What is Dosimetry?


I am a Medical Dosimetrist here at the Provision Center for Proton Therapy in Knoxville.  The reason for this blog post is to answer a question I have been asked by many friends and family. What is Dosimetry?  As I was writing this, I found it amusing that Microsoft Word underlines the word Dosimetrist in red due to the fact it doesn’t recognize the spelling.  To put it quite simply a Dosimetrist is one who measures dose.

So what are the responsibilities of a Medical Dosimetrist in the field of proton therapy?  A Dosimetrist is an individual that is part of a Dosimetry team that has many roles and duties.  Although the duties of a Dosimetrist are numerous, I am going to outline three of the major responsibilities of a Dosimetrist.  They are importation, contouring and planning.


The first of these is to import the CT scan into a treatment planning computer.  After a patient comes in on their first day and gets their treatment planning CT scan, the images are sent electronically to Physics where a Dosimetrist will import those images into a treatment planning computer and, if necessary, will fuse them with any other test they may have had, such as an MRI or a PET/CT.  Fusing is when a Dosimetrist overlays and aligns the treatment area found on a previous PET/CT or MRI with the treatment area on the treatment planning CT images.  Not all cases need to be fused and the necessity of whether to fuse or not is dictated by the Physician.


Once the images are fused, the second responsibility of a Dosimetrist is to contour in the “organs at risk” or as we refer to them: O.A.R.’s.  Organs at risk are defined as organs that are in the path of the beam and/or are in close proximity of the beam and have a low radiation dose tolerance.  An example of these would be the eye when treating the brain, the heart when treating a lung or the bladder when treating the prostate.  To contour these structures the Dosimetrist will use the treatment planning software to trace around the organs that are represented on the treatment planning CT images.  Once these organs and tissues are drawn the physician will come in and contour in the area that needs to be treated to their prescribed dose.   It is during this time that the number of beams and their angles are discussed.


After the Physician has contoured in the treatment area, sometimes referred to as the Region of Interest, and the Dosimetrist has contoured in the O.A.R’s, the Dosimetrist can start the most important part of their job, the planning.  How a patient will be treated is collectively decided by the Physician, the Dosimetrist and the Medical Physicist.  The Physician will let the Dosimetrist know how much radiation they want delivered to the treatment site and in how many fractions or days.  The Dosimetrist will then enter that information into the treatment planning system and create a plan that will deliver that dose to the cancer while sparing as much healthy tissue as possible. After the plan has been approved by the Physician and has passed Physics Quality Assurance testing it is then exported to the treatment console for the Radiation Therapists to deliver daily treatments to the patient until the prescription is fulfilled.

I hope this answers a few of your questions and spurs a few more.  Please feel free to ask me or any other staff members any questions you may have about your treatment.


Jeff Stamper is a Medical Dosimetrist at Provision Center for Proton Therapy.