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Master Thesis in Physics


We offer a master thesis position in the physics group at the development department at RaySearch Laboratories in Stockholm, starting as soon as possible. The topic is algorithms for computation of expected EPID-images based on plan and patient data, and comparison to the corresponding measured EPID-images. A possible second step in the thesis work is to use any difference between expected and measured EPID-image to compute an updated dose in the patient.

Tools to use are RaySearchs powerful treatment planning software RayStation, the RayStation scripting interface in Python, the code in C++ and C# where we develop RayStation, and measurement cooperation with a French clinic (Macon/Villeurbanne). The RaySearch crew in general, and the physics group at the development department in particular, will be your colleagues and your support.

Background and purpose

Before starting radiation treatment against cancer it is important to verify that the treatment actually received corresponds to what was planned, to avoid mistreatments. This is called QA, quality assurance. A linear accelerator, LINAC, can be equipped with a semiconductor plate, a so called EPID, with the sensitive surface perpendicular to the beam. When the EPID is irradiated, the signal is converted to an image, which can be compared with an expected image. When the physics model is good, and the LINAC behavior is well understood, the expected and the measured image agree. This is generally the case, and the plan can be accepted for treatment. If the images aren´t consistent, a clinic can decide to re-plan the treatment, since they cannot confidently know the dose distribution in the patient. Another possibility is that the EPID-image can be used to compute a new dose in the patient, which is the basis of a better informed decision. The concept of comparison between measurement and expectation is the same for all QA-tools, but EPID-QA has the potential to be considerably faster than other techniques. 

A common radiation treatment plan is divided into several treatment sessions, a typical number of such fractions is thirty. Between fractions, the healthy tissue is given time to recover, and more so than the tumor. However, if the time between fractions becomes too long the tumor may grow too much. If a clinic has problem with one of their LINACs, they can choose between delaying one fraction for all the patients planned for treatment on that LINAC and risk tumor growth, or they can move the patients to another LINAC. With LINAC-specific re-planning and quick QA, the move can be made while keeping the plan quality. Here EPID-QA is an important key.

During treatment, the tumor can shrink, the patient can gain or lose weight, in the prostate, bladder, rectum area it is common that organ positions and size vary. If such changes in the patient geometry leads to insufficient irradiation of the tumor, or inacceptable irradiation of sensitive organs, it is time to create a new plan for the remaining treatment. Also in this case, fast and efficient QA adds flexibility, and in the end leads to better care for the patient.


  • You are motivated and intelligent and you like physics analysis and programming.
  • This master thesis is the final part of a master education in physics, engineering physics, medical physics or a similar program, where you have excelled.
  • You are used to computers and programming, if the experience is less this is compensated with will and skill to learn.
  • You are not deterred by a large question, instead you find a way to handle the question by finding smaller, manageable parts that lead you toward the answer to the larger question.

Working at RaySearch

RaySearch, a world leader in the field of advanced software for radiation therapy, offers a workplace with an open and positive atmosphere, where learning, creativity and collaborations are highly valued. Located in the heart of Stockholm, RaySearch provides you with a unique opportunity to work in a fast growing, international company proud of the products and solutions they work towards. Read more about RaySearch.


You are welcome to send your application including a resume, marked “Master thesis in physics” by clicking on the button below. For questions contact Agnes Angerud.