Ideal SRS prescrioption IDL?

[evilbooyaa]

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As an aside to some stuff I was reading for Gfunk's other thread, I came across this paper:

Choosing a Prescription Isodose in Stereotactic Radiosurgery for Brain Metastases: Implications for Local Control - ScienceDirect

Basically saying, for GK SRS, prescribing to a higher IDL results in better LC due to higher doses around the gross tumor itself with a higher IDL. Maybe there's value in putting a margin on the tumor?

Random retrospective, I know, but figured it was worth discussion

 

[Palex80]

I use a 2mm margin for linac-SRS, what do you do? 1mm for CTV + 1mm for PTV.

 

[nkmiami]

For cavities, the stanford experience from what I understand was that less conformal plans had less failure, which led them to adopt 2mm margin on all cavities even for single fraction, and this is done in national protocols.

 

[evilbooyaa]

I use a 2mm margin for linac-SRS, what do you do? 1mm for CTV + 1mm for PTV.

1mm PTV for intact, 2mm for cavity. This is linac-based.

I meant more for those trained or gung-ho about GK and the (to me) insane 50% IDL prescriptions I routinely see from evaluating outside plans. I generally like ~80% IDL prescription (or 125% of prescription dose max) for linac based. All my experience is linac-based.

The failiure rates in the referenced paper were 12 and 11% on the strata below prescription IDLs of 80% or less, while they were 4% for 81%+. Wonder if any GKSRS folks are routinely prescribing to IDLs that high?

 

[medgator]

For cavities, the stanford experience from what I understand was that less conformal plans had less failure, which led them to adopt 2mm margin on all cavities even for single fraction, and this is done in national protocols.

Intuitively makes sense to me, probably why frst/partial brain conformal is the best way to go for post op treatment imo I just wish there was better data for it, aka give 40/15 to cavity plus 2-3 cm for some of these larger resected mets presenting with significant edema.

 

[nkmiami]

I havent done gamma knife in a while but prescribing to a higher isodose line would be like covering the same lesion with a bigger cone. It can be used for saving treatment time trading off conformality. I imagine it would be dependent on the shape and number of lesions.

 

[scarbrtj]

As an aside to some stuff I was reading for Gfunk's other thread, I came across this paper:

Choosing a Prescription Isodose in Stereotactic Radiosurgery for Brain Metastases: Implications for Local Control - ScienceDirect

Basically saying, for GK SRS, prescribing to a higher IDL results in better LC due to higher doses around the gross tumor itself with a higher IDL. Maybe there's value in putting a margin on the tumor?

Random retrospective, I know, but figured it was worth discussion

I might be brain farting, but if you change the IDL up or down, the dose at the margin (which I consider to be the Rx isodose) does not change; i.e., if 20 Gy is the Rx and it covers the tumor at the 50% IDL in one plan and 80% in another, it's 20 Gy at the tumor edge in both plans. And, as the IDL goes up, the "overall" dose delivered to the tumor goes down, and conformality index goes down. Thus this article seems to indicate less delivered tumor dose/less confrmality yields better LC. However, you're right, with higher IDLs, the dose does spread out outside the tumor edge more. But why would increased dose outside and "around the gross tumor itself" matter for LC? Good question. If tumor targeting were a problem (by problem I mean it's not quite as accurate as they think) in GK and the conformality was "too cute," one can see that less conformality might be better and (counter-intuitively) actually deliver more tumor dose.

 

[nkmiami]

I havent done gamma knife in a while but prescribing to a higher isodose line would be like covering the same lesion with a bigger cone. It can be used for saving treatment time trading off conformality. I imagine it would be dependent on the shape and number of lesions.

I might be brain farting, but if you change the IDL up or down, the dose at the margin (which I consider to be the Rx isodose) does not change; i.e., if 20 Gy is the Rx and it covers the tumor at the 50% IDL in one plan and 80% in another, it's 20 Gy at the tumor edge in both plans. And, as the IDL goes up, the "overall" dose delivered to the tumor goes down, and conformality index goes down. Thus this article seems to indicate less delivered tumor dose/less confrmality yields better LC. However, you're right, with higher IDLs, the dose does spread out outside the tumor edge more. But why would increased dose outside and "around the gross tumor itself" matter for LC? Good question. If tumor targeting were a problem (by problem I mean it's not quite as accurate as they think) in GK and the conformality was "too cute," one can see that less conformality might be better and (counter-intuitively) actually deliver more tumor dose.

agreed,here is an example- if you had a sausage shaped 8mm lesion, you could cover it with several adjacent/ slightly overlapping 3mm shots prescribed to around 50% or a single 1cm shot prescribed to 90%. Obviously covering a sausage with a large circle is not as conformal as 3 tiny adjacent circles and fall off is less, but it saves time and maybe there is benefit from additional dosimetric margin.

 

[Neuronix]

I do no margin on Gammaknife (single fraction). For small mets I often go up on the IDL to tighten the prescription isodose. This is a common practice and I think any analysis on this point is confounded by lesion size and shape.

When I fractionate on linac (30 Gy x 5 fractions) I also use no PTV margin per this paper:

Fractionated stereotactic radiation therapy for intact brain metastases

When prescribing to 30 Gy in 5 fractions with 0 margin it takes 2 mm to get back down to 25 Gy in 5 fractions anyway.

 

[nkmiami]

I do no margin on Gammaknife (single fraction). For small mets I often go up on the IDL to tighten the prescription isodose. This is a common practice and I think any analysis on this point is confounded by lesion size and shape.

When I fractionate on linac (30 Gy x 5 fractions) I also use no PTV margin per this paper:

Fractionated stereotactic radiation therapy for intact brain metastases

When prescribing to 30 Gy in 5 fractions with 0 margin it takes 2 mm to get back down to 25 Gy in 5 fractions anyway.

that is a really good paper. Fractionation probably blurs out some of the random error...

 

[scarbrtj]

I do no margin on Gammaknife (single fraction). For small mets I often go up on the IDL to tighten the prescription isodose.

I think you mean you go down on the IDL in order to tighten (increase conformality) the Rx isodose; just want to be sure.

 

[Neuronix]

An 80% IDL 4 mm shot has a smaller prescription dose volume than a 50% IDL 4 mm shot for the same prescription dose. So you go up on the IDL to tighten the shot for very small lessons.

 

[scarbrtj]

An 80% IDL 4 mm shot has a smaller prescription dose volume than a 50% IDL 4 mm shot for the same prescription dose. So you go up on the IDL to tighten the shot for very small lessons.

I have read this several times and I have no idea what you're saying. Can you elaborate. I am speaking from the standpoint that it's a truism that IDL% and conformality index (CI) are inversely proportional (a higher IDL lowers the CI & you get less rapid falloff outside the tumor). And when you say "tighten" I am thinking CI.
(EDIT: If by "tighten" you mean less dose heterogeneity, IDL% and "tightening" are directly proportional... I find discussing radiosurgery difficult; far easier to show than say.)

 

[Neuronix]

We're saying the same thing in different words.

Conformity index = prescription isodose volume / target volume (there are a few definitions but this is the simplest)

The prescription isodose volume decreases as you increase the prescription isodose. Therefore, increasing the prescription isodose decreases the conformity index.

When I write "tighten", I'm saying visually that the isodose lines are getting closer together. Or, you could say that the prescription isodose volume is getting smaller.