Spine vs Carina IGRT for locally advanced lung cancer

[Gfunk6]

I have always used spine IGRT, but a few articles in the Red Journal (here and here) were brought to my attention.

The authors of the 2nd paper (from Princess Margaret) apparently use a 2 step registration. They line up to the spine first then micro-adjust to improve carina/PTV alignment.

There are a couple of questions that come up in my mind:

1. Who is doing the shifts? If MDs are responsible, doesn't that mess up RT workflow?
2. Given the air-tissue interfaces and heterogeneity of lung cancer, are we really well served making large shifts?

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[medgator]

I have always used spine IGRT, but a few articles in the Red Journal (here and here) were brought to my attention.

The authors of the 2nd paper (from Princess Margaret) apparently use a 2 step registration. They line up to the spine first then micro-adjust to improve carina/PTV alignment.

There are a couple of questions that come up in my mind:

1. Who is doing the shifts? If MDs are responsible, doesn't that mess up RT workflow?
2. Given the air-tissue interfaces and heterogeneity of lung cancer, are we really well served making large shifts?

I've used carina a lot since training. The carina makes more sense as a "fiducial" for lung CA since you are matching soft tissue in lung cancer typically, not bone (unless it's some real aggressive paraspinal/rib mass). I'll review it with the therapists the first few times and then they generally get the hang of it.

 

[deleted4401]

Good question. Thought the point of IGRT is to be able to tighten PTV margins, so assumption was to match to CTV. But, then the spine is sometimes way off. I think for OARs, maybe you need to have a margin around them, as well (maybe this has been studied and there is a number - perhaps 0.7cm?), so that you aren't constantly splitting the difference between the tumor and the spine/cord, and stick to matching to tumor.

But, then I looked at these papers. I didn't know that there was significant IO variability. I seem to agree with my partners when we look at it. If you guys are matching to spine or carina, what's your PTV for lung? You guys gating or what?

S

 

[Gfunk6]

I've used carina a lot since training. The carina makes more sense as a "fiducial" for lung CA since you are matching soft tissue in lung cancer typically, not bone (unless it's some real aggressive paraspinal/rib mass). I'll review it with the therapists the first few times and then they generally get the hang of it.

What happens if aligning the carina leads to spine and (more importantly) cord mis-alignment? Do you have a tolerance for shifts in millimeters?

 

[Gfunk6]

. I seem to agree with my partners when we look at it. If you guys are matching to spine or carina, what's your PTV for lung? You guys gating or what?

We perform a 4DCT, contour an ITV and then expand that by 5 mm to generate a PTV.

 

[ShirleyT]

Interesting point, i actually read that paper from PMH and I have to agree that sometimes the bone alignment does not line up to the soft tissue when doing CBCT...... Carina may indeed be a better surrogate.

Having said that, I still usually align to bone when doing daily kv, but I also look at the carina as well on the kv images and compare to DRR, if any big inconsistencies between the two i will ask for a CBCT. On the CBCT, i also have dosimetry import in the 100% and 95% IDL and then i make sure that target is covered by at least 95% IDL.

 

[Palex80]

We use carina/PTV alignment and not the spine.

If you are worried about spine tolerance dose, there is one trick we have been using:
After you are done planning export critical isodoses above your spine tolerance per fraction and create a new volume, then import this new volume into your CBCT station. During treatment then look if these new volumes go into the spine during daily CBCT.
This is not 100$% bullet-proof, especially for IMRT, since by changing the position you are probably changing the isodoses too, but still it's a rather good estimation, of what you are doing.

I hope you understand what I mean...

 

[medgator]

What happens if aligning the carina leads to spine and (more importantly) cord mis-alignment? Do you have a tolerance for shifts in millimeters?

Yeah I put a PRV on the cord and have the therapists call me if there is a 0.5 cm or greater shift in the spine. IF there is a really big discrepancy beyond that, it's time for a CT to figure out what's going on

 

[seper]

In image guidance for advanced NSCLC, I ask therapists to try to align both carina and spine. In a difficult match, carina's more important for me.

 

[deleted4401]

I like that 50% isodose line idea, am going to try that on the next patient.