Dow R. Wilson,
President & CEO,
Varian Medical Systems (left),
and Dr. Arkadi Stolpner,
founder, DTC.

There is more to proton shielding than meets the eye

October 12, 2015
by Philip F. Jacobus, CEO
Dr. Arkadi Stolpner founded the International Institute of Biological Systems Diagnostic and Treatment Center, named after Dr. Sergey Berezin, in 2003, with one used Siemens MRI.

Today, the CEO operates a fleet of 89 MRIs in 62 cities in Russia and eastern Europe. After noticing that a high percentage of patients were diagnosed with cancer, expanding into oncology was a natural evolution for the company.
First it was an Elekta Gamma Knife treatment unit, then an Accuray CyberKnife. Next, came a Varian TrueBeam linear accelerator and currently, Dr. Stolpner is about six months away from completing construction on a two gantry proton beam center in suburban St. Petersburg.
In fact, Dr. Stolpner and I are business partners and I am so very familiar with the challenges that he has faced in providing radiation therapy treatment options in Russia. He seemed like the perfect person to talk to about shielding.
Philip F. Jacobus: When you decided that there was a need for radiotherapy, what did you know about shielding?

Arkadi Stolpner: My very first purchase was an Elekta Gamma Knife. Actually, I did not think too much about the therapy room construction shielding because I was more worried about the regulations surrounding cobalt. Our government is very strict about the use of cobalt and other radioactive materials and has a lot of regulations surrounding it. We had to follow a strict protocol when we installed our first Gamma Knife.
This was my first exposure to the shielded therapeutic room shielding. In many ways, I am self-taught, and by visiting with other physicians and experts in the field I learned that the treatment room had to be protected with about 20 to 25 centimeters of ordinary, simple concrete. I went on to learn that in general, the more powerful the treatment option (higher energy of the radiation used), the thicker the concrete has to be.
PJ: What are the thickness requirements for the different delivery systems?

AS: Of course, every country is different in its regulations for radiation protection. Russia is different from Europe which is different from North America, as I know, but here in Russia, the Gamma Knife requires the least amount of shielding.
For instance, for our CyberKnife our shielding was two meters of concrete in certain directions whereas for our Varian TrueBeam, it was two and a half meters of concrete and thickness. As for proton therapy, sometimes the walls are 15 feet thick! In other places, the walls for proton therapy can be only 6.5 feet thick but proton walls are definitely thicker. And of course the construction of the treatment rooms and auxiliary environment, the infrastructure, is planned by a special team of engineers and physicists. There are a lot of rules which have to be taken into account when dealing with radiation protection. When the high-energy treatment modalities are used, radioactive or ionized particles in the air arise, so special requirements for ventilation should be considered.
Routing of the cables is important, their exposure to the radiation (primary, at least) should be minimized. In certain cases a decontamination layer is necessary. It is the part of the inner wall which can be relatively easily dismantled and utilized as radioactive waste. So care should be taken about the materials used in this decontamination layer, because the materials should be minimally activated by radiation. As for the protons and high energy cyclotrons, special mathematical modeling algorithms are used (Monte Carlo method) which allow us to create a statistically- proven picture of the dose distribution during operation of this equipment. Further, if we have a dose distribution, we can calculate the necessary shielding geometry and most effective materials. 




The radiation oncology center
being constructed in Russia by DTC;
the facility features concrete
shielding walls for both
proton therapy suites
and linac suites.

PJ: Is the thickness of the wall the only difference between the proton site and a linear accelerator or Gamma Knife site?

AS: No, absolutely not. In the case of a linear accelerator, only the room with the linear accelerator needs to be shielded. But in the case of a proton center, the room with the cyclotron must be shielded as well. Also, when the beam travels out of the cyclotron and travels down the beam line it has to turn at least twice. Once to get behind the proton gantries and again to get from the beam line into the proton gantries.
In these areas where the turning takes place there is a considerable amount of neutron scatter, and in these areas the concrete walls must be very thick. As I said, about 15 feet. For effective deceleration and further shielding from neutrons, additional materials like plastic or boronated polyethylene can be used. Penetration capacity of the neutrons is very high, since they are uncharged particles, and such additional shielding gives very good results when installed in the direction of high neutron flux.
Along the walls, on both sides of the beam line and around the gantry rooms, concrete walls are required. Also, in the case of a linear accelerator, the walls might be 15 feet high but in the case of the proton gantry room, the concrete walls must be almost 60 feet high. So between the cyclotron, the beam line, where the beam lines turn and the three story tall gantry room, there is a lot more concrete involved in a proton system than in a linear accelerator or Gamma Knife.
As you may know, there is a procedure for pouring and curing concrete, and pouring more concrete into walls which are thicker and taller requires more time to pour the concrete and to allow it to cure. Therefore, it is more expensive in many ways. The large mass of concrete also emits a great amount of heat, so thermal cracks can occur in the wall, which degenerates the shielding effectiveness dramatically. In order to prevent this, the concrete has to be of a high grade and specially cured, as well as thoroughly vibrated after the pouring.

PJ: Is the only issue comparing a proton site to a conventional linear accelerator the thickness and quantity of concrete?

AS: Unfortunately, there is more to it than that, Phil. There are miles and miles of conduit that must be placed inside the frame
of the concrete before the concrete is poured. This conduit is used for cables and water and other components used for the proton system. It is considerably more complicated than a simple linear accelerator. Also, this conduit must turn and bend like a snake to help keep the necessary shielding effectiveness. Multiple bends increase scattering, which is good for shielding purposes. There should be no straight run for the radiation. So, proton walls take much longer, not only because they are wider and higher but because they have a complicated infrastructure inside the walls.
PJ: Are there any other factors to consider?

AS: Preparation. We have a saying here in Russia that “he who fails to prepare is preparing to fail.” I can tell you that a lot of thought and prep work has to go in to preparing the areas where the concrete will be poured, even before the concrete is poured. This was so much more time consuming for us than when we poured our linear accelerator or Gamma Knife walls.
PJ: What about the floors and ceilings?

AS: According to our regulations here in Russia, we cannot place any areas occupied by the staff below or above the linear accelerator or the proton system. Also, because of the weight of the system, it normally goes on concrete which is on top of the sub floor which rests on earth. Typically, our floors are 6.5 feet thick and so are our ceilings. In every machine we’ve installed, we installed it with the idea that we might someday have to remove it, and much of our equipment has been installed under a roof hatch. Sometimes even stationary cranes may be installed in the technical area of a treatment room in order to be able to maintain or repair the unit in the future.
PJ: What about specialized concrete blocks that are used to stack and build a shield brick by brick?

AS: Certainly, this will save time during the construction phase but I am not sure that anyone has already used them for the construction of a real proton therapy site. But yes, this technology is tested and validated for radiation modalities like protons and neutrons, not only high energy photons. It is relatively expensive and not for everybody. But on the other hand one can gain some time on the construction, which can be important. Also I can imagine that conduit construction could be easier when such modules or bricks are used.
PJ: What about construction time?

AS: We have now worked on two Gamma Knife centers and they take about a month to produce. We have installed multiple linear accelerators as well, and generally it takes about two to three months to prepare the site for a linear accelerator. However, a proton site can take up to a year and even longer. On a proton site, between the treatment rooms, the cyclotron, the beam line and the equipment room, almost 40 percent of the building is dedicated to equipment.
PJ: Do you have any other advice for someone considering a proton site?

AS: Establishing this proton center has been one of the most difficult projects of my career. I have already spent nine months visiting the site every day and I expect to be working, to one degree or another, over the next 24 months. However, I believe in proton therapy. I feel there is a place for different treatment options and there is definitely a place for proton therapy. I am very excited to be involved in this project and I am quite sure that our efforts here will help to save lives in the years to come.