WBRT dose for diffuse subcentimetric BM secondary to melanoma

[evilbooyaa]

Speaking of treatment time with GK, does anyone treat to a higher isodose line (80%) to shorten treatment time when dealing with high number of mets? Obviously this will give less heterogeneity inside your PTV and will result in less sharp dose falloff but can save quite a bit of time compared to treating everything to the 50% IDL.

Not familiar with the GK system, but my concern was that it's an all day affair for the patient to go through GKRS, rather than about a 1 hour affair for linac-based.

I can't speak to difference in isodose prescription lines.

---

Members don't see this ad.

 

[medgator]

I dont see how that is ok- wouldnt you be altering your conformality as well? When I used to treat with gamma knife (>7 years ago), I did not always prescribe to the 50%, - sometimes another isodose line would cover the target. You should prescribe to what isodose line best covers the target- this typically is around the 50%, but not always.

Yes I think gk is usually around 50% while cyberknife is usually 80%, without huge variances from that

 

[Neuronix]

Speaking of treatment time with GK, does anyone treat to a higher isodose line (80%) to shorten treatment time when dealing with high number of mets?

For small metastases I do this routinely.

I would suggest that since Linac based typically treats to 80% IDL and the control is the same, the control should be the same with GK at 80% IDL. Tumor control is primarily driven by the margin dose, not the hot spot dose.

The conformality isn't very different in these cases--almost certainly not clinically meaningful.

 

[nkmiami]

Prescribing to an isodose line that is not a best fit (i.e. standard) defeats the point and effort of the gamma knife. The machine is very expensive and you are putting the patient through the trauma of a head frame. Why not just treat the patient on a truebeam/trilogy which can have an identical dosimetry?

Hiistrorically, 70-80% isodose line is a typical prescription line for cones based on conformality of the cones' penumbra (for example, if you covered a 5mm spherical target with a 5 mm cone, 70-80% would be the first line to include your target. (A 1 cm cone could cover your target with a higher isodose line but overall worse dosimetry and would never be accepted as standard practice.) Same rational for mlcs and gamma knife. Stylistically, I just wouldnt use a larger collimator than necessary to save time.

I agree that maybe for small targets, we are talking about very small differences in v12 etc, but if you ever have a complication, maybe that extra 15 minutes on the machine would have prevented it?

 

[Neuronix]

Prescribing to an isodose line that is not a best fit (i.e. standard) defeats the point and effort of the gamma knife. The machine is very expensive and you are putting the patient through the trauma of a head frame. Why not just treat the patient on a truebeam/trilogy which can have an identical dosimetry?

IMO, the dosimetry would only be comparable for a tiny metastasis with cone based treatment on Triology/Truebeam. I do not have a cone based linac setup. I think it would be reasonable to treat with microMLC (i.e. Novalis). I do not believe that the dosimetry would be comparable with 5 mm MLC leaves.

Also, some do treat on Truebeam/Triology with head frames and some treat on GK without headframes. This is a separate question.

Hiistrorically, 70-80% isodose line is a typical prescription line for cones based on conformality of the cones' penumbra (for example, if you covered a 5mm spherical target with a 5 mm cone, 70-80% would be the first line to include your target. (A 1 cm cone could cover your target with a higher isodose line but overall worse dosimetry and would never be accepted as standard practice.) Same rational for mlcs and gamma knife. Stylistically, I just wouldnt use a larger collimator than necessary to save time.

My physicist looked at this question and found essentially no difference on the Perfexion system for V12 based on different IDL prescriptions. I think he submitted an ASTRO abstract on this. The relationship here between dose falloff and IDL prescription is not straightforward, particularly in the modern era of mixed shots.

I agree that maybe for small targets, we are talking about very small differences in v12 etc, but if you ever have a complication, maybe that extra 15 minutes on the machine would have prevented it?

What complication? RT necrosis? For a sub-cm metastasis the V10 and V12 are going to be so small that the difference is negligible.

 

[nkmiami]

I dont think many people use cones (myself included) anymore. I was just trying to illustrate the point:Conceptually you are using a larger aperture (whether it be cone/collimator/mlc) than necessary to enable a prescription to a higher isodose line. It very well may not make a difference for tiny mets- I havent used a gamma knife in many years.

 

[seper]

would love to see data on that... I still take faster dose fall off when normalizing to a lower IDL as the central dogma of teletherapy..

IMO, the dosimetry would only be comparable for a tiny metastasis with cone based treatment on Triology/Truebeam. I do not have a cone based linac setup. I think it would be reasonable to treat with microMLC (i.e. Novalis). I do not believe that the dosimetry would be comparable with 5 mm MLC leaves.

relationship here between dose falloff and IDL prescription is not straightforward

.

 

[hot sauce]

Prescribing to an isodose line that is not a best fit (i.e. standard) defeats the point and effort of the gamma knife. The machine is very expensive and you are putting the patient through the trauma of a head frame. Why not just treat the patient on a truebeam/trilogy which can have an identical dosimetry?

Hiistrorically, 70-80% isodose line is a typical prescription line for cones based on conformality of the cones' penumbra (for example, if you covered a 5mm spherical target with a 5 mm cone, 70-80% would be the first line to include your target. (A 1 cm cone could cover your target with a higher isodose line but overall worse dosimetry and would never be accepted as standard practice.) Same rational for mlcs and gamma knife. Stylistically, I just wouldnt use a larger collimator than necessary to save time.

I agree that maybe for small targets, we are talking about very small differences in v12 etc, but if you ever have a complication, maybe that extra 15 minutes on the machine would have prevented it?

The reason for not using linac based SRS is that I do not have that technology at the moment (we are looking into it but not a top priority when we have a GK). For me the consideration is often when we find more mets on the planning MRI than we expected. With double dose Gd and thin slice MRI I may find more mets after the frame has been placed. If feasible I will still treat all of them even if there are more than 10. The patient is already there and the frame has already been placed. The time savings in this situation is not going to be 15 minutes. The difference in treatment time can be hours. It could be the difference between having a treatment that treats all the mets in a time frame the patient can tolerate and not being able to treat all the visible disease.

I have not formally analyzed the difference in the low dose falloff but the difference between 4 and 8 mm collimators appears quite small (and I am not employing this strategy in situations where I would need the 16 mm collimator). I cannot speak to how the dosimetry compares to truebeam or other linac solutions.

 

[nkmiami]

For 4 or 8 mm collimator I doubt it makes a difference. of course, you always try to minimize the number of shots and changing the collimator.- this was especially true in the days before the APS. You dont simply fill up the target with a bunch of 4 mm shots. I never really thought about this as precribing to a higher isodose line, but I guess it would be

I think I misinterpreted the question.