Cone radiation?

Cone beam CT is what they use for image guidance at the start of each treatment. They match up the image with the planning imaging before they start. They were maybe waiting for gas to clear the rectum or the bladder to fill, so they could get a better match up. There is another kind of radiation technique called Tomotherapy where the x-rays spiral around you and the imaging is done with "waste" X-rays - but it doesn't sound like that's what you're having.

I had pelvic radiation to 50.4 Gy, and they said they would then "cone down" to concentrate from there only on the prostate. That took the prostate to 79.2 Gy. To "cone" is to concentrate the radiation to a specific area by a collimator. The Varian Trilogy has a multi-leaf collimator, dynamically changing the beam shape to allow the accuracy needed for such high doses.

Here's a link defining this use of the term. I think it's what your therapy actually was doing.

cone down

Also, for alignment at the start of each of my treatment sessions they simply used two orthogonal x-rays to locate the implanted markers. The only CT I had was at the initial "simulation session" when designing the x-ray field. They fused that CT with the 3T MRI I also had to accurately define the treatment field. CT is not used in the x-ray therapy suite to my knowledge.

[Edit: a little more about the x-rays at the setup of my treatments. Just reporting my experience.]

The machine they use on me is a Varian Truebeam. Does that help?

I should be seeing my RO tomorrow for the weekly visit. So I will ask him.

I think Redwing has it right.

In my case (positive pelvic nodes) they initially irradiated the entire pelvic floor (44.5Gy), then "coned down" to focus the remaining energy on the prostate bed (68.4Gy). The initial treatments during the "cone down" took a little longer at first as they aligned me with the xrays / simulation CT, and then recalculated the radiation field to focus only on the prostate bed.

I was also on a Varian True Beam, and I think it had to do with programming the collimaters to get the radiation focused in the right place. I didn't have any implant markers - just the alignment tattoos. They started with those, and used the two xrays to get me lined up with the simulation images.

No, the x-ray beam doesn't leave any radiation behind, on anything - you, stuff in you, the table you're lying on, the room, etc...

and you won't develop x-ray vision or glow in the dark.

sigh, I was really hoping for the x-ray vision...

Redwing-

You had primary RT with both the pelvic nodes and the prostate treated. Thus they had to "cone down" from the pelvic nodes to the prostate. DJBearGuy had salvage RT of the prostate bed, so there is no coning down. Some machines have stereo X-rays, others use cone beam CT - that is an optional add on - every machine is configured according to the specs of the institution.

HardlyRob-
You had positive lymph nodes (N1) so they treated both the pelvic LN area and the prostate bed. I don't think DJBearGuy had his LNs treated since they were negative, he was not high risk, and he had a known positive margin in the prostate bed.

OK, I just talked to my RO.
-Cone refers to the Cone CT that they do nominally 1x/week. More often if something seems odd. Otherwise they do daily orthogonal Xrays. Similar to what TA said, except they try not to do cone CT so much.
-The radiation treatment itself is the same every time. Might be slightly different if they put me on a different machine, which they haven't done yet.