Tuesday, 2015/12/15 – Even More Nuclear Physics

OK…. I confess to being a total technerd, and fascinated by the Linac (Medical Linear Accelerator) that irradiated me five days a week for seven weeks. I was also fascinated by the imaging tech. they used to diagnose me, and to plan where to send the death rays. I made such a pest of myself asking the therapists about the science, that they arranged for a Medical Physicist to talk with me and answer some of my weird questions.

I alaready posted about the PET in September:

I published some of the actual imagery from my scans here:

I now have some clarification about PET. Here is some of the expanded information Dr. X gave me, with a few extra details his guidance led me to dig up::

The annihilation of the electron and positron generates two gamma rays, each about 0.5MV (half a Million Volts equivalent energy) that travel in opposite directions, on a line through the locus of the collision. Some of the rays hit a ring of phosphor crystals, making them glow. That light is picked up by very sensitive photocells, and analysis of which crystals get hit gives the computer a line along which the annihilation occurred. The computer records hundreds of millions of these lines, and uses the points at which they cross to tell where the radioactive marker is. Adding information on the TOF (Time Of Flight) of the rays can improve the accuracy, but is not in common use yet. So far, we just can’t measure the delays of the rays (picoseconds) traveling at the speed of light, accurately enough to make much use of the information. You can read more on TOF:

Now more on the LINAC:

The following will make better sense if you have read what I already wrote about the Medical Linear Accelerator on 9//1:

…and on 10/9:

Now Dr. X has helped me understand it even better, and here is some of that new information and clarity:

As I lay on my back on the couch, the therapists would sometimes tell each other to move me “ten millimeters more X,” or “just nudge him down a little on the Y axis.” I realized that they were referring to the directions of movement in Cartesian co-ordinates, with the abscissa (Y axis) being vertical to me (from my head to my toe) and the Ordinate (X axis) across my body, from one shoulder to the other. Of course, I had to ask if the Z axis was from my front to my back, but the therapists didn’t seem to understand, so I asked Dr. X later on. Indeed, that is how the Z axis is defined. I drew a picture to illustrate:


Of course, with me lying on my back, the whole thing rotated like this:


The coronal plane is perpendicular to the Z axis, and the coronal slices of a scan (CT, MRI, or PET) will be parallel to that plane. Imagery is most often “viewed” from the front of the subject (coronal) from below the subject’s feet (axial) or from their Left side (Sagittal) depending on which plane one selects.

This means that as one lies in a scanner or the Linac, these are the axes:


The imaginary line down the center of my body is called the Axial Isocenter. It is the one they most oft3en refer to, so they sometimes do not specify “axial.” The Linac gantry rotates around this axis, and the beam remains centered on the Isocenter as the gantry rotates. The standard radius of most medical linacs is 100cm, from the target to the Isocenter. The target is a piece of Tungsten the electrons hit to generate the X-rays, and it’s up inside the treatment head. For tumors on the skin or only slightly below it, the target is replaced by a scattering foil (very thin aluminum or copper) and the electron beam itself is directed to the treatment area.

Clinac_isocenter_960x500Here is a sketch of the basic configuration of the Varian Clinac that delivered the radiation therapy to my tumors:VarianKlystronLinacAfter the electrons are accelerated in the waveguide, some Linacs use a magnetic slalom path to further focus the beam.  This video shows such a beam path:

Varian Linacs, like the Clinac that was used on me, instead use a single 270° Achromatic bending magnet (dipole) to bend and focus the beam, as seen in these illustrations:



In the 270° turn through the dipole, the beam is narrowed to a millimeter (1/26 inch, or 0.0393701 inch) in diameter.

The electron beam velocity (near light speed) determines the energy level of the rays produced. I got 6MV (MegaVolt) X-rays (photons), because my tumor was not deep inside my body, but not at the surface either. The Clinac can also produce a 15MV photon beam for tumors deeper inside the body. The system can also do digital X-ray imaging, at either 6MV, or 12KVP (KiloVolt Peak)

To make sure the right amount of radiation is delivered, the computer uses two ion chambers in the beam path between the dipole and the final collimator. The first one tells the computer when the dose has been administered, and the second one can sense if the first one has failed, and cause an emergency shut down.

Prior to each patient getting their first dose of radiation, the mapcheck array is used to align the system and adjust the dosage to match their specific treatment plan. The mapcheck array has 475 diodes as sensors, sending dosage information to the computer in real time.

If you’ve read this far, then you are a true TechnoGeek like me. Congratulations, for the geek shall inherit the earth.