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Where Does Transit Dosimetry Stand Today?

October 13, 2015

Andiappa Sankar and Geoff Budgell

For the Proposition
Against the Proposition


Yes, Transit Dosimetry Has Come of Age 

By Andiappa Sankar

Over the last three decades, researchers from around the world have put in the work to understand the concept of transit dosimetry. That work has been translated into technological improvements and commercial solutions.

Has transit dosimetry come of age? I believe it has. Of course, it can be improved, but in its current form it provides access to information we've never had before—information that can enhance patient safety.

A Rich History of Research and Development

Transit dosimetry is not a brand new concept. Its development started in the 80's to early 90's when electronic portal imaging devices were developed to replace film based treatment field verification. Camera-based systems, liquid-filled ion chamber matrix assemblies, and early versions of silicon diode array systems were the early stages of usage and development.

Even then, physicists were tuned in to the potential of EPID technology, but we still had major hurdles to overcome. For example, there was the fact that proper transit dosimetry requires photon fluence to be converted to dose. Professor Rogers made significant progress in overcoming this hurdle using the EGS3 Monte Carlo algorithm.

In 1990, we saw two interesting studies. In the first, the authors predicted portal dose image calculated during the treatment planning, and compared it with the image taken during the treatment to verify geometric alignment and delivered dose.

The second study used an iterative approach to match the calculated DRR with the measured portal dose image. CT data was modified to represent the actual dose transient path and new dose calculations were performed on the new CT dataset. This is interesting because the described workflow is similar to modern cone beam CT-based dose calculation.

This work happened in 1990. The authors were not waiting for commercial entities to develop the technology, or for money to be pumped in. They laid the ground work, knowing the most important goal was improving the quality of treatment, and improving the methodology to measure treatment quality.

By 1998, we learned more about how EPIDs behaved, how they could be used for dose verification, and how accurate the results could be;


  • EPIDs used as dosimeters show a linear response and good dynamic rage with doses that can be as accurate as diodes or ion chambers.
  • EPIDs show good response to changes in field size and gantry position.
  • A point spread function successfully connects doses with grey scale values measured with CCD camera based EPID.
  • Accurate portal dose measurements from a CCD camera based EPID when compared with ion chamber measurements.
  • The liquid ion chamber matrix system shows potential for use in on-line radiotherapy dose verification.


Also during this time, Ben Mijneer and several of his colleagues at the Netherlands Cancer Institute really set themselves apart with their excellent work in this area. For example, they figured out the constraints preventing good agreement of EPID-measured dose rate for different geometries and inhomogeneities. They also discovered that it is possible to measure the midplane dose from exit dosimetry and evaluate the differences in patient anatomy.

At the turn of the century, physicists took what they learned about EPIDS, and applied it to 3-dimensional dose reconstruction. This was important work that helped Transit Dosimetry become a viable tool to study in a clinical setting.


Transit Dosimetry vs. Pre-Treatment verification

When IMRT and VMAT techniques were introduced, quite a lot of pre-treatment verification was carried out. To generate the data for pre-treatment verification, you need access to the treatment unit, and at most centers, there isn't enough time or manpower during work hours to carry this out. If you have transit dosimetry, this problem can be eliminated.

Transit dosimetry gives you a double advantage— you get extra confidence that the treatment unit is performing adequately, but you also have the added advantage of knowing the actual patient position and delivered dose.

This is supported by the findings of another study from the Netherlands Cancer Institute. They found that combining information from three fractions of EPID in vivo dosimetry was the best way to distinguish systematic errors from non-clinically relevant discrepancies, save hours of quality assurance time per patient plan and enable verification of the actual patient treatment.


Gaining Confidence in Transit dosimetry

As with any new technology, especially in medical physics, it has to be vetted and validated before we feel comfortable with it. This work was done in two studies.

The first uses multiple 2D planes within the patient volume and reconstructed a 3D dose grid. When compared with the TPS, the results were within 2% at the dose prescription point. Within 50% isodose surface of the prescribed dose, at least 97% percent were in agreement, evaluated with a 3D gamma method with a 3% 3mm criteria. However, their dose reconstruction model didn't include tissue inhomogeneities.

The second study addresses this gap. In evaluating in vivo PDI measurements behind actual patients, researchers found that on average, 87% of the pixels inside the field fulfilled the 3% local dose and 3mm DTA. That is wonderful!

These studies provide evidence for us to gain confidence in the practice of transit dosimetry. For such a complex technology to meet stringent QA criteria is further evidence that the technology has come of age.

Sure, further improvements could be made, and that will happen over a period of time. However the technology and knowledge exists today. If, as a clinician, you can improve the quality of your patients' treatments, even incrementally, it is worth taking advantage of that tool.

Transit Dosimetry in a Busy Clinic? It is Possible

We’ve been doing Transit Dosimetry at the Edinburgh Cancer Center since 2012. In 2013, we implemented a rigorous transit dosimetry program for all conformal VMAT radical treatments. This excludes plans with a treatment field larger than our EPID.

I have been collecting data on these patients who go through transit dosimetry, and so far, that totals about 3,250 patients. This is only to show that yes, we have implemented transit dosimetry on a large scale, so it is possible.

Interpolation and results from this data are still forthcoming, but I have been amazed at what we can see with transit dosimetry. Quite a lot of change is happening during treatment, but we have not been truly aware of these changes, even with some of the modern imaging technologies available today.

Today, this information comes through with transit dosimetry. And if there is an error, it forces you to look into different avenues of treatment--not just the treatment delivery machine. Therefore, transit dosimetry enhances our global understanding of treatment delivery, and that is a boon for our patients.

Where Does Transit Dosimetry Stand?

Let's think of four parameters to gauge the status of transit dosimetry. It can provide either true or wrong information, and it can be either precise or imprecise in identifying errors in all clinical situations.

From all the research that has been done, we know transit dosimetry does not give false information. Researchers have developed and tested 3D dose distributions, and verified it in heterogeneous and homogeneous phantoms, as well as various clinical sites. They’ve tried different clinical techniques like conformal, VMAT and hypofractionated SABR. Through this work, it’s been validated that transit dosimetry is a truthful technique.

The precision in transit dosimetry still needs to be improved so that it can detect all errors all the time. As it stands, in lung and breast cases, transit dosimetry may not show actual differences depending on what algorithm is used—some algorithms need a better solution for heterogeneity corrections. The ideal would be a Monte Carlo based calculation. This would shift transit dosimetry from being true and imprecise, to being true and precise.

However, in order to get to that point, the medical physics community needs to embrace transit dosimetry in its current state. Only by using this technique can we identify specific problems and think of solutions. More interest also shows vendors that transit dosimetry is a sustainable market in which they can invest research and development resources.

In the meantime, transit dosimetry provides insight that we've never had before—valuable insight that can improve the quality of treatment delivery.




No, Transit Dosimetry Has Not Come of Age

By Geoff Budgell


When I started in medical physics, back in 1994, the new technology was MLC and EPID. At my clinic back then, we were very proud because we had two EPIDs! Even then, people were already thinking about how to use EPID for doing dosimetry. But still, 21 years later, the development of transit dosimetry has lagged behind.

Therefore, I argue: no, transit dosimetry has not (yet) come of age.

Today, still very few people are using transit dosimetry, especially when compared to all the technologies we now have in routine clinical use—FFF, IMRT, and VMAT among others. In addition, manufacturers have been quite slow to get on board. So, why is that?


Why Has it Taken So Long to Develop?

For the first ten years of the transit dosimetry lifespan, we didn't really have the right technology. We were using fluoroscopic devices or liquid-filled ionization devices. It wasn't until amorphous silicon devices came along that we got a stable, undistorted dosimeter.

But during that period, transit dosimetry was outshone by seemingly more important technologies. IMRT, cone beam CT, VMAT, and SABR had much more immediately appealing benefits for the patient.

I think also that there were doubts about commercial viability of transit dosimetry. Some equipment manufacturers wondered whether it was worth putting money into developing it, because they didn't know whether they could sell it. Beyond that, they were already invested in developing technologies like VMAT and CBCT.


Transit Dosimetry has a lot of growing up to do

So, has transit dosimetry come of age? Well, what do we actually mean by come of age?

There are some dictionary definitions. "Something that's come of age has reached full, successful development," or we might say it's "reached maturity; attained prominence, respectability, recognition or maturity; or developed completely."

How do we apply those kinds of definitions to technology? I would suggest there are 4 ways we can judge the maturity of technology:

  1. Is the technology reliable and robust? If the comments from different speakers at the IPEM Transit Dosimetry meeting this year are any indication, transit dosimetry applications have several problems that still need to be ironed out. Comments on problems found with some of the currently available applications were very wide-ranging, varying from the software failing to recognized images, plans or patients to “the VMAT method is clunky” amongst many others.
  2. Is it a highly automated process that operates smoothly? More comments from the IPEM meeting indicate transit dosimetry is not yet automated enough: for example presenters told us that there is “Almost no automation!”, “Long processing time” “Time consuming, need to automate the internal steps used in the software”
  3. Has it been widely adopted? Most people today are interested in transit dosimetry but we know that it has not yet been widely adopted.
  4. Is it well-understood? People aren't quite sure what they'd do with transit dosimetry, and there are a few myths that cause confusion.


3 Myths about Transit Dosimetry

  1. Transit dosimetry replaces diodes. With transit dosimetry, you're measuring something different than you are with diodes. Transit dosimetry measures exit dose. It also measures 2D or 3D dose distributions, as opposed to 1D with diodes. Transit dosimetry is potentially very useful, whereas a diode measurement is almost useless, telling you very little about the plan that you're delivering to the patient.
  2. Transit dosimetry is going to save you time and money With transit dosimetry, you're inevitably going to see more problems because we are starting to look into 2D and 3D. Investigating those issues will take more time, and physics time costs money. Transit dosimetry might save radiographer (therapist) time vs diodes, but not necessarily if you think the purpose is to check the dose every day. Positioning the panel for each field could potentially take longer than putting diodes on the patient for each fraction. However, transit dosimetry may save time since it could help you reduce the number of pre-treatment verifications you are doing.
  3. Transit dosimetry makes life easier Radiotherapy is getting more complex. We can now see what we're treating. We are now able to treat the right volumes. We can now account for movement, and now with things like cone beam and transit dosimetry we can actually see and measure what we're treating. We need automated tools to do these complex things, and in that sense, transit dosimetry is going to be important in the future. I don't think we're there yet, but I think transit dosimetry is necessary. For one thing, we're going to eventually need it for adaptive planning on the fly. If you're taking images and re-planning live, then we're going to need tools that can quickly measure the impact of those actions.


Where Does Transit Dosimetry Stand?

When technology is first introduced, there's a lot of excitement, and you get what's called "the peak of inflated expectations." Then reality starts to set in when people find out what the problems are. They then fall into the trough of disillusionment, thinking perhaps it's not worth it after all.

But then, once people have realized what the technology is capable of and we start to sort out the problems, we move up the slope of enlightenment. Then is becomes widely used and very productive.

Transit dosimetry, I would say, is somewhere on the slope of enlightenment, we're moving towards where we can use it, but we're not there yet.

I don’t believe transit dosimetry will come of age until; first of all, we have tools that are robust, fully automated, and widely adopted; secondly, until the radiotherapy community actually knows what it's for and what it's going to be for in the future; and thirdly, when it's become a standard purchase with every linac that we buy.


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