Prostate IGRT Questions

[Haybrant]

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Just curious about strategies that people use for prostate IGRT and what you do with someone that tends to have a good amount of stool on a day to day basis and when you might hold treatment for this. We use fiducials but curious what people without fiducials are doing too. Thank you

 

[Gfunk6]

We perform daily CBCT so have a very good idea of rectal caliber. In some cases, you can shift the field anteriorly by a few millimeters and get good coverage/rectal sparing. Other times, the level of prostate deformation is so great that you have to defer treatment until the patient has a BM.

We don't use enemas during simulation because this creates an artificial and irreproducible scenario unless the patient is taking an enema prior to every single treatment.

 

[Mandelin Rain]

Daily CBCT. Can see bladder filling and intervene as you go along. Additionally, we use balloons. I was skeptical at first, but I love it. Consistent rectal caliber and anecdotally, much less rectal toxicity. Guys don't mind it too much after the first couple days. My therapists actually convinced me to do it, so they don't have a problem with it.

 

[BobbyHeenan]

We perform daily CBCT so have a very good idea of rectal caliber. In some cases, you can shift the field anteriorly by a few millimeters and get good coverage/rectal sparing. Other times, the level of prostate deformation is so great that you have to defer treatment until the patient has a BM.

We don't use enemas during simulation because this creates an artificial and irreproducible scenario unless the patient is taking an enema prior to every single treatment.

We do the same thing in my practice regarding CBCT, etc. If a good amount of stool therapists come and get me and we make shifts together. If too much stool or major deformation we wait until BM to treat.

 

[Brim]

We do the same thing in my practice regarding CBCT, etc. If a good amount of stool therapists come and get me...

I need to get some of these "stool therapists" that you speak of.

 

[Burt Radnolds]

We use daily CBCT. Regarding stool/gas, the therapists will call me back when they think a scan looks iffy. I will review the deformed anatomy of the PTV, bladder, and rectum and make a judgement call. I can't give you any thresholds I use though. If it is a routine problem, we recommend dietary modification and laxatives.

I currently still recommend an enema prior to CT simulation. Here is my personal reasoning.

1) I think it is hard to justify using an asymmetrical PTV expansion, ie skimping posteriorly, unless you do this.
2) The smaller rectum makes your dosimetrist work harder to properly spare the rectum.
3) If you have large stool filled rectum the day of CT and it happens to be an atypical day for you, then for the rest of your treatment, your rectum is small and the prostate/target is falling posteriorly out of the high dose region. Yes you can correct for this, but I would rather be correcting in the opposite direction.
4) Even if you create a situation that is a bit atypical the day of CT (small rectum), at worst the rectum will be larger during treatment pushing the prostate into the high dose region, which can also be accounted for by small shifts.

I will acknowledge that there is a small degree of counterintuitiveness to this. In general you want to simulate the patient in a position where there anatomy (both internal and external) will be in the most reproducible position. You could argue that "no enema" represents this. However, for the reasons above, I feel comfortable doing the enema as I feel that it minimizes the chance of marginal miss the most. Further, we coach our patients on the rectal caliber throughout treatment and may recommend dietary changes or laxatives if necessary and can normally get things ironed out without torturing them.

I find these conversations to be very interesting though and enjoy them. In residency you might learn one very specific way of doing things, and may even believe that's the only right way. I don't skin cats, cause I'm not a sicko, but there are many reasonable ways to complete the same task.

 

[jezyk]

...I currently still recommend an enema prior to CT simulation. Here is my personal reasoning.


2) The smaller rectum makes your dosimetrist work harder to properly spare the rectum....
.

What kind of PTV coverage do you try to maintain? Even with bowel prep, some patients have difficulty with the rectum V75 < 15% (when prescribing 7920cGy to 98% of PTV)

Also, does anyone look at the max point dose for bladder and rectum?

 

[scarbrtj]

One thing I don't see mentioned above is the following. What is the #1 determinant of the rectal DVH metrics one sees in IMRT inverse planning for prostate cancer? It is the choice of CTV(prostate)-to-PTV expansion margin.

 

[jezyk]

One thing I don't see mentioned above is the following. What is the #1 determinant of the rectal DVH metrics one sees in IMRT inverse planning for prostate cancer? It is the choice of CTV(prostate)-to-PTV expansion margin.

0.6 cm posterior margin is fairly standard I would think.

 

[medgator]

0.6 cm posterior margin is fairly standard I would think.

I believe the rtog study looking at hypoFx (70/28) had a 0.4 cm posterior margin. I think 0.4-0.6 cm is what most people are using these days with daily igrt (either kv with fiducials or cbct)

 

[BobbyHeenan]

I believe the rtog study looking at hypoFx (70/28) had a 0.4 cm posterior margin. I think 0.4-0.6 cm is what most people are using these days with daily igrt (either kv with fiducials or cbct)

I agree.

From a couple of years ago at one of the ASTRO education sessions they had a slide on this. Sorry for the poor quality, but here are some of the margins some of the bigger centers use.

 

[scarbrtj]

Food for thought. Toxicity of MV X-rays in the average ~2-8 MeV range is related to 1) dose/fractionation 2) anatomic site and 3) volume treated, correct? Let's take a 4x4x4 cm spherical prostate with a volume of roughly 32 cc's. In the extreme cases from above, 3 mm margin minimum, 1 cm margin maximum, tx volumes would be:
max 6x6x6 cm volume = ~110 cc's
min 4.6x4.6x4.6 cm volume = ~50 cc tx volume

The 1 cm margin treats more than twice as much volume as the 3 mm margin. Why am I the only guy I know of who ever mentions this? Margins matter. And while that list above lulls one into thinking everyone generally does it the same so we should expect generally equivalent clinical results institution to institution, I'm quite sure that's not the case.

 

[Gfunk6]

Food for thought. Toxicity of MV X-rays in the average ~2-8 MeV range is related to 1) dose/fractionation 2) anatomic site and 3) volume treated, correct? Let's take a 4x4x4 cm spherical prostate with a volume of roughly 32 cc's. In the extreme cases from above, 3 mm margin minimum, 1 cm margin maximum, tx volumes would be:
max 6x6x6 cm volume = ~110 cc's
min 4.6x4.6x4.6 cm volume = ~50 cc tx volume

The 1 cm margin treats more than twice as much volume as the 3 mm margin. Why am I the only guy I know of who ever mentions this? Margins matter. And while that list above lulls one into thinking everyone generally does it the same so we should expect generally equivalent clinical results institution to institution, I'm quite sure that's not the case.

I concur. Most of us make extensive use of IGRT and it is cognitively dissonant to continue to use 1 cm margins in this era.

 

[medgator]

I concur. Most of us make extensive use of IGRT and it is cognitively dissonant to continue to use 1 cm margins in this era.

I've seen old timers in a previous practice I worked for use those kind of margins on the rectum. Had to see a few of their prostate follow ups in clinic. Lots of late proctitis

 

[Reaganite]

I hear ya. Took over for a two person practice... both old-timers. One used 1cm margins; other used NO posterior margin. Proctitis, recurrence, proctitis, recurrence...literally every patient I see in follow up. Makes you wonder how much of the hate against rad oncs by older surgeons and med oncs is justified....

 

[Neuronix]

I agree.

From a couple of years ago at one of the ASTRO education sessions they had a slide on this. Sorry for the poor quality, but here are some of the margins some of the bigger centers use.

Wow what a flashback. I was in the crowd for that talk too and I loved that talk. As a resident I learned *THIS ONE VERY SPECIFIC WAY TO DO THINGS*, and that session helped me to realize that there is absolutely no consensus on much of what we do.

Did you photograph that slide? I'd love to get that slideset if you have it. I don't remember who gave that talk or even which ASTRO it was . Do you recall/have that info?

 

[BobbyHeenan]

Wow what a flashback. I was in the crowd for that talk too and I loved that talk. As a resident I learned *THIS ONE VERY SPECIFIC WAY TO DO THINGS*, and that session helped me to realize that there is absolutely no consensus on much of what we do.

Did you photograph that slide? I'd love to get that slideset if you have it. I don't remember who gave that talk or even which ASTRO it was . Do you recall/have that info?

I was in the crowd too and then integrated that slide into a talk I gave later. I just pulled the slide from the virtual meeting powerpoint (or took a Print Screen screen grab) when the virtual meeting became available a few months after ASTRO. I think it was 2013, but not sure.

I took that slide (and talk) the way you did - lots of well respected places do things slightly differently. Margins definitely matter here, as mentioned above a DVH will look wildly different even for a 7mm posterior margin vs. a 4 mm posterior margin.

For now in practice I generally do 6mm circumferential margins with 4mm posterior. I feel like with CBCT and/or fiducials (I contour the fiducials and put a structure that is fiducial plus 2mm around it and make sure the kV image has the fiducials inside that structure) then I'm well within the 6mm/4mm post set up error margin. When I do SBRT (rarely now in practice, but used to enroll patients on a clinical trial) I use 5mm with only 2-3 mm posterior.

 

[Neuronix]

Yeah, there's also the dirty little secret that many people with the super high doses or the big PTVs cheat on coverage of the posterior PTV to meet bowel constraint.

7mm circumfrential / 5mm posterior with 2 arcs and fiducials is how I was trained. Heck, drop it to one arc. Moonlighting taught me that urorads can treat 60 prostates a day like that. Agree with SBRT margins (5 / 3 mm).

This has kind of been mentioned, but we learned the hard way that you have to be really careful that your therapists know what they're doing/aligning to. Fiducials are kind of fool-proof assuming the bowel and bladder aren't doing wild and crazy things.

I hear ya. Took over for a two person practice... both old-timers. One used 1cm margins; other used NO posterior margin. Proctitis, recurrence, proctitis, recurrence...literally every patient I see in follow up. Makes you wonder how much of the hate against rad oncs by older surgeons and med oncs is justified....

Second this. Where I trained, I almost never saw proctitis or recurrence. Went to another practice where an old timer retired (7560 cGy IMRT no IGRT), and same thing 😱

 

[Haybrant]

Ive sim'ed a prostate guy 3 times now, enema'd him, put him on stool softners/laxatives and even mag citrate etc. But he keeps on having air throughout the rectum right behind prostate. The best Ive gotten is 4cm of air. He has fiducials in. What do you do? Can you plan it and treat?

 

[RadOncDoc21]

Ive sim'ed a prostate guy 3 times now, enema'd him, put him on stool softners/laxatives and even mag citrate etc. But he keeps on having air throughout the rectum right behind prostate. The best Ive gotten is 4cm of air. He has fiducials in. What do you do? Can you plan it and treat?

Seems consistent, you tried. If you have CBCT and fiducials, use that and beam on! Maybe go to 77 instead of 79 if that helps you sleep a little easier.

 

[Chartreuse Wombat]

I believe the rtog study looking at hypoFx (70/28) had a 0.4 cm posterior margin. I think 0.4-0.6 cm is what most people are using these days with daily igrt (either kv with fiducials or cbct)

RTOG 0415 required 4-10mm CTV to PTV margin. Non-uniform margins allowed but not required.Median overall was 7mm; median posterior was 5mm

 

[Burt Radnolds]

If it really is just air and not stool, if you wanted to and haven't tried yet, you could use a red rubber catheter before sim which I've found to be effective in relieving air pockets. Sometimes we cut multiple holes in the end to aid in accessing the air.

However, at this point you would definitely not be wrong to move forward with planning, given you will be using IGRT and have done due diligence. I would consider not skimping on the posterior PTV expansion. Also, you could have your dosimetrist identify the gas pocket and assign a tissue density halfway between air and water as sometimes a huge air pocket can create some weird planning phenomena and this may give you something closer to day to day reality. I will say I have no idea if there is any actual true value to this, and the counterargument is that you should just plan it as air as that is how it's been showing up on 3 consecutive sims.