Radiation Begins

Yet another new concept is in the learning mode as Tim begins the radiation treatment. The simulation was completed and a treatment plan computed this past week. Tim will get 36 Gy total dosage spread out over 20 equal treatment sessions. This is at the low end of the range previously discussed since Tim continues to receive the cyclophosphamide during this time. In fact, this week radiation started and he will also get the whammy treatment. Perhaps it will only be a double whammy of cyclophosphamide and vincristine this time because of the radiation therapy being coincident. These 20 days are done Monday through Friday with weekends off. The dose is additive and destroys cancer cells of any micro-tumor pieces left in his neck.

The equipment used is essentially a high powered x-ray machine that can deliver 0.6 Gy per minute of x-ray energy (they call it photon emission). The machine is a mini linear accelerator about one meter long that accelerates electrons and shoots them at a tungsten target which converts them to photons. The resulting beam is bent and then sent through a collimator device that is dynamically configured during treatment to focus the beam to the desired shape and location. Tim’s treatment plan involves exposing him from nine different angles and only takes about 10 minutes to actually administer.

Getting him positioned on the table in the same place is really important and so this is why a mask was made and crosshairs placed on the mask. Tim lies on a table and the mask anchors his head to the table. A laser guidance system is used to place the table in three axes and then an x-ray picture is taken. The process is to compare the baseline locating x-ray to one just taken to see if any adjustments need to be made. All these x-ray pictures are taken digitally, so they are immediately available to be viewed on the computer screen. That allows the technician to line up the baseline and the new x-ray and compute the correction factors to move the table. So far, things have been repeatable to 3mm or less. From there the nine doses are delivered.

The machine used is a Varian 21ex. There is a lot of information on their site for the technically interested at http://www.varian.com/. This link has a video you can look at that explains the methods used to administer Tim’s radiation therapy. http://www.varian.com/pinf/res005.html

--bob