Pointless

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I am all ears too.
My main concern is that we are indicating IMN-RT based on evidence produced by MA20 and the EORTC trial, which irradiated with very "generous" field arrangements, but are allowing underdosing at the same time. Will the (small) benefit shown in these trials still materialize?



Indeed.

That's how its defined in a large trial we currently have running (over 1600 patients to be recruited):

CTV-IMN (internal mammary nodes):
This volume connects cranially to the CTV-"medial supraclavicular nodes" and includes include both the internal mammary
vein and artery with a 5 mm margin. Dorsal border is the pleura. Caudally this volume ends at the cranial side of the fourth rib,
in case of caudal/medial located tumor at the cranial side of the fifth rib.
For the planning target volume (PTV) an additional margin is to be added to the respective CTV in
order to take intra-fraction, inter-fraction motion and machine uncertainty into account.
These margins depend on institutional standards. Generally, the minimum of 5 mm CTV-to-PTV
expansion is recommended.
For planning reasons the PTV should be cropped 2-3 mm beneath the skin in case of breast
conserving surgery and 2 mm beneath the skin in case of post-mastectomy radiotherapy. In case
of skin involvement the ventral border expands to the skin surface.



Yes we are.
How do you compensate for breathing motion with VMAT? This has come up whenever we entertain VMAT for difficult cases... how do you replicate the flash that you get form standard 3D plans? If the answer is breath hold, there is still considerable variability depending on the threshold that you accept. I am genuinely curious about this because I've never heard an answer that makes me feel OK with VMAT.
 

scarbrtj

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how do you replicate the flash that you get form standard 3D plans?
One way you could do that is place part of the ptv uniformly outside the skin and force the TPS to make it water equivalent. Old trick, but effective.
How do you compensate for breathing motion with VMAT?
Lots of in vitro data says that breathing affects IMRT/VMAT doses in voxels randomly, sometimes high, sometimes low, by as much as 20%; over a course of multi-fx tx, the random effects "central limit theorem"-ize to within a few percent on the Rx dose. (You can work out for yourself that if you Rx 4 Gy in 2 fx and deliver 2.5 Gy one day and 1.5 Gy the next, the radiobiological effects are exactly the same as 2 Gy times 2 fx.) Turning the dose rate delivery down helps this too. Also, I'm not aware of any clinical data where IMRT/VMAT has been used in motion-y scenarios where people can point and say: "See, the motion was too much, and it has harmed our clinical outcomes versus non-IMRT or non-VMAT approaches."
 

Palex80

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The interplay effect compensates for a lot of dose uncertainties becuase of breathing motion. It works for fractionated treatments quite good.

The dose modulation towards the skin seems quite good.

Our physicists have tested these dose uncertainties with a "breathing phantom" and found very small variations.
 
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Pointless

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One way you could do that is place part of the ptv uniformly outside the skin and force the TPS to make it water equivalent. Old trick, but effective.

Lots of in vitro data says that breathing affects IMRT/VMAT doses in voxels randomly, sometimes high, sometimes low, by as much as 20%; over a course of multi-fx tx, the random effects "central limit theorem"-ize to within a few percent on the Rx dose. (You can work out for yourself that if you Rx 4 Gy in 2 fx and deliver 2.5 Gy one day and 1.5 Gy the next, the radiobiological effects are exactly the same as 2 Gy times 2 fx.) Turning the dose rate delivery down helps this too. Also, I'm not aware of any clinical data where IMRT/VMAT has been used in motion-y scenarios where people can point and say: "See, the motion was too much, and it has harmed our clinical outcomes versus non-IMRT or non-VMAT approaches."
I guess I'm less concerned with the variability from motion within the modulated field and more concerned with outright missing your target if motion takes the breast outside of your field. You need to replicate the flash that you get in 3D plans, probably with at least a cm. The idea of extending the PTV outside of the breast and optimizing to this is an interesting one. What volume do you use to evaluate your target coverage then?
 

scarbrtj

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I guess I'm less concerned with the variability from motion within the modulated field and more concerned with outright missing your target if motion takes the breast outside of your field. You need to replicate the flash that you get in 3D plans, probably with at least a cm. The idea of extending the PTV outside of the breast and optimizing to this is an interesting one. What volume do you use to evaluate your target coverage then?
The PTV is an artificial-ish, non-clinical, and purely geometric construct to help us properly deliver doses to CTVs and GTVs. So evaluate target coverage using PTVs as a surrogate. Per Van Herk (“Errors and margins in radiotherapy” as I recall), well designed PTVs give a minimum of 95% Rx dose to 90% of patients’ target volumes. Thus there will always be chances of misses, underdoses, etc. But in general if the worry of target being outside the PTV is a big one, the PTV has not been well designed. That is to say people who worry about target miss have not studied and “tested” their PTV constructions.