Getting ready for radiation

This week we started down the path to understanding the radiation treatment coming up. When the tumor was removed it was not yet clear if it was cancerous. In fact, the previous needle biopsy had shown it to be benign so the surgeons really started out the surgery under the pretext that they were dealing with just a tissue growth that could be solid or cystic but not cancerous. Of course when the pathology report came back with the diagnosis, we then knew it to be cancerous. In a known cancerous resection the surgeons would have removed additional tissue around the tumor to make sure that as much as is possible all the cancerous cells were removed. In Tim’s case, the tumor was up against the skull and spinal column so there wasn’t much more tissue they could remove. After talking with the radiation oncologist, I don’t think there would have been much difference in the surgical approach even if they knew the tumor was cancerous.

So at this point the belief is that there are “microscopic” tumor pieces in the area of the surgery. These are the target for the upcoming radiation treatment. The process is quite fascinating as to how the treatment gets set up. The idea is to irradiate the tumor site and surrounding tissue/bone to create the margin that was not done during surgery and to also make sure that no other tissue receives radiation. The team creates a treatment plan that involves creating a 3 dimensional object to be irradiated. This process is called simulation and in addition to deciding on what exactly to irradiate, also produces the plan as to how to go about doing it. The radiation used in this case are x-rays and they will be administered from several (8-10) different angles, intensities, and beam sizes. In the volume to be irradiated, these different beams will become additive and outside of the volume they do not. So essentially in the volume, the full dose gets administered and outside very little. This is of course a computer assisted process, but it also takes a lot of consultation of experts to get right. It’s supposed to take 10 working days to compute.

The total dose will be in the range of 36-50 gray. They have to nail this down based on the continuation of chemo that Tim has chosen. It is likely to be at the lower end. Radiation measurements are weird I have to say. A gray is a measure of absorbed radiation and although related to an amount of energy, it is really energy absorbed per mass of tissue that is absorbing it. If your whole body took 36 gray you would be dead in minutes. But this amount of radiation will be administered over 5 weeks in daily sessions into a small mass. If the same amount of energy to be used for the localized area were administered to the whole body at once it would be a very small number of gray, about 0.15.

The other part of the simulation process is to make a thermoplastic mask so Tim’s head can be precisely positioned each time he goes in for the radiation dose. That was also done this week just prior to the most recent CT scans being done. With his head positioned on the CT table with the mask a new scan was done to assist in the simulation process. This scan will be combined with previous scans of the tumor so that the correct volume can be identified for radiation treatment. The mask making and new scan visit was pretty short, only about 45 minutes.

So now, it’s wait for the plan to be created. In about two weeks the sessions will start and last for about 5 weeks of weekday daily visits. These should take about 20 minutes in and out. Tim might be able to just drive himself over after school depending on schedules.
Next week is the first evaluation scan timeframe (3 months out). New scans will be done to see if any other cancer sites have developed. I think it will primarily be looking at the lungs to see if any cells have set up house there and started to grow. The expectation from the doctor is that nothing will be found but we continue to pray that that is the case.

--bob