (closely packed capillaries just below Bruch’s membrane) and then the larger vessels of the choroidal stroma.
Here I have annotated the image, with the green line overlying Bruch’s membrane. The choriocapillaris lies between Bruch’s membrane and the blue line.
The RPE and Bruch’s membrane can be useful anatomical landmarks, even in disorganised eyes.
In this lower power view of the choroid and retina, there is microscopic evidence of retinal detachment. The inner and outer photoreceptor segments are absent, there is some subretinal fluid, and some retinal pigment epithelial cells have migrated into the subretinal space. There’s also retinal gliosis.
As is the case with basement membranes in general, PAS stain can be quite useful for delineating Bruch’s membrane.
The sub-RPE space is not choroid
This case has multiple small tumour deposits.
On higher power, we can see clusters of tumour cells below the RPE, but above Bruch’s membrane. The choriocapillaris is uninvolved. This does not amount to choroidal invasion.
This case is a bit more tricky. There is a single tumour deposit at the level of the RPE (including a couple of rosettes). I think it’s still superficial to Bruch’s membrane and therefore this particular deposit is not within the choroid, but I’m concerned about the single cells within the choroid, not all of which are inflammatory cells. It so happens that this case had clear choroidal invasion elsewhere.
Choroidal invasion
In contrast to the previous case, in this case tumour clearly involves the choroid extensively, although the choroid is only slightly thickened. The RPE and Bruch’s membrane are still identifiable.
This case also demonstrates massive choroidal invasion, with markedly thickened choroid. The RPE and Bruch’s membrane are still visible.
This case is a little more tricky, due to distortion of the architecture by fibrosis. The choroid is towards the bottom right of the image. On low power, there is a large tumour, with a band of fibrosis between the tumour and deeper choroid.
On higher power, there is a line of pigmented cells which I interpreted as being RPE cells.
I’ve highlighted them here. As the tumour remains the vitread side of the RPE, I would not view this as choroidal invasion.
I hope that these three blog posts covering various aspects of retinoblastoma histopathology have been useful. I would welcome comments, questions or suggestions as to any further topics you might like to see. And of course, please let me know if you think I’ve got something wrong!
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