Sunday, October 9, 2011

What I've been up to: Residency, first rotation

Last week marked the end of the twelve weeks I spent in my first rotation as a resident.  Because radiation therapy is not something you would necessarily have any knowledge of without having actually experienced it yourself, I get a number of questions like "what exactly do you do?"  It's been a long time since I last posted about what goes on in grad school (and things have changed quite a bit) so I figured I'd give it a whirl.  As is true with any person's specialty, it's hard to adequately explain the ins and outs with a brief description (without boring you completely) but I can attempt to elaborate on the questions I typically get from Mike. 

Radiation therapy is used predominately to treat cancer and usually in some combination with chemo and/or surgery.  The two major ways of delivering radiation are through external beam (ie. with a linear accelerator) or brachytherapy (implanting tiny radioactive seeds directly to the site of the tumor).  In our clinic, we have four linear accelerators (linac for short).

this is one of the linacs we have- radiation shoots out of the yellow circle source
The best way to think of these how these machines deliver radiation is to equate it to getting a diagnostic x-ray (ie. for a broken leg)- the difference being the energy of the beam in a typical x-ray is around 100 kilo(=thousand)volts while our machines deliver x-rays between 6 and 18 mega(=million)volts.  Our linacs treat with either high energy x-rays (referred to as photons) or electrons of various energies.  The machine can rotate 360 degrees and the table rotates around as well.  This allows us to shoot the beam of radiation through the patient at pretty much any angle.

The normal course of events is as follows:  The patient comes to the clinic and undergoes a CT scan.  The physicist will "fuse" any other scans the patient has had such as a PET scan or MRI.   This allows us to combine or overlay the two scans so we can determine the location of the tumor that might not be visible on a CT scan.

first image: PET scan, second: CT scan, third: PET-CT fusion- Bright area indicates location of disease source

The physicist and/or MD will contour (basically color in 3-D) the major organs in the treatment area.

organ contours source
The radiation oncologist will contour the tumor volumes and supply us with a prescription dose of radiation to deliver to the site.  Along with this, we have a list of radiation dose limits that the healthy tissues and organs can tolerate.

At this time, the physicist takes over and does the bulk of our work: treatment planning.  The idea is to use our treatment planning software to experiment with different radiation beam angles, energies, and techniques to design a plan that delivers the maximum amount of dose to the tumor while limiting the dose to all of the healthy tissues.

one of my treatment plans for a brain cancer case
Radiation effects from treating healthy organs can be acute (ie. radiating the bowel causing GI symptoms) or long term (possibly inducing secondary cancers) so limiting their dose is extremely important.  Using multiple beams at different angles is the most obvious way to make this happen.  By intersecting the beams at the tumor site, low radiation doses are spread out over more normal tissue and the cumulative dose in the tumor is greater.

six beams to treat the prostate at center source

Once the treatment plan meets the goals (or gets as close as possible), we present the plan to the radiation oncologist for approval.  Finally, the plans are exported to the linac consoles so that the radiation therapists can treat the patients.

plan for treating a lung cancer

Patients are generally treated once a day for several days (traditionally ranging from 10 to 44 treatments).  The idea behind this is to allow the normal tissue some time to recover between each treatment.  Normal tissue recovers more quickly from radiation and will partially heal itself between treatments while the cancerous tissues do not experience this same recovery.

The rotation I just completed was IMRT/Rapid Arc.  These are two special techniques used to allow us to better spare dose to normal tissue.  A conventional treatment can treat with any size box or rectangle.  The IMRT technique makes use of the set of 120 "leaves" inside the linac.  These leaves are 5 mm wide lead blocks that each move independently (and very quickly) across the radiation field while the beam is on to selectively block out and treat desired areas. 

this is a snapshot of the treatment- blue lines indicate the leaf position at this point in time, yellow box jagged outline indicates the starting and ending position of the leaves
Rapid Arc is similar to IMRT except that instead of treating several angles, the radiation remains on while the machine rotates and the leaves move.
two 180-degree rapid arc fields
If there's any chance you haven't given up on this blog post yet and are still reading (haha), hopefully that gave you some insight into what I do without boring you to tears!

ellie's opinion of this post


  1. Wow sound complicated. I love the picture of Ellie. :)

  2. This is a great post. I was looking for something like this and you gave me what i wanted to know about Radiation Therapy in precise. As it helpful in most of the cancer treatment a precise knowledge is necessary.

    Prostate Cancer Treatment