Brachial Plexus in H&N planning

[gmsquid]

Granted, but I might argue that a global plan hotspot of 74 Gy is less likely to lead to significant hotspot than 74 Gy to <4%. I also have concerns about relative volumetric constraints when the plexus is in general meant to look like this.
View attachment 341257View attachment 341258
Given the extent to which the plexus goes into the vertebral canal and out into the supraclavicular area/axilla, the vast majority of the plexus should already be outside of the low dose PTV, even in the over-contoured above PTV, somewhat negating the relevance of a <10% and <4% constraint, though you may draw yours differently (this is just the pic I got from the paper). Lastly, I question how much confidence we can have in the day-to-day location of a volume that measures an average of 8.5 cc (range 1.2-17.7 cc) (ten fold difference in size!?!) and spans >10 cm. I'd be more worried that the 78 Gy hotspot just outside the plexus on sim is in the plexus on a daily basis.

This is a fair point. usually I constrain my plexus max to way below the 74 at 4% if tumor is touching it. The reference to me just tells me that doses of 70 Gy may be reasonable if necessary. Though 70 Gy isn’t always necessary and there is probably increasing probability of plexopsthy with increasing volume over the 66-70 Gy. Simply constraining the global hot spot may not optimally spare the amount getting 66-70 Gy. And you may be able to constrain the V70 to under 4% or even go lower.

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[gmsquid]

Agree, w/VMAT.. it’s like trying to eat low carb Indian - mostly a naan issue.

Good pun…but chawal is carby too

 

[RadOncDoc21]

Agree, w/VMAT.. it’s like trying to eat low carb Indian - mostly a naan issue.

We love our Indian food here!

 

[Ray D. Ayshun]

This is a fair point. usually I constrain my plexus max to way below the 74 at 4% if tumor is touching it. The reference to me just tells me that doses of 70 Gy may be reasonable if necessary. Though 70 Gy isn’t always necessary and there is probably increasing probability of plexopsthy with increasing volume over the 66-70 Gy. Simply constraining the global hot spot may not optimally spare the amount getting 66-70 Gy. And you may be able to constrain the V70 to under 4% or even go lower.

I'd be interested to know how frequently parts of the sim plexus have zero overlap with a single days treatment plexus.

 

[gmsquid]

I'd be interested to know how frequently parts of the sim plexus have zero overlap with a single days treatment plexus.

It’s likely irrelevant. Unless it’s systematic in the same way. Whatever constraints are they feather over time and the outcome is the same. Do we really believe 2 Gy everyday is necessary or is the toxicity likely a some of 2.2s and 1.8/ reaching 66? All the constraints reflect treated patients who never set up perfectly the same way

 

[Ray D. Ayshun]

It’s likely irrelevant. Unless it’s systematic in the same way. Whatever constraints are they feather over time and the outcome is the same. Do we really believe 2 Gy everyday is necessary or is the toxicity likely a some of 2.2s and 1.8/ reaching 66? All the constraints reflect treated patients who never set up perfectly the same way

Sure, but I wonder if it's a matter of shifting hotspots around with arcs also means that daily setup differences also mean that the hotspot data from the plan is also irrelevant. I feel like you'd have to make a plexus prv.

 

[TheWallnerus]

Do we really believe 2 Gy everyday is necessary or is the toxicity likely a some of 2.2s and 1.8/ reaching 66?

I am not 100% sure I am picking up what you're throwing down here, but if I am...

Mathematically, the acute/late effects of 66 Gy/33 fx are equal to 16 fx of 2.2 Gy fx, 16 fx of 1.8 Gy, and a fx of 2.0 Gy.

Here is a spreadsheet to prove it. So if the daily dose truly varies randomly by ±10% in OARs or tumors or what have you, do not worry... the clinical outcomes will be the same. This "law" begins to break down the farther plus/minus percent you go from the daily dose, but as shown for plus/minus 10% the "law" holds.

 

[gmsquid]

I am not 100% sure I am picking up what you're throwing down here, but if I am...

Mathematically, the acute/late effects of 66 Gy/33 fx are equal to 16 fx of 2.2 Gy fx, 16 fx of 1.8 Gy, and a fx of 2.0 Gy.

Here is a spreadsheet to prove it. So if the daily dose truly varies randomly by ±10% in OARs or tumors or what have you, do not worry... the clinical outcomes will be the same. This "law" begins to break down the farther plus/minus percent you go from the daily dose, but as shown for plus/minus 10% the "law" holds.

Yes, thats essentially my point. Daily setup probably matters in as much as it keeps dose within 10%, which in a HN plan should be feasible. So even though the heat moves around day to day due to changes in setup even of a small structure, using a single point max such as 66 Gy is meaningful because it likely represents the average equivalent dose to that point during the entire course. Hence we don’t need a prv from this empiric data that only looks at the max. The prv is extra conservative, and may result in under dosing.
This is different in sbrt spine per se where heterogeneity is far more severe.

 

[gmsquid]

This is why systematic error is far more critical than random error. Random error should balance out, but if you’re systematically wrong in the same place you will under or over dose. This is why I’m keen on looking at coverage fall off near oars such as the parotid. If the dose falloff is too severe, you will systematically miss the ptv (or worse gtv) and thus underdose leaving a potential for failure

 

[gmsquid]

Also why a wishlist or summary of dvh numbers does not tell the whole story. If you have 95% coverage but miss in the same 5% next to the parotid every day, your plan sucks

 

[Ray D. Ayshun]

Now, I want to make some plans with the plexus with and without constraints.

 

[TheWallnerus]

This is why systematic error is far more critical than random error. Random error should balance out, but if you’re systematically wrong in the same place you will under or over dose. This is why I’m keen on looking at coverage fall off near oars such as the parotid. If the dose falloff is too severe, you will systematically miss the ptv (or worse gtv) and thus underdose leaving a potential for failure

Also why a wishlist or summary of dvh numbers does not tell the whole story. If you have 95% coverage but miss in the same 5% next to the parotid every day, your plan sucks

Who wrote this on Wikipedia. Who knows?

 

[SubserviantToABR]

Also why a wishlist or summary of dvh numbers does not tell the whole story. If you have 95% coverage but miss in the same 5% next to the parotid every day, your plan sucks

Don't miss: Hazards of sparing the ipsilateral parotid gland in the node-positive neck with intensity modulated radiation therapy: Spatial analysis of regional recurrence risk

 

[medgator]

Don't miss: Hazards of sparing the ipsilateral parotid gland in the node-positive neck with intensity modulated radiation therapy: Spatial analysis of regional recurrence risk

Dosimetrist definitely seems worried when i nuke a parotid sometimes....

 

[deleted1111261]

Dosimetrist definitely seems worried when i nuke a parotid sometimes....

Totally
Gotta remind them the point of what we are doing

 

[elementaryschooleconomics]

Dosimetrist definitely seems worried when i nuke a parotid sometimes....

As long as you're only nuking one parotid - that's why God gave us a spare!

 

[medgator]

As long as you're only nuking one parotid - that's why God gave us a spare!

Oh I've nuked both before.... Bad npc

 

[TheWallnerus]

As long as you're only nuking one parotid - that's why God gave us a spare!

Oh I've nuked both before.... Bad npc

Does anybody here remember when Peter Johnstone used to be all about acupuncture for xerostomia. Some people swear by it. PJ did a parlor trick where he'd do the needling and then have the patient lean forward and saliva would start dripping from their mouth.

 

[evilbooyaa]

A reiteration of the key point.

Right now I am planning a prostate. Met many people in life who insist on contouring the hips. When they do, don't see them using hips as a constraint though (although probably used to in old 5-field IMRT days). Why contour the hips? "You must." You be over there doing your thing, I'll be over here doing mine. We'll both be taking great care of patients.

I always give a constraint to the femoral heads when creating prostate plans. Is that what you mean when you say contour hips?

What you have put together seems to be well within constraints as well. The end product is what matters.

The algorithm literally never will (dump a ton of MUs in the hips) in a multi-arc VMAT.

I have seen VMAT plans in our weekly chart rounds from inexperienced dosimetrists who were pushing hard on bladder and rectum without worrying about femoral heads that required re-planning.

The initial algorithm will not if you're just looking for a conformal circle without any concavities to carve out organs, but if you are pushing hard on rectum and bladder the dose will go laterally and may go over femoral head constraints. Depends on patient anatomy as well.

 

[Ray D. Ayshun]

Bumped into this looking for something else:

https://ro-journal.biomedcentral.com/track/pdf/10.1186/1748-717X-8-173.pdf

In any case, it's OPC, which when considering the deescalation trials we're performing, does make me ponder contouring and constraining the plexus in HPV+ cancers.