Normalization vs Isodose Line Prescription

[RadOncMegatron]

Anyone else confused with this?

Using Eclipse, we don't prescribe to an isodose line, but have the normalization option. Someone told me these are the same, but I don't think so.

Take for instance a VMAT based SBRT lung plan that meets all DVH criteria. Max 125% Min 100% Normalization 102%. What isodose line does this correspond to?

I'm pretty sure the difference has to do with inverse planning and/or volume based planning that is different from the old school isocenter with resultant isodose line planning.

Thoughts?

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[seper]

I think for VMAT planning "normalization to isodose line" does not apply. Plan as you described roughly corresponds to 3D CRT normalized to 75% IDL (25% hot as the center), IMHO

 

[RadOncMegatron]

I think for VMAT planning "normalization to isodose line" does not apply. Plan as you described roughly corresponds to 3D CRT normalized to 75% IDL (25% hot as the center), IMHO

That's what I was thinking too b/c no other number would come close and I suppose that hot spot could (but often does not) represent the iso where the prescription used to be calculated back in the day. I think the rub is the "roughly" part, which I think is "fairly" rough (sorry for another vague term) because of the difference between VMAT and static beam.

 

[seper]

My physicists used to tell me, "in VMAT planning we don't think in terms of normalization, we think of absolute doses"

That's what I was thinking too b/c no other number would come close and I suppose that hot spot could (but often does not) represent the iso where the prescription used to be calculated back in the day. I think the rub is the "roughly" part, which I think is "fairly" rough (sorry for another vague term) because of the difference between VMAT and static beam.

 

[Haybrant]

Normalization has to do with a reference point; sometimes that is of max dose sometimes that is of mean dose. Mean dose normalization doesnt really tell you all that much i think but this is how some people report their prescriptions. Its not like prescribing to a particular isodose line which is what you might do to heat up or cool down a plan.

TBH im a bit confused about this, would be great to hear what other people know about this. thanks for asking about it

 

[scarbrtj]

There are two things in Eclipse: normalization and prescribed isodose. The former changes the isodose % isolines, the latter does not. Both change the MUs and delivered grays and absolute gray hotspot etc.

At the outset, whenever confused, show dose and DVHs and isodoses in absolute grays. Then there can be no confusion.

One might perform a RapidArc optimization to deliver 50 Gy/5 fx to a lung nodule. Then one will calculate the plan, converting "idealized fluences" into a more actual fluence pattern. The computer might tell you that 95% of the PTV is getting 45 Gy or more. At this point, you would see the ~90% line covering the PTV. So, Dr. Old School will type 90% in the prescribed isodose box in Eclipse. He will see no isodose change and the % hotspot will be the same. Dr. New School will go into the normalization dialog box and ask Eclipse to give 100% of the dose to 95% of the PTV. He will then see the %isodose lines change and the % hotspot go up, likely from ~110% to ~122% (just a total generalization here). In both plans, one will see an absolute hot spot of ~60 Gy if one looks at dose in grays* instead of %. Dr. Old School and Dr. New School are doing the same thing... generally neither one sort of understands that the other is doing that though. It's Eclipse's fault, also I somewhat blame dosimetrists and physicists for not showing the MDs what's happening under the hood.

One can easily "go off the rails" if you do both: normalize, and prescribe an isodose. Don't do that. It's too confusing, possibility for mistakes higher.

In practice, I almost exclusively use normalization and expect >100% hotspots in all cases, IMRT or no. In cranial radiosurgery, I normalize the plan such that the absolute dose gray max is 100%. Then, I start prescribing different isodoses to cover the target (so that my "radiosurgery lingo" matches others'). This is the exception to my rule of not normalizing and doing a Rx isodose % line at the same time.

* never capitalize gray

 

[seper]

why not?

 

[scarbrtj]

Anyone else confused with this?

Using Eclipse, we don't prescribe to an isodose line, but have the normalization option. Someone told me these are the same, but I don't think so.

Take for instance a VMAT based SBRT lung plan that meets all DVH criteria. Max 125% Min 100% Normalization 102%. What isodose line does this correspond to?

I'm pretty sure the difference has to do with inverse planning and/or volume based planning that is different from the old school isocenter with resultant isodose line planning.

Thoughts?

If you normalize to 102%, the initially calculated plan showed a larger 100% isovolume, although 100% still equals the Rx. After normalizing, the 100% shrunk (ie the plan is delivering about 2% less MUs overall) but 100% still equals Rx. If you went the prescribed isodose route, the Rx dose becomes whatever % isoline you type in for the % prescription. So, again, my point being that normalization changes % isolines, and prescribing a % does not, but both change the MUs. It is true that typing 102% for normalization does EXACTLY the same thing as typing 102% for Rx % in terms of MUs per beam, gray dose etc. *IF* you type 102% for normalization *and* 102% for % dose, you will deliver 1/(1.02^2)= 96.12% less absolute dose.

 

[scarbrtj]

why not?

because gray is a derived SI unit like a newton, which we never capitalize

 

[scarbrtj]

I think for VMAT planning "normalization to isodose line" does not apply. Plan as you described roughly corresponds to 3D CRT normalized to 75% IDL (25% hot as the center), IMHO

125% hotspot = 80% RxIDL, as 1/0.8=1.25. 133% hotspot = 75% IDL, as 1/0.75=1.333.

 

[MedPhys2MD]

In regards to 3D plans - in Eclipse, plan normalization value can actually mean different things based on what calculation algorithm you're using ie if using PBC vs AAA. for example, PBC automatically normalizes each field individually while AAA's plan normalization value is a function of all of the input fields (this is how it defaults before the planner changes their normalization method)

In reference to IMRT/VMAT it's a little different than 3D in that after you finish a round of optimizing/doing full dose calculation, the 'plan normalization value' returns to 100% automatically no matter it was before. If you change it to 102% and then reoptimize, the plan normalization will return to 100%. This is because when you optimize, your not normalizing to a point, you're modulating based on volumetric values.

With electrons, if you have a single field, having a plan normalization of 100% will give you the prescription dose at Dmax. If you'd like to prescribe to the 90% isodose line you can do so by changing either the prescribed percentage or the plan normalization value to 90 and you'll get the same result.

But short answer, adjusting your plan normalization value and prescribe percentage can produce the same exact result. However, depending on how your initial plan normalization is, you might have to do a little math if you'd rather keep your prescribed percentage at 100%

 

[scarbrtj]

With electrons, if you have a single field, having a plan normalization of 100% will give you the prescription dose at Dmax. If you'd like to prescribe to the 90% isodose line you can do so by changing either the prescribed percentage or the plan normalization value to 90 and you'll get the same result.

Visually, you will not get the same result. After you calc a beam en face on a flat surface, and let's say you're using a 9MeV and you type 1 Gy / 1 fx as the prescribed dose, you will get a beam of 100 MUs (let's say for example). You will see the 100% isodose at ~1.5 cm; the plan parameters in the 4x4 GUI window will be showing a global dose maximum of 100%. You could type 90% as the prescribed dose in the prescription dialog box and... visually, the isolines will not shift at all. You will not see a 110% isoline. The beam will now show 111 MUs, however. Now, you could reset Rx% to 100% and you could type 90% as the normalization. The lines shift! You now see a 110% isovolume. All the isovolumes less than that expand out slightly. BUT... the beam shows 111 MUs, just as before. Clinically, this is the same, although two people doing it these different ways will report different % isovolume distributions... absolute isovolume distributions will be equal, you can confirm by displaying everything in absolute doses.

CAUTION: If physicist normalizes to 90% and shows you the plan, and you the MD say "prescribe to 90%," you will now be giving 123 MUs i.e. about 20% more dose per day than perhaps you intended to give.

 

[MedPhys2MD]

You are correct that the isodose lines do not change when in relative mode. If you change it to absolute mode, you will see the dose values change.

But this happening is a function of how Eclispe defines the prescription value when prescribing to an isodose line. It's not intuitive when the values are relative.

One thing I do when showing plans to attendings in Eclipse is "bold" the prescribed percentage line... This makes it easier visually to evaluate a plan

Visually, you will not get the same result. After you calc a beam en face on a flat surface, and let's say you're using a 9MeV and you type 1 Gy / 1 fx as the prescribed dose, you will get a beam of 100 MUs (let's say for example). You will see the 100% isodose at ~1.5 cm; the plan parameters in the 4x4 GUI window will be showing a global dose maximum of 100%. You could type 90% as the prescribed dose in the prescription dialog box and... visually, the isolines will not shift at all. You will not see a 110% isoline. The beam will now show 111 MUs, however. Now, you could reset Rx% to 100% and you could type 90% as the normalization. The lines shift! You now see a 110% isovolume. All the isovolumes less than that expand out slightly. BUT... the beam shows 111 MUs, just as before. Clinically, this is the same, although two people doing it these different ways will report different % isovolume distributions... absolute isovolume distributions will be equal, you can confirm by displaying everything in absolute doses.

CAUTION: If physicist normalizes to 90% and shows you the plan, and you the MD say "prescribe to 90%," you will now be giving 123 MUs i.e. about 20% more dose per day than perhaps you intended to give.

 

[MedPhys2MD]

Just wanted to add a comment that there a practical reasons why some planners change the prescription percentage instead of the plan normalization value - some of those reasons are historic, some are based on the 2nd MU verification process at the center, and some are for the purpose of controlling the dose distribution.

 

[scarbrtj]

As a final thought, which should have been my initial thought...

When you normalize in Eclipse, the 100% line, if it exists*, ALWAYS equals the Rx dose in grays (normalizing to %'s less than 100% creates newer, progressively hotter % isovolumes above the inital Dmax % and expands the 100% isovolume if it exists; mathematically, all the reverse will be true, too).

When you prescribe to a % in Eclipse, the 100% NEVER equals the Rx dose in grays and new % isovolumes are NEVER created, NEVER change.

In Eclipse, both of these statements are the same thing


* you can type 10 Gy as the prescription dose in Eclipse before doing any IMRT planning. Then in the IMRT optimization process, you could "prescribe" or optimize only 5 Gy to the PTV and limit the plan max to 5.01 Gy. Eclipse does what you tell it. When you calc initially after optimization, you will not have a 100% isovolume anywhere, maybe 50-55% tops. Or... you could be perverse and optimize 9 to 11 Gy to 99.9% of the PTV and have one cubic mm of the PTV interior getting 20 Gy. This sort of possibility can really skew normalization, % isodose line, etc., discussions if "your prescription" were for 10 Gy.

 

[scarbrtj]

This topic came up again recently; reviewed a few basic concepts with some med students and I recalled this pedantic thread lol. ICRU 50 and 62 told us that at a minimum we should report min and max absolute PTV doses. Also it highly recommended dose heterogeneity in the PTV between +7% and -5% of Rx dose. The reports likewise defined the term "hot spot": an entity OUTSIDE the PTV. AAPM TG-101 report on SBRT modified the view a little: "Dose prescriptions in SBRT are often specified at low isodoses, e.g., 80% isodose... this practice increases dose heterogeneity within the target." Back to the original question:

Take for instance a VMAT based SBRT lung plan that meets all DVH criteria. Max 125% Min 100% Normalization 102%. What isodose line does this correspond to?

This would correspond to the (100% minimum divided by 125% maximum) 80% isodose line. The 102% normalization is a moot point to some extent in terms of dose reporting; it changed the absolute gray min/max coverage, and ultimately the gray prescription, on the PTV, but it did not alter the prescribed percent isodose (which by definition in radiosurgery is the minimum isodose covering the target). Had the plan been normalized at 100%, the PTV max would have been 127.5% and the minimum 102%; the prescribed isodose still would be 102/127.5 = 80%. Note that the prescribed isodose is not 100% in your example (nor is it 102%). It is always in the context of the plan maximum being the 100% line--nothing can be hotter than 100% (I say so!), so you have to conceptualize that 125% maximum as being 100% and then mathematically work it out from there. Somewhat similar (and confusing) concepts arise in electron prescribing... a topic for another day.

 

[Palex80]

One major problem in my humble opinion when reporting minimal and maximal doses is the volume of the PTV that is receiving these doses.
With modern planning, we may be adressing only minimal volumes of a pair of voxels, which are clearly clinically irrelevant.

 

[evilbooyaa]

Prescribing to the 80% isodose line is equivalent to having a 20% hotspot (which, despite scarbtj's link, I've been taught as being essentially equal to 'dose heterogeneity' within the PTV).

Am I mistaken in my above line? Had a long discussion with one of our physicists a while ago about GK-SRS vs Linac-based SRS regarding recommended isodose prescription lines and what that really meant for the patient.

 

[hot sauce]

Prescribing to the 80% isodose line is equivalent to having a 20% hotspot (which, despite scarbtj's link, I've been taught as being essentially equal to 'dose heterogeneity' within the PTV).

Am I mistaken in my above line? Had a long discussion with one of our physicists a while ago about GK-SRS vs Linac-based SRS regarding recommended isodose prescription lines and what that really meant for the patient.

It would be 25% hotspot (1/0.8 = 1.25)

 

[evilbooyaa]

It would be 25% hotspot (1/0.8 = 1.25)

Oh, really? I was extrapolating from electrons prescribing to the 95% isodose line, where that just meant that the 100% region would receive 105%, and the 95% isodose line would get 100% of dose.

However, doing 1/0.95 = 1.05xx so that makes sense. In GK, people prescribe 18Gy x 1 to the 50% isodose line. Does that mean patients are getting 36Gy point doses in single fraction to the brain? All the GK-SRS plans I've seen (imported from other facilities) show a max point dose < 27Gy. Does that mean they weren't prescribing to the 50% isodose line?

I was under the impression that people who plan based on max hotspots were OK with a 120% hotspot (for Linac-based SRS). Given that the recommendation seems to be to prescribe to the 80% isodose line for Linac-SRS, people are OK with delivering 125% of prescription dose?

 

[radiaterMike]

Yes 2 fold hot spots are possible with gamma knife. There is variability from plan to plan and smaller and/or more spherical targets can have less heterogeneity.


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[Palex80]

Prescribing to the 80% isodose line is equivalent to having a 20% hotspot (which, despite scarbtj's link, I've been taught as being essentially equal to 'dose heterogeneity' within the PTV).

Am I mistaken in my above line? Had a long discussion with one of our physicists a while ago about GK-SRS vs Linac-based SRS regarding recommended isodose prescription lines and what that really meant for the patient.

Well the 20% hotspot, when prescribing to the 80% isodose tends to get rather big.
I usually ask my dosimetrists to cover the GTV with the 95% isodose, when prescribing the 80% isodose to the PTV for example. But that's just me...

 

[Neuronix]

In GK, people prescribe 18Gy x 1 to the 50% isodose line. Does that mean patients are getting 36Gy point doses in single fraction to the brain?

That's exactly what this means. It may not be EXACTLY 36 Gy (it may be slightly more or slightly less) and it may be to a tiny point. I'm looking at a print out right now from a GK I did recently where I prescribed 21 Gy to 50% IDL to multiple small brain mets. The max points were: 42.0 Gy, 42.4 Gy, 42.1 Gy, 42.7 Gy.

All the GK-SRS plans I've seen (imported from other facilities) show a max point dose < 27Gy. Does that mean they weren't prescribing to the 50% isodose line?

It is possible.

I was under the impression that people who plan based on max hotspots were OK with a 120% hotspot (for Linac-based SRS). Given that the recommendation seems to be to prescribe to the 80% isodose line for Linac-SRS, people are OK with delivering 125% of prescription dose?

I would be fine with that in most cases.

 

[evilbooyaa]

That's exactly what this means. It may not be EXACTLY 36 Gy (it may be slightly more or slightly less) and it may be to a tiny point. I'm looking at a print out right now from a GK I did recently where I prescribed 21 Gy to 50% IDL to multiple small brain mets. The max points were: 42.0 Gy, 42.4 Gy, 42.1 Gy, 42.7 Gy.



It is possible.



I would be fine with that in most cases.

Thanks for the info. We do a fair amount of SRS but seem to fractionate the vast majority of the time given how easy it is to with Linac-based SRS, and none of the attendings seems to accept more than a 115% max hotspot.

 

[BobbyHeenan]

Thanks for the info. We do a fair amount of SRS but seem to fractionate the vast majority of the time given how easy it is to with Linac-based SRS, and none of the attendings seems to accept more than a 115% max hotspot.

Just FYI - hot spots haven't correlated much with risk of necrosis for a huge sum of single fraction brain met literature. For both mets and AVM's, moderate isodose volumes (the 10-14 Gy volumes) seem to correlate best with necrosis risk. In addition, some authors suggest gradient index (50% isodose volume divided by 100% volume) < 3 or <4 is most ideal, and that's hard to achieve unless you're getting hot spots to promote more rapid dose fall off. So sometimes (not all the times, but it can happen) by wanting the middle of tumor "less hot," you're actually increasing that moderate dose spill which may increase necrosis risk.

We had a very active CNS radiosurgery program where I trained and did a TON bit of both gamma knife and linac SRS (both single and 3-5 fraction) and routinely accepted hot spots of 130-150% of Rx for all kinds of cases - brain mets, meningiomas, etc.

 

[evilbooyaa]

Thanks Heenan. I'm aware of the peak vs plateau effect. We still respect published guidelines for normal brain dose in brain mets (V12 < 5cc for single fx, V18 < 30cc for 3 fraction), but we fractionate everything above 1cm (and quite a few things below 1cm, although that's from older attendings mostly) so it's not routinely an issue. I know that the volume of the tumor is more important than the single dimension length for picking fractionation, but in general they correlate pretty well. Conformality Index is generally not an issue either with our planners. I'll have to look deeper into gradient index for our plans - I'm not sure if we're looking at that on every single SRS plan. We definitely look at that with our single fraction, but not routinely for 3 or 5 fraction.

Are there any published constraints for 5 fx SRT?

Stuff like this is what I really like this forum for. Hope there's more topics like this. Thanks for the info!

 

[Neuronix]

Personally, I don't believe in fractionation for tumors less than 3 cm. I think 3 cm - 4 cm is controversial. We have a wealth of data on single fraction treatments and very limited data on fractionated therapy. The lack of safety and efficacy data in multi-institutional settings especially compared to SRS doesn't mean that it's safer or more efficacious.

I know there are some people who strongly disagree with me. I'm sure someone will retort my post very strongly. I think fractionated brain radiotherapy is based on limited single institution data compared to the vast history of SRS. I also think there are a lot of people out there who don't want to invest in a real SRS setup with cones and/or microMLCs, SRS level technology and physics support, and head frames or other real time image guidance, but still claim to do SRS/FSRT and bill for it. I've run into these people in person and had them try to bite my head off, I think because I'm threatening a big revenue stream for them.

I've reviewed the 5 fraction literature and what I think is the largest 5 fraction series was done where I trained when I was training there and I have some concerns with the data. There are almost no 5 fraction constraints. One paper from Germany suggested V20Gy (in 5 fractions) correlated with necrosis. But you can treat 20 Gy to the whole brain in 5 fractions! This makes no sense to me. I avoid it if I can. I think this area is ripe for a multi-institutional prospective trial, but good luck getting it funded.

 

[medgator]

Personally, I don't believe in fractionation for tumors less than 3 cm. I think 3 cm - 4 cm is controversial. We have a wealth of data on single fraction treatments and very limited data on fractionated therapy. The lack of safety and efficacy data in multi-institutional settings especially compared to SRS doesn't mean that it's safer or more efficacious.

Completely agree. I send my smaller mets out for GK. I trained on GK and it really is an elegant, simple and effective solution for what it does, and for what I believe fractionated linac-based SRS doesn't

 

[evilbooyaa]

Personally, I don't believe in fractionation for tumors less than 3 cm. I think 3 cm - 4 cm is controversial. We have a wealth of data on single fraction treatments and very limited data on fractionated therapy. The lack of safety and efficacy data in multi-institutional settings especially compared to SRS doesn't mean that it's safer or more efficacious.

I know there are some people who strongly disagree with me. I'm sure someone will retort my post very strongly. I think fractionated brain radiotherapy is based on limited single institution data compared to the vast history of SRS. I also think there are a lot of people out there who don't want to invest in a real SRS setup with cones and/or microMLCs, SRS level technology and physics support, and head frames or other real time image guidance, but still claim to do SRS/FSRT and bill for it. I've run into these people in person and had them try to bite my head off, I think because I'm threatening a big revenue stream for them.

I've reviewed the 5 fraction literature and what I think is the largest 5 fraction series was done where I trained when I was training there and I have some concerns with the data. There are almost no 5 fraction constraints. One paper from Germany suggested V20Gy (in 5 fractions) correlated with necrosis. But you can treat 20 Gy to the whole brain in 5 fractions! This makes no sense to me. I avoid it if I can. I think this area is ripe for a multi-institutional prospective trial, but good luck getting it funded.

That is certainly a very strong opinion.

Single-Fraction Versus Multifraction (3 × 9 Gy) Stereotactic Radiosurgery for Large (>2 cm) Brain Metastases: A Comparative Analysis of Local Contr... - PubMed - NCBI

Higher rates of radionecrosis in 2cm tumors with single fraction regimen compared to 3 fraction regimen. My attendings are certainly on the opposite end of the spectrum for you, but I wouldn't just ignore this paper entirely. Unless the argument is going to be "GK or bust for SRS".

I'd be interested in that Germany paper, if you have it.

 

[Neuronix]

Single institution data. Single fraction SRS is underperforming in that paper IMO.

But this is what I mean--this topic is ripe for a prospective multi-institutional clinical trial. Until that happens, why interfere with something that works well and is widely available.

I'd be interested in that Germany paper, if you have it.

Phase II trial of hypofractionated stereotactic radiotherapy for brain metastases: results and toxicity. - PubMed - NCBI

 

[medgator]

That is certainly a very strong opinion.

Single-Fraction Versus Multifraction (3 × 9 Gy) Stereotactic Radiosurgery for Large (>2 cm) Brain Metastases: A Comparative Analysis of Local Contr... - PubMed - NCBI

Higher rates of radionecrosis in 2cm tumors with single fraction regimen compared to 3 fraction regimen, with equivalent control. My attendings are certainly on the opposite end of the spectrum for you, but I wouldn't just ignore this paper entirely. Unless the argument is going to be "GK or bust for SRS".

I'd be interested in that Germany paper, if you have it.

The sf srs control rate in that paper was awful. GK is more like 90-95% in the real world at a year.

Gamma Knife Radiosurgery for 5 to 10 Brain Metastases: A Good Option for Upfront Treatment | Cancer Network

 

[Haybrant]

I think this discussion got quite convoluted. Can someone give us a summary of the original question in as simple terms as possible. To me many posters were confusion 'prescribing to an IDL' (like in SBRT) with 'normalization' which Ive understood are two different things.

 

[evilbooyaa]

The sf srs control rate in that paper was awful. GK is more like 90-95% in the real world at a year.

Gamma Knife Radiosurgery for 5 to 10 Brain Metastases: A Good Option for Upfront Treatment | Cancer Network

But is it 90-95% in tumors that are 2cm+ in size? 85-90% in the real world is what I expect for any SRS in all comers. I don't see any portion of your link that refutes that or confirms 90-95% control at a year. Can you point out which study you're using to support your argument? All of the numbers range from 65% at 1 year (or lower), as high as 80%.

Regardless, my main discussion point (and the main reason I believe in hypofractionation in this setting) is about the decreased risk of radiation necrosis in large lesions when you do hypofrac vs single fraction.

But this is what I mean--this topic is ripe for a prospective multi-institutional clinical trial. Until that happens, why interfere with something that works well and is widely available.

Completely agree to the first sentence. But in regards to the bolded, that's the thing, I don't think single fraction works all that well for large tumors. Everyone's cut-off is different. Your cut-off is 3cm - Mine would certainly be anything 2cm or bigger = hypofrac. Honestly, it'd be probably be things > 1cm = hypofrac. Unless where I'm practicing has a GK, because nobody wants to put patients through an 8 hour day multiple times in a week, despite that one paper that does multi-fraction GK-SRS.

 

[scarbrtj]

Prescribing to the 80% isodose line is equivalent to having a 20% hotspot (which, despite scarbtj's link, I've been taught as being essentially equal to 'dose heterogeneity' within the PTV).

Am I mistaken in my above line? Had a long discussion with one of our physicists a while ago about GK-SRS vs Linac-based SRS regarding recommended isodose prescription lines and what that really meant for the patient.

Yeah I'm a stickler. Hotspot is defined per ICRU as a dose entity outside the PTV; you indeed are talking about PTV dose heterogeneity only.

 

[medgator]

But is it 90-95% in tumors that are 2cm+ in size? 85-90% in the real world is what I expect for any SRS in all comers. I don't see any portion of your link that refutes that or confirms 90-95% control at a year. Can you point out which study you're using to support your argument? All of the numbers range from 65% at 1 year (or lower), as high as 80%.

Gamma knife radiosurgery for metastatic brain tumors from lung cancer: a comparison between small cell and non-small cell carcinoma. - PubMed - NCBI

Stereotactic radiosurgery in the treatment of brain metastases: the current evidence. - PubMed - NCBI

65% at 1 year is low IMO for local control from GK-SRS for a brain met.

Regardless, my main discussion point (and the main reason I believe in hypofractionation in this setting) is about the decreased risk of radiation necrosis in large lesions when you do hypofrac vs single fraction.

Not really clear, and it's potentially less efficacious, I agree with Neuronix that more studies need to be performed.

Unless where I'm practicing has a GK, because nobody wants to put patients through an 8 hour day multiple times in a week, despite that one paper that does multi-fraction GK-SRS.

With a GK perfexion, you can treat a lot of mets quickly (assuming you don't have old cobalt!)

 

[nkmiami]

regarding local control for gk, the paper that is often cited- Vogelbaum/Cleveland Clinic experience one year local control 85% for tumors up to 1-2 cm. For bigger tumors the local control really drops off to around 50% at one year. Is this size (more tumors cells) vs the lower dose? (I thought John Suh used to show a slide that the 85% local control, was really for tumors under 1cm...)

Recent series in hypo fractionation suggest maybe you can achieve higher local control in bigger tumors with fractionation.

For most tumors (VMAT on trilogy), I try to get the fall off 5 mm away to be 50%- (obviously this depends on the size/number of tumor, but often involves 40-50% hot spots). For tumors over 1cm or multiple tumors, 50% within 5 mm signifies to me that this is usually the best the planner/system can go - and taking after scarbrtj- I am personally pushing the optimizer on these cases. (edit- this is in cases of normilization to 95% coverage.)

 

[evilbooyaa]

Not really clear, and it's potentially less efficacious, I agree with Neuronix that more studies need to be performed.

Is there any data is there to at all suggest that local control is WORSE with hypofractionated, rather than single-fraction? Do you believe that RN rates are truly identical between single fraction and hypofractionated radiosurgery? Up to what size (or volume) of tumor?

I'm all for more studies, and I certainly don't want to make this go the way of the hypofrac prostate discussion that ravaged this forum for a few weeks.

I suppose we'll agree to disagree. I imagine more studies will come out over time (nkmiami, if you have any additional series' pubs, please post links) showing benefit of multiple fraction in terms of both tumor control and RN rates in anything over 2cm. Stuff under 1cm will likely be safe for single fx. I think 1-2cm will be the controversial area.

*EDIT* - Final question, this one is simply for my edification - When treating multiple brain mets, with single fraction, does the global V12 limitation < 5cc? Or is it V12 < 5cc (per treated lesion)? Similar question for V18 < 30cc from the Minitti paper (if anybody else out there does hypofrac).

 

[nkmiami]

regarding V12- it has never really been well defined, but it definitely per lesion if there are multiple lesions. some people calculate v12 as (V12- Gtv) For a single fraction of 20 Gy, in my experience the V12 is often about 3 x the volume of the tumor.

 

[Palex80]

We calculate V12 always as (treated volume-GTV). In case of multiple lesions we consider each V12 for every lesion separately, as long as they are not directly adjacent and thus creating a V12 in between with "2xV6". If the latter is the case we treat those two close lesions on different days, leaving a couple of days / weekend in between.
Our constraints are V12<10cc and V10<12cc. In my experience this means that most "round" GTVs of about 2-2.5 cm diameter can be treated with 1x20Gy to the 80% isodose covering the PTV, while using a 2mm GTV-PTV-margin. Lesions >2.5cm often produce V10/12s beyond these constraints. We fractionated then.

 

[scarbrtj]

Oh, really? I was extrapolating from electrons prescribing to the 95% isodose line, where that just meant that the 100% region would receive 105%, and the 95% isodose line would get 100% of dose.

However, doing 1/0.95 = 1.05xx so that makes sense. In GK, people prescribe 18Gy x 1 to the 50% isodose line. Does that mean patients are getting 36Gy point doses in single fraction to the brain? All the GK-SRS plans I've seen (imported from other facilities) show a max point dose < 27Gy. Does that mean they weren't prescribing to the 50% isodose line?

I was under the impression that people who plan based on max hotspots were OK with a 120% hotspot (for Linac-based SRS). Given that the recommendation seems to be to prescribe to the 80% isodose line for Linac-SRS, people are OK with delivering 125% of prescription dose?

Again, good examples of why we should all use a common terminology with SRS (or electrons). If you have a 10x10 9MeV electron field and want 2 Gy at Dmax, you'll give ~200 MUs to do so. However if you prescribe to the 80% line, you'll give 2/0.8= 2.5 Gy at Dmax (and 250 MUs) for a 125% "hot spot" (2.5/2). However, I would never call the Dmax the "hot spot." It's just Dmax.

Likewise, in SRS, *THERE IS NEVER ANYTHING HOTTER THAN 100%*. This is a basic concept, and it's plumb surprising how this gets jumbled. Now your Eclipse may say 125% "hot spot" or whatever when doing linac SRS but that is only one way to display things. In Eclipse, you can choose to have the maximum dose in the plan *always* display at 100%. Then you would look graphically at your target coverage, choose an isodose that best covers the target, and then *prescribe* (not normalize per se in Eclipse parlance) to that isodose. As a final step, one would go from percent isodoses to absolute Gy isodoses to see that everything is kosher. SBRT people and SBRT papers etc. have not been so particular about reporting things this way. But one can not submit a cranial SRS paper to a journal where there are >100% "hot spots," or whatever you'd like to call an absolute Gy dose hotter than the Rx dose within the chosen target, without the reviewer rejecting the paper.

If you prescribe to the 80% and there's a 125% hot spot... that's confusing/ambiguous. I have no idea what that could mean--is the 80% eighty percent of 125% or 80% of 100%? Is the max 100% or 125%? If the max is 125% and you prescribe to 80%, does the 125% become 125/80 = 156%? Who knows. I do know that, yes, if the Rx is 18 Gy in SRS and the Rx% is 50%, the dose max will be 36 Gy. And I also know that if the dose max is 27 Gy and the Rx was 18 Gy, in SRS, the RX% was 67%. And yes, people are very comfy with prescribing to the 80% in linac SRS, or lower as the opportunity arises. Intensity-modulating the field, however, can provide for more target homogeneity than is available with open-field or cone based approaches. (These are broad generalizations, YMMV.) Linac SRS users who are prescribing at isodoses much higher than 80% are probably not achieving as high as a conformity index as possible, which is one of the driving desires of the SRS planner.

 

[nkmiami]

http://connect.physicsworld.com/bio...benefit-surgeons-and-patients/2003747.article

really good stereo lecture with free SAM credit from UAB and Varian. We basically implemented this.

 

[Brim]

http://connect.physicsworld.com/bio...benefit-surgeons-and-patients/2003747.article

really good stereo lecture with free SAM credit from UAB and Varian. We basically implemented this.

Thanks for posting that. It's a nice educational resource, but just to let others know, the ability to earn CME credit is expired. However, the narrator/host has a great accent which makes up for that. (and it's fun to hear a British person say "Y'all.")

 

[evilbooyaa]

Intensity-modulating the field, however, can provide for more target homogeneity than is available with open-field or cone based approaches. (These are broad generalizations, YMMV.) Linac SRS users who are prescribing at isodoses much higher than 80% are probably not achieving as high as a conformity index as possible, which is one of the driving desires of the SRS planner.

Yes, all of the (Linac-based) SRS at my institution is done with intensity modulation of the beam. Are people doing Linac SRS without IMRT technology?

 

[Neuronix]

Cones! Not just for ice cream

 

[medgator]

Cones! Not just for ice cream

Honestly aren't they safer/better for functional stuff compared to micro mlc?

 

[Gfunk6]

Honestly aren't they safer/better for functional stuff compared to micro mlc?

Yes. The original SRS disaster in NYT was due to MLC error.


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[OliveTree]

Yes. The original SRS disaster in NYT was due to MLC error.


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The Dec. 2010 NYT article (among other 2010 articles) has one victim turning into a vegetable because the cone was smaller than the field size, with radiation spilling around the cone to much of the brain. The Jaws should have been closed further; the light field was blocked by the mount so it wasn't immediately obvious in a clinical check.

 

[seper]

As far as understand the deal with LINAC-based SRS, cones have sharper penumbra.

 

[radiaterMike]

As far as understand the deal with LINAC-based SRS, cones have sharper penumbra.

Yes. Also no leakage between leaves with cones ( which is pretty small effect with MLC). With cones there is less clearance between the LINAC and patient so some orientations may be limited.


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[scarbrtj]

Yes, all of the (Linac-based) SRS at my institution is done with intensity modulation of the beam. Are people doing Linac SRS without IMRT technology?

Sure. I'm one of those people. A tiny met doesn't really need IMRT. Just do open fields with MLC border at target edge. There are minimal, clinically inconsequential dosimetric differences with 5mm leaves vs 3mm leaves vs 2 mm leaves vs cones in the vast majority of cases. That is to say, I can show you a plan, and you wouldn't be able to tell if it was a 5mm MLC or cone based in most cases.

* technically (early) Cyberknife is linac SRS without IMRT e.g.