Brain Met Dosing

[Reaganite]

As far as I can tell, for oral boards it seems safe to follow the 90-05 dosing scheme of 24Gy, 18Gy, and 15Gy, but you guys in the real world...are you following these doses? Some observations I've made having spent time at several US centers:

1. Majority of centers I've been to rarely treat to a marginal dose of 24Gy, even if the lesion is small (<2cm). More commonly I'm seeing 20Gy and occasionally 22Gy to the margin.

2. For lesions >3cm, people seem to throw their hands up a bit. Nobody thinks 15 Gy is enough. One center I trained at used a hypofractionated scheme for these lesions (5-6Gy x 5).

3. 90-05 dosing scheme is in the context of prior fractionated treatment (including patients who were treated up to 60 Gy). Are the single fraction radiosurgery doses from the trial really appropriate for radiosurgery in a patient who is radiation naive? For example, if a patient has a lesion >3cm, can we safely treat with a single fraction dose >15Gy if no prior radiation therapy has been given?

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[Palex80]

1. We give 20 Gy to the 80% isodose.

2. For lesions >3cm we use a fractionated schedule like the one you described. Usually it's 5x6 Gy.

3. That's a tough question. We rarely change the dose given with SRS/FSRT because of previous delivered dose, as long as the target lesion is not in a highly sensible region (for example brainstem).

 

[Cancerdancer]

Any good papers describing the 6 Gy x 5 approach/outcomes? This one suggests 6 Gy x 6, but 6 Gy x 5 seems reasonable, always nice to have some good references...

PMID: 20800386

 

[Gfunk6]

It depends what technology you are using to deliver SRS. If using Gamma Knife or cones (fixed collimators) then you are probably prescribing to the 50% isodose line. So if you prescribe 18 Gy, the mean tumor dose is probably 24 Gy.

If you are using primary/tertiary collimation then you are better off following RTOG 90-05, though 20 Gy to the 80% IDL is probably fine.

We don't recommend single fraction SRS for lesions > 3 cm. We usually do 5 G x 5 fractions but prescribe to 70-80% IDL so the inside of the tumor is hot.

 

[Palex80]

A further point, which needs to be taken into account, is what kind of fixation you use and what your resulting CTV-PTV-margin is.

One benefit of fractionated treatmeent is, that you can avoid invasive ring fixation (although there are clinics, which perform SRS without the ring). In my personal experience the ring fixation was the most traumatic experience of brain met stereotactic treatment.

If you use a 2-3 mm CTV-PTV margin, because you are not using the ring, then you should be more careful with the dose outside of the CTV. This may have implications on deciding to prescribe to the 80% or 95% isodose line.

 

[medgator]

In my personal experience the ring fixation was the most traumatic experience of brain met stereotactic treatment.

But a very useful device for SRS when treating, for example, trigeminal neuralgia when you are delivering very high doses near the brainstem

 

[Gfunk6]

Having worked with GK during residency, I was eager to bring a ring fixation system into our SRS practice. However, our neurosurgeons were adamantly against it, preferring non-invasive systems. We use a specialized thermoplastic mask from Brainlab and use a 2 mm PTV. We try to keep the V12 of normal brain under 5 cc ideally to minimize the risk of symptomatic necrosis.

For ring fixation, we never even bothered with a PTV. GTV=CTV=PTV.

 

[Reaganite]

A further point, which needs to be taken into account, is what kind of fixation you use and what your resulting CTV-PTV-margin is.

One benefit of fractionated treatmeent is, that you can avoid invasive ring fixation (although there are clinics, which perform SRS without the ring). In my personal experience the ring fixation was the most traumatic experience of brain met stereotactic treatment.

If you use a 2-3 mm CTV-PTV margin, because you are not using the ring, then you should be more careful with the dose outside of the CTV. This may have implications on deciding to prescribe to the 80% or 95% isodose line.

This is the exact issue I'm wrestling with. With the move toward these frameless systems with a 2mm ptv margin are the "standard" rs dosing schemes really appropriate? If I remember correctly, the doses from 9005 were marginal doses with no ptv expansion.

 

[TarHeelRadOnc]

As far as I can tell, for oral boards it seems safe to follow the 90-05 dosing scheme of 24Gy, 18Gy, and 15Gy, but you guys in the real world...are you following these doses? Some observations I've made having spent time at several US centers:

1. Majority of centers I've been to rarely treat to a marginal dose of 24Gy, even if the lesion is small (<2cm). More commonly I'm seeing 20Gy and occasionally 22Gy to the margin.

2. For lesions >3cm, people seem to throw their hands up a bit. Nobody thinks 15 Gy is enough. One center I trained at used a hypofractionated scheme for these lesions (5-6Gy x 5).

3. 90-05 dosing scheme is in the context of prior fractionated treatment (including patients who were treated up to 60 Gy). Are the single fraction radiosurgery doses from the trial really appropriate for radiosurgery in a patient who is radiation naive? For example, if a patient has a lesion >3cm, can we safely treat with a single fraction dose >15Gy if no prior radiation therapy has been given?

Would not exceed RTOG 90-05 doses. Obviously, toxicity data in that phase II is influenced by prior brain RT, but you wouldn't have a leg to stand on if you encountered high grade toxicity using a dose higher than the MTDs of that study.

Agree with previous posters that doses <24Gy are reasonable for lesions <2cm, provided you prescribe to a low isodoses line. Also agree with fractionated stereotactic tx for lesions >3cm. I use 6Gy x 5 prescribed to 80-85% IDL with small PTV margin (dependent on method of immobilization). Dose to periphery of GTV ends up being close to 7Gy x 5. There is a Korean study in Red J showing excellent in field LC rates with 7Gy x 5 for lesions ineligible for single fx SRS.

 

[subatomicdoc]

For ring fixation, we never even bothered with a PTV. GTV=CTV=PTV.

The concept of GTV=CTV=PTV flies in the face of all other areas of radiation oncology.

There may be a needed margin for microscopic disease --> http://1.usa.gov/UlMMd0
Adding 1mm may improve local control for SRS. --> http://1.usa.gov/NGtr7n
Postop cases may do better with 2mm. --> http://1.usa.gov/OOyaCx

Perhaps if combined with whole brain there isn't a need for a CTV, but there are still issues with image coregistration, patient movement, and different types of error for a PTV. Less so with ring than frameless, but still some.