IMRT and image guided technologies.

[Pewl]

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IGRT and tomotherapy are pretty new. Though I'm not totally won over by tomotherapy. I rememeber the exhibit that Tomotherapy Inc had at last year's AAPM convention in Seattle. It has amazing accurate dose delivery because it's pretty much a megavoltage CT scanner. However, treatment times are so long and in today's world of churning patients out as quickly as possible, this modality might not be too popular. Also, if you're just trying to irradiate a small area, you might be better of with a regular linac. (The cyber knife is pretty sexy too, but for some reason my professors here at ucla are all against it. They think it's too flashy).

 

[stephew]

All IMRT takes a long time. Tomo is one of the best image guided therapies out there. Cyber knife is very good at what it does but things like novalis can do more or less the same thing. "too flashy" isn't much of good complaint about cyberknife. The sag concerns it initially raised are better ones but I believe this is fixed.

 

[Mulletfluf]

Treatment time is a large concern for any center. With IGRT / IMRT becoming more common place, I think most would agree that the overall treatment time has increased. However, I think we've started to address this issue. Simple solutions would be MLC and dynamic wedges for traditional cases. For IMRT cases you could always remove the flattening filter. While that would mean you would have a dedicated IMRT machine and you would have to collect beam data again, it would reduce the treatment time significantly. I have also seen IMRT cases were the dmlc's compensate for tumor motion so the beam can be constantly on rather than on only at one stage of the gated cycle. I think we're getting there, but lets face it we're doing more than we ever were before.

 

[G'ville Nole]

Treatment time on Tomo is typically a function of both the volume irradiated and the dose per fraction. Hence, if a community practice wanted to bring in a Tomo unit to do conventional prostate treatments (2 Gy per fraction, relatively small volume), the treatment time wouldn't be significantly longer than an IMRT approach. Add to that the fact that they can bill for "Image guidance" weekly, and any potential "throughput" losses could be pretty easily offset.

As far as popularity goes, I think the majority of the interest in Tomotherapy (for now) arises from academic centers. Tomotherapy's MVCT capability makes it extremely well suited for dose- and dose-per-fraction escalation, since one can comfortably begin to reduce the PTV for these treatments. Dose-per-fraction escalation is not something that's highly likely to catch on in community practices anytime soon. But, in the same way that IMRT disseminated throughout private practices 5-7 years ago (if they didn't get it, they feared their neighbors would; then it's a race to put up the biggest billboard), we'll probably see the same thing happen with IGRT.

This comes from someone who is training at the birthplace of Tomotherapy, so take my commentary for what it's worth in light of my institutional bias...

 

[Houston500]

Not trying to instigate an argument, but how does tomotherapy (using the Peacock or Tomo Unit) ensure dose accuracy on a target that is moving, like a lung tumor, when you're delivering radiation using a field that's equivalent to a small narrow beam.

Another question, how does the Tomo unit's MV imaging compare to other IGRT platforms that use kV imaging?

 

[stephew]

Not trying to instigate an argument, but how does tomotherapy (using the Peacock or Tomo Unit) ensure dose accuracy on a target that is moving, like a lung tumor, when you're delivering radiation using a field that's equivalent to a small narrow beam.

Another question, how does the Tomo unit's MV imaging compare to other IGRT platforms that use kV imaging?

actually tomo has a suprising presence in the community.

As for accuracy; i suspect from your question that you aren't clear on how the beam is delivered. Its a fan beam that is slim, and the key is the pitch of the unit. That is rotation of the beam while the couch moves.

In terms of respiratory movement in general including imrt if no gating is used or techniques like breath hold, the assumption is thatthe margins are wide enough and over time the error is "smeared out" this has been an issue in rt treatment over history.

 

[G'ville Nole]

Not trying to instigate an argument, but how does tomotherapy (using the Peacock or Tomo Unit) ensure dose accuracy on a target that is moving, like a lung tumor, when you're delivering radiation using a field that's equivalent to a small narrow beam.

Another question, how does the Tomo unit's MV imaging compare to other IGRT platforms that use kV imaging?

Using lung as a specific example, we are using 4D-CT planning to create a tumor "motion envelope". This is a high speed scan that uses a radiocamera to register the breathing cycle and create ten separate "bins" into which the acquired images are placed. We then contour on several of these binned groups to signify the tumor position at various phases in the respiratory cycle. These contours are fused to create the motion envelope, which serves as the de facto GTV.

When you look at the fields generated using this technique, they end up (after CTV and PTV expansion) a little smaller than what we used to do (i.e. 2 cm C-C, 1.5 cm lat). There is definitely normal lung included in this volume, but the mean residual lung NTDs using tomotherapy are very low (we usually come in much lower than our constraint of 18.5 Gy), and incidence of pneumonitis has been virtually absent.

To answer your second question, there's no doubt that the kVCT images are prettier. MVCT does work pretty well for lung, where there is a good contrast between normal lung and tumor. It's a little fuzzier for things like prostate, but wth practice, quite doable.

 

[medgator]

actually tomo has a suprising presence in the community.

Do you forsee tomo eventually catching on the way IMRT did in most rad onc centers? Or is it too cost-prohibitive?

 

[G'ville Nole]

Do you forsee tomo eventually catching on the way IMRT did in most rad onc centers? Or is it too cost-prohibitive?

I think the cost of a unit is around $3M, so most facilities could likely swing it. However, the physics support required to generate the plans may be prohibitive at smaller centers. Catching on like IMRT is a bit deceiving, since IMRT is a technique which is applicable to any number of different treatment platforms (of which Tomo is one), but I think gradual expansion into the community is inevitable, and not really cost-prohibitive.

 

[stephew]

Do you forsee tomo eventually catching on the way IMRT did in most rad onc centers? Or is it too cost-prohibitive?

i think eventually there will be more helical tomo or comparative things. The big issue isnt the cost so much as the maintenance; tomo has to be able to service. but remember: there is no ONE best machine.

 

[Rahl22]

IMRT has been so widely implemented because, in many cases, clinics needed only outfit current machines with MLCs, etc., to make them IMRT-capable. This has an inherently smaller overhead than buying a whole new tomo unit. As is, I'm not sure the advantages outweigh replacing current Linacs with tomos, however, as new clinics open, one could spend initial startup money on tomo units. This will probably be seen more and more. So it won't be as fast, but it will still happen, especially as tomotechnology improves and education about it becomes more wide-spread.