A dosimetry team member looks at MRI of brain

What is dosimetry and why is it crucial for successful proton therapy treatment?


Many of proton therapy patients ask, “What is dosimetry and what role does it play in my proton therapy treatment?” So, we passed those questions along to the best person to answer them – an actual Medical Dosimetrist at a proton center developed by Provision. The remainder of this blog is his explanation.

Explaining the role of dosimetry

It’s not surprising that patients ask about dosimetry a lot, because I also get asked these questions by many friends and family. In fact, as I was writing this, I found it amusing that Microsoft Word underlines the word “dosimetrist” in red because it doesn’t recognize the spelling.

So, what is dosimetry? To put it quite simply, a dosimetrist is a person who measures dose.

As for the responsibilities of a Medical Dosimetrist in the field of proton therapy, there are many roles and duties.  Although the dosimetry team has numerous duties, in this blog I’ll outline three of our major responsibilities: Importation, Contouring, and Planning.


Importation is the process of importing 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 our Medical Physics department. Then 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 “OARs” as we refer to them. 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. Examples of OARs include 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 OARs, the dosimetrist can start the most important part of their job – treatment planning. Exactly how a patient will be treated is collectively decided by the physician, dosimetrist, and medical physicist. The physician tells the dosimetrist how much radiation they want delivered to the treatment site and in how many fractions or days. The dosimetrist then enters that information into the treatment planning system and creates 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 Quality Assurance testing by the physicist, it is then exported to the treatment console for the Radiation Therapists to deliver daily treatments to the patient until the prescription is fulfilled.