In Gamma Knife multiple isocentres are placed (ahem!) to create dose heterogeneity.
Is it justified to keep a single isocentre for say spherical target? Or multiple isocentres for the same? Or any other recommendation?
Reality is merely a persistent illusion
For Gammaknife (or Perfexion) radiosurgery, unless you have a very small, absolutely spherical target, you would need more than one isocentre. most tumors have irregular and non-geometric shapes, hence, you usually end up adding multiple spheres with different sizes. There is no minimum or maximum number, but you don’t want too many as it can extend the treatment duration. Usually start with one large one that covers the whole target, and then keep adding smaller ones, depending on the helmet sizes available, over the periphery to get the conformality index to be, ideally, below 1.2. You can add in blocks to change the shape of the isodose for each of the spheres, and block out the lenses etc. Lexell Gamma plan and the perfexion planning systems, have a software wizard tool that can do the planning and is very helpful.
The only time I use a single isocentre is for Trigeminal neuralgia, thalamotomy, and the occasional tiny brain met.
Hi abhishek,
During my tenure I was fortunate enough to have a hands on experience with the first Perfexion in Asia in PGI. Its really a great machine - no need of changing collimator helmets, no blocking required. The planning is very simple and as Gaurav pointed out the number of isocentres required is a balance between plan conformity and treatment time.
In LINAC based SRS using mMLC as you would be doing for X Knife a single isocentre is usually adequate for most leisons except for the most irregular ones. Adding additional isocentres does little to improve the conformity and adds significantly to the setup and QA time. We in TBCC usually choose the 80% isodose line for prescription and most GKS plans I have seen tended to be around 60-50%. The natural consequence of this is that we have a far greater dose heterogenity in case of GKS - which may be good or bad depending on which side of the camp you are on. So far there is little emperical data to prove that the increased dose heterogenity in GKS leads to an increased morbidity or increase control. The precision of GKS however is undoubtedly greater which lends to its use in treating smaller leisons near nerves like schwannomas with greater accuracy and confidence. The ability to do SBRT, FSRT however are something that LINAC based RS can offer you.
Is there any data that says a higher conformity index (= poorer conformity) improves the local control? - In our series at TBCC at least it came out as a significant factor influencing LC in brain metastasis. There has been a publication with these findings for meningiomas[1]. Note that a higher CI means a greater dose spillage outside the tumor almost always in the setting of SRS unless you are missing the tumor (using the definition CI = Volume of Reference dose / Target Volume). Why this is so in meningiomas ? Probably as these guys treated the dural tail (Oh and this was for GKS).
There is also data from a french group for LINAC based RS where they found that a high CI leads to higher risk of complications. However we did not see this in our series. That said it is logical to assume that with a higher CI greater amounts of normal brain are treated to presecribed doses - so getting radionecrosis there is quite understandable.
So is a single isocentre good for a spherical lesion - No hard data but I would say yes.. I believe in the KISS principle and specially for LINAC based RS where the number of variables that can go wrong in a slight isocentre shift are more than GKS. So while having multiple isocentres can potentially improve the CI (by reducing dose spillage) the incremental advantage obtained when using mMLC based planning is really little unless you are dealing with a large irregular tumor. Even in moderately irregular lesions using dynamic conformal arc therapy and mMLC a single isocentre is usually sufficient for a lesion. Patients with multiple lesions obviously need multiple isocenters.
Why we should not attempt to increase the heterogenity in a target with multiple isocentres in LINAC based RS? The reason I feel is a fundamental one that I pointed out earlier and it is where do I place the higher dose. It is usually assumed that leisons that are spherical have the greatest disease burden plumb in the centre - but that may not be true necessarily.
Thanks Gaurav and Santam!
I think the major issue with using X Knife with multiple isocentres could perhaps be widening of the penumbra which does not seem to be a major issue with Radiosurgery.
Inherently, I am veering towards the idea of a heterogeneity of dose within a tightly confined volume but as Santam rightly points out, we might as well be hitting in the dark. But then, keeping the hot spots well in the target is not harming anyone; only we don
Now, is there any objective evidence that mutliple isocentres increase the conformality say in a given spherical volume without spilling off the excess dose? I can foresee a simple experiment to set up a spherical phantom with dosimetry to the be done at the edges and say to measure the distance between the prescription isodose and 50% as cut off. I could not find any objective published reference to my assertions.
One more issue. Planning multiple isocentres would probably increase the integral dose; not an issue in palliation but definitely for the benign cases. Has the use of mMlcs solved this problem? Or perhaps the use of newer TPS? In that case how do you go about it to reduce the integral doses in case multiple isocentres are required?
Reality is merely a persistent illusion
Hi Abhishek,
Good points.
I think the major issue with using X Knife with multiple isocentres could perhaps be widening of the penumbra which does not seem to be a major issue with Radiosurgery.
I dont think using multiple isocentres can increase the penumbra which is primarily a function of radiation source size, distance and the location of the tertiary collimator as well as the beam energy. I think what you are referring to would be an increase in the dose spillage due to the effect of the intersecting beams
Now, is there any objective evidence that mutliple isocentres increase the conformality say in a given spherical volume without spilling off the excess dose
A very simple experiment to do on a TPS actually. I will see if any of my physicists will agree to waste some time on this so I can get back at you with concrete results. However that said I dont think using multiple isocenters would lead to a straightforward increase in the integral dose. Taking an example of brain metastasis using LINAC prescribing 18 Gy - during planning the objective is to give a peripheral dose of 18 Gy such that 18 Gy covers the entire target. So if you are using 1 or 3 isocenters the planner will adjust the dose at each isocentre so that 18 Gy covers the target with minimal tissue overlap i.e. in other words the dose gets divided between isocenters. A same thing happens in case of gamma knife. The fixed multiple beam orientation of gamma knife will not lead to a significant increase in integral dose as well the dose gets distributed all across the brain. In case of LINAC based radiosurgery for a single leison where you will have to use say 4 arcs if using 2 isocenters instead of 3 if using 1 yes the integral dose can increase - not because you are increasing the marginal dose but because you are irradiating a greater volume of tissue using a higher number of arcs. That is one of the reason we try to minimize the number of arcs in LINAC based stereotaxy. Another important reason is QA time - both pretreatment and on treatment. It takes about 45 min to deliver 18 Gy to 2 leisons using frame based sterotaxy in our center using Novalis using 2 -3 arcs on each leison. This time is less than what would be incurrred if we were doing a framless stereotaxy as then we would tend to image them with exac trac before each arc (while in frame based treatment we tend to image them once for each isocenter). The QA time is alos about 1 hour or so. I know time should not be a consideration when it comes to treatment but in terms of patient comfort and logistics it makes a lot of sense to reduce the number of isocenters if you can achieve the same plan quality.
I agree with you Santam; although the rationale for avoiding multiple isocentres may boil down to saving time.
Importantly, I feel that any planning done by linac based radiosurgery system should probably be compared to the Gamma Knife system in terms of conformality index.
There is an interesting paper (published in the Red Journal recently) which speaks about similar issue while planning for VMAT.
http://www.scribd.com/full/47239321?access_key=key-1ifh4qt91exjg3l5l3le
Correct me in my conclusion then:
For a spherical lesion if it can be included in a single arc, it's worthwhile to include this in a single isocentre because any gain perhaps in terms of multiple isocentres would be marginal and increase set up time including hassles for QA.
If the lesions are wide apart, multiple isocentres make sense but one has to set up the machine for individual isocentre.
If the lesions are irregular multiple isocentres are required to ensure complete coverage of the tumor.
Reality is merely a persistent illusion
Hi abhishek.
There is aleady sufficient body of literature comparing conformity in LINAC RS vs GKS. Plans compared just on basis of conformality index have a serious flaw that it doesnot tell you about what dose you have prescribed and what is the degree of homogenity in the target dose. So a CI of 1.2 can occur for a dose of say 12 Gy to 10 cc volume or 24 Gy to 20 cc. The risk of complications and the outcome are markedly different in the two senarios. Besides it makes little sense to keep one of the other as a benchmark simply because the two modalities are so different in the way the dose is built up. Further LINAC based RS has much broader applicability in terms of areas of use unlike GKS. And as I have reiterated earlier there is really little to discriminate between the two modalities in terms of outcome or toxicity.
Your conclusions are valid but in here for Novalis RS we do treat all seperate leisons with seperate isocentres primarily due to limitation of the machine in terms of its inability to deliver a VMAT. However that said a single isocentre single arc VMAT will probably be inadequate for irregular shaped structures.
Another thing when doing dynamic conformal arc is that one of the planning goals is to ensure that arc doesnot hit the other target in its exit so arcs are placed in a non overlapping fashion for targets.
